Super liquid-repellent surfaces –

Interactions and gas capillaries

Mimmi Eriksson

Academic Dissertation which, with due permission of the KTH Royal Institute of

Technology, is submitted for public defense for the Degree of Doctor of Philosophy

on Friday 9 October 2020, at 10:00 a.m. in Kollegiesalen, Brinellvägen 8, Stockholm.

Doctoral Thesis in Chemistry

KTH Royal Institute of Technology

Stockholm, Sweden 2020

i

Abstract

Super liquid-repellent surfaces have attracted a lot of interest in recent

years. In addition to the large scientific interest there are many potential

technological applications ranging from self-cleaning materials to

microfluidic devices. In this thesis, interactions between liquid-repellent

surfaces in liquids were studied, with the aim to investigate the detailed

mechanisms of super liquid-repellence, such as superhydrophobicity and

superamphiphobicity. An atomic force microscope (AFM) was used to

measure the interaction forces between super liquid-repellent surfaces and

a microsphere in different liquids. Additionally, a setup combining AFM

with laser scanning confocal microscopy (LSCM) was used, which enabled

simultaneous imaging in order to capture the microscopic events between

the sphere and the surface during a force measurement. The confocal

images successfully visualized how the strongly attractive forces measured

between liquid-repellent surfaces are due to the formation of a gaseous

capillary bridge between the two surfaces. Similar long-ranged forces with

capillary formation and growth were observed both in water and in lower

surface tension liquids. Additionally, the confocal images enabled

determination of the capillary shape and volume, and the data showed an

increase of the capillary volume during the major part of the process of

separating the surfaces. A gaseous layer underneath the liquid at super

liquid-repellent surfaces was also visualized with LSCM, and it was

concluded that this gaseous layer is responsible for the formation and

growth of large gas capillaries. It was found that an increased amount of

available gas in the gaseous layer influenced the interactions and allowed

the capillary to grow larger during separation. Further, theoretical

calculations based on the size and shape of the capillary suggested that a

small under pressure in the capillary drives the gas to flow from the

gaseous surface layer into the capillary, facilitating growth during

separation.

Keywords: superhydrophobicity, superamphiphobicity, wetting, capillary

forces, AFM, LSCM.

ii

Sammanfattning

Extremt vätskeavvisande ytor har väckt stort intresse de senaste åren.

Förutom det stora vetenskapliga intresset finns det många potentiella

tekniska tillämpningar, allt från självrengörande material till mikrofluidala

system. I denna avhandling studerades hur vätskeavvisande ytor

interagerar i vätskor, detta i syfte att undersöka de detaljerade

mekanismerna bakom extrem vätskeavvisning. Ett atomkraftmikroskop

(AFM) användes för att mäta interaktionskrafterna mellan vätskeavvisande

ytor och en mikrosfär i olika vätskor. En uppställning som kombinerade

AFM med laserkonfokalmikroskopi (LSCM) möjliggjorde samtidig

avbildning för att fånga de mikroskopiska händelserna mellan partikeln

och ytan under en kraftmätning. Konfokalbilderna visualiserade

framgångsrikt hur de starkt attraktiva krafterna mellan vätskeavvisande

ytor orsakas av bildandet av en gasformig kapillär mellan de två ytorna.

Liknande långväga krafter med kapillärbildning observerades både i vatten

och i vätskor med lägre ytspänning. Dessutom möjliggjorde

konfokalbilderna beräkning av kapillärens form och volym och detta

visade en ökning av kapillärvolymen under huvuddelen av

separationsprocessen. En gasformig film under vätskan vid extremt

vätskeavvisande ytor visualiserades med LSCM och slutsatsen drogs att

denna gasfilm är ansvarig för bildandet och tillväxten av stora

gaskapillärer. Det visade sig att en ökad mängd gas i denna gasfilm

påverkade interaktionerna och tillät kapillären att växa sig större under

separationen. Vidare visade teoretiska beräkningar utifrån kapillärens

storlek och form att ett litet undertryck i kapillären driver gasen att

strömma från den gasformiga ytfilmen in i kapillären och detta bidrar till

tillväxten under separationen.

Nyckelord: superhydrofobicitet, superamfifobicitet, vätning, kapillär-

krafter, AFM, LSCM.

iii

List of publications

This thesis is based on the following papers:

I Mimmi Eriksson, Mikko Tuominen, Mikael Järn, Per M. Claesson,

Viveca Wallqvist, Hans-Jürgen Butt, Doris Vollmer, Michael Kappl,

Patrick A.C. Gane, Joachim Schoelkopf, Hannu Teisala and Agne

Swerin. Direct Observation of Gas Meniscus Formation on a

Superhydrophobic Surface. ACS Nano, 2019, 13, 2246-2252.

II Mimmi Eriksson, Per M. Claesson, Mikael Järn, Mikko Tuominen,

Viveca Wallqvist, Joachim Schoelkopf, Patrick A.C. Gane and Agne

Swerin. Wetting Transition on Liquid-Repellent Surfaces Probed by

Surface Force Measurements and Confocal Imaging. Langmuir,

2019, 35, 13275-13285.

III Mimmi Eriksson, Per M. Claesson, Mikael Järn, Viveca Wallqvist,

Mikko Tuominen, Michael Kappl, Hannu Teisala, Doris Vollmer,

Joachim Schoelkopf, Patrick A.C. Gane, Jyrki M. Mäkelä and Agne

Swerin. Gas capillaries and capillary forces at superamphiphobic

surfaces: Effects of liquid surface tension. Submitted, 2020.

IV Mimmi Eriksson, Per M. Claesson, Mikael Järn, Viveca Wallqvist,

Mikko Tuominen, Michael Kappl, Hannu Teisala, Doris Vollmer,

Joachim Schoelkopf, Patrick A.C. Gane, Jyrki M. Mäkelä and Agne

Swerin. Superhydrophobic surfaces: Effects of gas layer thickness on

capillary interactions. Manuscript.

The papers are referred to in the text by their Roman numerals and the full

versions are appended at the end of the thesis.

iv

Contributions to the included publications

Paper I All experimental work and data analysis. Major part of

manuscript preparation.

Paper II All experimental work and data analysis. Major part of

manuscript preparation.

Paper III Major part of experimental work (except LFS-coating, cross-

sectional SEM and XPS) and data analysis. Major part of

manuscript preparation.

Paper IV Major part of experimental work (except LFS-coating and

cross-sectional SEM) and data analysis. Major part of

manuscript preparation.

Related work not included in this thesis

V Mimmi Eriksson and Agne Swerin. Forces at Superhydrophobic

and Superamphiphobic Surfaces. Current Opinion in Colloid &

Interface Science, 2020, 47, 46-57.

VI Haiyan Yin, Maziar Sedighi Moghaddam, Mikko Tuominen,

Mimmi Eriksson, Mikael Järn, Andra Dédinaité, Magnus Wålinder

and Agne Swerin. Superamphiphobic Plastrons on Wood and their

Effects on Liquid Repellence. Materials & Design, 2020, 195,

108974.

v

Summary of included publications

Paper I

Laser scanning confocal microscopy was combined with colloidal probe

atomic force microscopy to obtain microscopic images of gas capillaries

during force measurements between a superhydrophobic surface and a

hydrophobic microsphere in water. The confocal images provided visual

proof that the long-range attractive interactions acting on separation are

due to capillary formation and volume growth. The capillary shape and size

were extracted from confocal images allowing direct calculations of the

Young-Laplace capillary pressure. It was concluded that the pre-existing

gaseous layer at the superhydrophobic surface facilitates the formation and

growth of the capillary, and that an under pressure in the capillary drives

the gas flow from this gaseous layer into the capillary allowing the volume

to increase during separation.

Paper II

The relation between wettability and interaction forces of a nanostructured

superhydrophobic and a smooth hydrophobic surface was studied by

adding ethanol to water at different concentrations. Colloidal probe atomic

force microscopy measurements between a hydrophobic microsphere and

the superhydrophobic surface showed attractive interactions consistent

with the formation of a large and growing gas capillary in water and when

ethanol was introduced at 20 vol%. Laser scanning confocal microscopy

confirmed the presence of a gaseous layer at the superhydrophobic surface

consistent with a Cassie-Baxter type wetting state for both water and 20

vol% ethanol. For the ethanol concentration 40 vol% where no force curves

related to a growing capillary were observed, confocal images indicated

that the surface structure was wetted by the liquid with no or small amounts

vi

of trapped gas. This indicates that a gaseous layer at the surface is needed

for large gas capillaries to form and grow. Additionally, no force curves

with attractions in the micrometer range were observed between the

hydrophobic microsphere and a smooth hydrophobic surface. However, in

this case, interactions consistent with the formation of a small gas capillary

with constant volume during separation were observed in water and 20

vol% ethanol, where the macroscopic contact angles were larger than 90°.

Paper III

The setup combining colloidal probe atomic force microscopy and laser

scanning confocal microscopy utilized in Paper I, was used to study

interactions involving a superamphiphobic surface and to investigate

whether and how surface interactions and gas capillary formation were

affected by the surface tension of the liquid. Force measurements between

a hydrophobic microsphere and a superamphiphobic were performed in

three liquids with different surface tensions: water (73 mN m-1), ethylene

glycol (48 mN m-1) and hexadecane (27 mN m-1). Attractive interactions

due to bridging gas capillaries were observed in all three liquids, and the

range and magnitude of the forces as well as the capillary size decreased

with decreasing liquid surface tension. While the wetting properties were

similar on the superamphiphobic surface for all three liquids, it was found

that the wettability of the probe particle highly influenced the interactions.

When this contact angle was below 90°, a repulsion due to deformation of

the liquid-gas interface at the superamphiphobic surface was observed

prior to capillary formation. Calculations of the free energy due to capillary

formation from the shape of the capillary meniscus and comparing with

force measurements, suggested a small under pressure in the capillary

during the dynamic measurements.

vii

Paper IV

In this paper, it was investigated how the coating thickness and the

thickness of the gaseous layer on superhydrophobic coatings influence the

interactions and gas capillary size and shape. Superhydrophobic samples

with different coating thicknesses were prepared by applying an increased

number of liquid flame spray coating cycles. With laser scanning confocal

microscopy, it was confirmed that the thickness of the gaseous layer

increased with increasing coating thickness. During colloidal probe atomic

force microscopy measurements between the superhydrophobic samples

and a hydrophobic microsphere, attractive capillary forces with the

formation of bridging gas capillaries were observed for all five coatings. It

was found that the range of the attractive force and the capillary size

increased with increasing coating thickness. The results indicated that the

amount of available gas in the gaseous layer is influencing the capillary

formation and growth.

viii

ix

Acknowledgements

I want to express my sincere gratitude to all people who in any way have

helped and supported me during this thesis work.

First, I would like to thank my supervisors, Agne Swerin and Per Claesson,

for giving me the opportunity to join this project and for your valuable

support, engagement and scientific guidance. To my co-supervising-team,

Mikael Järn, Mikko Tuominen and Viveca Wallqvist, thank you all for

your encouragement and support as well as contributions to interesting

discussions during our project meetings.

To all my co-authors on the papers, without you this thesis would be a

completely different story. Thank you all for great collaborations.

Joachim Schoelkopf and Patrick Gane, thank you for always showing such

great engagement and enthusiasm in the project. Hans-Jürgen Butt, Doris

Vollmer, Michael Kappl and Hannu Teisala, thank you for inviting me to

Mainz and for sharing your knowledge. Thanks to Jyrki Mäkelä for

inviting us to Tampere and to Janne Haapanen and Paxton Juuti for

preparing excellent coatings.

I would also like to thank everyone at the Max Planck Institute for Polymer

Research for making me feel welcomed and for all help and technical

support during my stays.

To all colleagues (past and present) at RISE, thanks for all help, support

and stimulating discussions (both scientific and non-research-related) as

well as contributing to a nice working atmosphere. Thanks to everyone at

the Division of Surface and Corrosion Science at KTH for help, nice

discussions and a friendly environment.

Thanks to all my friends and family for your love and support, and Lisa,

thank you for being my greatest support during this work and in life.

x

Finally, I would like to acknowledge the Swedish Foundation for Strategic

Research and Omya International AG for funding this project, as well as

Knut and Alice Wallenbergs stiftelse for travel grants supporting my

attendance at an international conference.

xi

Abbreviations

2D Two dimensional

3D Three dimensional

ACA Advancing contact angle

AFM Atomic force microscopy

CA Contact angle

CAH Contact angle hysteresis

CB Cassie-Baxter

CVD Chemical vapor deposition

LFS Liquid flame spray

LSCM Laser scanning confocal microscopy

LV-SEM Low vacuum scanning electron microscopy

FOTES 1H,1H,2H,2H-perfluorooctyl-trietoxysilane

FOTCS 1H,1H,2H,2H-perfluorooctyl-trichlorosilane

PMI N-(2,6-diisopropylphenyl)-3,4-perylene dicarboxylic

acid mono imide

RA Roll-off angle

RCA Receding contact angle

SEM Scanning electron microscopy

TEOS Tetraethyl orthosilicate

TPCL Three-phase contact line

TTIP Titanium tetraisopropoxide

XPS X-ray photoelectron spectroscopy

xii

xiii

Contents

Introduction 1

Theoretical background 3

Wetting ................................................................................................. 3

Surface forces ..................................................................................... 12

Experimental methods 23

Super liquid-repellent coatings ........................................................... 23

Force measurements ........................................................................... 31

Imaging ............................................................................................... 37

Results and discussion 43

Superhydrophobic and superamphiphobic surfaces ............................ 43

Interactions involving superhydrophobic surfaces and observations of

gas capillaries ...................................................................................... 48

Interactions involving superamphiphobic surfaces and the effect of

liquid surface tension .......................................................................... 57

Capillary growth and the effect of the amount of available gas ......... 65

Calculations of capillary forces and comparison to measurements .... 69

Concluding remarks and future perspectives 79

References 83

xiv

1

Chapter 1

Introduction

Extremely non-wetting or liquid-repellent surfaces have been a known

phenomenon for centuries [1, 2]. Although water-repellence has been a

well-known property in nature [3, 4], the interest in liquid-repellent

surfaces was rather limited before 1997 when the origin of the water-

repellent and self-cleaning properties of the lotus (Nelumbo nucifera) leaf

was explained [5]. Researchers and scientists have always found

inspiration from nature, which through billions of years of evolution has

found its way of developing smart and creative solutions. Just like many

other technological advances have been developed by mimicking the

brilliant solutions already found in nature, the design of artificial water-

repellent surfaces was originally inspired by the many natural surfaces with

special wettability [6]. In recent years, scientists have also succeeded to

produce surfaces which can repel other liquids such as oils [7, 8]. To create

super liquid-repellent surfaces, the detailed surface topography and

chemistry are important. So far, the successful approach has been to

combine a specific microstructure with a low surface energy material [9].

Since the late 1990s, the research interest in liquid-repellent surfaces has

increased rapidly. In addition to a large scientific interest in extreme liquid-

repellence there are many potential technological applications such as self-

cleaning materials, corrosion protection and prevention of ice-formation or

bacterial growth [10, 11]. However, there are still challenges that need to

2

be addressed in order to bring super liquid-repellent surfaces into real-

world applications. First, the complex surface structures are highly

susceptible to mechanical wear, and abrasion can lead to loss of the liquid-

repellent properties [12]. A good mechanical durability is therefor of prime

importance for any practical applications [13]. Second, fluorinated

chemicals are commonly used to achieve the low surface energy [14], and

many of these substances have been shown to have majors concerns for

both the environment and human health [15]. To solve these challenges,

there is a need for more research in the area of liquid-repellence in order to

understand the underlying mechanisms. In particular, an extended

fundamental understanding of the interplay between microscopic and

macroscopic wetting properties and the interactions between surfaces and

liquids is needed. With a complete fundamental understanding, the

appropriate surface structure and chemistry can be combined in the

optimization of future super liquid-repellent surfaces. Most importantly,

with these insights, unwanted chemicals (such as perfluorinated

compounds) can be avoided, and mechanical durable materials and

coatings can be developed by safe and environmentally friendly processes.

Thus, the work in this thesis may contribute to the UN sustainable

development goals and in particular to goal number 6, clean water and

sanitation and goal number 12, responsible consumption and production.

This thesis work elucidated the detailed mechanisms of super liquid-

repellence with the focus on how such surfaces interact in liquids. The

outline of the thesis is structured as follows: The following chapter,

Chapter 2, provides the reader with a theoretical background of the

wettability of super liquid-repellent surfaces and the relevant surface forces

needed to understand interactions between such surfaces. The most

important instrumental techniques and procedures that were employed

during the work are described in Chapter 3. In Chapter 4, the key results

and findings are summarized and discussed. Finally, Chapter 5 presents the

main conclusions and implications of the presented work together with

suggestions for further studies.

3

Chapter 2

Theoretical background

Wetting

Wetting on smooth and rough surfaces

Wetting of ideal surfaces – the Young equation

The wettability of a solid surface is defined by the shape of a liquid droplet

resting on the surface. The contact angle (CA) θ, where the liquid, solid and

vapor meets in the three-phase contact line (TPCL), is most often used for

characterization of wettability. On an ideal (perfectly smooth, inert and

chemically heterogeneous) surface the thermodynamic equilibrium contact

angle can be described by Young’s equation [16]:

𝛾LV cos 𝜃Y = 𝛾SV − 𝛾SL (1)

Here, θY is the Young contact angle and SV, SL and LV are the interfacial

tensions of the solid-vapor, solid-liquid and liquid-vapor interfaces,

respectively (illustrated in Figure 1). The maximum contact angle of a liquid

drop on a smooth surface is obtained if the surface free energy of the liquid

(LV) is as high as possible and the surface free energy of the solid (SV) is as

low as possible. This can be achieved for a droplet of water (LV = 72 mN

4

m-1) on a surface of hexagonally packed -CF3 groups (SV = 6.7 mN m-1),

resulting in a contact angle in the order of 120° [17]. This value can be seen

as the chemical upper limit of contact angles for a liquid drop on a smooth

surface.

Figure 1. A liquid droplet on an ideal surface.

Real surfaces are not ideal

It is important to know that Young’s equation (Eq. 1) is generally not

applicable for real surfaces. First, the condition of thermodynamic

equilibrium is generally not fulfilled in practice. For instance, evaporation

of the droplet can take place even if the atmosphere is saturated [18].

Second, most real surfaces are not ideal and generally display both chemical

heterogeneity and surface roughness. Even if a surface may appear

macroscopically smooth, it typically exhibits micro-, nano- or even

molecular scale roughness. It is well-known that surface roughness may

enhance (or reduce) wettability, and contact angles on real surfaces can

exceed the upper limit (120°) predicted by Young’s equation [19, 20]. When

considering real surfaces, it is also important to distinguish between the

macroscopic apparent contact angle and the microscopic contact angle. The

apparent contact angle θapp, is obtained from the macroscopic shape of the

drop and is typically the angle measured experimentally by goniometry and

the sessile drop method. The measured angle typically describes an average

of the contact angles along the three-phase contact line. On the microscale,

the contact angle may deviate from the apparent contact angle, e.g. due to

surface roughness or chemical heterogeneity. The microscopic contact angle

is equal to the contact angle measured on a smooth and homogeneous flat

surface of the same material. The microscopic contact angle can vary along

5

the contact line and cannot be easily measured. On an ideal surface the

microscopic contact angle equals the Young contact angle.

Wetting of rough surfaces – the Wenzel and Cassie-Baxter models

Wetting of rough surfaces is often described by the two opposing wetting

models by Wenzel [19] and Cassie and Baxter [20]. When a liquid is

described to be in the Wenzel wetting state, the liquid penetrates the surface

depressions and fully wets the structure (Figure 2).

Figure 2. Liquid droplets in the Wenzel and Cassie-Baxter states.

The Wenzel equation relates the apparent contact angle of a droplet in the

Wenzel state (𝜃appW ) to the Young contact angle:

cos 𝜃appW = 𝑟 cos 𝜃Y (2)

The roughness factor r is defined as the ratio between the real surface area

and the projected surface area of a flat surface. According to the Wenzel

equation, surface roughness will enhance the hydrophilicity or

hydrophobicity. A hydrophilic material (θY < 90°) will be even more wetted

when surface roughness is increased. Similarly, for a hydrophobic material

(θY > 90°) the apparent contact angle will increase with increasing

roughness.

In opposite to the Wenzel state, a liquid droplet can be suspended on top of

the surface features with pockets of air (or vapor) trapped underneath

6

(Figure 2). A liquid taking this configuration is described to be in the Cassie-

Baxter (CB) wetting state. In the CB state, the droplet rests on a composite

interface (in this case consisting of patches of air and solid) and the apparent

contact angle (𝜃appCB ) relates to the Young contact angle according to the CB

equation [20]:

cos 𝜃appCB = 𝑓s cos 𝜃Y + 𝑓s − 1 (3)

Here, fs is the liquid-solid area fraction, i.e. the ratio between the area where

the liquid is in contact with the solid and the projected composite area. In

contrast to the Wenzel equation, the CB equation predicts that high apparent

contact angles (θapp >> 90°) can be achieved not only if θY > 90° but also if

θY < 90°, provided that the liquid-solid area fraction is small enough.

Validity of the Wenzel and Cassie-Baxter equations

The Wenzel and CB equations (Eqs. 2 and 3) are often used in the literature

to determine the present wetting state of textured surfaces, and good

agreement between experimentally measured contact angles and

theoretically calculated values using the Wenzel or CB equations are often

reported. However, the validity of the equations is debated and it has been

especially emphasized whether the apparent contact angle can actually be

predicted by interactions within the contact area beneath the droplet or at the

three-phase contact line [21]. The validity of the Wenzel and CB equations

was early questioned [22-24] and later experiments designed to test the

validity have disproved them [25, 26]. A debate on the topic was started

after Gao and McCarthy published their paper with the provocative title

“How Wenzel and Cassie were wrong” in 2007, where they stated that

contact angles is only determined by interactions at the TPCL and that the

interfacial area within the contact perimeter is irrelevant [26]. In an

extensive review by Erbil, both views of the TPCL and interfacial contact

area were presented [21]. Several important points from published papers

supporting the two sides were summarized, and it was concluded that most

data found in the literature are inconsistent with the Wenzel and CB theories.

7

However, while the use of the Wenzel and CB equations is questioned and

in general should be avoided, the Wenzel and CB wetting states (Figure 2)

are well established concepts and can still be valid as visual descriptions of

wetting states on textured surfaces.

Contact angle hysteresis

From the models described above it appears as wettability of a liquid on a

solid surface can be described by one (equilibrium) contact angle.

Experimentally, this sole value of the contact angle is often referred to as

the “static” or “equilibrium” contact angle of a drop “as placed”. In reality,

however, the situation is more complicated and there is rarely a single

“static” contact angle. In fact, when a liquid droplet is placed on a solid

surface the contact angle can take any value between an upper and a lower

limit, depending on how the droplet was placed on the surface. The

minimum value is given by the receding contact angle (RCA) θrec, measured

when the liquid front is receding over the solid surface. Similarly, the

maximum value is determined by the advancing contact angle (ACA) θadv

as measured when the liquid advances over the surface. The advancing

contact angle is larger than the receding one, and the difference between

ACA and RCA is called the contact angle hysteresis (CAH). Contact angle

hysteresis arises from chemical and/or topographical heterogeneities in the

surface and on an ideal surface the CAH is zero. The contact angle hysteresis

may be a rough measure on the drop adhesion to the surface, as a larger

CAH suggests that the drop adheres stronger to the surface.

Figure 3. Measuring advancing and receding contact angles.

8

The advancing and receding contact angles (and thus also the contact angle

hysteresis) may be measured by either increasing (liquid advancing) and

decreasing (liquid receding) the volume of a sessile drop or by tilting the

surface so that the drop starts moving downhill (Figure 3). By tilting the

surface, the roll-off (or sliding) angle (RA), that is the tilt angle at which the

drop starts moving, can also be measured. The lower roll-off angle the lower

liquid adhesion to the surface.

Super liquid-repellent surfaces

Definitions and terminology

Super liquid-repellence is still a relatively new research field and the

terminology is not very well-defined. In the vast and increasing number of

publications on super liquid-repellence over the last decades, many terms

have been created and used to describe different surfaces of special

wettability. Although there have been attempts to create a common and

accurate terminology [27], the use of different definitions and terms are still

found. A surface which exhibit extreme water-repellence is commonly

called superhydrophobic. A water droplet on a superhydrophobic surface

will take an almost spherical shape. In addition, the droplet will not adhere

to the surface and easily rolls off, leaving a completely dry surface behind.

The commonly used definition of a superhydrophobic surface is a high

apparent water contact angle of ≥ 150° with a low contact angle hysteresis

and roll-off angle of ≤ 5-10° [13, 28-31]. This definition is, however, not

entirely unambiguous. As mentioned, “static” contact angles depend on how

the droplet is placed on the surface and can in principle take any value

between the receding and advancing contact angles. The roll-off angle on

the other hand depends on the droplet volume. Therefore, other definitions

have been proposed and one suggestion is to use a single criteria of a high

apparent receding contact angle (≥150°) [32]. The advantage with this

definition is that the receding contact angle determines the roll-off angle and

it does not, when accurately measured, depend on droplet size [33].

9

Superamphiphobic is commonly used to describe a surface which is super

repellent to both water and oily liquids (or other polar or nonpolar lower

surface tension liquids) [34, 35]. The commonly used definition for

superamphiphobicity is generally the same as for superhydrophobicity, i.e.

a high apparent contact angle and a low roll-off angle, with the extension to

include also liquids with lower surface tension in addition to water. Another

frequently used term to describe such surfaces exhibiting both water- and

oil-repellence is superomniphobic [9, 36] and other less frequently used

terms includes superhygrophobic [37, 38] and superlyophobic [39, 40].

Surface design

When designing super liquid-repellent surfaces there are two key aspects to

take into consideration: surface chemistry and surface structure. From the

point of surface chemistry, the strategy has been to achieve a surface energy

as low as possible in order to maximize the non-wettability. As mentioned,

the lowest known surface energy is achieved by using fluorine chemistry

and perfluorinated compounds are still typically used in the literature [14,

41, 42]. However, there have been recent attempts towards fabricating

fluorine-free super liquid-repellent surfaces [43, 44]. For instance, Liu and

Kim reported that using a specific surface morphology, any material can be

made super-repellent even to the lowest known surface tension liquids

(fluorinated alkanes) regardless of the surface chemistry [44]. For future

sustainable and fluorine-free super liquid-repellence it is highly interesting

to find surfaces of special wettability in nature, as nature cannot synthesize

perfluorinated chains. Examples like the springtail (Collembola) skin [45]

proves that it is possible to obtain surfaces with oleophobic properties

without using fluorinated materials. Another interesting observation from

nature, indicating that the surface chemistry might not be the decisive

parameter, is a study suggesting that the wax on the lotus leaf is actually

moderately hydrophilic [46]. The water contact angle on a smooth carnauba

wax (assumed to be similar to the wax on the lotus leaf) surface was found

to be 74°.

10

As for surface structure, the goal is to minimize the contact between the

liquid droplet and the solid substrate and to maintain a CB wetting state.

Once the liquid penetrates the surface depressions and a transition to the

fully wetted Wenzel state occurs, the droplet is pinned, and the super liquid-

repellent property is lost. This wetting transition is typically a reversible

process [47]; thus, it is critical that the surface design can maintain a stable

CB wetting state. For water it is considerably easier to design a structure

which can maintain the CB state than for low surface tension liquids. For

instance, a simple microstructure of cylindrical pillars can be sufficient for

water. A liquid drop is placed on the structured surface, resting on top of the

pillars with air trapped underneath, i.e. CB state (cross-section in Figure 4).

Figure 4. Wetting on model structures.

The liquid-air interface between the pillars will be curved and the curvature

depends on the pressure difference across the interface, P. If the

microscopic contact angle between the liquid and the pillar θm (Figure 4), is

smaller than the advancing contact angle of the material θadv the TPCL is

pinned and the CB state is maintained; this, of course, provided the pillars

being high enough so that the curved interface does not touch the surface

between the pillars. In this case the fully wetted Wenzel state will occur even

if θm < θadv. In contrast, if θm > θadv, the TPCL will slide down the pillar walls

as the liquid wets the material until the structure is fully wetted. Following

the argument above, we see that for simple pillar structures the liquid can be

maintained in the CB state if θadv > 90°. In the case where the liquid is water,

this simple structure is sufficient if using a hydrophobic material. However,

for oils this is not sufficient due to the fact that θadv < 90° for oils on all

known materials. To repel liquids when θadv < 90° it is necessary to design

the surface structure with a re-entrant or overhang morphology (Figure 4).

11

With this type of structure, it is possible to maintain the CB state for liquids

with θadv ≈ 30° [44]. For a liquid which will completely wet the material (θadv

≈ 0°), a doubly re-entrant structure is needed in order to maintain the CB

state (Figure 4). This type of microstructure has been shown to repel all

liquids (even liquids with very low surface tension < 20 mN m-1) regardless

of the surface energy of the material [44]. Again, we note that with the right

surface design, the surface chemistry is not decisive for achieving super

liquid-repellent properties. The model structures shown in Figure 4, have

been proven to show super liquid-repellence both experimentally (e.g. [8,

44]) and using computer simulations (e.g. [48, 49]). These ordered and well-

defined structures can be fabricated by using e.g. photolithographic

techniques [8, 44, 50] or 3D printing technology [51-53].

In addition to using well-defined model structures, re-entrant morphology

can also be realized by randomly ordered structures. One drawback of using

random structures is that it is more challenging to achieve a surface design

with doubly re-entrant structures. Hence, surface chemistry is important for

achieving superamphiphobicity for random structures and fluorine

chemistry is still most often used. A major advantage, on the other hand, is

that random structures often form a hierarchical structure which is

advantageous for designing robust superamphiphobic surfaces [54]. A

hierarchical structure exhibits topography variation in two (or more) length

scales. For hierarchical superamphiphobic surfaces, typically one is in the

micrometer scale and one in sub-micrometer scale. Hierarchical structures

are commonly found in nature to achieve robust and mechanical durable

water-repellence. One example of a natural hierarchical structure is the well-

known lotus leaf [5]. Its surface is covered by micron-sized protrusions of

epidermal cells which are further covered by epicuticular wax tubules of 200

nm in diameters. Hierarchical surface designs can also enhance the

mechanical robustness. Low mechanical robustness is still the main issue

for liquid-repellent surfaces to be used in real applications [12, 13].

Microscale (and macroscale) structures are more mechanical robust and can

protect the weaker submicron structures in between, with retained

antiwetting properties [55-57].

12

Random structures are typically fabricated by bottom-up processes e.g.

deposition of nanoparticles [58-61] or nanofilaments [62, 63]. This is

typically advantageous as these processes can be applied on a variety of

substrates and materials and can be easy up scalable. It is also possible to

utilize the underlying microstructure of the substrate in order to create

overhanging re-entrant morphologies on e.g. wood [64], textile [65, 66] or

paper [67, 68]. Another approach is to combine top-down fabricated

microstructures with bottom-up randomly deposited nanostructures [69, 70].

Randomly ordered structures can also be realized using top-down processes

such as laser texturing [71, 72] or different etching techniques [73-76].

Surface forces

In this section, the most relevant surface forces for this work will be

presented: van der Waals interactions, interactions between hydrophobic

surfaces and capillary forces.

van der Waals interactions

The van der Waals force is a result of interactions of electromagnetic nature

between molecules and typically includes contributions from dipole-dipole

(Keesom orientation interactions), dipole-induced dipole (Debye inductive

interactions) and instantaneous dipoles due to fluctuations in the distribution

of electronic charge (London dispersive interactions). The van der Waals

force is always attractive between identical materials but can in some cases

be repulsive for dissimilar materials. A simple expression for the van der

Waals force (FvdW) between macroscopic bodies can be obtained by a pair-

wise summation of the interactions between the molecules in the two bodies

via integration. For interactions between a sphere (radius R) and a flat

surface at a distance D the expression is given by [77]:

𝐹vdW = −𝐴𝑅

6𝐷2 (4)

13

Here, A is called the Hamaker constant and depends on the materials and

interacting media involved. The Hamaker constant can be calculated from

the dielectric properties of the two surfaces and the intervening medium

using the Lifshitz theory. An interesting feature of the Lifshitz theory is that

it, unlike the earlier Hamaker approach, ignores the atomistic nature of the

interacting bodies and the separating medium. Instead it just considers the

fluctuating electromagnetic fields that extend from every surface and can be

related to their frequency-dependent dielectric properties. For two identical

materials “1” interacting across a medium “3”, the equation is given as [77]:

𝐴 = 3

4𝑘𝑇 (

𝜀1−𝜀3

𝜀1+𝜀3)

2+

3ℎe

16√2

(𝑛12−𝑛3

2)2

(𝑛12+𝑛3

2)3 2⁄ (5)

where i is the dielectric constant, ni the refractive index for medium i, e

the electronic absorption frequency in the UV region (typically assumed to

be the same in all media, 3 1015 s-1), T the absolute temperature, k the

Boltzmann’s constant and h the Planck’s constant. In Eqs. 4 and 5

retardation effects due to the finite speed of light have been ignored, which

does not introduce any significant error at small separations (below a few

tens of nanometers).

Interactions between hydrophobic surfaces

Smooth hydrophobic surfaces

The first measurements of interactions between hydrophobic surfaces in

aqueous solution was reported almost 40 years ago by Israelachvili and

Pashley [78]. The measured interactions showed a long-range attractive

force, much stronger than the expected van der Waals force and it decayed

exponentially with separation distance. This first report was soon followed

by many others observing similar long-range (in the tens to hundreds of

nanometers range) interactions [79-89]. The mechanisms of this long-range

“hydrophobic force” puzzled scientists for many years and the suggestions

explaining the origin of the attraction were several. Some suggested

14

explanations include water structural effects [79, 90, 91], hydrodynamic

forces [92, 93] or contamination from hydrophobic species [94, 95].

However, the formation of bridging air or vapor capillaries has become the

most widely accepted explanation [80, 96-100]. The theory of a bridging gas

capillary (also called cavity, bridge, bubble or meniscus) is supported by e.g.

direct visual observations [80, 101], effects of de-gassing the water [102-

105] and by the similarity to liquid capillary bridges between hydrophilic

surfaces in humid atmosphere [100]. The theory of capillary forces will be

further explained in the following section.

Figure 5. Schematic of the typical shape of a force-distance curve

measured between smooth hydrophobic surfaces and illustrations of

the corresponding capillary formation and break-up.

Figure 5 shows a schematic of the typical shape of a force-distance curve

measured between two smooth hydrophobic surfaces. The force-distance

curve is obtained by measuring the interaction forces when two surfaces are

brought together (approach, black line) and separated (retraction, red line).

On approach, the force is zero at large separation distances with no

interaction between the surfaces. When the separation becomes sufficiently

small, a sudden attraction (defined as a negative force) starts to appear (A).

15

This attraction is assigned to the formation of a bridging air/vapor capillary

between the two surfaces (B). After the surfaces make contact at zero

distance, the separation is again increased upon retraction and the attractive

force is decreasing due to elongation of the capillary while keeping a (close

to) constant capillary volume (C-D). At a certain separation, the capillary

ruptures and the force returns to zero (E).

Rough hydrophobic and superhydrophobic surfaces

While interactions between smooth hydrophobic surfaces have been widely

studied over the last decades, studies on interactions between

superhydrophobic or topographically structured hydrophobic surfaces are

few. Singh et al. reported the first measurements on interactions between

superhydrophobic surfaces in 2006 [106]. They observed interactions

extending into the micrometer range, i.e. much longer in range than

previously observed on smooth surfaces. Optical imaging revealed a

bridging capillary between the two surfaces giving rise to the strong

attraction, and the authors argued the capillary formation being caused by

capillary evaporation of confined water. Furthermore, the shape of the force

curve was distinctly different from what was previously seen on smooth

hydrophobic surfaces. The same kind of shape, which clearly did not follow

the assumption of a constant capillary volume, was later observed on

topographically structured hydrophobic surfaces [107]. It was suggested that

the capillary would grow due to an inflow of air from the reservoir trapped

in the rough surface during separation. The theory was supported by more

detailed studies on superhydrophobic surfaces, which also showed that

capillary growth type of force curve increased in frequency going from

interactions between hydrophobic-hydrophobic to superhydrophobic-

hydrophobic to superhydrophobic-superhydrophobic surfaces [108, 109].

Figure 6 shows a schematic of force-distance curves measured between two

topographically structured (super)hydrophobic surfaces. As in the case for

smooth hydrophobic surfaces, a sudden attraction is observed at a certain

separation distance on approach (A) and assigned to the formation of a

16

bridging gas capillary (B). However, the striking difference in the shape as

compared to the case of smooth hydrophobic surfaces is seen on retraction.

Rather than decreasing right after contact (C), the attractive force is

increasing due to a growing capillary caused by an inflow of gas from the

reservoir of trapped gas in the structures of the superhydrophobic surfaces

(D). After the attraction reaches a maximum value the force starts to

decrease before the capillary finally ruptures whereby the attraction

disappears (E).

Figure 6. Schematic of the typical shape of a force-distance curve

measured between two topographically structured

(super)hydrophobic surfaces and illustrations of the corresponding

capillary formation, growth and break-up.

Capillary forces

It is well-known that hydrophilic particles can adhere to each other due to

an attractive force caused by a liquid capillary bridge. The capillary can form

by capillary condensation or by accumulation of adsorbed liquid. However,

a capillary bridge can also form as a liquid bridge in another immiscible

liquid or, as mentioned in the previous section, as a gas/vapor bridge in a

17

non-wetting liquid. Most literature on capillary forces is focused on liquid

capillary bridges [110-116], however the theory describes the shape of the

capillary and is analog with the case of a gas capillary [117, 118]. In this

thesis, if nothing else is specifically stated, the capillary is assumed to be a

gas bridge surrounded by liquid.

Figure 7 shows the schematic of an axisymmetric capillary bridge between

a sphere (radius R) and a plane separated by a distance D. The capillary

position is described by the contact radius rs on the flat surface and the angle

on the sphere. The contact angles p and s are the contact angles of the

capillary on the spherical particle and the flat surface, respectively, and by

convention, the contact angles are on the liquid side of the interface.

Figure 7. Illustration of a bridging capillary between a spherical

particle and a flat surface.

The capillary (sometimes called meniscus or pendular ring) causes an

attractive force between the two surfaces, a capillary force. The capillary

force in the normal (vertical) direction, includes two contributions. The first

one (F) is due to the surface tension acting on the wetted perimeter:

𝐹𝛾 = 2π𝑟c 𝛾 sin 𝜃 (6)

The second contribution (FΔP) is caused by the capillary pressure P:

𝐹Δ𝑃 = π𝑟c2Δ𝑃 (7)

18

The total capillary force Fcap is calculated as:

𝐹cap = 𝐹Δ𝑃   − 𝐹𝛾 (8)

The capillary force can be evaluated on either the sphere or the flat surface,

and the magnitude should be the same under equilibrium conditions. On the

flat surface the contact radius rc is equal to rs and for the sphere the contact

radius is given by rp = R sin (Figure 7). Similarly, the contact angle is the

contact angle on the flat surface s or particle p, respectively. If the contact

radius, contact angle and capillary pressure are known, the capillary force

can be directly calculated using Eqs. 6-8. The capillary pressure can be

calculated from the shape of the capillary liquid-gas interface using the

Young-Laplace equation.

Young-Laplace capillary pressure

The Young-Laplace equation relates the curvature of a liquid interface to

the pressure change across the interface, i.e. the difference in pressure P

between the two phases. In the absence of gravitation, or when gravity is

negligible, the Young-Laplace equation is given by:

Δ𝑃 = 𝛾 (1

𝑟1+

1

𝑟2) (9)

Here, r1 and r2 are the principal radii of curvature of the interface. There are

two principal curvatures at any given point on a 2D surface. The two

principal radii of curvature are given by the radius of the curved surface in

two perpendicular normal planes at that point. For instance, for a spherical

drop (or bubble) of radius rd, the two radii are r1 = r2 = rd and the curvature

is 2/rd. In the case of a capillary bridge, the two principal radii are given by

the radius in the vertical plane (r1) and radius in the horizontal plane (r2),

illustrated in Figure 7. In this case, r1 describes the concave curvature of the

interface and is defined as negative, while r2 is positive since it describes the

convex curvature. The form of the Young-Laplace equation as given in Eq.

9, uses an important approximation, called the circular (or toroidal)

19

approximation. Using the circular approximation, it is assumed that the

shape of the interface in the vertical plane (the meridional profile) can be

described by a circle of radius r1. In many cases, the exact shape of the gas-

liquid interface is rather described by other classes of geometrical curves

e.g. nodoids or unduloids [110, 119]. However, for small capillaries, the

difference between numerical calculations of the exact shape and the

circular approximation are generally small and can be neglected [113].

When the circular approximation is not applicable, the full Young-Laplace

equation needs to be solved in order to obtain the exact shape of the gas-

liquid interface. For an axisymmetric capillary bridge and when the gravity

effect is negligible, the following form of the Young-Laplace equation is

valid [110, 119, 120]:

2�̃� =ℎ′′

(1+ℎ′2)

3 2⁄ +ℎ′

𝑟(1+ℎ′2)

1 2⁄ (10)

where �̃� is the constant mean curvature of the liquid-gas interface and

2�̃� ≡∆𝑃

𝛾 , ℎ′ ≡

𝑑ℎ

𝑑𝑟 , ℎ′′ ≡

𝑑2ℎ

𝑑𝑟2 (11)

Here, h is the height of the interface and r the distance from the central axis.

The full Eq. 10 is difficult to solve analytically, however it has been solved

in the limit of 𝑟 ≪ 𝜅, where 𝜅 = √𝛾 𝜌𝑔⁄ is the capillary length; the liquid

surface tension, the density of the liquid and g = 9.82 m s-2 the gravitational

acceleration. In this limit several approximate analytical formulas to

describe the meniscus shape have been derived [121-124]. An approximate

formula to describe the shape of a liquid meniscus around a spherical

microparticle have been proposed by Schellenberger et al. [125]:

ℎ(𝑟) = 𝑟p sin 𝛼 [ln (4𝜅

𝑟+√𝑟2−𝑟02 sin2 𝛼

) − 0.577] + 𝑏 (12)

20

Here, rp = R sin is the contact radius on the particle, α = p – the angle of

the gas-liquid interface with the horizontal and 0.577 the Euler-Mascheroni

constant. The constant b has no physical meaning and is added as the

equation otherwise diverges for large r. Eq. 12 is valid for 𝑟 ≪ 𝜅 and Bond

number Bo ≪ 1 (Bo ≡ 𝑅 𝜅⁄ ).

Constant capillary volume

An explicit expression for the capillary force of a bridging capillary of

constant volume V between a sphere and a plane (as illustrated in Figure 7)

has been derived by Butt and Kappl [113]:

𝐹cap = 4π𝛾𝑐𝑅 (1 −𝐷

√𝑉

π𝑅+𝐷2

) (13)

where

𝑐 =cos(𝜃p+𝛽)+cos 𝜃s

2 (14)

In the derivation of Eq. 13, it is assumed that the circular approximation is

applicable and that r2 >> r1 (which is valid if R >> r1). Fitting of Eq. 13 have

been shown to agree with measurements between smooth hydrophobic

surfaces (force curve as illustrated in Figure 5) [107, 126, 127]. However,

as Eq. 13 is only valid for constant capillary volume, force curves measured

between rough (super)hydrophobic surfaces (force curve as illustrated in

Figure 6) often cannot be fitted to Eq. 13 [107-109, 126].

Free energy approach

Another approach to determine capillary interactions is to calculate the free

energy change due to capillary formation [96, 128]. The total free energy

change Gcap includes contributions from the surface tension GA and the

capillary pressure GPV:

21

Δ𝐺cap = Δ𝐺𝛾𝐴 − Δ𝐺𝑃𝑉 (15)

The surface tension contribution is calculated from the free energy cost of

creating the gas-liquid interface and the change in free energy due to de-

wetting of the sphere and flat surface:

Δ𝐺𝛾A = 𝛾(𝐴i + 𝐴p cos 𝜃p + 𝐴s cos 𝜃s) (16)

Here, Ai is the capillary surface area of the gas-liquid interface, Ap and As

are the capillary surface areas on the sphere and the flat surface,

respectively. The free energy contribution from capillary pressure GPV is

calculated from the pressure difference across the gas-liquid interface P

and the capillary volume V:

Δ𝐺𝑃𝑉 = Δ𝑃𝑉 (17)

Contributions from three-phase contact lines

Other contributions may also influence the capillary force, such as

properties at the three-phase contact line (TPCL), e.g. line tension. The line

tension is the energy required to form one length unit of a TPCL and enters

the picture through a distortion of the contact angle due to a highly curved

TPCL [129, 130]. Line tension is expressed from the difference between

the macroscopic ∞ and the microscopic m contact angles by the modified

Young’s equation [131]:

𝜏 =𝛾LV

𝜅g(cos 𝜃∞ − cos 𝜃m) (18)

Here, 𝜅g =cos 𝜑

𝑟 is the geodesic curvature of the TPCL; the angle between

the surface and the plane containing the wetting perimeter and r the radius

of curvature of the TPCL. For a circular contact line of radius r on a flat

surface 𝜅g =1

𝑟 and on a spherical surface (of radius R) 𝜅g =

1

𝑅 tan 𝛽, where

is the angle describing the position of the TPCL (see Figure 7) [132]. The

22

line tension contribution becomes important for a highly curved TPCL, i.e.

for a radius of a few micrometers or less [133]. Even though the concept of

line tension is well defined and understood, its magnitude and sign are still

disputed. Values of several orders of magnitude difference have been

reported and even of different signs [129]. Second, when surface roughness

is introduced, the situation will be even more complex as roughness may

cause pinning (and de-pinning) of the TPCLs. Pinning forces may be

estimated from the change in contact angle from an initial quasi-equilibrium

contact angle e [134-136]:

𝐹pin = 𝛾LV(cos 𝜃m − cos 𝜃e) (19)

23

Chapter 3

Experimental methods

Super liquid-repellent coatings

The superhydrophobic and superamphiphobic coatings used in this work

were prepared using the following general approach: first, surface

roughness was created by applying a micro-/nanostructured coating on a

flat substrate and second, the surface energy was lowered by surface

modification with a fluorosilane.

Micro-/nanostructured coatings

I utilized two different types of micro-/nanostructured coatings during this

thesis work. In Papers I and II, a nanoparticle coating was applied using a

dip coating process. In Papers III and IV, a nanostructured coating was

applied by using a thermal aerosol-assisted deposition technique. One

advantage of using both types of coatings is that they can be applied on

different substrates, which was specifically advantageous for confocal

imaging which requires the substrate to be a thin cover glass.

Dip coated nanoparticle coating

The silica (SiO2) nanoparticle coatings used in Papers I and II were

prepared using a simple dip coating method as described previously [108,

24

109, 137]. Dip coating is a fast and easy approach to induce randomly

ordered surface roughness. High-precision (No. 1.5H, thickness 170 ± 5

μm) thin microscope cover glasses (Paper I) and silicon wafers (Paper II)

were used as substrates for the coatings. In the coating procedure, the

substrate was dip coated in a dispersion containing 0.5 wt% silica

nanoparticles, 12.5 wt% perfluoroalkyl copolymer and 87 wt%

hydrofluoroether solvent. The substrate was vertically dipped in the

dispersion 3 times and between each dipping cycle the solvent was allowed

to evaporate, leaving a rough composite coating of silica nanoparticles and

fluoropolymer. In order to increase the mechanical durability, the coated

sample was heat treated at 450-500 °C for 2 hours. During the calcination

process the fluoropolymer decomposes and evaporates and a rough coating

of silica nanoparticles is left on the substrate. This coating is then

fluorosilanized to achieve a low surface energy.

Liquid flame spray coating

In Papers III and IV, a nanostructured titanium dioxide–silicon dioxide

(TiO2/SiO2) coating was prepared using a thermal aerosol-assisted

deposition called the liquid flame spray (LFS) technique. In LFS, a liquid

solution containing organometallic precursor molecules is injected though

a hydrogen-oxygen turbulent, high-temperature (>2500 °C) flame [138].

After exiting the burner spray nozzle, the precursor solution is atomized

into small (micrometer-size) droplets. The droplets evaporate in the hot

flame and the precursor organometallic molecules react and form

nanoparticles. The nanoparticles aggregate and finally deposit on the

substrate that is moving through the flame. As a result, a nanostructured

coating layer is formed. The final coating composition and morphology

can be controlled by adjusting process parameters such as flow rate of the

combustion gases (hydrogen and oxygen), composition, concentration and

feed rate of the precursor solution, distance between burner and substrate,

and sample moving velocity. The working principle of LFS is shown in

Figure 8.

25

LFS has been proven to be a highly suitable coating technique for

achieving super liquid-repellence as it produces coatings with a highly

porous surface structure with high level hierarchical roughness [58, 64,

139]. One of the greatest advantages of using LFS is that it is a fast coating

process and can be applied in a continuous process, e.g. in high-speed roll-

to-roll processes [138-140]. In addition, it is suitable for coating of a wide

range of substrates and different materials. Since the coating velocity

through the high temperature flame is fast, even cellulose-based materials

such as wood or paper can be coated using LFS [64, 141].

Figure 8. Schematic illustration showing the working principle of

LFS.

In this work, LFS coatings were prepared following a method previously

described by Teisala et al. [58]. High-precision (No. 1.5H, thickness 170

± 5 μm) thin microscope cover glasses were used as substrates. In order to

26

achieve the turbulent, high temperature flame, the combustion gases were

fed into the LFS burner at flow rates of 50 L min-1 (hydrogen) and 15 L

min-1 (oxygen), respectively. The precursor solution consisted of tetraethyl

orthosilicate (TEOS) and titanium tetraisopropoxide (TTIP) dissolved in

isopropanol and was injected at a rate of 12 mL min-1. The total Ti+Si

concentration in the precursor solution was 50 g L-1 with a Ti/Si weight

ratio of 99/1. The coatings were applied by passing the substrate through

the flame spray with a velocity of 0.8 m s-1 at a distance of 6 cm away from

the burner face. In Paper III, the final coating was achieved by

subsequently passing the substrate 5 times through the flame spray. In

Paper IV, different coating thicknesses were prepared by passing the

substrates through the flame spray different number of times (1, 2, 3, 4 and

5 coating cycles).

Surface modification

Growth of thin silica layer to protect from photodegradation

Titanium dioxide is well-known for its photocatalytic activity and can

break down organic compounds when exposed to irradiation with energy

corresponding to its band gap [142, 143]. In order to prevent photocatalytic

degradation of the fluorosilane, a thin passivating silicon oxide layer was

grown on the TiO2/SiO2 nanostructured coatings prior to silanization. The

silica layer was applied via a gas-phase reaction, where the samples were

placed in a desiccator together with TEOS and ammonia in two open vials

at atmospheric pressure and room temperature for 4 hours. This process

results in the growth of a few nanometers thick silica shell covering the

nanostructured coating, and such a thin layer will not alter the coating

morphology and the liquid-repellent properties of the final coating and is

sufficient to diminish photodegradation [58].

27

Fluorosilanization

In order to lower the surface energy of the micro-/nanostructured coatings,

the samples were surface modified with fluorosilanes.

Perfluoroalkylsilanes are the frequently used for decreasing the surface

energy when preparing superhydrophobic and superamphiphobic surfaces

[14]. One advantage of using deposition of silanes is that they may form a

very thin coating layer, and hence it will not change the morphology of the

surface structures. In this work, two similar tri-functional silanes with a

fluorinated tail was used: 1H,1H,2H,2H-perfluorooctyl-trietoxy-silane,

FOTES (Papers I and II) and 1H,1H,2H,2H-perfluorooctyl-trichlorosilane,

FOTCS (Papers III and IV). The small difference between the two silanes

is their functional groups: ethoxy groups in FOTES and chloride in FOTCS

(Figure 9).

Figure 9. Chemical structure of silanes used in this work.

Substrates suitable for silanization include silicon, glass and metal oxides,

since their surfaces are covered by hydroxyl groups that can react with the

functional group of the silane, forming a covalent bond. To increase the

number of hydroxyl groups on the surface and hence maximize the

reactivity, the substrate is often activated prior the silanization process. In

this work, air (Papers I and II) or oxygen (Papers III and IV) plasma were

used for activation. The silanes can be applied via solvent deposition or

chemical vapor deposition (CVD). In this work, surface modification by

CVD was applied by using two different approaches: at elevated

temperature (70 °C for 24 hours at P = 1 atm) and at reduced pressure (100

mbar for 2 hours at room temperature).

28

Surface characterization

The superhydrophobic and superamphiphobic coatings were characterized

mainly in terms of wettability, which will be described in more detail in

this section. Additionally, techniques used to characterize surface

topography/morphology and surface chemistry of the coatings will be

briefly described.

Wettability

The most common way to characterize wettability of a surface is to

measure contact angles by goniometry using the sessile drop method.

Typically, a microliter-sized drop is gently placed on the surface using a

motorized syringe and a thin needle. The profile of the sessile drop is

captured using a high-resolution camera and the shape of the profile is

analyzed by the software to determine the contact angle. A schematic

image of a contact angle goniometer setup is shown in Figure 10.

Figure 10. Schematic of a contact angle goniometer setup and an

optical image of a sessile water droplet on a superhydrophobic

surface.

Advancing and receding contact angles can generally be measured by two

different approaches. The first is by increasing and decreasing the drop

volume by slowly pumping liquid in and out with the needle placed inside

the liquid drop close to the sample surface. A video is recorded during the

pumping and each image is analyzed to obtain the contact angles when the

liquid is advancing and receding over the surface. To avoid effects of the

29

needle distorting the drop shape, the contact angles should be determined

using sufficiently large drop volumes depending on the contact angle

hysteresis (about 10-15 µL is in most cases adequate for a hysteresis of less

than 10°) [33]. Alternatively, the advancing and receding angles can be

determined by tilting the sample so that the drop is rolling or sliding down-

hill. When the drop is in motion, the advancing contact angle is determined

from the downhill side and the receding one from the uphill side [144]. The

main advantages of using contact angle goniometry are that it is a relatively

fast, simple and straight-forward technique. However, there are some

issues to be aware of in order to obtain meaningful, reliable and

reproducible results. For instance, operational procedures such as

illumination, camera settings, baseline position and fitting method can

highly influence the results [145]. Additionally, for surfaces displaying

very high contact angles (such as super liquid-repellent surfaces), contact

angle measurements by sessile drop goniometry has been identified to

involve large errors [29, 146, 147]. Since the gap between the solid and the

liquid close to the contact point is small, it is difficult to accurately

determine the drop shape and the position of the baseline. In fact, for

contact angles over 150°, the error will increase drastically if the baseline

is misplaced by only one pixel. For highly liquid-repellent surfaces (CA

close to 180°), the error can even be larger than 10° [146, 148].

An alternative technique for wettability characterization of structured and

super liquid-repellent surfaces is laser scanning confocal microscopy

(LSCM) [32, 149-151]. The use of an inverted LCSM to image a sessile

droplet allows observation of the three-phase phase contact line at high

resolution (often < 1 µm) [149]. From the high-resolution confocal image,

the contact angles can be determined at a higher precision than using

optical goniometry. In addition, confocal imaging can provide insight on

the wetting state on super liquid-repellent surfaces. LSCM is described in

more detail in a following section.

30

Topography

The surface topography of the superhydrophobic and superamphiphobic

coatings was investigated using atomic force microscopy (AFM) and

scanning electron microscopy (SEM).

In AFM (described in more detail in the following section), a topography

image is generated by scanning a sharp tip attached to a cantilever across

the surface. The tip can either be in direct contact with, close above or

tapping the surface. Interactions between the tip and the surface due to

height variations are detected by deflection or changes in the oscillation

amplitude of the cantilever. Using a feed-back loop to the piezoelectric

scanner the height variations are measured and a 3D image of the surface

topography is created. In this work, AFM images were recorded by gently

tapping the surface.

In SEM, a focused electron beam is used to scan across the surface to

render an image of the sample. Compared to optical microscopy (which is

limited by the wavelength of light), SEM imaging provides much higher

resolution and depth of focus. Most commonly, an image is created by

detecting low-energy secondary electrons. Secondary electrons are

electrons ejected from the sample surface by the high energy of the electron

beam. Due to their low kinetic energy, only the electrons close to the

surface will reach the detector. In the detector, the electrons are converted

into an electrical signal and displayed as a two-dimensional intensity map,

which can be viewed as an image. The intensity in the signal depends on

the number of electrons reaching the detector. On surfaces tilted towards

the detector, the escape path is typically shorter, and a larger number of

electrons will reach the detector as compared to perpendicular surfaces.

This gives SEM images a perceived three-dimensional appearance with a

good perspective and sense of surface topography. Normally, a SEM

operates under high vacuum conditions (< 10-4 Pa) and samples needs to

be conductive to avoid surface charging. In order to produce good images

of electrically insulating materials, the samples need to be coated with a

thin layer of a conductive metal (e.g. gold or platinum). Alternatively, non-

31

conductive materials can be imaged using low vacuum SEM (LV-SEM)

without the use of any surface pre-coating. In LV-SEM, the pressure in the

specimen chamber is typically 1-2000 Pa and since a gas is present, the gas

molecules will ionize and neutralize any charge that may build up on a non-

conductive sample. In this thesis work, the nanostructured coatings were

sputter coated with a thin layer of gold prior SEM imaging. Additionally,

LV-SEM was used to image colloidal probes without surface pre-coating.

Surface chemistry

X-ray photoelectron spectroscopy (XPS) is a highly surface sensitive

method (analysis depth of typically 2-10 nm) which provides quantitative

information of surface chemical composition. In XPS, the sample is

irradiated with a beam of well-defined X-rays under ultra-high vacuum

conditions. When the X-ray photons interact with the atoms in the surface

region, photoelectrons are emitted. The kinetic energy of the

photoelectrons depends on the characteristic binding energy of the

element. XPS can provide both elemental composition and quantitative

information, as the number of detected electrons in each peak is directly

related to the amount of that element within the sampling volume. In

addition, since the characteristic binding energy of an atom is influenced

by the chemical environment, qualitative information of different chemical

states of an element (different functional groups, chemical bonding,

oxidation state etc.) can be obtained with XPS.

Force measurements

Atomic force microscopy

The atomic force microscope (AFM) was invented in 1986 [152] and has

developed into a very versatile tool for materials and surface

characterization. The main use of AFM is for topographical imaging and a

32

wide range of materials and surfaces can be imaged at high resolution, even

down to atomic resolution. Another great capability of AFM is to measure

interaction forces between different surfaces, which also is the main use of

AFM in this thesis work.

Figure 11. Schematic illustration showing the working principle of

AFM.

The working principle of AFM is illustrated in Figure 11. The sensing part

in an AFM is a cantilever spring with a probe attached to its free end. The

probe is usually a sharp tip but can be an object with other geometry, e.g.

a sphere as illustrated in Figure 11 and described later in this section. A

laser is focused on the back side of the end of the cantilever and the

reflected laser beam is directed to a detector. The detector is often a split

photodiode consisting of four sectors, which very sensitively monitors the

position of the reflected laser beam in the horizontal and vertical direction.

Movement of the reflected laser spot corresponds to deflection of the

cantilever (bending or twisting). A piezoelectric scanner is used to move

the sample relative to the cantilever in xyz directions. The scanner can

either move the sample (as illustrated in Figure 11) or the cantilever

(including the whole laser-detector system), and this generally depends on

the manufacturer of the instrument. Whether the sample or the cantilever

is moved, the principle is the same. The piezoelectric material enables very

33

precise movement and the position of the sample (or cantilever) can be

determined with high precision.

Normal force measurements using AFM

The AFM can be used for normal force measurements where the

interaction forces between the tip and the sample are measured [153-155].

During a force measurement, the cantilever is moved towards

(approaching) the sample in the normal (vertical) direction and back up

again (retracting or separating). The deflection of the cantilever (the

electrical signal from the photodiode in voltage) and the vertical position

of the cantilever (piezo scanner z-displacement) are recorded. The recorded

raw data can then be converted into a force-distance curve (in short “force

curve”), illustrated in Figure 12.

Figure 12. Schematic of set-up and raw data obtained during a

normal force measurement and corresponding force-distance curve

obtained using the deflection sensitivity and cantilever spring

constant.

The cantilever deflection is directly proportional to the interaction force

between the tip and the sample. In order to convert the photodetector signal

(VPSD) into force (F), first, the deflection sensitivity (s) needs to be

determined. The deflection sensitivity is the conversion factor (usually in

nm/V) of how much the cantilever deflects in units of distance for a certain

measured change in photodetector voltage. It is found by identifying the

constant compliance region where the tip and the sample are in “hard wall”

contact. The constant compliance region is linear for a hard surface/tip,

which means that the change in cantilever deflection is equal to the change

34

in piezo displacement. Hence, the deflection sensitivity is given as the

slope of the constant compliance region. For soft or fragile samples where

it is difficult to reach hard wall contact without destroying the sample, the

deflection sensitivity can be measured before or after the measurement on

e.g. a glass or mica surface. Once the deflection of the cantilever (x) is

known, the force can be calculated by multiplying the deflection with the

normal cantilever spring constant (kz) using Hooke’s law: F = kzx. Finally,

the distance between the sample and the tip (D) is calculated by adding the

cantilever deflection to the piezo position (Z).

Cantilever spring constant calibration

For quantitative force measurements, it is highly important to know the

spring constant of the cantilever as otherwise it is not possible to convert

the cantilever bending into force. Several different methods have been

proposed for calibrating the spring constant of cantilevers. For instance, by

measuring the change in resonance frequency when attaching a known

mass to the end of the cantilever the spring constant can be calculated

[156], or by using a reference cantilever with known spring constant the

spring constant of a unknown cantilever can be determined [157]. In this

work, the method proposed by Sader et al. [158] was used to determine the

spring constants. The Sader method is based on the principle that a viscous

fluid damps the thermal motion of an object. During calibration, the

cantilever is allowed to freely vibrate due to thermal motion in air (or other

fluid). The resonance frequency and the quality factor of the vibration are

measured and by knowing the width and length of the (rectangular)

cantilever, the normal spring constant kz is calculated from these values

and the density and viscosity of air.

Colloidal probe AFM

An important development for measurements of surface forces using AFM

is the colloidal probe technique introduced in 1991 [159, 160]. Typically,

35

a micron-sized spherical particle is glued to the cantilever (Figure 13) and

the interactions between the particle and the surface are measured. The

advantages of using spherical particles is a well-defined radius and the

force can be analyzed more quantitatively and with better sensitivity (since

the total force is higher than for a small tip). Different materials of the

microsphere can be used such as polymers, metals or metal oxides, but

most commonly silica or glass since such particles are available at different

sizes and typically have a very smooth surface. Glass or silica surfaces also

offer the advantage of easy chemical modification of the microsphere

surface by e.g. CVD. The colloidal probes can be attached by different

methods such as melting or sintering the particle onto the cantilever or by

using thermoplastics or two-component epoxy glue.

Figure 13. SEM image of a silica microsphere glued to the end of a

tipless AFM cantilever.

In this work, silica (typical radius 6 µm) and glass (typical radius 10-40

µm) microspheres were used as colloidal probes. The microspheres were

attached to tipless cantilevers by a two-component glue using a

micromanipulator under an optical microscope. The colloidal probes were

then surface modified by CVD fluorosilanization as previously described

for the super liquid-repellent coatings.

36

Measurements in liquid

AFM measurements can be performed in different environments such as

air, different gases, vacuum and liquid. For measurements in liquids a

liquid cell is typically used (Figure 14). A liquid cell is commonly made of

glass and includes a cantilever holder and a silicone O-ring sealing the cell

against the sample surface. The liquid cell used in Paper II can hold a few

mL of liquid and includes an inlet and outlet, which enables easy addition

and removal of liquid. In some AFM instruments it is possible to simply

trap a liquid drop between the AFM head cantilever holder and the sample,

and in this case a liquid cell is not needed. This approach was used for

combined force and imaging measurements in Papers I, III and IV.

Figure 14. Measurements in liquid using a liquid cell (left) or a

trapped liquid droplet (right).

External piezo setup

For force measurements using AFM, the range of the forces that can be

measured is limited by the z-range of the AFM piezo scanner (typically <

10-15 µm). In order to record full force curves where the interaction forces

would exceed the range of the AFM piezo scanner, an external piezo was

used in Papers III and IV. Here, the AFM was placed on an external piezo

which was used to move the AFM head towards and away from the surface.

During a measurement, the cantilever deflection was recorded by the AFM

as usual, and the tip position was determined from the external piezo

displacement.

37

Imaging

In order to image the events taking place during a force measurement, laser

scanning confocal microscopy was used in Papers I, III and IV.

Laser scanning confocal microscopy

Laser scanning confocal microscopy (LSCM) is widely used in the

biomedical sciences for three-dimensional imaging of fixed or living cells

and tissue [161]. The capabilities of LSCM have also been realized in

material and surface science [162, 163]. The basic concept of scanning

confocal microscopy was developed in the 1950’s [164], but it was not

until the late 1980’s, after important advances in computer and laser

technology, the interest for confocal microscopy was broadly increased

and LSCM along with the first commercial instruments were introduced

[165-167]. The major advantages of confocal microscopy over

conventional wide field optical microscopy are considered to be the

controllable depth of field, elimination of background out-of-focus

information and the capability to collect optical sections from thick

samples.

The basic working principle of LSCM is illustrated in Figure 15. An image

is created by scanning one (or more) focused laser beam(s) across a defined

area of the sample in a raster pattern controlled by scanning mirrors. After

the laser beam hits the target sample, the reflected light (and any

fluorescently emitted light) will follow the same path and pass through the

dichroic mirror. A pinhole aperture is situated in front of the photodetector

and the optics is designed so that the laser light focused on the sample is

“confocal” (having the same focus) with the point of light on the pinhole.

This means that only light from the focal plane of interest will pass through

the pinhole and reach the photodetector while all out-of-focus information

is eliminated. This makes it possible to image optical sections at high

resolution. Optical sectioning is a non-destructive method which is both

38

faster and simpler than sectioning the sample by physical means (i.e.

preparing thin slices) before imaging.

Figure 15. Schematic illustration of the working principle of laser

scanning confocal microscope (LSCM).

Different imaging modes can be utilized in LSCM. Fluorescence confocal

microscopy is most commonly used in biomedical science since it offers

high degree of sensitivity, combined with the ability to specially target and

label different components. Imaging in fluorescence mode typically

requires the sample being labelled by a fluorophore. The fluorophore is

excited by the high intensity light from the scanning laser (of a certain

wavelength) and will emit fluorescent light (of a certain wavelength). The

fluorescence wavelengths are then registered by the detector. The laser

source (exciting wavelength), type of fluorophore and detector

wavelengths are commonly adjusted for a specific system. In addition to

detect the fluorescence, another option is to detect reflected (backscattered)

laser light. By using reflection light imaging, also unstained samples can

39

be imaged. Reflection mode imaging is especially advantageous in

studying wetting and interfacial phenomena, as light will be reflected on

interfaces, allowing interfaces to be directly observed. By using several

photodetectors, which detect different wavelengths, both fluorescence and

reflected light can be detected simultaneously. Similarly, it is also possible

to collect images from multiple-labelled samples, i.e. simultaneously

collect several optical sections collected at different excitation

wavelengths. The basic image unit in LSCM is to collect single two-

dimensional optical sections in the xy-plane of a specific focal plane (z-

axis). By collecting a sequence of optical sections at stepwise changes in

focus, so-called z-series or z-stacks, can be combined and viewed as a

three-dimensional image. Additionally, by scanning along one line in the

x- or y-direction at different z-depths, a two-dimensional cross-section or

vertical slice can be obtained.

LSCM for studying wetting

In recent years, LSCM has been proven to be a powerful tool to study

wetting phenomena and super liquid-repellence [32, 125, 149-151, 168-

170]. The use of an inverted LSCM (sample is imaged from below) is

highly suitable for studying surface wetting as it allows observation of

interfaces and three-phase contact lines at high resolution (often < 1 µm).

The liquid phase can be visualized by adding a fluorescent dye, and

interfaces can be directly observed due to the reflected light. It is possible

to directly observe the wetting state below the droplet. Figure 16 shows an

example of an LSCM image of a water drop resting on a superhydrophobic

surface.

Figure 16. LSCM image of a fluorescently labelled water droplet

resting on a superhydrophobic surface.

40

Some limitations of using LSCM imaging for studying super liquid-

repellent surfaces include that the sample needs to be transparent and the

substrate cover glass of high precision (170 ± 5 µm in thickness). In

addition, thick and highly rough or void samples causes a high degree of

scattering and the signal reaching the detector will be reduced.

Fluorescent dyes

To fluorescently label the liquids, two different fluorescent dyes were used

in this work, ATTO 488 and N-(2,6-diisopropylphenyl)-3,4-perylene

dicarboxylic acid mono imide (PMI). The molecular structures of ATTO

488 and PMI are shown in Figure 17.

Figure 17. Molecular structures of fluorescent dyes used in this

work.

ATTO 488 is a polar commercial dye (ATTO-TEC GmbH) with a

maximum spectroscopic absorption wavelength at 500 nm and emission at

520 nm, and it was used to label water (Papers I, III and IV) and ethylene

glycol (Paper III). The non-polar dye PMI has a maximum spectroscopic

absorption wavelength around 505 nm and emission around 525 nm [149]

and was used to label hexadecane (Paper III).

LSCM combined with AFM setup

In this work, a home built inverted LSCM at Max Planck Institute for

Polymer Research in Mainz, Germany, was used for imaging during force

41

measurements in Papers I, III and IV. The main advantage of using this

setup in my thesis work was that it can be coupled with an AFM, which

makes it possible to collect images of the events taking place between the

tip and the sample during a force measurement [125, 171]. The confocal

microscope uses a 473 nm laser and a 40×/0.95 dry objective. The

fluorescence from the dyed liquid and the reflected light from the interfaces

were detected simultaneously with two different detectors. Typically, 2D

cross-sectional images were recorded by scanning the laser along one line

in the x-direction at different heights in the z-direction at an acquisition rate

of 1 frame s-1.

Image processing and analysis

The confocal images collected during force measurements in Papers I, III

and IV, were processed and analyzed in an automated manner using the

ImageJ software (see an example in Figure 18). The shape of the liquid-

gas interface was obtained from the fluorescence image (cyan). After

thresholding and binarization, the shape of the interface was extracted, and

a circle was fitted to the position of the spherical colloidal probe. The

reflection image (red) was used to extract the positions of the liquid-air and

glass-air reflections, which was used to identify the capillary base on the

super liquid-repellent surface.

42

Figure 18. Image processing and analysis of a confocal image with

light reflected from the interfaces in red and the liquid with

fluorescent dye in cyan. First, the fluorescence and reflection

images were converted to binary images, from where the shape of

the liquid-air interface (cyan symbols), positions of the liquid-air,

glass-air reflections (dashed red lines) and colloidal probe (black

circle) were extracted.

43

Chapter 4

Results and discussion

Superhydrophobic and superamphiphobic

surfaces

Nanostructured coatings

Two types of coating methods were used to prepare the nanostructured

super liquid-repellent surfaces studied in this thesis work: dip coating

(Papers I and II) and liquid flame spray (LFS) (Papers III and IV). Figure

19 shows an AFM topographical image of a dip coated sample. The dip

coating method produced a relatively thin coating with a thickness of

approximately 1−2 m.

Five different LFS-coated samples were prepared by applying an

increasing number of subsequent coating cycles. Figure 20 shows SEM

images of samples prepared using 1 and 5 coating cycles, respectively. An

increase in the number of coating cycles resulted in an increase in the

coating thickness from below 1 µm for samples prepared using 1 coating

cycle up to 7 µm for samples prepared with 5 cycles. The surface roughness

also increased with the number of coating cycles, where protrusions grow

larger with increasing number of coating cycles.

44

Figure 19. AFM topographical image (10 × 10 m2) of a dip coated

sample with height profile along the horizontal dashed line in the

center of the image.

Figure 20. SEM images of LFS-coatings: (a) 1 coating cycle and (b)

5 coating cycles. Scale bars 5 µm.

45

Wettability measured with contact angle goniometry

After surface modification with fluorosilane, all coatings studied in this

thesis work were superhydrophobic as water droplets were seen to adopt

an almost spherical shape with high contact angles (≳ 150°) and low roll-

off angles (≲ 5°) as measured by goniometry (Table 1). LFS-coatings

prepared by 2-5 coating cycles, showed excellent water-repellence as

droplets would roll away as soon as the goniometer needle was detached

resulting in non-measurable roll-off angles for these surfaces (< 1°). In

addition, the LFS-coating prepared by 5 coating cycles was

superamphiphobic as contact angles for hexadecane ( = 27 mN m-1) was

measured to be θadv = 160 ± 2°, θrec = 150 ± 5°, and roll-off angles RA = 2

± 1°.

Table 1. Summary of water contact angles measured by goniometry.

RA for 10 µL droplets.

Sample θadv (°) θrec (°) RA (°)

Dip coating 158 ± 1 148 ± 2 5 ± 1

LFS - 1 coating cycle 163 ± 1 154 ± 4 3 ± 2

LFS - 2 coating cycles 162 ± 1 156 ± 8 < 1

LFS - 3 coating cycles 161 ± 1 159 ± 2 < 1

LFS - 4 coating cycles 162 ± 1 160 ± 1 < 1

LFS - 5 coating cycles 161 ± 1 159 ± 1 < 1

The wettability of the dip coated superhydrophobic surface was further

studied by adding ethanol to water in order to lower the surface tension of

the liquid (Paper II). A wetting transition from super liquid-repellence (θ

> 150°) to complete wetting (θ < 5°) was observed (Figure 21). For an

ethanol content of ≤ 20 vol% ( ≥ 40 mN m-1), both the advancing and

receding contact angles were high (≳ 140°) and roll-off angles were low

(≲ 10°), while 30-40 vol% ethanol ( = 35-32 mN m-1) showed high

advancing contact angles (≳ 150°) and low receding contact angles (< 20°).

In this case, the droplets were pinned even when the samples were tilted

46

by 90°. This indicated that a transition from a low adhesive CB type of

wetting state to a high adhesive Wenzel state occurred between 20 and 30

vol% ethanol.

Figure 21. Advancing and receding contact angles for different

water/ethanol mixtures on the dip coated superhydrophobic surface.

Error bars show the standard deviation and for some data points they

are smaller than the symbols.

Surface wettability studied with LSCM

As mentioned in Chapter 3, contact angle measurements performed with

optical goniometry and drop shape analysis involve a large uncertainty for

high apparent contact angles, as the gap between the liquid and the solid

close to the contact line is small and can be difficult to identify in an optical

image. By LSCM imaging of sessile droplets, a more detailed insight into

the wettability of the super liquid-repellent surfaces was obtained in this

work. LSCM images revealed higher contact angles as compared to those

measured with optical goniometry. Water contact angles were determined

to approximately 165° for dip coated samples and approximately 170° for

the LFS-coated samples. It is worth noting that contact angles determined

47

by LSCM are measured for a sessile droplet “as placed” and the measured

values should be in between the ACA and RCA.

Figure 22. Laser scanning confocal microscopy images of liquid

droplets on a dip coated superhydrophobic surface: (a) a

fluorescently labelled water droplet in cyan with the light reflected

from interfaces in red, (b) an undyed water droplet, (c) droplet of

20 vol % ethanol, and (d) droplet of 40 vol % ethanol. The insets in

(b-d) show optical images of 5 L drops of respective liquids resting

on the surface (scale bar 1 mm). The dotted white line in (d) is added

as a guide to the eye to enhance the interface underneath the liquid.

Another great advantage of using LSCM to study the wettability of the

liquid-repellent surfaces, is that different wetting states can be visualized.

In Figure 22b, the two horizontal red lines underneath the water droplet are

from light being reflected at the interfaces. The lower reflection arises from

48

the interface between the glass substrate and gas and the upper reflection

from the interface between the liquid and gas. This demonstrates the

presence of a gaseous (air-vapor) layer between the liquid and the coated

substrate. Generally, the thickness of this gaseous layer was found to be of

the same order as the thickness of the nanostructured coatings, indicating

that the liquid was suspended on top of the protrusions of the coatings,

consistent with a CB type of wetting state. In contrast, if light only was

reflected from one interface underneath the droplet, as was the case for a

40 vol% ethanol droplet on the dip coated sample (Figure 22d), this

indicated that the surface structure was penetrated by the liquid with no or

a small amount of trapped gas.

Interactions involving superhydrophobic

surfaces and observations of gas capillaries

Force measurements between hydrophobic and super-

hydrophobic surfaces in water

Hydrophobic colloidal probes

Hydrophobized microspheres were used as colloidal probes in all force

measurements during this thesis work. Two types of microspheres were

utilized (silica and glass) and fluorosilanized in order to make the surface

hydrophobic. Figure 23 shows SEM images of colloidal probes with one

glass and one silica sphere. The silica spheres had a low polydispersity

with a typical radius of R = 3 µm and a somewhat smoother surface (Ra =

6 nm, Rq = 8 nm) as compared to the glass spheres (Ra = 39 nm, Rq = 49

nm). The glass spheres were mainly chosen because of their larger size (R

= 10-20 µm) in order to make sure the colloidal probe was visible during

confocal imaging.

49

Figure 23. SEM images of colloidal probes: (a) glass sphere (R = 16

m) and (b) silica sphere (R = 3 m). Scale bars 10 m.

Interactions between smooth hydrophobic surfaces

For comparison, the results of force measurements between smooth

hydrophobic surfaces in water will first be briefly described. Figure 24

shows an example of a force curve between a fluorosilanized silicon wafer

and a fluorosilanized silica microsphere (R = 3 µm). The shape of the force

curve has the characteristic features as described in Chapter 2.

Figure 24. Example of force measurement recorded on approach

(black) and retraction (red) between a hydrophobic microsphere (R

= 3 µm) and a hydrophobic flat surface. The dashed black line is the

theoretical van der Waals force (Eq. 4) and the solid black line is

fitting of the capillary force equation for constant capillary volume

(Eq. 13).

50

The dashed black line is the theoretical van der Waals force calculated

using Eq. 4. The non-retarded Hamaker constant A = 2.8 × 10-21 J was

calculated using Eq. 5 and dielectric data for water ( = 80, n = 1.333) and

fluorocarbon ( = 2.1, n = 1.359) [77]. As can be seen in Figure 24, the

measured forces are clearly inconsistent with the expected van der Waals

force, and the attraction is more likely caused by the formation of a

bridging gas capillary. Fitting of the theoretical capillary force equation for

constant capillary volume (Eq. 13) to the experimental data measured on

retraction, shows that the equation provided a good representation at large

separation (Figure 24, solid black line). The capillary volume extracted

from this fit was found to be V = 2 × 10-22 m3 (= 0.2 aL).

Interactions between hydrophobic and superhydrophobic surfaces

Force measurements between a hydrophobic microsphere and a

superhydrophobic surface performed in this thesis, frequently showed

strong attractive interactions ranging into the micrometer scale. The range

and magnitude of the measured forces varied depending on the specific

system. However, the measured force curves shared the main

characteristics as explained in Chapter 2. Figure 25 shows examples of

force curves measured on a dip coated and an LFS-coated

superhydrophobic surface, respectively. The force curves in Figure 25

cannot be fitted by assuming a gas capillary of constant volume, and rather

the shape suggests an increase in capillary volume during the major part of

the separation process. This type of force curve was observed in the

majority of cases on the dip coated superhydrophobic sample. In Paper II

it was concluded that 84% of the measurements resulted in capillary

volume increase type of force curves in water and in Paper I they were

observed in 90% of the cases. In contrast, for measurements on an LFS-

coating prepared with 5 coating cycles, this kind of force curves were

observed every time. Even though the total number of measurements was

fewer and no statistical analysis was made in this case. This indicates that

while how frequently the interactions showing increasing capillary volume

characteristics occur are likely related to local wetting characteristics (e.g.

51

local pinning sites), it may still be possible to predict by macroscopic

wettability measurements.

Figure 25. Examples of force curves measured between a

hydrophobic microsphere and a superhydrophobic surface. The

force has been normalized by particle radius. (a) Dip coated

superhydrophobic sample and hydrophobized silica sphere (R = 3

µm) and (b) LFS-coated superhydrophobic sample (5 coating

cycles) and a hydrophobized glass sphere (R = 16 µm). Note the

different scales in panel a and b.

52

Gas capillary imaging and development during force

measurements

Capillary imaging during force measurements

LSCM was used to record microscopic images of gas capillaries during the

AFM force measurements. The capillary images could be connected to the

corresponding observations in the force curves and previously proposed

events, such as capillary formation, growth and rupture, was now

visualized.

Figure 26. Examples of confocal images of gas capillary formation,

development and rupture. Scale bar 10 µm.

Figure 26 shows an example of a force measurement with corresponding

LSCM images. At the start of the measurement, when the force is zero, the

particle and the superhydrophobic surface are seen to be well-separated in

the corresponding confocal image (Point 1). When a strongly attractive

force is appearing, the confocal images demonstrate the formation of a gas

capillary between the two surfaces (Point 2-3). During the retraction of the

cantilever, the gas capillary persists as the attractive force is increasing

53

(Point 4-6), and at the point when the force returns to zero, capillary rupture

is observed (Point 6-7).

Capillary shape and development during separation

The confocal images did not only offer visualization of gas capillaries, but

also allowed us to quantify and monitor the development of the capillary

shape and size during separation. From the shape of the capillary obtained

from confocal images, capillary characteristics such as the capillary

diameter on the superhydrophobic surface d, the angle describing the de-

wetted area on the particle , and the microscopic contact angles at the

liquid−gas interface of the particle p, and superhydrophobic surface s,

were determined (Figure 27).

Figure 27. Illustration of a gas capillary (volume V) between a

spherical particle (radius R) and a flat surface at separation distance

D, with the diameter of the capillary on the flat surface d, the angle

defining the de-wetted area on the sphere β and the contact angles

at the gas-liquid interface of the flat surface θs and particle θp,

respectively.

To determine the gas capillary volume, it was assumed that the cross-

sectional confocal image was recorded at the center of the capillary and

that the capillary was axisymmetric. The capillary volume Vcap was then

calculated by:

𝑉cap = ∫ π(𝑟(𝑧))2

 d𝑧ℎ

0− 𝑉p (20)

54

where r is the capillary radius at each pixel in the z-direction and h is the

height of the capillary gas-liquid interface. Vp is given by the volume

occupied by the spherical cap of the particle and calculated by:

𝑉p =π𝑏(3𝑎2+𝑏2)

6 (21)

where 𝑏 = 𝑅(1 − cos 𝛽) is the height of the cap and 𝑎 = 𝑅 sin 𝛽 its radius.

Gas capillaries observed at superhydrophobic surfaces during this thesis

work, were typically determined to be in the order of Vcap ≈ 10-15 – 10-13 m3

(= 1 – 100 pL). It is worth noting that this is several orders of magnitude

larger than the volumes determined by fittings of the capillary force

equation in the case of the smooth hydrophobic surfaces (Vcap ≈ 10-22 m3).

By calculating the capillary volumes during the retraction of the particle,

it was observed that capillaries (initially) grow in volume during separation

(Figure 28b). Calculations of the Young-Laplace capillary pressure P

from the capillary shape (Eq. 9) suggested that an under pressure in the

capillary drives the inflow of gas from the pre-existing gaseous layer into

the capillary and this allows the volume to increase during separation

(Paper I).

In addition to visualize the capillary growth, the capillary development was

monitored by the change in positions of the capillary TPCLs and the

contact angles at the liquid-gas interface during separation. Figure 28

shows an example of capillary characteristics development during

retraction for a dip coated superhydrophobic surface. In general, when the

attraction increased during retraction (D < 6 µm in Figure 28), capillaries

were observed to spread on the superhydrophobic surface as observed by

an increase in d (Figure 28c). Different degrees of pinning to the surface

were observed for the different superhydrophobic surfaces. On the dip

coated samples, capillaries were observed to be either pinned to the surface

during the retraction, or intermittently pinned and spreading (as in Figure

28) reaching maximum values of d ≈ 30 µm. Whereas on the LFS-coated

samples a very low degree of pinning was observed, and capillaries were

55

observed to spread over the surface during the main part of the retraction.

Capillary widths reaching maximum values as high as d = 170 µm was

observed on an LFS-coated sample prepared using 5 coating cycles.

Figure 28. Example of gas capillary development during retraction

for a measurement on a dip coated superhydrophobic surface in

water.

The capillary TPCL on the particle surface was typically pinned when the

attractive force increased as observed by a relatively constant (Figure

56

28e). With the TPCL pinned to the particle surface, the contact angle

increased with separation until reaching a maximum value (Figure 28f) at

the distance where the attractive force was largest (D ≈ 6 µm in Figure 28).

Then, as the attractive force decreased, the TPCL would start to move over

the particle surface as observed by a decrease in and the particle slowly

became wetted again before the capillary finally ruptured.

Steps in force curves due to pinning/de-pinning of TPCLs

Small steps (a sudden reduction of the attractive force) were often observed

in the force curves recorded on separation. These steps in the attractive

force were identified as being due to pinning/de-pinning of the TPCLs on

either the superhydrophobic surface or hydrophobic particle. De-pinning

of the TPCLs was observed by a change in d or , and in many cases

directly observed in the confocal images.

Figure 29. Example of steps in a force curve measured on retraction

(a) with corresponding capillary menisci shapes before (c) and after

(b) the step. De-pinning of the TPCL is highlighted by the black and

gray arrows in (b) and (c).

Figure 29 shows an example where de-pinning of the TPCL was observed

on an LFS-coated superhydrophobic sample (5 coating cycles). In cases

57

where a step in the attractive force could not be linked with a distinct jump

of the TPCL, it is likely that de-pinning would occur outside the 2D plane

of the cross-sectional confocal image or it was too small to be resolved.

Interactions involving superamphiphobic

surfaces and the effect of liquid surface tension

Water-ethanol mixtures

In order to study the effect of liquid surface tension and the relation

between wettability and interaction forces on a dip coated

superhydrophobic surface, force measurements were recorded in

water/ethanol mixtures in Paper II. Figure 30 shows how the liquid surface

tension decreases when adding ethanol to water.

Figure 30. Surface tension of water/ethanol mixtures at 25°C.

Plotted using data from [172].

The effect of adding ethanol on the interactions measured on a dip coated

superhydrophobic surface was compared to those measured on a smooth

58

hydrophobic surface (a fluorosilanized silicon wafer). Typical force curves

recorded in different water/ethanol mixtures for the smooth hydrophobic

surface are presented in Figure 31.

Figure 31. Typical force curves recorded on (a) approach and (b)

retraction between a smooth hydrophobic surface and a

hydrophobic particle in different water/ethanol mixtures. The inset

in (a) shows the theoretical van der Waals force in water (dashed

black line) and ethanol (solid gray line), and the solid black lines in

(b) are the theoretical fits to the capillary force equation for constant

capillary volume (Eq. 13).

59

As seen in Figure 31, the magnitude of the attractive force was found to

decrease when the ethanol concentration was increased. A gas capillary

bridge connecting the two surfaces is expected to form if the contact angle

is > 90° [173, 174]. When ethanol was added at 20 vol%, the macroscopic

contact angles on the hydrophobic surface were adv = 96 ± 3°, rec = 86 ±

2°. Indeed, attractive interactions suggesting gas capillary formation were

typically observed, and interactions measured during retraction were

consistent with those expected due to a bridging gas capillary with a

constant volume. When the ethanol concentration was further increased to

40 vol%, the macroscopic contact angles were well below 90° (adv = 78 ±

2°, rec = 71 ± 2°) and a gas capillary connecting the two surfaces were not

expected in this case. However, attractive interactions were still typically

observed. There is no clear explanation for the observed attraction, but it

may be due to preferential adsorption of ethanol over water at the

hydrophobic surface [108, 175]. This may cause the formation of a

capillary bridge consisting of a liquid phase more concentrated in ethanol

than the bulk liquid, giving rise to the attractive interactions.

Force measurements between a dip coated superhydrophobic surface and

the hydrophobic particle showed that the presence of ethanol strongly

affected the interaction forces also in this case. As discussed in the previous

section, force measurements in water frequently show interactions

consistent with a capillary volume increase during separation. Such force

curves were also observed in 20 vol% ethanol (Figure 32) but the range

and magnitude of the attractive interactions decreased as compared to in

pure water. These observations are consistent with capillary growth being

facilitated by the pre-existing gaseous layer at the superhydrophobic

surface as such gaseous layer was visualized by LSCM for 20 vol% ethanol

(Figure 22). Additionally, at the ethanol concentration of 40 vol%, where

confocal images indicated that the surface structure was wetted by the

liquid with no or small amounts of trapped gas, no force curves consistent

with a large and growing capillary were observed. Hence, these results

support the view that the pre-existing gaseous layer is responsible for the

60

formation and growth of large gas capillaries at super liquid-repellent

surfaces.

Figure 32. Typical force curves showing interactions consistent

with a gas capillary volume increase recorded on (a) approach and

(b) retraction in water and 20 vol % ethanol.

In Paper II, it was also concluded that macroscopic wettability can be an

indicator to both if, and how frequently, the interactions showing

increasing capillary volume characteristics occur. Even if a gaseous layer

61

is present at the surface, it is likely that chemical or topographical

irregularities and pinning points can locally prevent the formation of large

capillaries. For an increased number of such pinning points, the formation

of a large gas capillary with increasing volume during separation will likely

occur less frequently. The number of such pinning points is reflected in the

macroscopic wettability by CAH. This may explain why interactions

consistent with capillary growth were observed only in 29% of the cases in

20 vol% ethanol where the CAH was larger (θadv = 155 ± 1°, θrec = 138 ±

8°) as compared to water (θadv = 158 ± 1°, θrec = 148 ± 2°), where such

interactions were observed in the majority of the measurements (84%).

Lower surface tension liquids

In Paper III, the interactions involving a superamphiphobic surface (LFS 5

coating cycles) in lower surface tension liquids were studied in order to

further investigate whether, and how, gas capillary formation was affected

by the surface tension of the liquid. Using LSCM imaging during force

measurements between the superamphiphobic surface and a hydrophobic

microsphere, it was found that bridging gas capillaries occurred also in the

lower surface tension liquids ethylene glycol ( = 48 mN m-1) and

hexadecane ( = 27 mN m-1), see Figure 33 for examples.

Figure 33. LSCM images of gas capillaries observed in (a) ethylene

glycol and (b) hexadecane.

62

Typical force curves with corresponding capillary menisci shapes recorded

in each liquid are shown in Figure 34. As seen, both the range and

magnitude of the attractive forces, as well as the capillary size, decreased

as the surface tension of the liquid was reduced.

On approach, a weak repulsion was observed prior to capillary formation

in hexadecane and in some cases in ethylene glycol. A repulsion was never

observed in water. It was concluded that the observed repulsion resulted

from deformation of the liquid-gas interface, and it was observed in the

confocal images by a compression of the gaseous layer at the

superamphiphobic surface. It was found that the ratio of the slopes of the

repulsion in ethylene glycol and hexadecane (≈ 1.7-1.8) was close to the

ratio of their surface tensions (1.8). This observation was supported by the

fact that the energy required to deform the gas-liquid interface is expected

to scale with the surface tension. It was suggested that the contact angle on

the particle was responsible for this effect as the lower the contact angle on

the particle the higher energy barrier for de-wetting. The contact angles on

the probe particle just after formation of the gas capillary were on average

found to be p = 87°, p = 73° and p = 50° for water, ethylene glycol and

hexadecane, respectively. These values were found to agree with receding

contact angles on a chemically similar flat surface (water rec = 84°,

ethylene glycol rec = 66° and hexadecane rec = 51°). These results show

that due to the large amount of gas in the gaseous layer on the

superamphiphobic surface, a gas capillary bridge can form even if the

particle is wetted by the liquid ( < 90°).

During separation, gas capillaries observed in ethylene glycol and

hexadecane followed a similar general evolution as described for water in

the previous section (Figure 28). That is, in the beginning, when the

attractive force increased, the capillary spread on the superamphiphobic

surface (increasing d) while the TPCL on the particle was pinned (constant

). Then, after the maximum attractive force was reached and the force

decreased, the TPCL position was moving over the particle surface

63

(decreasing ) while being close to constant on the superamphiphobic

surface (constant d).

Figure 34. Typical force curves recorded in water, ethylene glycol

and hexadecane with the corresponding shape of the capillaries.

Note the different scales in panels a, c and e.

64

Initial capillary growth was also observed in ethylene glycol and

hexadecane, however the volumes were much smaller, both initially and at

maximum size, as compared to those observed in water (Figure 35).

Capillaries grew up to maximum volumes on average 2.5 × 10-14 and 1.0 ×

10-14 m3 in ethylene glycol and hexadecane, respectively, compared to 1.2

×10-13 m3 observed in water. Similarly, the capillary diameter on the

superamphiphobic surface was smaller in the lower surface tension liquids,

maximum values were of on average d = 170 m, d = 100 m and d = 85

m in water, ethylene glycol and hexadecane, respectively.

Figure 35. Maximum capillary (a) volume and (b) width on a

superamphiphobic surface observed in water, ethylene glycol and

hexadecane.

Also on retraction it was observed that the wettability of the particle is

highly influencing the interactions. First, the TPCL on the particle is

pinned during separation, the contact angle on the particle is increasing

until a maximum value is reached. Then the TPCL is moving over the

particle surface and the de-wetted area becomes wetted again until the

capillary finally ruptures. As the liquid is advancing over the particle

surface, the contact angle is expected to be the advancing contact angle.

The largest observed contact angles on the particle in each liquid (p =

116°, p = 98° and p = 78° in water, ethylene glycol and hexadecane,

respectively) were indeed found to agree with the advancing contact angles

65

on a chemically similar flat surface (water adv = 116°, ethylene glycol adv

= 99° and hexadecane adv = 74°).

Capillary growth and the effect of the amount

of available gas

For the capillary to form and grow during separation it requires an inflow

of gas. The gas can enter the capillary by the means of two contributions:

due to gas dissolved in the liquid diffusing into the capillary or by gas being

transported from the pre-existing gaseous layer at the surface. The

individual contributions cannot be easily evaluated, but their effects will

be discussed in this section.

Influence of dissolved gasses

For measurements between smooth hydrophobic surfaces, dissolved gasses

have been shown to strongly influence the interactions in water [102, 104,

176-178]. As the range of the attractive interactions have been observed to

decrease for de-gassed water, it is likely that the capillary formation is due

to diffusion of the dissolved gassed. In the case of super liquid-repellent

surfaces with a pre-existing gaseous layer, however, it is expected that the

majority of gas enters the capillary by means of this layer. However, this

does not preclude the possibility that dissolved gas also may diffuse from

the bulk liquid, contributing to the capillary growth.

During this thesis work, force measurements have been performed in both

de-gassed (Paper II) and normal aerated (Papers I, III and IV) liquids.

However, the effect of de-gassing was not investigated per se, mainly due

to experimental challenges. First, as the air diffusion coefficient in water

is relatively high (2 × 10-9 m2 s-1) [179] and small amount of liquid is used

in the experiments, it is likely that the water will be rapidly saturated.

Second, and especially if a closed liquid cell is used in order to eliminate

66

the surrounding air to contribute in saturating the liquid, the gas initially

trapped in the rough surface structure will diffuse into the liquid. It has

been shown that this diffusion process can be very fast and that the gaseous

layer may even be completely dissolved within seconds or minutes

depending on the surface structure of the superhydrophobic surface [180].

Even if the gaseous layer is not completely dissolved, this diffusion process

will reduce the amount of gas present in the gaseous layer at the surface.

Hence, any differences in capillary volume and growth may be due to the

amount of accessible gas in the surface layer being reduced, and any effect

of dissolved gasses diffusing into the capillary cannot be evaluated in a

straightforward manner.

A further aspect of interactions involving superamphiphobic surfaces is

that gas is readily dissolved in different amounts in different liquids, and

the gas solubility is expected to increase with decreasing liquid surface

tension [181]. In Paper III, it was observed that gas capillaries were

smallest in hexadecane where the gas solubility was the highest and largest

in water with the lowest gas solubility. These results suggested that, while

gas diffusion from bulk liquid into the capillary might be more important

in hexadecane, most of the gas is however likely entering from the pre-

existing gaseous layer.

Influence of gaseous layer thickness

In Paper IV it was investigated how the thickness of the gaseous layer (and

thus the amount of available gas) influence the interactions and the

capillary size and shape. Superhydrophobic samples with different coating

thicknesses were prepared by applying an increased number of LFS

coating cycles, and LSCM confirmed that an increase in coating layer

thickness led to an increase in the thickness of the gaseous layer. The

thickness of the homogeneous part of the coatings and the height of the

highest protrusions determined from SEM images, and the gas layer

thickness below a sessile water droplet and when water is trapped between

the surface and the AFM, are summarized in Table 2.

67

In the force measurements, the range of the attractive forces and the size

of the capillaries were observed to increase with increasing coating

thickness (number of coating cycles), indicating that the amount of

accessible gas in the gaseous layer is influencing capillary formation and

growth. Gas capillaries at the sample prepared by 1 coating cycle were

observed to be considerably smaller both at the initial and final state as

compared to the other four coatings, suggesting that the amount of

available gas was a limiting factor for a coating that thin. For samples

prepared by 2, 3 and 4 coating cycles, only a small difference was observed

in capillary size and attractive force, which may be explained by the fact

that the gas layers at these coatings were of similar thickness. On the

sample prepared using 5 coating layers, gas capillaries were observed to

grow considerably larger, resulting in higher attractive forces.

Table 2. Approximate coating thicknesses and corresponding

gaseous layer thicknesses.

Coating layer Gaseous layer

Sample

Homo-

geneous

part

Pro-

trusions

Sessile

droplet

In

AFM

1 coating cycle 0.6 µm 1.2 µm 1 µm < 1 µm

2 coating cycles 1.2 µm 3.4 µm 3 µm 2 µm

3 coating cycles 1.9 µm 3.8 µm 4 µm 3 µm

4 coating cycles 2.3 µm 4.8 µm 5 µm 3 µm

5 coating cycles 3.9 µm 7.1 µm 7 µm 4 µm

The results suggested that, while the amount of available gas in the pre-

existing gaseous layer seems to influence the interactions, evaluating the

effect may not be only related to the increased layer thickness. That is

because the total amount of accessible gas is not only determined by the

layer thickness, but also the surface roughness and porosity of the coating

layer. If surface roughness and/or porosity is increased for a fixed layer

thickness, the volume ratio of gas to solid in the coating will be increased.

68

An increased surface roughness is likely to explain why, even though the

gas layer at the sample with 5 coating cycles are just slightly thicker as

compared to the sample prepared using 4 coating cycles, gas capillaries can

still grow considerably larger.

Figure 36. (a) Maximum attractive force encountered on approach

and retraction and (b) the range of attraction obtained from force

measurements, and (c) maximum capillary width on the

superhydrophobic surface dmax, and (d) maximum capillary volume

Vmax obtained from capillary images. Error bars show the standard

deviations.

69

Calculations of capillary forces and comparison

to measurements

Capillary pressure from meniscus shape and calculation of

the theoretical capillary force

For capillaries observed on the dip coated superhydrophobic sample, the

capillary pressure was determined using Eq. 9 (Paper I). The two principal

radii (r1 and r2) of curvature of the capillary gas-liquid interface were

determined from the confocal images. Once the capillary pressure was

determined, the theoretical capillary force was calculated using Eqs. 6-8

and compared to the maximum attractive force observed on separation. It

was found that theoretically calculated forces (Fcap = -4.1 N) were slightly

lower but in the same order of magnitude as the experimentally measured

values (F = -8.4 N). The observed difference may be explained by the

fact that force measurements are dynamic while the theoretical equation

describes an equilibrium situation.

In contrast, for gas capillaries observed on the LFS-coated samples (Papers

III and IV), the shape of the liquid-gas interface clearly could not be fitted

assuming the circular approximation, hence Eq. 9 cannot be used to

determine the capillary pressure in these cases. Instead, an attempt was

made to calculate the capillary pressure using Eqs. 10-11 from fittings of

Eq. 12 to the experimental data of the meniscus shape. Calculations were

performed using data from Paper III for capillaries observed in water,

ethylene glycol and hexadecane. The capillary lengths at 25°C for water,

ethylene glycol and hexadecane are = 2.7, = 2.1 and = 1.9 mm,

respectively. The Bond numbers in the experiments were in the range of

Bo ≈ 0.006-0.007. This means that the requirements for Eq. 12 to be valid

holds (𝑟 ≪ 𝜅 and 𝐵𝑜 ≪ 1) and the experimental data of the meniscus

shape could be fitted to Eq. 12. Figure 37 shows that fittings to Eq. 12 can

describe the meniscus shape well in all three liquids.

70

Using the fitted functions, P was estimated using Eqs. 10-11 for the three

cases and calculations arrived at values of P in the orders of ± 10-6 – 10-5

Pa. It was however noted that the result of these calculations, both values

and sign, sensitively depended on details of the fit, and likely included

large errors. The results show that the pressure difference clearly is very

small, however due to the errors likely being large, the calculated values

were not used in further calculations of the capillary forces.

Figure 37. Examples of capillary experimental data (cyan symbols)

with fit to Eq. 12 (red lines) in (a) water (fitting parameters rp = 13.8

m, = 69° and b = -50.5 m), (b) ethylene glycol (fitting

parameters rp = 10.7 m, = 41° and b = -26.5 m) and (c)

hexadecane (fitting parameters rp = 6.9 m, = 45° and b = -20.5

m).

71

Calculating capillary interactions using a free energy

approach

Integration of force curves

In order to compare calculations of the free energy of capillary formation

(Eq. 15) with measured values, the force curve needs to be integrated:

∆𝐺 = − ∫ 𝐹d𝐷𝑏

𝑎 (22)

In our case we want to compare the integral of the force curve measured

on retraction to the free energy calculated from capillary images during

separation. We need to account for the energy at zero distance by adding

the integral of the force curve measured on approach, and arrive at the final

equation for the energy integral:

∫ 𝐹d𝐷 = − ∫ 𝐹approachd𝐷0

𝐷a+ ∫ 𝐹retractd𝐷

𝐷r

0 (23)

Here, Da is the separation distance when an attractive force is first observed

on approach (when the capillary is formed) and Dr the separation when the

attraction disappears on retraction (when the capillary ruptures).

Figure 38. Example of integral of force curve recorded on an LFS-

coated superhydrophobic surface (5 coating cycles) in water.

72

Figure 38 shows an example of the integrated force curve ∫FdD for a

measurement on an LFS-coated sample (5 coating cycles) in water. The

capillary is only thermodynamically stable when the ∆G < 0, which is only

the case for small separations (D ≲ 4 µm in the example in Figure 38).

When ∆G > 0 at large separations, the capillary is in a metastable state but

remains due to a high energy barrier for rupturing [118]. Only at

sufficiently large separations the energy barrier is overcome and the

capillary ruptures.

Calculation of the surface tension contribution from the capillary

shape

From the capillary shape evaluated from the confocal images, the surface

tension contribution to the free energy change due to capillary formation

GA could be calculated using Eq. 16. The three surface areas of the

capillary were determined from capillary images. The capillary surface

area on the particle Ap was estimated from the particle radius R and the

angle describing the position of the three-phase line :

𝐴p = 2π𝑅2(1 − cos 𝛽) (24)

The capillary surface area on the super liquid-repellent surface As was

estimated from the capillary diameter on the surface d:

𝐴s =π𝑑2

4 (25)

Finally, the capillary surface area of the gas-liquid interface Ai was

calculated as:

𝐴m = 2π ∫ 𝑟(𝑧)ℎ

0√1 + (

d𝑟

d𝑧)

2d𝑧 (26)

where r is the capillary radius at each pixel in the z-direction and h is the

height of the capillary gas-liquid interface.

73

Figure 39. Calculations of GA (blue symbols) and comparisons to

the integral of the force curve ∫FdD (solid black line) measured on

retraction for measurements in (a) water, (b) ethylene glycol and (c)

hexadecane.

74

Figure 39 shows examples of GA calculated during separation and it is

compared to the integral of the force curve ∫FdD for measurements in

water, ethylene glycol and hexadecane (Paper III). A deviation between

measurements and calculations at large D was observed in water and

ethylene glycol, while in hexadecane there was a reasonably good

agreement. The observed difference suggests that the force measured in

water and ethylene glycol, cannot be described by the surface tension

contribution alone and likely includes contributions from the pressure-

volume work VΔP and/or properties at the TPCLs. However in

hexadecane, the surface tension term likely provides the major contribution

and the other contributions are insignificant.

Estimating the capillary pressure from the difference between

measured and calculated free energy values

Even for a small capillary pressure, there may still be a significant

contribution from a V∆P term to the measured forces because of large

capillary volumes. If the difference in free energy between the force curve

integral ∫FdD and calculations of GA are accounted for by only a VΔP

term, we can estimate the capillary pressure from:

Δ𝑃 = −∫ 𝐹d𝐷−Δ𝐺𝛾𝐴

𝑉 (27)

In Paper III it was found that, when estimating the capillary pressure using

Eq. 27, only a small under pressure in the capillary (< 0.02 atm) was needed

to account for the observed differences in water and ethylene glycol.

Additionally, similar results were obtained in water for LFS-coated

superhydrophobic surfaces prepared by 2, 3 and 4 coating cycles (Paper

IV). Such a small under pressure in the capillary was judged to be

reasonable during our dynamic measurements and it provides a mechanism

for gas flow into the capillary that facilitates capillary growth.

Figure 40 shows an example of P calculated using Eq. 27 for a

measurement at an LFS-coated sample (5 coating cycles) in water. As seen

75

in Figure 40, calculations suggest that the under pressure is increasing at

large separations when the capillary is stretched out and the volume

decreases before rupture. However, this is not a reasonable scenario as it

would indicate that gas flows from a lower pressure region to a higher

pressure region. This observation instead indicates that additional free

energy contributions arising from TPCL effects need to be accounted for

to describe the measured force. The results also suggest that these effects

are particularly important at large separations when the capillary decrease

in size just before rupture (at D ≳ 25 m in Figure 40). This is further

supported by calculations for the LFS-coating prepared by 1 coating cycle,

suggesting an even larger increase in under pressure (up to 0.1 atm) with

increasing separation. In this particular case the contact angle hysteresis on

this coating is larger than for the other LFS-coatings, suggesting that TPCL

effects may play an even larger role for gas capillaries observed on this

surface.

Figure 40. Example of the capillary pressure P calculated from the

difference between ∫FdD and GA for a measurement in water.

Effects from three-phase contact lines

The results presented here suggest that effects from TPCLs may be

important when theoretically describing the total measured forces

involving super liquid-repellent surfaces. It was found that TPCL effects

76

seem to be more important for measurements involving the dip coated

superhydrophobic surface and the LFS-coating prepared with 1 coating

cycle. These observations are in line with the fact that these two surfaces

exhibit macroscopically more pinning of water, as measured by a larger

CAH (and smaller RA), as compared to the other four LFS-coatings.

During this thesis work, and in a related work [182], attempts were made

to estimate contributions from TPCLs as in pinning force and line tension.

However, it was found that while TPCL effects most likely do play a role,

calculations are challenging. One issue is to determine which contact angle

that should be used for calculations using Eqs. 18-19. When calculating the

pinning force, it could be argued that the advancing contact angle should

be used. This is because as long as the contact angle is less than the ACA,

the TPCL is pinned to the surface. As the contact angle becomes equal to

the ACA, the TPCL is moving over the surface as the capillary contracts

and the pinning force is zero. For the line tension, one approach may be to

use the receding contact angle in this case. Line tension usually manifests

as a difference between the microscopic and macroscopic (or

“equilibrium”) contact angles caused by a highly curved TPCL. However,

in this case, when the capillary is stretched out, the contact angle is forced

to change from its “equilibrium” (the contact angle when the capillary was

formed, i.e. RCA) and this change causes the tension in the TPCL. It was

found that results highly depended on the values of the macroscopic

contact angles and even small differences (sometimes only 1°) could lead

to large differences in the calculated values. Thus, I had to leave it by

concluding that TPCL effects likely are important, but calculations to

prove the point could not be done with sufficient accuracy.

Error analysis

Calculations of the theoretical forces from capillary shape may involve

errors, particularly since 2D images are used to estimate properties of a 3D

situation. First, the assumption that the capillary is perfectly axisymmetric

and that the image is recorded in the exact center, likely leads to errors in

determining e.g. capillary volume and surface areas. Additionally, due to

77

error propagation, even small errors in determining capillary

characteristics from the confocal image can lead to relatively large errors

in the calculated values. As an example, the error in calculating GA was

estimated using propagation of error analysis in Paper III. It was found that

for absolute errors as low as d = 1 µm and s = p = = 1°, will lead

to errors in GA in the order of 1 – 3 × 10-11 J for a measurement in water.

If the errors were increased to d = 2 µm and s = p = = 2°, the total

errors in GA increased to 3 – 6 × 10-11 J. In Figure 41 it can be seen how

large these errors are in relative terms.

Figure 41. Example of calculations of GA with estimation of

errors for a measurement in water. The shaded areas represent

GA+(GA) and GA-(GA) and the inner (darker) area is

calculated from errors d = 1 µm and s = p = = 1° and the

outer (lighter) from d = 2 µm and s = p = = 2°.

78

79

Chapter 5

Concluding remarks and future

perspectives

This thesis aimed to elucidate how super liquid-repellent surfaces interact

across a non-wetting liquid in order to get a more detailed understanding

of super liquid-repellence. The use of LSCM imaging in this work offered

the opportunity to study wetting characteristics at the microscale and was

essential for reaching many of the main conclusions. First, confocal

imaging successfully captured the microscopic events during AFM force

measurements between a superhydrophobic surface and a hydrophobic

microsphere in water, clearly visualizing the previously proposed events

of gas capillary formation, growth and rupture. Attractive interactions due

to capillary growth were confirmed by quantifying the capillary volume

from the shape of the capillary obtained from confocal images. Moreover,

LSCM was used to show the presence of a gaseous layer underneath the

liquid consistent with a Cassie-Baxter type of wetting state. The results

presented in this thesis support the view that this pre-existing gaseous layer

is responsible for the formation and growth of large gas capillaries at super

liquid-repellent surfaces. That the gaseous layer facilitates capillary

formation and growth was further supported by studying superhydrophobic

surfaces with different coating thicknesses. It was found that an increased

amount of available gas in the gaseous layer influenced the interactions

and allowed the capillary to grow larger during separation. In addition,

80

similarly shaped long-range capillary forces were observed in lower

surface tension liquids, provided that a gas layer was present at the surface.

Further, it was found that, due to the large amount of gas present in the

surface layer, a large capillary can form and grow even if the liquid wets

the particle surface. Finally, the capillary images allowed theoretical

calculations of the capillary interaction from the meniscus size and shape.

It was found that theoretical calculated forces can be consistent with the

measured forces. However, it was concluded that several contributions

need to be considered including effects from the three-phase contact lines.

Further, the calculations suggested an under pressure in the capillary and

this drives the gas to flow from the gaseous surface layer into the capillary,

facilitating growth during separation.

While the work presented in this thesis can hopefully contribute to the

general understanding of super liquid-repellent surfaces and how such

surfaces interacts with liquids, it also acts as a groundwork for further

studies on interactions involving super liquid-repellent surfaces. These

findings demonstrate the use of experimental techniques to visualize the

microscopic events of gas capillary formation and how the shape can be

further used for theoretical calculations. Yet, several questions need to be

further investigated in order to increase the knowledge of super liquid-

repellence and to aid future design and development of sustainable super

liquid-repellent materials. The next steps could be to perform more

systematic studies including different types of coatings, particles and

liquids. By systematically vary the contact angles of both the super liquid-

repellent surface and the colloidal probe, as well as the surface tension of

the liquid, more details on how the interactions and capillaries are affected

by the wettability of the two surfaces and the surface tension can be

investigated. It would be interesting to include studies involving model

surfaces with ordered structures such as pillars or re-entrant structures. A

challenge might be to fabricate complex structures on thin cover glass

which is necessary in order to use LSCM for visualization. However, the

use of ordered structures offers the possibility to vary different geometric

parameters such as height, spacing and liquid contact area, which can

81

contribute to the understanding of how local wetting characteristics

influence the interactions. Additionally, knowing the exact geometry

enables determination of the amount of gas present in the gaseous layer.

Finally, more studies including different systems allows for continued

investigations in theoretically describing the measured forces and the

contributions from TPCLs can be further elucidated. With these insights it

might be even possible to predict interactions between liquid-repellent

surfaces from theoretical calculations.

82

83

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