The Pennsylvania State University

The Graduate School

Department of Mechanical and Nuclear Engineering

DESIGN AND FABRICATION OF BIO-INSPIRED MULTIFUNCTIONAL SLIPPERY

SURFACE COATINGS WITH NOVEL FUNCTIONALITIES

A Dissertation in

Mechanical Engineering

by

Jing Wang

2018 Jing Wang

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Doctor of Philosophy

August 2018

The dissertation of Jing Wang was reviewed and approved* by the following:

Tak Sing Wong

Assistant Professor of Mechanical Engineering and Bioengineering

Dissertation Advisor

Chair of Committee

Pak Kin Wong

Professor of Bioengineering and Mechanical Engineering

Donghai Wang

Associate Professor of Mechanical Engineering

Seong Kim

Professor of Chemical Engineering

Karen Thole

Distinguished Professor

Head of the Department of Mechanical and Nuclear Engineering

*Signatures are on file in the Graduate School

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ABSTRACT

Bio-inspired multi-functional liquid-repellent surfaces have been intensively studied for

the past decades because of their significant potentials for many industrial and biomedical

applications. To this end, state-of-the-art surfaces including superhydrophobic surfaces,

superoleophobic surfaces, and slippery liquid-infused porous surfaces (SLIPS) have been

developed, where these surfaces display a number of remarkable functions such as self-cleaning,

anti-biofouling, anti-icing, drag reduction, enhanced heat transfer and water harvesting. Despite

recent advances, surfaces that possess self-cleaning and anti-fouling properties while at the same

time are capable of repelling both liquids and viscoelastic solids or with enhanced mechanical

robustness are rare. A central feature of this dissertation is to develop design principles to create

such a multi-functional repellent coating, and develop a facile and scalable fabrication method to

apply the coating onto common materials such as glass, ceramics, metals, and plastics. Here, I

have developed a new form of cross-species bio-inspired slippery liquid-infused porous surfaces

(X-SLIPS) that can self-repair under thermal stimulation even under large area of physical and

chemical damages. These thermally self-healing omniphobic coatings can be applied onto a broad

range of metals, plastics, glass, and ceramics of various shapes, and show excellent repellency

towards aqueous and organic liquids. In addition, I have designed and fabricated liquid-

entrenched smooth surfaces (LESS) – a sprayable, anti-biofouling surface coating that is capable

of repelling both aqueous liquids and viscoelastic solids with dynamic viscosities spanning over 9

orders of magnitude, i.e., three orders of magnitude higher than previously reported. LESS can

significantly reduce adhesion from different viscoelastic solids up to ~90% compared to state-of-

the-art liquid-repellent materials. The amount of water required to clean LESS is only ~10% of

that required for untreated surfaces. The results have implications for significant reduction of

wastewater generation and biofouling in industrial and household settings, which may mitigate

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global water scarcity and transmission of the infectious diseases, associated with poor sanitation

facilities.

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TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................. vii

LIST OF TABLES ................................................................................................................... xii

ACKNOWLEDGEMENTS ..................................................................................................... xiii

Chapter 1 Introduction ............................................................................................................. 1

1.1 A Brief Overview of Bio-inspired Liquid Repellent Surfaces ................................... 1

1.2 State-of-the-art Liquid Repellent Surfaces ................................................................. 4

1.2.1 Superhydrophobic and Superoleophobic Surfaces .......................................... 4

1.2.2 Slippery Liquid-infused Porous Surfaces ........................................................ 10

1.2.3 Limitations of the State-of-the-art Liquid Repellent Surfaces ........................ 15

1.3 Objectives of the Dissertation .................................................................................... 16

Chapter 2 Slippery Liquid-infused Porous Surfaces with Self-healing Property..................... 17

2.1 Motivation of the Study ............................................................................................. 17

2.2 Fabrication of Cross Species Bioinspired Slippery Liquid-Infused Porous

Surfaces (X-SLIPS) .................................................................................................. 19

2.3 Characterization of Self-Healing Mechanism ............................................................ 20

2.4 Robustness Characterization ...................................................................................... 25

2.5 Chapter Summary ...................................................................................................... 29

Chapter 3 Viscoelastic solid repellent surfaces ........................................................................ 30

3.1 Motivation of the Study ............................................................................................. 30

3.2 Design of Viscoelastic Solid Repellent Surfaces ....................................................... 33

3.2.1 Design Criteria I: Interfacial Adhesion ........................................................... 33

3.2.2 Design Criteria II: Interfacial Wetting ............................................................ 37

3.3 Fabrication of Liquid-Entrenched Smooth Surfaces (LESS) ..................................... 41

3.4 Adhesion of LESS against Viscoelastic Feces ........................................................... 42

3.5 Water Consumption of LESS ..................................................................................... 45

3.5.1 Simulated Toilet Flushing ............................................................................... 45

3.5.2 Water Consumption of Various Surfaces ........................................................ 47

3.6 Adhesion against Human Feces ................................................................................. 48

3.7 Anti-Bacterial Performance of LESS ......................................................................... 50

3.8 Durability of LESS ..................................................................................................... 55

3.9 Chapter Summary ...................................................................................................... 58

Chapter 4 Summary and Discussion ........................................................................................ 60

Appendix A Experiments on Fabrication and Characterization of X-SLIPS.......................... 62

Appendix B Calculation of Work of Adhesion ....................................................................... 65

vi

Appendix C Identification of Bacteria in Rainwater .............................................................. 66

References ................................................................................................................................ 67

vii

LIST OF FIGURES

Figure 1-1: A brief history of bio-inspired liquid-repellent surfaces. ...................................... 2

Figure 1-2: Other examples of bio-inspired liquid-repellent surfaces. The scanning

electron microscope (SEM) images for butterfly wing, mosquito eyes, and

leafhopper brochosomes are obtained from references [10], [11], and [12],

respectively. ..................................................................................................................... 3

Figure 1-3: State-of-the-art designs for superhydrophobic or superoleophobic surfaces,

including surface structure, structure orientation, structure size, and surface

chemistry. ......................................................................................................................... 5

Figure 1-4: Illustration of re-entrant and double re-entrant structures of superoleophobic

surfaces............................................................................................................................. 6

Figure 1-5: Cacida-inspired jumping droplet superhydrophobic surfaces for enhance heat

transfer. Image credit: Reference [18]. ............................................................................ 6

Figure 1-6: Chemical structures of PTFE, PDMS, and POSS. ................................................ 7

Figure 1-7: Re-entrant and double re-entrant surface structures created by silicon etching.

SEM images credit: Reference [5] and [7]. ...................................................................... 8

Figure 1-8: Superhydrophobic or superoleophobic surfaces created by micro/nano beads

self-assembly. a. Schematic showing how the self-assembled structure can repel

liquids. b. SEM image of self-assembled candle soot. c. SEM image of self-

assembled TiO2 beads. Image credit: reference [25] and [27]. ........................................ 9

Figure 1-9: Application examples of superhydrophobic surfaces. a. An illustration of drag

reduction. b. A demonstration of anti-icing performance on superhydrophobic

surfaces. c. Nano-cone shape superhydrophobic surface used for anti-fogging. d. A

patterned superhydrophilic/superhydrophobic surface for fog harvesting. Image

credit: Reference [15], [35], and [32]. .............................................................................. 10

Figure 1-10: Overview of state-of-the-art development of SLIPS. .......................................... 11

Figure 1-11: Schematic drawings illustrating the interfacial energy at different wetting

status. Image credit: Reference [8]. .................................................................................. 12

Figure 1-12: Schematic of SLIPS and its self-healing performance. a. Fabrication of

SLIPS. b. Self-healing of the lubricating layer on SLIPS and the comparison of

crude oil repellency between SLIPS and a Teflon-coated flat surface. ............................ 13

Figure 1-13: Application examples of slippery liquid-infused porous surfaces (SLIPS). a.

The anti-icing and frosting comparison of uncoated surfaces and SLIPS. b. Anti-

bacteria fouling comparison of superhydrophobic surfaces and SLIPS. c. Anti-

marine fouling performance of liquid-infused PDMS. d. Blood repellency of SLIPS-

viii

coated glass. e. An ultra-sensitive SERS detection SLIPS platform. Image credit:

References [45], [54], [48], [47], and [53], respectively. ................................................. 14

Figure 2-1: Cross-species bio-inspired slippery coating: X-SLIPS. a. Optical images

showing a plant leaf and a Nepenthes pitcher plant. b. Schematic showing the

concept of slippery coating fabrication process inspired by the slippery rim of the

Nepenthes pitcher plant. c. Schematic showing the concept of self-repairing inspired

by the wax repair of plant leaves. The excess silane layer (hydrophobic wax) on a

substrate serves a reservoir to repair the surface hydrophobicity, and to prevent the

lubricant from being displaced by the foreign liquids. Insets showing the liquid

repellency comparison of octane droplets on lubricated substrates with undamaged

(left) and damaged (right) silane coating. ........................................................................ 18

Figure 2-2: Scanning electron micrographs showing the surface morphologies of different

industrial metals (a) before and (b) after etching processes. ............................................ 20

Figure 2-3: Mechanism of thermal self-repairing. a. The contact angle change before and

after oxygen plasma damage, as well as after the thermal self-repairing. b. XPS data

of silanized surface before and after oxygen plasma, as well as after thermal

treatment. The substrate is titanium, and the silane is perfluorinated silane. The

atomic percent of fluorine and titanium at each condition is shown in the inset chart

and is the average of three different measuring areas. c. Schematics of self-repairing

mechanism, which involves the vaporization of the silane molecules. ............................ 22

Figure 2-4: Robustness and thermal self-healing performance on the chemical

intermediate layer of X-SLIPS. a. Optical images showing the water repellency

comparison between a damaged and a self-healed chemical intermediate layer on

titanium. b. A plot showing the thermal self-healing repeatability of the intermediate

layer of X-SLIPS. A 20 µL octane droplet was used for measuring the sliding angle

on a tilting stage. The intact or self-healing surface with lubricant has a very small

sliding angle of around 10o; while the surface after damage has a 90

o sliding angle. c.

A plot showing the dependence of thermal-healing time on applied temperature.

Damaged intermediate layer can be healed around 280 min at 60 ℃, while less 1

min at more than 180 ℃. The semi-log plot shows that the self-healing rate and the

heating temperature fits the Arrhenius relationship. In the inset, t is the thermal

healing time in second, and T is heating temperature in Kelvin. The sample for this

experiment is silanized titanium. Error bars indicate standard deviations for three

independent measurements. ............................................................................................. 23

Figure 2-5: Demonstration of silane evaporation upon substrate heating. a. Silicon wafer

1 is etched with microstructures and is highly hydrophilic. b. Silicon wafer 2 is a

silanized surface with etched microstructures and is highly hydrophobic. c, d.

Schematic showing the experimental setup. Note that wafer 2 was in contact with

the hot plate and was heated at a temperature of at 60 oC for >3 hours or 200

oC for

10 min. e. After the heating process, the wafer 1 was observed to become

hydrophobic, confirming that part of the silane molecules were transferred from

wafer 2 to wafer 1. ........................................................................................................... 25

ix

Figure 2-6: Coating adhesion test for the silanized textured surface and a commercial

surperhydrophobic coating. a. Performance comparison of the coating adhesion

between the silanized textured surface and the superhydrophobic coating. The

adhesion test protocol includes the use of a scotch tape to adhere onto the testing

surface with a 5 N weight on top of it. The tape was then peeled off from the

surface. After each peeling test, we measured the sliding angle of a 20 μL droplet of

deionized water. Low sliding angle indicated that the surface is slippery. b. Real

time images showing water repellency performance on the two surfaces after the

scotch tape peeling test. After 20 times of peeling, the commercial superhydrophobic

coating completely lost its superhydrophobicity; while our silane coating still

maintained its liquid repellency. Water is colored with blue dye for visual

representation. .................................................................................................................. 26

Figure 2-7: Self-repair of X-SLIPS coating. a. A plot showing the physical abrasion

performance comparison between lubricated and non-lubricated samples. Each

abrasion cycle involved dragging a sandpaper of average particle size of 470 µm

along with a pressure of 15.6 kPa across the surface with a velocity of ~0.02 m/s. b.

Optical images showing the thermal repair of X-SLIPS on glass, polyethylene, and

ceramic. The coating was damaged by physical abrasion of sandpaper. ......................... 28

Figure 2-8: Liquid repellency of X-SLIPS coating on various materials. a. A plot showing

the contact angle hysteresis of various liquids on the slippery surfaces. Inset shows a

dodecane droplet sliding with a very small sliding angle (less than 3°) on a coated

carbon steel substrate, indicating the super slippery nature of the surfaces. Error bars

represent the standard deviations for at least four independent measurements. b. The

coating method can be applied to various substrates such as stainless steel,

aluminum, titanium, glass, copper, carbon steel, ceramic, and polyethylene. The

yellow droplet is 10 μL dyed octane. c. Comparison of untreated and X-SLIPS-

coated stainless steels of different surface geometries (flat plate, pipe section, and

sphere) for the repellency of dyed octane (in yellow). ..................................................... 28

Figure 3-1: Overview of state-of-the-art liquid and viscoelastic solid repellent surfaces.

(A) Schematic and optical images showing the comparison of adhesion between

viscoelastic solids and different engineered surfaces including a superhydrophobic

glass (left), a SLIPS-coated glass (center), and a LESS-coated glass (right). The

superhydrophobic glass was created using a commercially available

superhydrophobic coating (NeverWet, LLC). The SLIPS-coated glass has an

underlying surface roughness ~1 µm. Synthetic feces with a solid content percentage

of 30% (dynamic viscosity = ~2406 Pa·s) were used in these experiments. (B) A plot

showing the reported dynamic viscosity range of liquids and viscoelastic solids that

can be repelled by the state-of-the-art liquid repellent surfaces and the present work. ... 31

Figure 3-2: Schematics showing adhesion of viscoelastic solids on surfaces with different

surface treatments. Adhesion of viscoelastic solids on a) an untreated surface; b) a

silanized surface; c) a lubricated surface without chemical treatment; and d)

lubricated surface with silanization treatment. ................................................................. 34

x

Figure 3-3: Measured viscoelasticity of synthetic feces. The storage and loss moduli of

synthetic feces with different solid content fraction (e.g. 10%, 20%, 30%, 40%,

50%, and 60% of solids in synthetic feces). The solid and open symbols represent

the storage modulus and the loss modulus, respectively. ................................................. 35

Figure 3-4: Comparison of the Dahlquist criterion with the measured storage moduli of

different synthetic feces. The storage moduli of synthetic feces with different solid

content fraction (e.g. 10%, 20%, 30%, 40%, 50%, and 60% of solid in synthetic

feces) under frequency of 1 rad/s. The error bars represent standard deviations of

storage moduli from three independent measurements. ................................................... 35

Figure 3-5: Fabrication of liquid-entrenched smooth surfaces (LESS). (A) Schematic

illustration showing a two-step spray coating process to form LESS. (B) Optical

images showing the individual coating processes on glass. The lubricant used was

silicone oil and the blue testing liquid was dyed water. ................................................... 41

Figure 3-6: XPS characterizations of silicon element on untreated (e.g. glass) and PDMS-

grafted substrates. a, The peak of silicon element is originated from silicon dioxide,

which is the main component of glass. b, The peak of silicon element (Si 2p) is

originated from silicon dioxide and silicone, which confirmed the presence of the

PDMS on the PDMS-grafted substrate. ........................................................................... 42

Figure 3-7: Adhesion comparison among different surfaces. .................................................. 43

Figure 3-8: Adhesion comparison among surfaces with different roughness. ......................... 44

Figure 3-9: Surface roughness measurement on different surfaces. ........................................ 45

Figure 3-10: Water consumption on different surfaces under different flow rate with

impact of different feces................................................................................................... 48

Figure 3-11: Experimental setup of simulated flushing. .......................................................... 48

Figure 3-12: Comparison of adhesion on different surfaces with impact of human fecal

waste. ............................................................................................................................... 49

Figure 3-13: Adhesion on SLIPS with impact of human fecal waste. ..................................... 50

Figure 3-14: Rainwater bacteria concentration measurement. ................................................. 51

Figure 3-15: Bacteria adhesion measurement on different surfaces. ....................................... 52

Figure 3-16: Sliding angle of urine on different surfaces. ....................................................... 53

Figure 3-17: Sterilization of liquid-infused surfaces with different underlying roughness. .... 54

Figure 3-18: Rainwater bacteria identification. ....................................................................... 54

Figure 3-19: Durability measurement on LESS with contamination of different feces. .......... 55

xi

Figure 3-20: Durability and replenishment of the lubricant layer on LESS. ........................... 57

xii

LIST OF TABLES

Table 3-1: Compositions of the synthetic feces. ...................................................................... 37

Table 3-2: Surface energy components for materials at ambient environment. ....................... 40

Table 3-3: Calculated spreading parameters (S) and Hamaker constant (A) for liquids on

PDMS-grafted glass. ........................................................................................................ 40

Table 3-4: Physical parameters for the simulated-toilet flushing systems. .............................. 47

Table 3-5: Bacteria identification from mass spectrometry. .................................................... 51

xiii

ACKNOWLEDGEMENTS

I would like to thank my adviser Professor Tak Sing Wong for his guidance and

encouragement over the course of this research. His leadership has not only enabled me to

develop as a graduate student but also instilled in me a sense of passion for the work one is

committed to. I would also like to thank my committee Professor Pak Kin Wong and Dr. Leon

Williams of Crafield University, UK, for their input in this research.

As the first PhD student in Prof. Wong’s group, I am extremely fortunate to experience

the establishment and development of the laboratory and the research team. I am grateful that

Prof. Wong has brought so many talented researchers and friendly colleagues in the group. I

would like to thank former postdoctoral researchers in the group, specifically, Dr. Shikuan Yang

for sharing his research experience and encouraging me to continue my research journey, Dr.

Xianming Dai and Dr. Yu Huang for involving me in their research projects. I would also like to

thank my colleagues, Nan Sun, Birgitt Boschitsch, Ryan Sikorski, Lin Wang, Antonino

Turrigiano, for the help in experiments and many fruitful discussions.

I would like to gratefully acknowledge the support of the National Science Foundation

(Grant Nos. 1351462 and 1757165) and PPG Foundation for funding this research. I would like to

thank the Materials Research Institute, Pennsylvania State University for using of their facilities

for micro/nano fabrication and material characterization. Specifically, I would like to thank Vince

Bojan for his help on XPS measurement, Dr. Bangzhi Liu for his help on SEM imaging, Michael

Labella and Chad Eichfeld for their help on photolithography, Tim Tighe for his help on AFM

and optical profilometer, Josh Stapleton for his help on optical measurements, and Maria Dicola

for her help and supervise on the laboratory safety.

xiv

I am forever indebted to my parents Zhuangzhi Wang and Liping Ma for their selfless

love, support, and encouragement throughout my life. They are the ones who are wholly and

solely behind all that I am. Finally, I would like to thank my beloved wife Yiyu Xu and lovely

daughter Skylar Wang, without whom I cannot achieve this work.

Chapter 1

Introduction

1.1 A Brief Overview of Bio-inspired Liquid Repellent Surfaces

Liquid repellency is a crucial property for various living species in nature, ranging from

plants to insects to animals. Researchers have been inspired by these species to design and

fabricate various liquid repellent surfaces for industrial, biomedical, agricultural, and daily-life

applications. In 1944 (Figure 1-1) [1], Cassie and Baxter studied how ducks use their feathers to

stay dry when contacting water, and developed the well-known Cassie-Baxter equation, which

predicts the influence of surface structure and chemistry on liquid repellency. This equation

becomes one of the most important fundamental governing equations in surface wetting science.

The Cassie-Baxter equation is also a powerful tool to guide the surface structure design for

superhydrophobic surfaces. With the first synthesis of polytetrafluoroethylene (PTFE) in 1938,

which is also known as Teflon, researchers have been trying to create man-made

superhydrophobic surfaces since then. The first reported synthetic superhydrophobic surface was

developed using roughened PTFE in 1991 [2]. In the late 1990s, Barthlott and Neinhuis [3] first

revealed the mechanism of the superhydrophobicity on lotus leaf by studying the surface structure

using high resolution scanning electron microscope (SEM). Lotus leaf can repel water and self-

clean because of the trapped air within its hierarchical micro and nano surface structures. The

contact angle of a water drop on a lotus leaf is generally around 170o. Since then, people have

discovered that many plant leaves also exhibit superhydrophobicity with a water contact angle of

around 160o.

2

Figure 1-1: A brief history of bio-inspired liquid-repellent surfaces.

In the 21st century, superhydrophobic surfaces have been developed rapidly for various

applications. Specifically, the high demand of oil-repellent surfaces has greatly accelerated the

progress of the field. Though oil-repellent surfaces have been created with porous PTFE fibers

[4], the underlying mechanism had not been understood until the creation of re-entrant surfaces in

2007 [5]. The re-entrant surface structure, also known as hoodoo structure, has been focused in

the past decade for creating scalable and durable superoleophobic surfaces. Four years later after

the discovery of re-entrant surfaces, people found out that natural species also use this special

surface structure to keep their body clean from muddy environment [6]. These nature species are

springtails. Springtails live in a harsher environment than plant leave, and thereby require better

liquid-repellency. To adapt to the muddy underground environment, springtails have evolved

these re-entrant or even double re-entrant surfaces in different body parts. Inspired by these

underground hexapods, the double re-entrant surfaces were first fabricated in 2014 [7]. Different

from hierarchical and re-entrant structure, which requires low surface energy chemical coatings,

the double re-entrant structure does not require surface chemistry modification to achieve

superoleophobicity.

3

Both superhydrophobic and superoleophobic surfaces use surface structures to trap a thin

layer of air in order to create a minimal contact of water towards solid surfaces. In this case,

liquid drops typically maintain a near spherical shape due to capillarity and roll off these surfaces

easily. In 2011, another type of liquid-repellent surfaces was developed by infusing lubricants

into porous surfaces [8]. Through the lubrication, surfaces can achieve repellency towards

immiscible foreign liquids. This liquid-infusing strategy was inspired by the Nepenthes pitcher

plant, whose rim gets wetted in moisture and becomes super slippery. Insects, such as ants, use

viscous oils on their feet to walk on smooth surfaces [9]. Owing to the immiscibility of the

adhesive oil and the water film on rim surfaces, ants will slide off from rim of pitcher plant due to

hydroplaning. Bohn and Federle discovered this passive pray strategy on pitcher plant in 2004

[10], which inspired material scientists to create the slippery liquid-infused porous surfaces

(SLIPS) and open up a new research direction in liquid-repellent surface design.

Figure 1-2: Other examples of bio-inspired liquid-repellent surfaces. The scanning electron

microscope (SEM) images for butterfly wing, mosquito eyes, and leafhopper brochosomes are

obtained from references [11], [12], and [13], respectively.

4

With over ~8.7 million eukaryotic species predicted globally [14], there are many more

biological examples that could inspire us for designing liquid-repellent surfaces with special

functions. In Figure 1-2, I have listed a few well-known species that have been studied in recent

years. For example, the butterfly wing surfaces have anisotropic micro/nano structures, which

makes water droplets roll in the direction away from butterfly body [15]. These structures inspire

research of directional superhydrophobic surfaces and directional slippery rough surfaces for drop

transportation and condensation applications, respectively. Additionally, the mosquito eyes have

been proven to repel micro droplets, showing significant anti-fogging performance [12]. A recent

research has revealed that the nanoscale pillars play an important role of anti-fogging [16], which

is significant for astronauts’ face shields [17]. In another example, the leafhopper brochosomes

demonstrate strong water repellency [18] as well as ultra-anti-reflective property. Researchers

have successfully manufactured the artificial brochosomes recently and demonstrated their strong

anti-reflection property [13]. These nano porous micro balls could be utilized as a surface coating

to enhance solar energy absorption. In summary, nature has been inspiring us to create novel

surface designs for a variety of applications, especially for liquid repellent coatings.

1.2 State-of-the-art Liquid Repellent Surfaces

1.2.1 Superhydrophobic and Superoleophobic Surfaces

Superhydrophobic and superoleophobic surfaces have been intensively studied in three

major areas: design, fabrication, and applications. In this section, I will briefly introduce the state-

of-the-art superhydro(oleo)phobic surfaces in each of these areas.

5

Figure 1-3: State-of-the-art designs for superhydrophobic or superoleophobic surfaces, including

surface structure, structure orientation, structure size, and surface chemistry.

Design: To design a superhydrophobic or superoleophobic surfaces, researchers have

focused on different design parameters, including surface structure, structure size, structure

orientation, and low surface energy chemicals, (Figure 1-3). Since the discovery of the

micro/nano hierarchical structure on lotus leaf, various surface structures have been designed

based on fundamental thermodynamic principles or by direct mimicking of specific natural

species. Among various surface structure designs, the re-entrant and double re-entrant surface

structures are the most significant designs for robust superhydrophobic surfaces. The re-entrant

structure enables robust superhydrophobicity on hydrophobic materials; while the double re-

entrant structure extends the robustness liquid repellency to hydrophilic materials (Figure 1-4). In

addition to re-entrant and double re-entrant structures, many other surface structures have been

designed to address different application requirements, for example, cone-shape structure (Figure

1-5) has been designed to induce jumping droplet effect for enhanced heat transfer in

condensation [19]. This surface structure was inspired by the surface of cicada wing, which can

stay dry in high moisture environment. Furthermore, by tuning the size of the cone-shape

structure, researchers have achieved a number of significant improvements on superhydrophobic

6

surfaces for anti-fogging and anti-icing applications. With a close-packed nano-size cone-shape

structure, the surface has demonstrated an extraordinary anti-fogging performance with a good

optical transparency, since the structure size is smaller than the wavelength of the visible light

[20, 21]. When the cone-shape is sub-millimeter size, the contact time of an impacting water drop

is shortened by four times comparing to regular hierarchical structured superhydrophobic surfaces

[22]. This finding has a great potential for anti-icing applications.

Figure 1-4: Illustration of re-entrant and double re-entrant structures of superoleophobic surfaces.

Figure 1-5: Cacida-inspired jumping droplet superhydrophobic surfaces for enhance heat transfer.

Image credit: Reference [19].

In addition to the shape of the surface structures, researchers have studied the influence

of structure orientation on superhydrophobicity and the synthesis of new low surface energy

chemicals to improve superhydrophobicity. McCarthy and his co-workers [23] have studied the

influence of contact line distortion by the orientation of surface structures and discovered that a

random arrangement of the pillars on superhydrophobic surfaces reduces the contact angle

7

hysteresis of a liquid droplet. Since the synthesis of PTFE and PDMS around 1940, researchers

have synthesized numerous silanes and siloxanes as low surface energy chemicals to create

superhydrophobic surfaces. One of the well-known chemicals developed in recent years is the

polyhedral oligomeric silsesquioxane (POSS) (Figure 1-6) [5], which has been successfully used

to fabricate robust superoleophobic surfaces.

Figure 1-6: Chemical structures of PTFE, PDMS, and POSS.

Fabrication: To fabricate superhydrophobic or superoleophobic surfaces, many

manufacture methods have been developed, with a particular focus on fabrication scalability and

the coating robustness. One early conventional method to create superhydrophobic surfaces is to

deposit low surface energy chemicals (e.g. PTFE) on rough or porous surfaces [2]. With a better

understanding of superhydrophobic surface design, various methods have been developed, such

as direct replication of natural surfaces [24], silicon etching [25], self-assembly of micro and nano

beads [26], block copolymer formation [27], etc. For example, the re-entrant and double re-

entrant structures are examples of super/oleohydrophobic surfaces fabricated using silicon etching

method (Figure 1-7). However, the key drawback of direct replication and silicon etching

methods is that they are scalable and for silicon etching, the process is relatively expensive.

8

Figure 1-7: Re-entrant and double re-entrant surface structures created by silicon etching. SEM

images credit: Reference [5] and [7].

To realize scalable superhydrophobic surfaces, several methods have been developed in

past decades. One of the most widely adapted methods is the self-assembly of micro and nano

beads (e.g. SiO2 and TiO2) (Figure 1-8) [26, 28]. Micro and nano beads with a certain ratio are

suspended in a silane solution. With the help of super glue, this mixture is sprayed onto the glue

layer, and strongly adheres onto the surface. Hierarchical or even re-entrant surface structures can

be created by this method. In a recent study, an organic superhydrophobic coating was created by

using PTFE beads [29]. Although the robustness of these superhydrophobic surfaces are stronger

than silicon-based superhydrophobic surfaces, these surfaces still cannot withstand extremely

harsh working environments, such as strong mechanical abrasion, high hydrostatic pressure,

marine biofouling, etc.

9

Figure 1-8: Superhydrophobic or superoleophobic surfaces created by micro/nano beads self-

assembly. a. Schematic showing how the self-assembled structure can repel liquids. b. SEM

image of self-assembled candle soot. c. SEM image of self-assembled TiO2 beads. Image credit:

reference [26] and [28].

Applications: Superhydrophobic surfaces have shown a number of potential applications

(Figure 1-10), such as drag reduction [30, 31], enhanced heat transfer [32], fog harvesting [33],

anti-fogging [16, 34], anti-icing [35, 36], etc. All these applications are enabled by the thin air

layer created by the surface structure and chemistry of the superhydrophobic surfaces. A number

of superhydrophobic surfaces are currently being commercialized, e.g., NeverWet®. Although

there are many potential applications of the superhydrophobic or superoleophobic surfaces,

pressure stability, scalable fabrication, and coating robustness are still major issues that need to be

solved.

10

Figure 1-9: Application examples of superhydrophobic surfaces. a. An illustration of drag

reduction. b. A demonstration of anti-icing performance on superhydrophobic surfaces. c. Nano-

cone shape superhydrophobic surface used for anti-fogging. d. A patterned

superhydrophilic/superhydrophobic surface for fog harvesting. Image credit: Reference [16], [36],

and [33].

1.2.2 Slippery Liquid-infused Porous Surfaces

To address the poor pressure stability in superhydrophobic or superoleophobic surfaces,

another type of liquid repellent surface was developed by changing the air-cushion into a liquid

lubricating layer, which makes the foreign immiscible liquid drops float on this lubricant layer.

This surface was first developed by infusing lubricant into a porous surface in 2011 [8] and

showed strong omniphobicity and great potential for various applications. The surface was

11

referred to as slippery liquid-infused porous surfaces (SLIPS). After the development of SLIPS,

many slippery liquid-infused surfaces have been designed, fabricated, and applied to numerous

applications, shown in Figure 1-10. I will briefly introduce some of the state-of-the-art

development of SLIPS in design, fabrication, and application.

Figure 1-10: Overview of state-of-the-art development of SLIPS.

Design: Currently, the design of slippery liquid-infused porous surfaces (SLIPS) is

primarily based on thermodynamic analysis. From the initial design of SLIPS, Wong and his co-

workers compared the surface energy of different wetting status (Figure 1-11), including liquid A

on substrate (EA), liquid B on substrate (E2), and liquid B on substrate with the present of liquid A

(E1). When the interfacial energy of EA is larger than E1 and E2, the surface is more favorable to

be wetted by liquid B in the present of both air and liquid A. In another method, Smith et al. have

introduced spreading parameter (S) to make sure that the lubricant can spread and wet on the

substrate [37]. In 2017, Wang and her co-workers [38] developed design criteria purely based on

12

the spreading parameters under several different wetting conditions. In the same year, Daniel et

al. [39] introduced disjoining pressure into the design criteria, which further strengthens the

design criteria of SLIPS.

Figure 1-11: Schematic drawings illustrating the interfacial energy at different wetting status.

Image credit: Reference [8].

Fabrication: Since the inception of SLIPS in 2011 (Figure 1-12), various fabrication

methods to create slippery liquid-infused surfaces have been proposed. The initial SLIPS was

created by infusing lubricating oils into porous polymers with a pore size ranging from few

hundred nano meters to a few microns. Another approach to create SLIPS-like material was

proposed by Jiang and his co-workers, who took advantage of nano porous network of cross-

linking polymers to create an oil-infused organo-gel [40]. This organo-gel can retain lubricant

within the bulk material, which can replenish the lubricant lost at the surface and thereby

enhancing the durability of its slippery function. Other exploration of lubricant retention

systematically studied the influence of the surface roughness scale, finding out that the nano

structure has the best lubricant retention under shear stress [41]. Moreover, to further enhance the

robustness of SLIPS, autonomous self-replenishment properties have also been introduced to this

liquid-infused surface system [42]. In addition to lubricant retention, Yao et al. developed a

13

dynamic slippery liquid-infused surface, which has the capability of controlling droplet mobility

through a mechanical control of the exposure of surface roughness towards liquid drops [43].

Furthermore, Wong and his co-workers introduced the concept of slippery rough surfaces (SRS),

on which, droplets are still mobile in Wenzel state [44]. Different from SLIPS, the SRS remove

the lubricant between microscale roughness and retain the lubricant in nanoscale roughness,

which creates higher exposing area to the environment while maintaining slipperiness towards

various liquids. Besides the fabrication relating to lubrication and surface roughness, optical

transparency is another important factor for various applications. In 2013, Vogel et al. fabricated

the transparent slippery liquid-infused porous surfaces by inverse colloidal monolayers [45].

Figure 1-12: Schematic of SLIPS and its self-healing performance. a. Fabrication of SLIPS. b.

Self-healing of the lubricating layer on SLIPS and the comparison of crude oil repellency

between SLIPS and a Teflon-coated flat surface. Image reference: [8].

Application: Owing to the high mobility and pressure stability of the lubricating layer,

these liquid-infused surfaces show many advantages over superhydrophobic or superoleophobic

surfaces, in terms of reduced adhesion towards various fluids, rapid and repeatable self-healing,

14

high temperature, and extreme pressure stability (up to ~676 atm) [8, 46]. The development of the

liquid-infused surfaces has opened up novel solutions to many challenging problems including

icing and frosting [47, 48], biofouling [45, 49, 50], drag reduction [51], condensation [52-54], and

single molecule detection [55] (Figure 1-13).

Figure 1-13: Application examples of slippery liquid-infused porous surfaces (SLIPS). a. The

anti-icing and frosting comparison of uncoated surfaces and SLIPS. b. Anti-bacteria fouling

comparison of superhydrophobic surfaces and SLIPS. c. Anti-marine fouling performance of

liquid-infused PDMS. d. Blood repellency of SLIPS-coated glass. e. An ultra-sensitive SERS

detection SLIPS platform. Image credit: References [47], [56], [50], [49], and [55], respectively.

In the anti-icing application, slippery liquid-infused surface systems have shown great

reduction on ice adhesion strength, from ~100 kPa (conventional superhydrophobic approach) to

~10 kPa (liquid-infused gel) [57], even down to 2 Pa (magnetic fluids) [58]. With the pressure

stability and immiscibility to the bacteria solution, SLIPS have been demonstrated excellent anti-

15

bacteria performance, especially outperforms superhydrophobic surface over long time exposure

to bacteria [59]. In addition to repel bacterial fouling, SLIPS have found to exhibit great

performance on repelling mussel adhesion (Figure 1-13), which is considered as one of the

stickiest creatures in the world. Furthermore, SLIPS have been demonstrated to repel various

biological fluids, leading to many biomedical applications, such as tubing and catheters [49, 60],

endoscopy [61], dental care [62], etc. With high droplet shading rate, SLIPS with certain

roughness designs have also shown extradentary water collecting ability through condensation

and fog harvesting, for example, the bumpy SLIPS [63] and directional SRS [54]. In addition,

SLIPS can also be used as a detection platform for surface enhanced Raman scattering (SERS),

since any droplets would shrink and evaporate into a small dot without contact line pinning. This

process can dramatically increase the concentration of the targeting material, which then would

be easily detected by SERS.

1.2.3 Limitations of the State-of-the-art Liquid Repellent Surfaces

Current state-of-the-art liquid repellent surfaces usually have three components: surface

structure/roughness, chemical functional layer, and repellent functional layer (air-cushion or

lubricant layer). The damage of any components would cause the loss of the liquid repellency on

the surface. Although slippery liquid-infused porous surfaces (SLIPS) have already shown a great

pressure stability and self-healing of the lubricant, significantly enhancing the robustness of

repellent functional layer from superhydro(oleo)phobic surfaces, the chemical functional layer

can still be easily damaged through mechanical abrasion, chemical reaction (e.g., oxygen

plasma), high temperature (e.g., flame), etc. Introducing a robust and durable chemical functional

layer for SLIPS would further enhance the robustness of this liquid repellent coating system and

16

provide a slippery liquid-infused coating method more applicable for various industrial

applications.

Surface coatings that can repel various liquids are readily available; however state-of-the-

art coatings that can repel viscoelastic materials (e.g., feces) and exhibit strong anti-bacterial

properties are rare [50]. Viscoelastic materials usually exhibit strong adhesion towards various

surfaces and cause serious problems, for example, marine biofouling and congestion of oil

pipelines. Therefore, the design and fabrication of such viscoelastic solid repellent surfaces is

greatly desired and will further expand the repellency range of the current liquid repellent

surfaces.

1.3 Objectives of the Dissertation

This dissertation investigates the design and the fabrication for slippery liquid-infused

surfaces to enhance its robustness and to achieve super repellency towards both liquids and solids

for various applications. To accomplish this, the specific objectives are:

1. To develop a method to further enhance robustness and durability of slippery liquid-

infused surfaces in their chemical functional layer;

2. To explore the underlying mechanism of the self-healing;

3. To design and develop a coating method to repel viscoelastic solids for water-saving

application;

4. To develop a simple coating method for creating slippery liquid-infused surfaces on

common materials.

Chapter 2

Slippery Liquid-infused Porous Surfaces with Self-healing Property

2.1 Motivation of the Study

The recent development of Nepenthes pitcher plant inspired slippery liquid-infused

porous surfaces (SLIPS) has shown great promise in medical and industrial applications with

functions ranging from self-cleaning [8, 64], anti-icing [47], anti-fouling and anti-coagulation

[49, 56, 65, 66], anti-staining [67], and drag reduction [68, 69], to enhanced condensation heat

transfer [52, 53]. Applying SLIPS onto a substrate requires a micro/nano-textured surface with

proper surface chemical functionalization so that a liquid lubricant with lower surface energy will

completely wet the substrate. The lubricant layer is dynamically reconfigurable due to its liquid

state and lends SLIPS the ability to self-heal after damage. However, such self-healing requires

the presence of a molecularly continuous or nearly continuous underlying chemically

functionalized layer; if this layer is damaged, the lubricating liquid can no longer form a

continuous layer that adheres to the substrate, permanently damaging the overall liquid repellency

of the material. One method nature uses to address issues of self-repairing is exhibited in plant

leaves, which have a hydrophobic wax layer to protect them from biological and environmental

stresses, such as excessive water transpiration and irradiation from sunlight. If the protective layer

is damaged, the wax layer can be repaired by the plant to restore its function [70-75].

18

Figure 2-1: Cross-species bio-inspired slippery coating: X-SLIPS. a. Optical images showing a

plant leaf and a Nepenthes pitcher plant. b. Schematic showing the concept of slippery coating

fabrication process inspired by the slippery rim of the Nepenthes pitcher plant. c. Schematic

showing the concept of self-repairing inspired by the wax repair of plant leaves. The excess silane

layer (hydrophobic wax) on a substrate serves a reservoir to repair the surface hydrophobicity,

and to prevent the lubricant from being displaced by the foreign liquids. Insets showing the liquid

repellency comparison of octane droplets on lubricated substrates with undamaged (left) and

damaged (right) silane coating.

Inspired by the concepts of plant leaf wax repair [71-73] and the slippery surfaces of the

Nepenthes pitcher plant [10] (Figure 2-1), we report a new form of cross-species bioinspired

slippery liquid-infused porous surfaces (X-SLIPS) with a chemically functionalized layer that can

be self-repaired by heating, even after significant physical and chemical damage over large areas.

Our approach distinguishes from other state-of-the-art self-healing SLIPS coatings, which are

only capable to repair damages of physical size on the order of 1 mm [8, 76, 77]. We further

19

demonstrate that the thermally self-healing slippery coatings can be applied onto a broad range of

metals, plastics, ceramics, and glass of various shapes, and exhibit excellent repellency towards

various aqueous and organic liquids.

2.2 Fabrication of Cross Species Bioinspired Slippery Liquid-Infused Porous Surfaces (X-

SLIPS)

To create an X-SLIPS coating, we first created micro/nanoscale textures on a surface via

acid etching or sand blasting (Figure 2-2). [41] The textured surfaces are coated with

perfluorinated silane through liquid-phase silanization, making the surfaces highly hydrophobic

(Figure 2-1). The silane layer serves as an intermediate layer between the textured solid and the

lubricant to induce complete wetting of the liquid lubricant. While a monolayer of silane is

sufficient to create this intermediate layer, the important step here is that we prolong the

silanization process to ensure excess silane molecules are deposited onto the textured surfaces.

The excess silane molecules serve as a reservoir for the thermal self-repairing process (Figure 2-

1). After the silanization, we apply perfluorinated lubricant onto the chemically functionalized

surface by a spray coating or spin coating process. Since perfluorinated lubricant is immiscible to

both aqueous and organic phases, the resulting surface can then repel a broad range of aqueous

and organic fluids. The detailed fabrication process and materials list are in Appendix A.

20

Figure 2-2: Scanning electron micrographs showing the surface morphologies of different

industrial metals (a) before and (b) after etching processes.

2.3 Characterization of Self-Healing Mechanism

To characterize the thermal self-repairing process of the chemical intermediate layer (i.e.,

non-lubricated silanized substrate), we chemically damaged the silane layer on textured titanium

surfaces using oxygen plasma prior to lubrication. Applying oxygen plasma across the whole

surface (surface area: 4 cm2) critically damages the silane layer, as is evident from the dramatic

change in static contact angle; the static contact angle was greater than ~130o (highly

hydrophobic) prior to damage and less than 5° (superhydrophilic) after damage, as shown in

Figure 2-3. This is further confirmed by the X-ray photoelectron spectroscopy (XPS) analysis

(Figure 2-3), which showed that the atomic percent of the fluorine element drops significantly,

from 50.9% to 13.1%, indicating that a significant portion of the silane coating (C-F bond) has

been destroyed. The superhydrophilic nature of the damaged sample is attributed to the

21

generation of hydrophilic Si-O-chain network at the silane surface layer, produced during a

chemical reaction between the silane and oxygen plasma. [78] The undamaged perfluorinated

silane in the valleys of the textures, as well as those present underneath the network of Si-O

chains could serve as a reservoir to replenish the perfluorinated functional groups at elevated

temperatures.

To demonstrate that these reservoirs could facilitate functional layer repairing, we heated

the substrate at 150 ℃ for 5 min. After this heating process, we observed that the contact angle

returned to more than 150o (Figure 2-3), indicating the regeneration of the surface hydrophobic

property. Furthermore, the fluorine concentration increased and returned to the initial level after

the heating process. Specifically, XPS analysis demonstrated that the atomic percent of fluorine

returned to 54.4% after the heating process. If this increase in fluorine is a result of fluorine re-

functionalizing the damaged regions of the substrate, we would expect that it will cause a

decrease in non-functionalized substrate (here, titanium). We observed that the change in atomic

percent of titanium is indeed converse to the change of fluorine, confirming that the fluorine

reattached to the damaged substrate areas during the thermal process. The absence of titanium

after self-repairing suggests that the silane has become uniformly distributed again on the

textured titanium surface with a thickness larger than 10 nm–the penetration depth of the X-ray in

the X-ray photoelectron spectroscopy. This observation indicates the complete recovery of

perfluorinated silane on the whole titanium surface. The repaired silane layer is fully functional as

the perfluorinated lubricant can stably wet the repaired layer without being displaced by external

fluids, such as octane (surface tension ~21.6 mN/m).

22

Figure 2-3: Mechanism of thermal self-repairing. a. The contact angle change before and after

oxygen plasma damage, as well as after the thermal self-repairing. b. XPS data of silanized

surface before and after oxygen plasma, as well as after thermal treatment. The substrate is

titanium, and the silane is perfluorinated silane. The atomic percent of fluorine and titanium at

each condition is shown in the inset chart and is the average of three different measuring areas. c.

Schematics of self-repairing mechanism, which involves the vaporization of the silane molecules.

To further explore the thermal self-repairing performance, we investigated the healing

repeatability at a given temperature (Figure 2-4). In these tests, the silanized titanium pieces were

chemically damaged by 3 min oxygen plasma, and then heated at 250 ℃ for self-repairing. The

titanium pieces can last for at least seven damage-healing cycles. To further explore the

maximum number of damage-healing cycles on silanized titanium surfaces at a given

temperature, we repeated the damage-and-healing cycles, and counted the number of successful

healing until the samples failed to repair after 2 hours of continuous heating. At a healing

23

temperature of 150 ℃ and 180 ℃, the coating can fully repair for 18±1 and 16±2 times,

respectively. If the applied temperature is reduced to 120 ℃, the number of re-healing cycles is

25±2. The maximum number of self-healing cycles reduces as the applied temperature increases.

This reduction is attributed to the vaporization of the silane molecules such that some of these

molecules do not redeposit onto the substrate. The detailed experiment process refers to Appendix

A.

Figure 2-4: Robustness and thermal self-healing performance on the chemical intermediate layer

of X-SLIPS. a. Optical images showing the water repellency comparison between a damaged and

a self-healed chemical intermediate layer on titanium. b. A plot showing the thermal self-healing

repeatability of the intermediate layer of X-SLIPS. A 20 µL octane droplet was used for

measuring the sliding angle on a tilting stage. The intact or self-healing surface with lubricant has

a very small sliding angle of around 10o; while the surface after damage has a 90

o sliding angle. c.

A plot showing the dependence of thermal-healing time on applied temperature. Damaged

intermediate layer can be healed around 280 min at 60 ℃, while less 1 min at more than 180 ℃.

24

The semi-log plot shows that the self-healing rate and the heating temperature fits the Arrhenius

relationship. In the inset, t is the thermal healing time in second, and T is heating temperature in

Kelvin. The sample for this experiment is silanized titanium. Error bars indicate standard

deviations for three independent measurements.

Furthermore, we investigated the relationship between the repairing temperature and the

time for the complete healing of the chemical functionalized layer (Figure 2-4). To investigate

this relationship, we first treated the silanized titanium surface with oxygen plasma to damage the

surface functional group, and then heated the substrate at different temperatures for specific time

intervals until the surface hydrophobicity was recovered. Thermal-healing time is highly

dependent on the heating temperature. For example, it takes less than 5 min at temperature higher

than 150 ℃ for complete healing, whereas the healing would take ~300 min at temperature of 60

℃. The temperature dependence of healing time follows the Arrhenius equation (Figure 2-4),

which can be expressed as,

1

ln( ) ln( )aEk A

R T

(2-1)

where k is the self-healing rate at the absolute temperature T (in kelvins) – here we

consider the reciprocal of time (s-1

) to be the self-healing rate. A is the pre-exponential factor, Ea

is the activation energy, and R is the universal gas constant (8.31 kJ/kmol·T). This equation

allows us not only to predict the time required for the self-healing to occur at a specific

temperature, but also to understand the thermal-healing mechanism. The activation energy of self-

healing (Ea = 53.3 kJ/mol) is in line with the vaporization enthalpy (of the perfluorinated silane

on the surface (i.e., on the order of 50 – 65 kJ/mol), [79] which suggests that vaporization of the

silane molecules is the dominant driving force for the thermal-healing process. We further

confirmed the vapor phase transfer of silane experimentally and showed that the process can

occur for temperatures from 200 ℃ down to 60 ℃. (Figure 2-5)

25

Figure 2-5: Demonstration of silane evaporation upon substrate heating. a. Silicon wafer 1 is

etched with microstructures and is highly hydrophilic. b. Silicon wafer 2 is a silanized surface

with etched microstructures and is highly hydrophobic. c, d. Schematic showing the experimental

setup. Note that wafer 2 was in contact with the hot plate and was heated at a temperature of at 60

oC for >3 hours or 200

oC for 10 min. e. After the heating process, the wafer 1 was observed to

become hydrophobic, confirming that part of the silane molecules were transferred from wafer 2

to wafer 1.

2.4 Robustness Characterization

In addition to the chemical damage of the functionalized layer, we also applied physical

damages to the surfaces and demonstrated the thermal self-repairing of the substrate. In the first

physical test, we investigated the coating adhesion of the silanized textured surfaces as compared

to that of a commercially available spray-coated superhydrophobic surface. We used a scotch

tape to attach the surfaces with a weight of 5 N (pressure of 12.5 kPa) and subsequently detach

the tape from the surfaces. For the commercially available superhydrophobic surfaces, the coating

lost its superhydrophobicity after 15 times of peeling-and-detachment cycles (see Figure 2-6). As

a comparison, the silanized textured surfaces can last for at least 50 peeling-and-detachment

cycles without degrading the liquid-repellent performance. This indicates that the chemical

functionalization layer has a very strong adhesion with the base textured substrate.

26

In the second physical test, we investigated the scratch resistance of the X-SLIPS coating.

We used a sandpaper to create mechanical abrasions on a lubricated and non-lubricated nano-

textured aluminum substrate (see Supporting Information). Specifically, the lubricated coatings

can survive over 200 physical damage cycles than the non-lubricated coatings (~ 5 cycles) due to

the greatly reduced friction (Figure 2-7). In addition, with harder materials, for example titanium,

the surface textures have higher strength and therefore higher scratch resistance. By heating at

150 ℃ for 10 min, the X-SLIPS on aluminum substrate restore its liquid repellency shown in the

images in Figure 4a. The thermal healing process for lubricated aluminum surfaces generally

takes longer time than those of non-lubricated surfaces because the silane is limited to thermal

migration (i.e., the lubricant layer protects the silane from vaporizing). Unlike the chemical

damage where the silane layer is uniformly reacted or removed across the whole substrate,

mechanical abrasion removes the silane by destroying the surface texture of the substrate. The

undamaged silane coatings present near the mechanically damaged areas then serve as reservoirs

to replenish the silane coating upon heating. We have further demonstrated that the physically

damaged substrate can be thermally self-healed on different materials, such as glass, ceramic and

polyethylene (Figure 2-6).

Figure 2-6: Coating adhesion test for the silanized textured surface and a commercial

surperhydrophobic coating. a. Performance comparison of the coating adhesion between the

27

silanized textured surface and the superhydrophobic coating. The adhesion test protocol includes

the use of a scotch tape to adhere onto the testing surface with a 5 N weight on top of it. The tape

was then peeled off from the surface. After each peeling test, we measured the sliding angle of a

20 μL droplet of deionized water. Low sliding angle indicated that the surface is slippery. b. Real

time images showing water repellency performance on the two surfaces after the scotch tape

peeling test. After 20 times of peeling, the commercial superhydrophobic coating completely lost

its superhydrophobicity; while our silane coating still maintained its liquid repellency. Water is

colored with blue dye for visual representation.

Our coating method can be extended on most materials, such as metals (stainless steel,

aluminum, copper, titanium, etc.), glass, ceramics, and plastics (e.g., polyethylene) and a variety

of surface geometries (Figure 2-8). In Figure 2-8, commonly used industrial materials, stainless

steel, aluminum, titanium, glass, copper, carbon steel, ceramic, and polyethylene, are coated with

a perfluorinated silane and lubricated with perfluorinated oil (Dupont Krytox®). These surfaces

can repel aqueous fluids, hydrocarbons and other organic fluids with a broad range of surface

tensions ranging from ~21.6 mN/m (octane) to ~72.8 mN/m (water). The contact angle hysteresis

of these liquids on the lubricated surfaces is typically less than 5 degrees. In supporting videos 4

and 5, a coated ceramic can repel viscous complex liquids, such as ketchup (aqueous-based) and

mustard (oil-based). We have shown that these coated materials can be thermally healed under

large area chemical or physical damage. The robustness and self-repairing nature of our surfaces

will provide a robust and universal solution that could address many of the “sticky” problems in a

broad range of industrial, medical and daily life applications.

28

Figure 2-7: Self-repair of X-SLIPS coating. a. A plot showing the physical abrasion performance

comparison between lubricated and non-lubricated samples. Each abrasion cycle involved

dragging a sandpaper of average particle size of 470 µm along with a pressure of 15.6 kPa across

the surface with a velocity of ~0.02 m/s. b. Optical images showing the thermal repair of X-

SLIPS on glass, polyethylene, and ceramic. The coating was damaged by physical abrasion of

sandpaper.

Figure 2-8: Liquid repellency of X-SLIPS coating on various materials. a. A plot showing the

contact angle hysteresis of various liquids on the slippery surfaces. Inset shows a dodecane

29

droplet sliding with a very small sliding angle (less than 3°) on a coated carbon steel substrate,

indicating the super slippery nature of the surfaces. Error bars represent the standard deviations

for at least four independent measurements. b. The coating method can be applied to various

substrates such as stainless steel, aluminum, titanium, glass, copper, carbon steel, ceramic, and

polyethylene. The yellow droplet is 10 μL dyed octane. c. Comparison of untreated and X-SLIPS-

coated stainless steels of different surface geometries (flat plate, pipe section, and sphere) for the

repellency of dyed octane (in yellow).

2.5 Chapter Summary

Drawing inspirations from the wax repair of plant leaves and the slippery surfaces of the

Nepenthes pitcher plants, we have developed a cross-species bioinspired slippery coating that can

perform self-repair function upon thermal stimulation. We stress the importance of healing the

intermediate chemical functional layer as it is crucial for the proper function and longevity of the

slippery surfaces. The resulting surfaces can be applied onto a range of metals, glass, and plastics

with different surface geometries, and can self-repair even with critical large area physical

damage. The robustness and self-repairing nature of our surfaces will provide a robust and

universal solution that could address many of the “sticky” problems in a broad range of industrial,

medical and daily life applications.

Chapter 3

Viscoelastic solid repellent surfaces

3.1 Motivation of the Study

Water shortage is one of the most pressing global issues [80]. In 2016, a report showed

that 4 billion people in the world are facing severe water scarcity [81]. Meanwhile, it is estimated

that over 141 billion liters of water – nearly 6 times the daily water consumption of the entire

population in Africa [82] – is consumed globally everyday by toilet flushing alone [83]. A

number of approaches have been proposed to reduce fresh water consumption for toilet flushing

that range from the use of rainwater for flushing [84] to the use of self-contained dry toilets [85].

Owing to the great variety of complex factors such as local environment [86], resource

availability [87], and user preference [88], none of these approaches can completely address the

water consumption issue. A relatively unexplored approach is to engineer the material interface of

toilet surfaces to significantly weaken the adhesion of human feces and urine to reduce the

consumption of fresh water.

Surfaces that can repel various aqueous fluids (e.g., urine) are widely available such as

superhydrophobic surfaces and slippery liquid-infused porous surfaces (SLIPS) [5, 7, 8, 49, 56,

89-91]; however, it is more challenging to develop a surface to repel sticky viscoelastic solids

such as human feces whose viscosities can span from watery-like liquid to hard solid [92]. Owing

to the large variations of liquid and solid compositions of human feces, most types of feces

exhibit highly complex viscoelastic behaviors, leading to their strong stickiness towards various

surfaces. To the best of our knowledge, no state-of-the-art liquid repellent surfaces (i.e.,

31

superoleophobic or superhydrophobic surfaces or SLIPS) have demonstrated repellency against

viscoelastic solids with dynamic viscosity higher than ~100 Pa·s (Figure 3-1).

Figure 3-1: Overview of state-of-the-art liquid and viscoelastic solid repellent surfaces. (A)

Schematic and optical images showing the comparison of adhesion between viscoelastic solids

and different engineered surfaces including a superhydrophobic glass (left), a SLIPS-coated glass

(center), and a LESS-coated glass (right). The superhydrophobic glass was created using a

32

commercially available superhydrophobic coating (NeverWet, LLC). The SLIPS-coated glass has

an underlying surface roughness ~1 µm. Synthetic feces with a solid content percentage of 30%

(dynamic viscosity = ~2406 Pa·s) were used in these experiments. (B) A plot showing the

reported dynamic viscosity range of liquids and viscoelastic solids that can be repelled by the

state-of-the-art liquid repellent surfaces and the present work.

Here, we report the design and fabrication of liquid-entrenched smooth surfaces (LESS),

a sprayable, multifunctional surface coating designed to repel both aqueous fluids and

viscoelastic solids of dynamic viscosities spanning over 9 orders of magnitude (i.e, from ~10-3

Pa·s to ~105 Pa·s, see Figure 3-1). Distinct from the conventional liquid repellent surface design

principles [5, 7, 8, 89], our surface coating is designed based on the Dahlquist criterion (1966)

and interfacial adhesion analysis, together with rigorous thermodynamic considerations, in order

to achieve excellent viscoelastic solid repellency. LESS consist of a smooth solid surface that is

chemically functionalized to retain a microscopically thin layer of lubricant. Specifically, LESS

can be easily applied onto various hydrophilic surfaces within minutes, including materials

commonly used for toilets (e.g., ceramic, vitreous china, steel, etc.). Compared to traditional toilet

surfaces, LESS have shown significant reduction of adhesion to a wide range of viscoelastic

solids. In addition, we have shown that LESS can maintain non-stickiness toward human feces,

which outperforms other commercial and state-of-the-art materials such as ceramic, silicone,

Teflon, and SLIPS. Specifically, surfaces coated with LESS consume only ~10% of the water

required to completely clean an uncoated surface. We have also developed a displacement

wetting scheme that provides a simple and efficient mechanism to renew the surface for enhanced

coating robustness and longevity. Moreover, we have demonstrated that LESS possesses

excellent anti-biofouling properties comparable to the state-of-the-art antifouling coatings such as

SLIPS [56].

33

3.2 Design of Viscoelastic Solid Repellent Surfaces

3.2.1 Design Criteria I: Interfacial Adhesion

Adhesion between a viscoelastic solid and the underlying solid surface can be quantified

by the adhesion energy at the solid-to-solid interface. Specifically, the adhesion energy, Gc, can

be expressed as [93]:

Gc = wa Φv(da/dt, T, ϵ) (3-1)

where Φv is a mechanical loss function, which depends on crack growth rate da/dt,

temperature T, and strain ϵ of the viscoelastic solid, and wa is thermodynamic work of adhesion.

Since the crack growth rate and strain are inherit properties of the viscoelastic solid, reducing the

adhesion of the viscoelastic solid and the substrate surface would require lowering the work of

adhesion. Specifically, the work of adhesion can be expressed as wa = γ13 + γ23 – γ12, which can be

further simplified by Girifalco and Good equation [94] as:

wa = 2(γ13 • γ23)1/2

(3-2)

where γij is the interfacial energy at the i-j interface, and 1, 2, and 3 refer to the

underlying solid substrate, the viscoelastic solid, and air, respectively. In order to reduce the work

of adhesion, the interfacial energies of the underlying solid-air interface (γ13) and the viscoelastic

solid-air interface (γ23) would need to be reduced. Traditionally, there are two methods to

decrease these interfacial energies (γ13 and γ23, see Figure 3-2). The first method involves surface

chemistry modification of the underlying solid substrate (e.g. silanization), which can effectively

reduce γ13. The second method involves lubrication between the viscoelastic solid and the

substrate surface. As reported previously [95], the lubricant could be absorbed by the viscoelastic

solids instead of being adhered onto the substrate surface, resulting in the reduction of γ23. In

order to reduce both γ13 and γ23 concurrently, the lubricant would need to stably adhere on the

34

underlying surface so as to retain a thin layer of lubricant (Figure 3-2). In this specific case, the

adhesion interface changes from a solid-to-solid interface (i.e., viscoelastic solid-to-underlying

solid substrate) to a solid-to-lubricant interface (i.e., lubricant-infused viscoelastic solid-to-

lubricant-coated solid substrate). The detailed calculation can be found in Appendix B.

Figure 3-2: Schematics showing adhesion of viscoelastic solids on surfaces with different surface

treatments. Adhesion of viscoelastic solids on a) an untreated surface; b) a silanized surface; c) a

lubricated surface without chemical treatment; and d) lubricated surface with silanization

treatment.

In addition, the total work of adhesion between two surfaces is directly proportional to

their respective contact area, which could be significantly increased by the presence of roughness

[94]. In 1960s, Carl A. Dahlquist [96, 97] showed experimentally that when the storage modulus

of an adherent material is below a certain critical value (i.e., typically 0.3 MPa), the material will

begin to flow and form conformal contact with the surface roughness of the adherent regardless

of the applied pressure. This is widely known as the Dahlquist criterion, which has been the basis

for the design of pressure sensitive adhesives. For a viscoelastic material that satisfies the

Dahlquist criterion, any roughness present on the adherent would further increase the surface

adhesion. Therefore, reducing the surface roughness of the underlying solid substrate will be

another important method to further reduce the surface adhesion.

35

Figure 3-3: Measured viscoelasticity of synthetic feces. The storage and loss moduli of synthetic

feces with different solid content fraction (e.g. 10%, 20%, 30%, 40%, 50%, and 60% of solids in

synthetic feces). The solid and open symbols represent the storage modulus and the loss modulus,

respectively.

Figure 3-4: Comparison of the Dahlquist criterion with the measured storage moduli of different

synthetic feces. The storage moduli of synthetic feces with different solid content fraction (e.g.

36

10%, 20%, 30%, 40%, 50%, and 60% of solid in synthetic feces) under frequency of 1 rad/s. The

error bars represent standard deviations of storage moduli from three independent measurements.

To verify the adhesion mechanics of different surfaces, we prepared synthetic feces that

have organic solid content very similar to that of human feces [98] for adhesion characterization

on surfaces of varying roughness. The recipe of synthetic human feces was developed from the

original recipe developed at the University of KwaZulu Natal at South Africa. The synthetic

human feces are composed of yeast, psyllium, peanut oil, miso, polyethylene glycol, calcium

phosphate, cellulose, and water. All solid components are expressed as dry mass, and the

corresponding percentages are shown in Table 3-1. The compositions of the synthetic feces are

biologically very similar to human feces [98]. The viscosity of the synthetic feces can be tuned by

the percentage of solid contents. We made synthetic feces with solid percentage of 10%, 20%,

30%, 40%, 50%, and 60%. These synthetic feces were used within 5 hours of preparation for

viscoelasticity measurements, adhesion tests, and water consumption tests. Human feces can be

classified into 7 categories based on their solid content percentage and viscosities [92]. This

specific classification, known as Bristol stool scale, ranges from Type 1 (hard solids) to Type 7

(entirely liquid). A key reason for the use of synthetic feces is that one can control their

viscoelastic properties systematically by varying the corresponding solid content percentage

(Figure 3-3). The solid contents of the synthetic feces range from 10 wt% to 60 wt%, which

correspond to storage modulus of ~ 1 Pa (~10% solid content) to ~105 Pa (~60% solid content).

These values closely emulate those of the fresh human feces [99, 100]. Based on the Dahlquist

criterion, most of the synthetic feces used here would be conformally contacting the rough

surfaces, particularly for the synthetic feces whose storage moduli are much less than 0.3 MPa

(Figure 3-4). Our adhesion measurements showed that surface adhesion of the synthetic feces

increases with the surface roughness (with average roughness, Ra ranges from 0.87±0.06 nm to

4.12±0.26 µm), indicating a relatively smooth surface would be desirable in reducing surface

37

adhesion with feces. In accordance to Dahlquist criterion, LESS requires a relatively smooth

substrate for adhesion reduction which is in contrast to SLIPS where rough substrate is essential

for lubricant retention.

Table 3-1: Compositions of the synthetic feces.

Ingredients % dry mass Nutrition

Yeast 32.49 Biomass

Psyllium 10.84 Fibre

Peanut oil 17.31 Fat

Miso 10.84 Fibre/Protein/Fat

Polyethylene glycol 12.14 Carbohydrate

Calcium phosphate 10.84 Biomass

Cellulose 5.53 Carbohydrate

3.2.2 Design Criteria II: Interfacial Wetting

To satisfy the design criteria (i.e., forming a stable lubricant layer on a smooth substrate),

the lubricant has to preferentially adhere to a relatively smooth substrate, which would require

matching surface chemical affinity [49, 101]. Thermodynamically, to achieve this condition the

lubricant should have a positive spreading parameter S on the solid substrate and a positive

disjoining pressure П(e) [38, 39, 102] in the presence of both air and the foreign immiscible

liquid of interest (i.e., S > 0 and П(e) > 0), respectively. Here, we define the spreading parameters

of the lubricant on the solid substrate in the presence of air as Sls and that in the presence of the

foreign immiscible fluid droplet as Slsf, respectively. Specifically, these spreading parameters can

be expressed as Sls = σs – (σls + σl) and Slsf = σsf – (σls + σlf), where σs, σls, σl, σsf, and σlf are the

38

interfacial tensions of solid-air, lubricant-solid, lubricant-air, solid-immiscible fluid, and

lubricant-immiscible fluid interfaces, respectively.

In addition, the disjoining pressure of the lubricant film can be expressed as П(e) =

A/6πe3, where e is the lubricant film height, and A is the Hamaker constant expressed as[94]:

2 2 2 2

1/2 1/2 1/2 1/22 2 2 2 2 2 2 2

33

4 8 2

s l f lf ls l e

s l f ls l f l s l f l

n n n nhvA kT

n n n n n n n n

(3-3)

where k is the Boltzmann constant, T is the absolute temperature, h is the Planck’s

constant, and ve ~ 4×1015

s-1

is the plasma frequency of free electron gas, while ϵs/l/f and ns/l/f are

respectively the dielectric constants and refractive indices of the solid, lubricant, and immiscible

fluid of interest (air or water in our case), respectively.

Based on these criteria, we have designed a number of LESS coatings for silica-based

materials, such as glass, silicon, and china, etc. These substrates were chosen based on their

inherent smoothness (with Ra ~ 1 nm) and their hydroxyl groups availability, which made surface

functionalization facile [103]. Using these smooth substrates, we further functionalized these

surfaces with dimethyldimethoxysilane to create polydimethylsiloxane (PDMS)-grafted chemical

layer. In addition, silicone oil was chosen as the lubricating fluid due to its strong chemical

affinity towards the surfaces as well as its excellent chemical stability and low environmental

impact [104]. Our calculations of spreading parameters and Hamaker constants have shown that

this specific material combination (i.e., silicone oil and the PDMS-grafted surface) satisfies the

requirement for a stable lubricant film formation (i.e., S > 0 and A > 0, Tables 3-2), which are

consistent with our experimental observations. The formation of stable lubricant layer on a solid

surface requires that the lubricant should completely wet the solid surface and cannot be

displaced by immiscible foreign liquids. To achieve this, the spreading parameter S and the

disjoining pressure Π(e) of the lubricant on the solid surface should be positive under air and

39

foreign immiscible liquids of interest. Specifically, the spreading parameters under air (Sls) and

immiscible liquid (Slsf) conditions can be calculated using a method proposed by van Oss,

Chaudhury, and Good (vOCG) introduced in a recent publication [38], which are shown below:

( ) 2 2 2 2 4LW LW LW

ls s ls l s l l s s l l l lS (3-4)

( )

2 2 2 2 2 2

2 2 2 4

lsf fs ls fl

LW LW LW LW

s l f l l s s l l f f l

LW LW

f s s f f s l l

S

(3-5)

Note that σLW

indicates the Lifshitz-van der Waals component of the interfacial tension at

an interface; whereas σ+ and σ

- represent the acid and base terms of the polar component of the

interfacial tension, respectively [38]. The solid surface here is grafted-PDMS-coated glass. The

lubricant is silicone oil, and the foreign liquid is water or a perfluorinated fluid such as DuPont™

Krytox 101. Interfacial tensions of different material interfaces of consideration are listed in

Table 3-2. The sign of disjoining pressure can be determined by Hamaker constant, which can be

calculated with the dielectric constants and refractive indices of the materials of interest (see

Table 3-3). Based on the calculations from the spreading parameters and the disjoining pressures,

silicone oil can spread completely on PDMS-grafted smooth surfaces, and be able to repel

immiscible liquids, such as water and DuPont™ Krytox 101. These predictions are consistent

with our experimental observations.

40

Table 3-2: Surface energy components for materials at ambient environment.

Materials σ (mN/m) σLW

(mN/m)

σ+

(mN/m)

σ -

(mN/m)

PDMS 23.1 22.9 0 3.05

water 72.8 22.6 25.5 25.5

silicone oil 20.7 20.7 0 0

Krytox 101 16.6 11.9 0.019 0.155

Note: The interfacial tension values listed in the table were obtained from reference [38].

Table 3-3: Calculated spreading parameters (S) and Hamaker constant (A) for liquids on PDMS-

grafted glass.

solid lubricant foreign

fluid

ϵs ns ϵl nl ϵf nf A

(10-21

J)

S

(mN/m)

stable

lubrication

PDMS silicone air 2.32 1.4 2.6 1.41 1 1 2.07 1.85 Y

PDMS silicone water 2.32 1.4 2.6 1.41 80 1.33 0.233 29.7 Y

PDMS silicone K101 2.32 1.4 2.6 1.41 2.1 1.3 0.564 34.4 Y

PDMS water air 2.32 1.4 80 1.33 1 1 -8.42 -84.1 N

Note: K101 indicates Dupont™ Krytox 101 perfluorinated oil. All dielectric constants and

refractive indices listed in the table were obtained from reference [94] and [39].

41

3.3 Fabrication of Liquid-Entrenched Smooth Surfaces (LESS)

Figure 3-5: Fabrication of liquid-entrenched smooth surfaces (LESS). (A) Schematic illustration

showing a two-step spray coating process to form LESS. (B) Optical images showing the

individual coating processes on glass. The lubricant used was silicone oil and the blue testing

liquid was dyed water.

To form LESS on silica-based smooth substrates, we developed a two-step spray coating

process (Figure 3-5). The first step generates a chemical layer on the substrate, and the second

step creates an overcoat lubricant layer. Before the first step of surface functionalization, the

surfaces need to be rinsed with isopropanol and deionized water in order to remove the surface

contaminants and expose the hydroxyl groups. Once the surface is clean, a solution containing

silane molecules (e.g., dimethyldimethoxysilane) is sprayed onto the surface, allowing these

molecules to react with the hydroxyl groups and forming a PDMS-grafted layer on the substrate.

The silanized substrate then becomes hydrophobic and can repel both water and alkanes [91]. The

formation of the new surface functional group was confirmed by X-ray photoelectron

spectroscopy (XPS) measurements showing the formation of Si-O bonds associated with

dimethyldimethoxysilane (Figure 3-6). In the second step, the PDMS-grafted surface is

42

preferentially wetted by silicone oil, forming a stable lubricating layer. The two-step fabrication

process generally takes less than 10 minutes, and can be applied to other relatively smooth

hydrophilic materials such as ceramic, carbon steel, titanium, etc. LESS exhibited strong

repellency towards both aqueous liquids (contact angle = 105.5o±0.3

o and contact angle hysteresis

= 0.8o±0.2

o) and viscoelastic synthetic feces (see Figure 3-3) as compared to the uncoated surface,

superhydrophobic surface, and SLIPS-coated surface (Figure 3-5).

Figure 3-6: XPS characterizations of silicon element on untreated (e.g. glass) and PDMS-grafted

substrates. a, The peak of silicon element is originated from silicon dioxide, which is the main

component of glass. b, The peak of silicon element (Si 2p) is originated from silicon dioxide and

silicone, which confirmed the presence of the PDMS on the PDMS-grafted substrate.

3.4 Adhesion of LESS against Viscoelastic Feces

To test the anti-adhesion performance of LESS-coated surface, we performed systematic

surface adhesion measurements of the synthetic feces at different solid contents against other

control surfaces. In the adhesion measurements, the synthetic feces were brought into full contact

and then completely detached from the surfaces. The adhesion was quantified by the work of

debonding [105]. First, we investigated the importance of a stable lubricant layer for reducing

43

surface adhesion. The control surfaces in these tests include uncoated bare glass with and without

lubrication of silicone oil, and PDMS-grafted glass. We normalized the work of debonding of the

synthetic feces on lubricated surfaces (lubricated glass and LESS) by that of the uncoated bare

glass. Our results showed that lubrication on bare glass (without chemical functionalization) can

reduce the surface adhesion by ~41% for synthetic feces with 40% solid content (i.e., the stickiest

sample in the test). In comparison, grafted-PDMS glass can reduce the surface adhesion by

~75%, while the LESS coating can reduce the adhesion by ~90% (Figure 3-7). Qualitatively,

these experimental results are consistent to the predictions by Eq. 2. Our results indicate that the

presence of stable lubricating layer on the underlying solid substrate is critical to reduce surface

adhesion.

Figure 3-7: Adhesion comparison among different surfaces.

44

Figure 3-8: Adhesion comparison among surfaces with different roughness.

In the second set of tests, we investigated the influence of underlying surface roughness

of the lubricated substrates to the overall adhesion performance. The control surfaces in these

tests include uncoated glass and silicone-oil infused SLIPS samples with either microscale (Ra ~ 4

µm) or nanoscale roughness (Ra < 1 µm) on the underlying substrates (see Figures 3-7, 3-8). Our

measurements showed that the adhesion increases with increasing underlying surface roughness

(Figure 3-8). In general, the LESS-coated surface outperforms other control surfaces, including

uncoated surfaces with or without lubrication, and SLIPS with different underlying roughness.

According to these tests (Figure 3-6), LESS coating treatment can lead to 90% adhesion reduction

45

in synthetic feces (~40% solid content, with a viscoelasticity similar to a Type 3 – 4 healthy

human feces [99, 100]) compared to those of the uncoated surfaces.

The glass surface (VWR) was intrinsically smooth with a roughness less than 1 nm. We

created nanoscale porosity into the glass surfaces using an established protocol [90] by immersing

them into 0.5 mol/mL sodium bicarbonate solution, and boiling for 48 hr. The micro-roughened

glass was etched by two steps: first by using a universal laser systems VLS 2.3 (35% power and

90% cutting speed) and then by buffered oxide etch (6:1 40% NH4F in water to 49% HF in

water) for 20 minutes at room temperature. The surface roughness of all these glass slides and

other materials (e.g., ceramic, carbon steel, titanium, etc.) was measured either by an atomic force

microscope (AFM) in peak-force-tapping mode or an optical profilometer from Zygo, shown in

Figure 3-9.

Figure 3-9: Surface roughness measurement on different surfaces.

3.5 Water Consumption of LESS

3.5.1 Simulated Toilet Flushing

To simulate the condition of toilet flushing, we designed an open channel flow system

with 50 mm width, in which the flow rate can be precisely controlled. After measuring the height

46

of the flowing water level where the sample (i.e., synthetic feces) is located, we calculate the

hydraulic diameter as:

4

4 =h h

AD R

P (3-6)

where A is the flow area cross section, and P is the wetted perimeter. Based on the

hydraulic diameter, we can then estimate the Reynolds number as:

Re hD

vD

(3-7)

where v is the flow velocity, ρ is the water density, µ is the dynamic viscosity. Since all

the open-channel flow under investigations have Reynolds numbers ≥2500, these flows are in

turbulent flows and the wall shear can be estimated as:

.0.2 0.8 1.8

2 2

0.0225( ( 2 ))

4w

a h V

a h

(3-8)

where a is the flow width, and h is the flow height[106]. We also measured the height of

the flowing water level in a real toilet where the tilting of the surface is ~45o and performed the

same open channel flow calculation. The toilet we measured consumes 1.6 gallon per flush, and

each flush take ~5 s. The parameters of the flow were estimated as follows: 600 mm flow width,

300 mm from flushing outlet, and 3.97 mm flow height. Based on these parameters, the Reynolds

number is estimated as ~177000, and the corresponding wall shear is 0.74 Pa. The wall shear in

our open channel system under different flow rate is calculated as shown in Table 3-4. Note that

the wall shear in our system is in the same order of magnitude as those calculated for the real

toilet. Therefore, the open channel flushing system can realistically simulate the real toilet

flushing condition.

47

3.5.2 Water Consumption of Various Surfaces

A standard toilet in the United States uses ~1.6 gallon (~ 6 L) of water per flush [107]. To

investigate the amount of water that would be required to clean LESS after it is contaminated

with the synthetic feces, we built a simplified open-channel experimental setup to emulate the

toilet flushing process. Specifically, our setup is capable of generating a flow rate from 1 gallon

per minute (i.e., 3.8 L/min with Reynolds number, Re ~ 4570, calculated based on the hydraulic

diameter [106], see Supplementary section 6) up to 2.8 gallon per minute (i.e., ~10.6 L/min with

Re ~ 13100). The estimated wall shear stresses generated by these flows range from 0.093 Pa (at

1 gallon per min) up to 0.78 Pa (at 2.8 gallon per min), which are similar to those of typical toilets

(Table 3-4). To simulate the adhesion of feces to toilet surfaces during defecation, we dropped the

synthetic feces (~5 grams) from a height of ~400 mm onto the test surfaces at a tilting angle of

45o. We then put the surfaces inside our open channel flow system to determine the amount of

water required to completely remove the feces and their residues (Materials and Method, and fig.

S8). We verified the complete removal of the feces residues using fluorescent trace dye, which

was mixed with the synthetic feces during our tests (Figure 3-11). Compared to uncoated glass

surfaces, LESS-coated surfaces reduce water consumption by up to 90% for various synthetic

feces at different solid contents (Figure 3-12). We have also conducted similar characterizations

on SLIPS samples and found that the water consumption increases with increasing underlying

substrate roughness – a trend that is consistent with the observations in the adhesion tests (Figures

3-10 and 3-11).

Table 3-4: Physical parameters for the simulated-toilet flushing systems.

Flow Rate (gpm) 1 1.5 2 2.5 2.8

Height (mm) Re

Wall Shear (Pa)

4.75 4578 0.093

3.62 7136 0.33

5.11 9049 0.28

4.27 11632 0.60

4.15 13081

0.78

48

Figure 3-10: Water consumption on different surfaces under different flow rate with impact of

different feces.

Figure 3-11: Experimental setup of simulated flushing.

3.6 Adhesion against Human Feces

Finally, we compared the adhesion performance of human feces on LESS and other state-

of-the-art and commercially available surfaces (Figures 3-12, 3-13). Specifically, we used glazed

ceramic (a typical toilet material), Teflon, and silicone as the control surfaces. For these tests, we

49

used a setup that allows human feces samples to be released from a drop rig at the same height

onto an acrylic support where the test coating is placed. When the support pin for the acrylic

surface is removed, the surface drops from a horizontal position to a vertical position, where the

feces are expected to slide down the face of the surface. In our tests, all the commercial surfaces

show extreme stickiness towards the human feces samples. LESS-coated glass, however, was the

only surface showing non-stickiness towards the feces sample and left no noticeable residue

behind (Figure 3-12). Furthermore, we have shown in a different set of feces impact tests that

traces of feces were left on a SLIPS-treated surface (with underlying surface roughness ~ 1 µm)

(Figure 3-13). Therefore, our tests further demonstrate that our LESS coating outperforms various

state-of-the-art surfaces on repelling viscoelastic human feces.

Figure 3-12: Comparison of adhesion on different surfaces with impact of human fecal waste.

50

Figure 3-13: Adhesion on SLIPS with impact of human fecal waste.

3.7 Anti-Bacterial Performance of LESS

One important reason that urinals or toilets need to be flushed and cleaned regularly is to

prevent the growth of bacteria and spread of infectious diseases. In certain regions (e.g., Brazil),

rainwater is used as the source for toilet flushing. However, rainwater can contain bacteria that

may contaminate the sanitation facilities [84]. Owing to the presence of the mobile lubricant

interface of LESS, we hypothesize that LESS may have comparable anti-biofouling performance

as state-of-the-art materials [56]. To verify this, we performed biofouling analyses on LESS-

coated substrate and other control surfaces using natural rainwater. Specifically, we collected

rainwater from a house roof in State College, PA, USA, and measured its bacteria content and

concentration (Figure 3-14).

51

Figure 3-14: Rainwater bacteria concentration measurement.

Table 3-5. Bacteria identification from mass spectrometry.

Organism

(best match)

Score

Value

Staphylococcus aureus 2.358

Enterobacter cloacae 2.281

Escherichia vulneris

Escherichia hermannii

Enterococcus mundtii

Acinetobacter calcoaceticus

2.256

2.212

2.219

2.077

Note: Any score value >2 indicates secure genus identification.

We identified the bacteria in the rainwater using a MALDI Biotyper system; and these

bacteria were identified as Staphylococcus aureus, Enterobacter cloacae, Escherichia vulneris,

Escherichia hermannii, Acinetobacter calcoaceticus, Enterococcus mundtii (Figures 3-15, 3-18,

Table 3-5), which are commonly found in rainwater [108]. We rinsed the LESS-coated substrate,

two SLIPS samples (one with underlying microscale roughness and the other with nanoscale

52

roughness), and uncoated bare glass with the collected rainwater for 1 min, and then immediately

incubated the substrates by attaching solid agar onto the surfaces in an incubator. After 36 hours

of incubation, we counted the bacterial colonies on these surfaces. Specifically, no observable

bacteria colonies were found on all SLIPS-coated and LESS-coated substrates; whereas the

untreated glass surfaces were contaminated with the bacteria in the rainwater (Figure 3-15). Our

results showed that the anti-biofouling performance of the LESS-coated substrate is comparable

to existing state-of-the-art anti-biofouling materials [56]. In addition to the rainwater tests, we

further characterized these surfaces with a mixture of E. coli and synthetic urine. 10 mL of this

contaminated urine was sprayed onto the test surfaces to simulate the urination process, followed

by the aforementioned procedures for the biofouling characterizations. Our test results are similar

to those found in the rainwater tests, where all of the SLIPS and LESS-coated samples showed no

observable bacteria colonies while the glass substrate showed significant contamination with

bacteria (Figure 3-15). We have further shown that SLIPS and LESS-coated substrates can repel

all bacteria-contaminated synthetic urine with a sliding angle of a droplet (10 µL) less than 5o

(Figure 3-16).

Figure 3-15: Bacteria adhesion measurement on different surfaces.

53

Figure 3-16: Sliding angle of urine on different surfaces.

It is interesting to note that in the case where the lubricant is fully depleted in SLIPS,

bacteria biofilm can attach onto rougher substrates persistently. Even after sterilization with

bleach and 70% of alcohol, there are still bacteria attached on the rough surfaces. In comparison,

the biofilms that are initially attached onto the lubricant-depleted LESS-coated surface can be

completely removed after the bleach sterilization (Figure 3-17). All of these results indicate that

LESS has excellent anti-biofouling performance and, therefore, could minimize the use of

disinfectants or other aggressive chemicals currently used for cleaning and sterilization.

54

Figure 3-17: Sterilization of liquid-infused surfaces with different underlying roughness.

Figure 3-18: Rainwater bacteria identification.

55

3.8 Durability of LESS

We have also investigated the durability of the LESS coating against realistic water flow

(i.e., 1 gallon/min to 2.8 gallon/min) and impact of synthetic feces. Specifically, the LESS coating

(with silicone oil of viscosity of ~20 cSt) can remain functional even after >100 flushes of water

at a flow rate of 2.8 gallon/min (Figure 3-20), as well as ~8 to 36 impact-and-flush cycles of

synthetic feces of various solid contents before further replenishment is required (Figure 3-20 and

Figure 3-19).

Note that typical thickness of LESS is <~3 µm and the estimated volume of lubricant

required to coat a typical toilet would be ~170 µL assuming a total surface area of 600 cm2 (as

compared to ~6 L of water per flush). Based on these characterizations, it is estimated that the

concentrations of the silicone oil in the flushed water is on the order of ~0.05 parts per million

[109] – a concentration that has shown with negligible impact to the environments [110].

Figure 3-19: Durability measurement on LESS with contamination of different feces.

Since LESS requires the use of lubricant for its function against aqueous liquids and

viscoelastic solids, it will be important to develop a lubricant replenishment strategy to renew the

surface in case the coating is depleted over time during toilet flushing. Since the PDMS-grafted

56

substrate of LESS is designed to adhere the silicone oil as opposed to aqueous liquids, it is

possible to replenish the lubricant layer by incorporating small amounts of silicone oil in the

flushing water so that the silicone oil can preferentially wet the surface through displacement

wetting (Figure 3-20). During the lubricant replenishment process, the following thermodynamic

condition has to be satisfied so that it is energetically favorable for the lubricant to displace the

aqueous liquid-wetted surface,

ΔE1 = σlcosθl – σfcosθf + σf – σl > 0 (3-9)

where σf represents the interfacial tension of the immiscible fluid-vapor interface, and θl

and θf, are the equilibrium contact angles of the lubricant and the foreign immiscible fluid on a

flat solid surface, respectively. Our experimental measurements (i.e., σf = 71.1±0.2 mN/m (water),

σl = 20.7±0.3 mN/m, θf = 106.5o±0.4

o, θl = ~ 0

o) have shown that Eq. (3-9) is satisfied (i.e., ΔE1 =

91.3 mN/m > 0) for the displacement wetting of the lubricant. Experimentally, we have further

shown that silicone oil wets the PDMS-grafted ceramic surface even when the surface has been

pre-wetted by water, and subsequently forms a functional layer to repel the water (Figure 3-20).

57

Figure 3-20: Durability and replenishment of the lubricant layer on LESS.

A critical criterion for displacement wetting is that the solid must be preferably wetted by

the lubricant rather than by the foreign liquid one wants to repel. To determine whether a solid

will be wetted by liquid B when liquid A already adheres onto the surface, we compare the total

interfacial energy of the individual wetting configurations. Specifically, configuration A refers to

the state where the smooth solid is wetted by the foreign liquid, and configuration B refers to the

state where the smooth solid is wetted by the lubricant. EA and EB represent the total interfacial

energies per unit area of the wetting configuration A and B, respectively. And σsf, σsl, σf, and σl

represent the interfacial tensions of the solid-immiscible fluid interface, solid-lubricant interface,

immiscible fluid-vapor interface, and lubricant-vapor interface, respectively. In order for the

58

lubricant to displace the foreign liquid and preferentially attach onto the solid substrate, wetting

configuration B should be at a lower energy state. To derive a thermodynamic relationship for the

displacement wetting from configuration A to configuration B, we have EA – EB > 0, which can

be further expressed as,

σsf – σsl > 0 (3-10)

In particular, Eq. (3-10) is reduced to experimentally measurable quantities with the use

of the Young equation [102], where we have,

σlcosθl – σfcosθf + σf – σl > 0 (3-11)

where θl and θf, are the equilibrium contact angles of the lubricant and the foreign

immiscible liquid on a flat solid surface, respectively.

3.9 Chapter Summary

In summary, we have developed a set of design criteria based on Dahlquist criterion and

interfacial adhesion and wetting analyses to design highly effective viscoelastic solid repellent

coatings against solids with dynamic viscosities over 9 orders of magnitude. Specifically, we

have created a facile and scalable LESS coating specifically designed to reduce adhesion to soft

viscoelastic solids (including both synthetic and human feces) with applications aimed to

minimize water consumption for sanitation and waste management. Our LESS coating can be

applied onto various clean hydrophilic surfaces (e.g. glass, ceramic, china, aluminum, titanium,

and carbon steel, etc.) within minutes through a simple two-step spray coating without the need to

access advanced deposition and fabrication facilities. More importantly, our LESS coating has

shown significant adhesion reduction (up to ~90%) for soft viscoelastic solids (with storage

modulus on the order of 1000 Pa) and requires only 10% of the cleaning water required for an

untreated control surface. The excellent anti-biofouling property of LESS could minimize not

59

only the bacteria attachment, but also the use of any aggressive chemicals currently used for

sterilization, thus reducing environmental impacts. In addition, the LESS coating can be

replenished through the displacement wetting mechanism, which offers a simple yet effective

surface renewal method. With over ~1 billion toilets and urinals around the world, it is

anticipated that the utilization of LESS in toilets and other sanitation facilities worldwide would

lead to significant water saving, allowing fresh water to be used for other essential human

activities such as irrigation for food production and sanitation to promote global health.

Chapter 4

Summary and Discussion

In this dissertation, I have addressed two of the major issues in some of the state-of-the-

art liquid repellent surfaces. The first one is the robustness of the slipper liquid-infused porous

surfaces (SLIPS). While SLIPS have shown great pressure stability and strong durability with a

self-healable lubricating layer, SLIPS could lose their liquid repellency if the intermediate

chemical layer is damaged. At the meantime, no suitable solutions have been proposed to enhance

the robustness of this chemical functional layer. Here, I have introduced a self-healing

mechanism into the chemical functional layer of slippery liquid-infused porous surfaces (SLIPS)

by a bioinspired approach. Inspired by the wax regeneration ability of plant leaves and the

slippery surfaces of the Nepenthes pitcher plants, I have developed a cross-species bioinspired

slippery coating (X-SLIPS) that can perform self-repair function upon thermal stimulation even

under large area physical and chemical damage. By characterizing the thermally self-healing

performance, I have explored the underlying mechanism of this phenomenon, providing a

valuable guidance of designing and fabricating such self-healable coating. With the enhanced

robustness, X-SLIPS will provide a universal solution that could address many of the durability

problems in a broad range of harsh working environment in industrial, medical and daily life

applications.

The second one is the viscoelastic solid repellency. Conventional liquid repellent surfaces

mostly require surface structure/roughness to either create a thin air layer or retain lubricant.

While this surface roughness requirement is beneficial to reduce contact or friction towards

liquids, it would instead dramatically increase the adhesion when the viscoelastic solid

61

conformally contacting the surface. In this dissertation, I have, for the first time, designed and

fabricated a slippery smooth surface coating that can repel not only liquids but also viscoelastic

sticky solids. I have developed a set of design criteria based on Dahlquist criterion and interfacial

adhesion and thermodynamic wetting analyses to design highly effective viscoelastic solid

repellent coatings against solids with dynamic viscosities over 9 orders of magnitude.

Furthermore, I have created a facile and scalable coating specifically designed to reduce adhesion

to soft viscoelastic solids, named as liquid-entrenched smooth surfaces (LESS) for saving water

from toilets. I have shown that LESS have significant adhesion reduction (up to ~90%) for soft

viscoelastic solids (with storage modulus on the order of 1000 Pa) and requires only 10% of the

cleaning water required for an untreated control surface. The excellent anti-biofouling property of

LESS could minimize not only the bacteria attachment, but also the use of any aggressive

chemicals currently used for sterilization.

To this end, many engineering opportunities emerge from this dissertation, leading to

some of the future studies for liquid/solid repellent surfaces. For example, state-of-the-art liquid

repellent surface manufacturing has covered most of the materials, except for some chemically

inert plastics. Plastics are one of the most widely used materials in industrial and biomedical

applications, and liquid repellency is usually a desired property for these applications. It remains

very challenging to chemically functionalize a plastic surface in order to achieve desired liquid

repellency. Inspired from a recent study about bio-adhesives [111], we may acquire the ability to

tune the surface chemistry of these plastics to make them more reactive to our functional low-

surface-tension chemicals. In this way, a novel fabrication method can be developed, and may

open up plenty of significant industrial and biomedical applications for liquid repellent surfaces,

for example, aeration membranes, face shield, ostomy bag, blood transfusion, etc.

62

Appendix A

Experiments on Fabrication and Characterization of X-SLIPS

Materials: Stainless steel 304, stainless steel 316, carbon steel, titanium, aluminum,

copper, glass, ceramics, polyethylene, polypropylene, polystyrene, and ABS were purchased from

McMaster-Carr Inc. 4-inch silicon wafers were purchased from Semiconductor Solutions, LLC.

Perfluorinated silane used in the experiment was (heptadecafluoro-1,1,2,2-

tetrahydrodecyl)trichlorosilane from Gelest Inc. Other types of silanes used in the experiments

included 1H,1H,2H,2H-perfluorodecyltriethoxysilane, trimethoxy(octyl)silane, and

trimethoxy(propyl)silane, all purchased from Sigma Aldrich Corporation. The chemical etchants

used for surface roughening were hydrochloric acid and ferric chloride acid both from Sigma

Aldrich Corporation. Sandpaper (grit 700) was purchased from 3M Company. The fluorinated

lubricant was DuPont Krytox oils (100 to 107) purchased from TMC Industries, Inc.

Surface Roughness Formation: Nanostructures can be formed on titanium surface

through etching using concentrated hydrochloric acid for 1 hour at 40 ℃. Stainless steel (304 and

316), carbon steel, and copper surfaces were etched with ferric chloride acid for 1 hour at 40 ℃ to

produce micro/nano structures. Aluminum surfaces were reacted with water vapor at 100 ℃ for

15 to 30 min, forming boehmite nanostructures.[112] We formed micro-structures on plastics,

including polyethylene, polypropylene, polystyrene, and ABS, through physical abrasion using

sandpaper. The glass surfaces were etched using a slow etching process where the sample was

immersed in 0.5 mol/mL of boiling sodium bicarbonate solution for 48 hours. The etching

process can form nano-porous structures on the surface.[113] Micro/nanostructures on silicon

were created by photo-lithography and deep reactive ion etch (DRIE). The ceramic was

intrinsically rough as received.

63

Chemical Functionalization and Lubrication: Before functionalizing the surface, the

roughened industrial metals, glass, ceramic, plastic, and silicon were cleaned and surface-

activated using oxygen plasma for 10 min. The materials were immersed in a silane solution

consisting of 1 – 4 mM of (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane in ethanol.

During the silanization process, the solution was exposed to air. The surface chemical

functionalization process was completed upon the full evaporation of the solution which typically

took less than 24 hours. By heating the solution at 70 ℃, the evaporation can be accelerated to

less than 5 hours. The average thickness of the silane coating is ~ 2.40±0.01 µm on flat titanium

samples based on a 4 mM (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane in ethanol.

The silane deposited onto the substrates is the byproduct of the reaction between

(heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (C10H4Cl3F17Si) and ethanol

(C2H5OH):1H,1H,2H,2H-perfluorodecyltriethoxysilane (C16H19F17O3Si).[114] The chemical

functional intermediate layers created by these types of silane (e.g., 1H,1H,2H,2H-

perfluorodecyltriethoxysilane, trimethoxy(octyl)silane, and trimethoxy(propyl)silane) can all self-

repair under thermal stimulation (Figure S2). After chemically functionalizing the surface, the

surface was lubricated with perfluorinated oils by spraying or spin coating.

Thermal-Repairing Characterizations: We damaged the intermediate chemical layer of

X-SLIPS using oxygen plasma for 3 min. A plasma cleaner from Harrick Plasma was used for

oxygen plasma treatment in all of the experiments. The pressure in the plasma chamber is ~700

mTorr, and the maximum power is 30 W. The frequency is in the 10MHz range, and the

maximum voltage is 750 V. After the surface damage, we lubricated the surface with Krytox 101,

and then measured the sliding angle of a liquid droplet (i.e., 15 µL of octane) using a tilting stage

(Fowler adjustable v-block set). To repair the chemical layer, we heated up the substrate at

elevated temperature (e.g., from 60 ℃ to 200 ℃) for 5 to 15 min, and repeated the

aforementioned sliding angle measurement. A 20 µL octane droplet was used for measuring the

64

sliding angle on a tilting stage. The thermally repaired surface with lubricant has a very small

sliding angle of ~10°; while the surface after damage has a 90° sliding angle.

Physical Abrasion Test: The abrasion test was performed on lubricated (Krytox 100)

and non-lubricated aluminum surfaces, which were boehmitized and silanized. Each abrasion

cycle involved dragging a sandpaper (solid fraction of ~0.5) of average particle size of 470 µm

along with a pressure of 15.6 kPa across the surface with a velocity of ~0.02 m/s. After each 5

cycles, we measured the sliding angle of a 20 μL droplet with our tilting stage. The liquid

droplets used for the tests were decane for lubricated surfaces and deionized water for the non-

lubricated surfaces.

65

Appendix B

Calculation of Work of Adhesion

The work of adhesion, wa, at the feces and substrate interface can be simplified as wa =

2(γ13 • γ23)1/2

by the Girifalco and Good equation [94]. Health human feces contain ~70% of water

[92] with the rest consisting of organic matters. Therefore, we can estimate the upper bound of

their surface energy (γ13) to be similar to that of water [102] (i.e., γ13 ≤~72 mJ/m2). From previous

literature, glass surface [102] (γ23) has surface energy as 310 mJ/m2. Based on the aforementioned

equation, the work of adhesion between human feces and glass surface (wa0) is ≤299 mJ/m2.

There are three different methods to reduce the work of adhesion (Figure 3-2). The first method

involves silanization of the glass surface with grafted-polydimethylsiloxane (PDMS). Since the

grafted PDMS has nearly identical chemical structure as the silicone oil, one can assume their

surface energies to be similar. The surface energy of silicone oil is measured to be ~21 mJ/m2.

Therefore, the work of adhesion between the synthetic feces and a PDMS-grafted glass (wa1)

would be ~78 mJ/m2. In the second method, a lubricating layer (silicone oil) can be added in

between the feces and an untreated glass surface to reduce the adhesion. Since silicone oil is

preferably to be absorbed to feces, only the surface energy of feces (γ13) changes. Note that the

silicone oil could be partially or fully infused into the feces, therefore we assume that the surface

energy of feces (γ13) to be ~72 mJ/m2 ≥ γ13 ≥ ~21 mJ/m

2. As a result, the work of adhesion

between the synthetic feces and a silicone-oil lubricated glass (wa2) would be ~299 mJ/m2 ≥ wa2 ≥

~161 mJ/m2. The third method involves coating a lubricating layer between the feces and a

chemically treated glass so as to retain a thermodynamically stable lubricant layer on the

substrate. In this case, we have ~72 mJ/m2 ≥ γ13 ≥ ~21 mJ/m

2, and γ23 ≈ 21 mJ/m

2. Therefore, the

work of adhesion on the LESS-treated glass (wa3) is ~78 mJ/m2 ≥ wa3 ≥ 42 mJ/m

2. Overall, we

have wa2 > wa1 ≥ wa3 and this trend is consistent with the experimental measurements.

66

Appendix C

Identification of Bacteria in Rainwater

The bacteria in collected rainwater were identified in following processes.50 µL of

rainwater was spread uniformly onto four different agar mediums: Thermo Scientific™ Blood

Agar (TSA with Sheep Blood) Medium, Thermo Scientific™ MacConkey Agar Medium, BD

BBL CHROMagar Orientation (Figure 3-18), and BD BBL MHB agar. Then the bacteria were

incubated for 24 hours. Isolates were prepared for analysis using a direct transfer method

following a standard Bruker protocol. Individual colonies from 24-hour cultures were transferred

onto a MALDI target plate using a sterile pipette tip and allowed to dry. The cells were lysed by

applying 1 µL of 80% formic acid solution in water, samples were allowed to dry and 1 µL of 10

mg/mL HCCA matrix solution in 50% aqueous acetonitrile containing 2.5% trifluoroacetic acid

was applied to each sample and allowed to dry. A bacterial test standard (BTS; Bruker Daltonics)

was used for instrument calibration and as a positive control. Matrix blank spots were included in

each analysis to ensure that the target plate was thoroughly cleaned and there is no carryover

signal. MALDI mass spectra were acquired on a Bruker Ultraflextreme MALDI TOF/TOF mass

spectrometer in the linear, positive-ion mode. Spectra were processed using a factory default

processing method for the Biotyper application and searched against a Bruker library containing

entries of 6903 cellular organisms using MALDI Biotyper Version 3.1 software (Bruker). The

results are shown in Table 3-5.

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VITA

Jing Wang

November 14, 1989 Born, Xi’an, Shaanxi, China

2008 – 2012 Bachelor of Engineering

Department of Precision Instrument and Mechanology

Tsinghua University

Beijing, China

2012 – 2018 Graduate Student Researcher

Department of Mechanical and Nuclear Engineering

The Pennsylvania State University, University Park

Pennsylvania, the United States of America

2013 – 2014 PPG Graduate Research Fellow

The Pennsylvania State University, University Park

Pennsylvania, the United States of America

PUBLICATIONS

1. Jing Wang, Keiko Kato, Alexandre Blois, and Tak-Sing Wong, “Bioinspired Omniphobic

Coatings with Thermal Self-Repair Function on Industrial Materials”, ACS Applied

Materials & Interfaces, vol. 8, pp. 8265 – 8271 (2016).

2. Yu Huang, Birgitt Boschitsch Stogin, Nan Sun, Jing Wang, Shikuan Yang, and Tak-Sing

Wong, “A Switchable Cross-Species Liquid-Repellent Surface”, Advanced Materials,

vol. 29, 1604641 (2017).

3. Shikuan Yang, Nan Sun, Birgitt Boschitsch Stogin, Jing Wang, Yu Huang, and Tak-Sing

Wong, “Ultra-antireflective Synthetic Brochosomes”, Nature Communications, vol. 8,

1285 (2017).

4. Xianming Dai, Nan Sun, Birgitt Boschitsch Stogin, Jing Wang, Shikuan Yang, and Tak-

Sing Wong, “Hydrophilic Slippery Rough Surfaces for Water Harvesting”, Science

Advances, 4: eaaq0919 (2018).

PATENTS

1. Jing Wang and Tak-Sing Wong, United States patent application US 15/302,315 (2014).

2. Xianming Dai, Birgitt Boschitsch, Jing Wang, Nan Sun and Tak-Sing Wong, United

States patent application US 62/152,532 (2015).

3. Jing Wang and Tak-Sing Wong, United States patent application US 62/424,062 (2016).

4. Jing Wang, Birgitt Boschitsch, and Tak-Sing Wong, United States Patent Application No.

62/671,054 (2018).