Comments on “An Essay on Contact Angle Measurements” by Strobel and Lyons

Debate - Discussion

Comments on ‘‘An Essay on Contact AngleMeasurements’’ by Strobel and Lyons

Michaela Muller,* Christian Oehr

The potential of contact angle measurements (CAM) as an analytical tool to characterizesurface treatments or modifications is often not fully exploited. Agreeing with Strobel andLyons, comparing contact angles is oftenmuchmore reasonable than comparing deduced datalike surface energies, because the latter are based on models, in turn involving the influenceand knowledge of intermolecular forces at the respective interfaces. For a comprehensivepicture, the measurement of contact angles itself has to be considered together with theappropriate model and the available techniques to carry out CAM. An appropriate measure-ment procedure will be given and a brief discussion of some models to derive free surfaceenergy from CAM.

Preliminary Remarks

In the technical community that deals

with surface modification of materi-

als, contact angle measurements

(CAM) are an important and a power-

ful tool for evaluating effects intended

by any surface treatment. Unfortu-

nately, there are different customs for

carrying out such measurements, on

the one hand,while on the other hand,

different theoretical approaches are

developed for calculating the surface

energy from CAM. Both the use of

different measurement procedures, as

well as the use of different models for

M. Muller, C. OehrFraunhofer Institute for InterfacialEngineering and Biotechnology,Nobelstrasse 12, 70569 Stuttgart,GermanyFax: þ49 711 9704200;E-mail:[email protected]

Plasma Process. Polym. 2011, 8, 19–24

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhe

surface energy calculations, often

result in apparent incomparability of

results from different laboratories. In

addition, some of the published mea-

surements are neither useful for dedu-

cing application-relevant properties

(where knowledge of the equilibrium

contact angle is sufficient) nor for

calculating surface energies. These

are the facts about which Strobel

et al. (this issue) complain.

With this present paper, we would

like to support the opinion that the

potential of CAM is often not fully

exploited and, regarding the under-

lying models, deductions are some-

times drawn prematurely. For a com-

prehensive picture, the measurement

of contact angles itself has to be

considered together with the appro-

priate model and the available tech-

niques to carry out CAM. We will first

focus on the contact angle measure-

ment, describing the procedureweuse

and will then briefly discuss some

im wileyonlinelibrary.com

models how free surface energy can be

derived fromthemeasurements. Thus,

the first part of this contribution is

dedicated to CAM, and the second to

the calculation of free surface energy.

Part I Contact AngleMeasurement (CAM)

To underline the problem, a short

description of CAM is now summar-

ized: Placing a droplet of any liquid on

a solid surfacewill in all cases result in

a contact angle between the two,

provided the respective surface ten-

sions of the materials involved are

different. But, it is not only the surface

tensions that determine the resulting

contact angle, because the latter is

additionally influenced by inhomo-

geneities in chemical composition

and/or structure (e.g., roughness) at

the contact line of the three phases

(solid–liquid–gas). Beside this, the

DOI: 10.1002/ppap.201000115 19

20

M. Muller, C. Oehr

purity of test liquid, electrostatic

charge at the surface, as well as

humidity in the gas phase, and sample

preconditioning, have to be controlled.

The inhomogeneities are, by defini-

tion, not representative of the whole

surface area of interest. The actual

contact angle that represents the solid

phase contains information relating

only to 10nm laterally and often less

than 1nm in depth. Thus, it can be

readily understood that uniformity

based on such a restricted volume

cannot be extrapolated for extended

surface areas of technical materials.

For comparison, small-spot XPS draws

its information from volumes that are

10mm laterally and a few nanometers

in depth. Thus, CAM is ‘‘more sensi-

tive’’ to differences in surface chem-

istry and structure, and it should be

taken with comparable care. In our

opinion, one should carry out CAM as

accurately as XPS measurements, for

example, by controlling the (liquidand

gas) phases in contact with the exam-

ined (solid) surface, by avoiding char-

ging, by considering suitable reference

materials, etc. In contrast with other

refined analytical techniques, contact

angles can be measured quite easily,

and people do so; we arewell aware of

this courseofaction fromourownfield

of plasma application: A glow dis-

charge is created with ease, and in all

cases leads to somemeasurable results

on anygiven surface. A complete set of

experimental parameters should be

controlled and quoted if any compar-

ison is to take place, but for the case of

plasma treatments this is often not

done, or at least not accurately docu-

mented.

Formeaningful CAM, the conditions

of the subsequent application as well

as temperature, electric fields, and

surface charges have to be taken into

account; furthermore, polymeric sur-

faces (reorientation of surface func-

tionalities) will adapt their surface

properties according to the environ-

ment, if the material is not highly

cross-linked. Due to this complex

situation it is of importance to mea-



sure the static advancing and receding

contact angles, which represent the

upper and lower limits of the ‘‘true’’

(meaning ‘‘equilibrium,’’ or Young’s)

contact angle, a value which lies

between the two (near the advancing

angle). In addition, it is generally

agreed that the advancing angle is

more strongly influenced by low-

energy domains in the surface, while

the receding angle is more influenced

by domains of higher surface energy.

This, again, represents valuable infor-

mationformanyapplicationareasand

it should be used. If the observed

contact angle hysteresis is small, this

signals that the surface is quite homo-

geneous with respect to chemical

composition and/or structure. The

interpretation of contact angle hyster-

esis must be done carefully since also

regular nanostructured surfaces can

show very low contact angle hyster-

esis, e.g., those which follow Cassie–

Baxter regime of wettability.

Strobel and co-workers[1] prefer the

Wilhelmy plate technique, which of

course is less ‘‘operatordependent,’’ but

with this method both sides of the

given sample must first undergo an

identical treatment, something that is

oftennot guaranteed. Second, theback-

side of the second sample is supposed

to have exactly the same composition

and structure as the untreated mate-

rial, but that is also not easy to

guarantee. Third, this method is

restricted to flat sheets or single fibers,

but it is not applicable to three-dimen-

sional samples. Fourth, fortunately an

often-fulfilled requirement is that the

contribution of the edges be insignif-

icant. Nevertheless, all of the methods

used do have some difficulties or other.

Therefore, it is of great importance that

the measurement procedures be used

in a standardized way if meaningful

comparisons are intended.

In our laboratory, we proceed with

the sessile drop technique as follows:

Prior to measurements, the samples

are storedandequilibratedunderwell-

defined humidity conditions. In the

case of insulating materials, possible

im

electrostatic charges are neutralized,

e.g., by treatment with an antistatic

gun.Asmall dropletof the selected test

liquid is dosed at the tip of a hollow

needle connected to a syringe and

brought into contact with the test-

surface. The droplet volume is then

carefully raised to about 10mL, so that

the contact line moves across the

substrate surface. This addition of

liquid is halted and when the contact

angle has stablilized, it is taken as the

value of advancing contact angle,

considering both sides of the projected

drop shape. Care must be taken to

avoid possible disturbance of the drop

shape by the inserted needle. In some

cases, the needle must even be with-

drawn prior to CAM. Then the liquid is

withdrawn until no further change in

the value of contact angle is observed,

and this is the measured receding

contact angle. This procedure is carried

out for each sample and each test

liquid at aminimumof three-different

positions on the sample’s surface.

Sometimes, one can observe different

angles on either side of the projected

droplet due to local differences in

surface composition and/or structure.

Therefore, we take both angles (left

and right side of the droplet) and use

the arithmetic mean value of 2. In

some cases, the duration between the

instant when the droplet volume no

longer changes to that when the

contact angle is read is also important,

especially when kinetic hysteresis has

to be considered. In this case, some

possible dissolution processes at the

surface or some swelling must be

taken into account. Such situations

should be avoided, because the final

measuredangleswill notbe stable and

true equilibrium will not have been

reached. Therefore, the test liquids

have to be chosen in a proper way

such that no swelling, dissolution, or

reorganization at the solid surface

occurs, at least none that are relevant

to the time-frame of the measure-

ment’s duration.

For surfaces which are employed in

contact with humid or fluid environ-

DOI: 10.1002/ppap.201000115

Comments on ‘‘An Essay on Contact Angle Measurements’’ . . .

ments, and/or which are subject to

thermodynamicandkinetichysteresis

(surfaceswhichareoftenof interest for

biomedical applications), the captive

bubble method[2] should be used,

instead. These samples are completely

immersed in the test liquid, with the

side to bemeasured facing downward.

An air bubble is then brought in

contact with the solid surface from

below. Depending on the wettability,

the bubble must be fixed in place by

the position of the needle. After a few

seconds, the static contact angle near

the triple phase line ismeasured.With

this method only the displacement of

liquid from the solid–liquid interface

by air can be analyzed, which corre-

sponds to the receding contact angle

by the sessile drop method.

Unfortunately, lessand lessworkers

now resort to measurements of both

advancing and receding contact

angles. Instead, they prefer to take

only readings (at least, hopefully) at

different surface locations; neverthe-

less, they obtain more-or-less stochas-

tic values between those of the advan-

cingand recedingangles, but these can

oftencover ranges that farexceed10 8Con technical surfaces. Unfortunately,

in Germany this tendency is encour-

aged by precisely those authorities

that define industrial standards (DIN).

In their recently published normative

papers,[3] they advise users to employ

static contact angle devices to observe

sets of ten droplets and to calculate a

mean angular value from these data,

with no mention whatever of advan-

cing or receding angles. It is merely

stated that within an area of

10 cm� 10 cm, three droplets should

Table 1. Important molecular interaction for

Interaction force

Coulomb

Keesom–van der Waals

Debye–van der Waals

London–van der Waals

Hydrogen bonding



be placed and thus three (randomly)

taken angles should be measured.

With this type of developments in

mind, it is important to restate the

meaning and utility of CAM from time

to time.

An often not considered topic is that

which deals with the segment-size of

sample surface responsible for the

observed contact angle.[4] This should

be clarified before certain ideas regard-

ing the influence of dimensions of

inhomogeneities be discussed. Again,

the contact angle represents only the

equilibriumof forces at the triple-phase

contact line,andnotofanysurfacearea.

In order to characterize an area, the

contact linemust bemoved, preferably

at very low capillary numbers, and the

angle has to be measured during this

constantmovement.Models have been

developed for this case, and they have

been used in industry experienced in

thedepositionof thinorganicfilms, e.g.,

for photographic purposes.[5] Here, the

dynamic advancing anglewould be the

appropriate one for the calculation of

surface energy.

On the microscopic level, the con-

tact line is determined by the inter-

molecular forces (Table 1) acting at the

point were the three phasesmeet, and

it is influencedby therangeofactionof

these forces. Beside the Coulomb

interaction, which scales with r�1

(r being the distance between inter-

acting molecules/atoms), all other

forces of interaction scale with r�6

and are therefore very short-ranged.

The interrelation between line energy,

advancing, receding, and Young’s con-

tact angles has recently been well

described in a paper by Tadmor.[6]

ces (r represents intermolecular distance).

Type of interaction

Electrostatic interaction

Dipole–dipole interaction

Dipole-induced dipole interaction

Dispersive interaction

im

In summary, measured contact

angles are dependent on how the

measurement is carried out. To com-

pare results of different working

groups it is therefore essential to

describe the measurement conditions

andprocedure indetail. Eachmeasure-

mentmethodmentioned, sessile drop,

tilting angle, and Wilhelmy plate

(static or dynamic), has some merits

and some disadvantages. But, if mea-

surements are carried out in a stan-

dardizedmanner, theywill be compar-

able. In order to obtain valuable

information about a surface by CAM,

the advancing and receding angles

from several locations on the surface

should be recorded, so as to ensure

some statistical significance. The

operator should be aware that his

measurement is even more surface-

sensitive than comparable techniques

like XPS, etc. and that sample prepara-

tion therefore has to be done at least

with the samedegree of accuracy. If all

this is respected, papers published in

Plasma Processes and Polymers will

maintain the high ranking of the

journal. Accurate measurements with

some statistical validation are in all

cases sufficient if the main topics of a

paper are depositionof filmsor surface

treatment and their application, on

the one hand, and demonstration of

the potential of plasma processes to

createnewsurfaces, on theotherhand.

Part II Surface Free Energy

If thermodynamic properties and

molecular kinetics of surfaces are the

focus of a paper, then certain addi-

Interaction range

�r�1

�r�6

�r�6

�r�6

�r�6

www.plasma-polymers.org 21

22

M. Muller, C. Oehr

tional aspects have to be taken into

account. In such a case, contact angles

are used to derive some material

properties that may be expected from

theoretical considerations. Thus, the

surface tension of a solid material can

be derived by starting with a macro-

scopic thermodynamic approach (top–

down approach), as well as by con-

sidering intermolecular forces on a

microscopic level (where both bot-

tom–up and top–down approaches

are used). Accordingly, to the authors’

knowledge, these different concepts

are not unified and are the objects of

controversial discussion. This situa-

tion explainswhy, for example, for the

samemeasured contact angles and for

the same triplet of (gas–liquid–solid)

materials that coincide at a contact

line, different surface energy values

might be calculated. This is due to the

use of different algorithms with dif-

ferentbasic analytical concepts,which

may result in different values of sur-

face free energies. The reason lies in

the dilemma that the surface tension

of a solid is not directly accessible. To

decide which calculation model is

more appropriate than the others,

one has to make assumptions regard-

ing intermolecular forces acting at

interfaces. The main molecular inter-

action forces are London-, Debye-,

Keesom-, Coulomb-, and hydrogen

bonding forces already mentioned in

Table 1. These forces originate from

chemical entities (OH-groups, CF3-

groups, etc.) at the interface, which

contribute to the total surface free

Table 2. Concepts to separate contributions

Concept Split-up of fr

Fowkes[11] gtotal ¼ gdispersiv

van Oss et al. [13] gtotal ¼ gLifshitz�



energy. To derive a complete picture

and to calculate amacroscopic contact

angle, the chemical functions respon-

sible for these forces should be known,

more specifically their densities, their

distributions, and orientations at the

surface. Furthermore, one has to take

into account the additional fact that

technical surfaces are rough, at least

on the nanometer scale, and that the

mentioned functionalities will be

mobile, depending on cross-linking/

mobility of the supporting network

structure. One can readily envisage

that complete information regarding

surface composition and structure is

usually not available.

Since the beginning of the past

century, going back to Einstein’s first

paper,[7] when people were convinced

it was possible to deduce macroscopic

behavior via a bottom–up approach

starting with a material’s stoichiome-

try, several attempts were made to

calculate surface tension.One concept,

developed during the 1920s by Sud-

gen,[8] defines the so-called parachor,

which correlates the density differ-

ence of two phases with the surface

tension. The parachor of a molecule,

which has the dimensions of volume,

was divided into volume equivalents

of segments of the molecule. This

approach was successful in a practical

way, but its theoretical foundation

was very weak. Due to strong criti-

cisms[9] in the 1960s, this approach

now appears to have been forgotten,

but a comprehensive overview on this

topic has appeared[10]; however, other

of molecular interaction forces to the surface f

ee surface energy g Commentary

e þ gpolar gdispersive is th

Waals forces

subsumes all

vanderWaals þ gacid�base gacid�base cont

(electrostatic

gLifshitz�vanderW

Debye- and Lo

im

molecularly oriented approaches,

dividing a calculated surface energy

in portions due to intermolecular

forces, seem to be more effective.

Thereby, surface energy is divided into

dispersive and polar components

(Fowkes,[11] Girifalco/Good, Owens/

Wendt[12]); later, Fowkes added Lif-

shitz/van der Waals and acid/base

components, the acid /base compo-

nent then being further analyzed by

van Oss et al.[13]

These top–down approaches result

in more and more complex mathema-

tical formulae (Table 2) and at least

three-different test liquids have to be

used to obtain the respective contribu-

tions to the overall value of surface

energy. Here, the main challenge is to

choose the appropriate organic liquids

that will not cause swelling of the

polymer surface, but inwhichall of the

various contributions (dispersive, acid

base contribution, etc.) nevertheless

differ among one another. Thus, the

liquids for the measurements must be

carefully chosen so as not to interact

with the solid, and must be used only

in a fresh state because any deteriora-

tion (e.g., due to oxidation, relevant for

the often-used methylene diiodide)

will change the test liquid’s polar

component. In Table 3 are shown the

methods we mainly use in our own

laboratory.

Our description up to this point is

based on the assumption that no

motions of molecules or functional

groups are involved on a scale of some

nanometers or belowat the interfaces.

ree energy.

e contribution of the London–van der

to surface free energy, and gpolar

other interaction force contributions

ains the strong interaction forces

and hydrogen bond interactions),

aals the much weaker Keesom-,

ndon-van der Waals forces

DOI: 10.1002/ppap.201000115

Table 3. Some frequently applied models for calculation of surface free energy.

Model/authors Equationsa) Commentary

Separation between polar and

dispersive contributions;

harmonic mean; Wu[14]

1þ cos#ð Þg l ¼ 42gd

sgdl

gds þ gd

l

þ 2gps g

pl

gps þ g

pl

!From our own experience,

suitable for non-ionic

surfaces with surface free

energies below ca. 35mNm�1

Separation between polar and

dispersive contributions;

geometric mean; Owens,

Wendt, Rabel and Kaelble[12]

1þ cos#ð Þg l

2ffiffiffiffiffigdl

q ¼ffiffiffiffiffigds

qþ

ffiffiffiffiffigps

q ffiffiffiffiffigpl

gdl

sFrom our own experience,

suitable for non-ionic

surfaces with surface free

energies above ca. 35mNm�1

Separation between

Lifshitz–van der Waals and

acid–base contributions;

van Oss et al. [13]

1þ cos#ð Þg l ¼ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigLWs � gLW

l

qþ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigþs � g�

l

pþ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffig�s � gþ

l

q� �Suitable for surfaces

with acid–base contributions

a)W, contact angle; g , surface (free) energy; subscripts l, s, surface energy of liquid and solid; superscripts d, p, LW, þ, �, dispersive, polar,

Lifshitz–van der Waals, acid, and base parts of surface energy.

Comments on ‘‘An Essay on Contact Angle Measurements’’ . . .

But this is not valid in many cases,

especially for polymer surfaces. A

polymer surface will rearrange its

structure and composition depending

on the properties of the contacting

phase, its chain mobility, internal free

volume, and on the temperature.

Beside these more and more com-

plex microscopic models, the purely

thermodynamic approach is also still

in use.[15]

Due to rapid advances in computer

science, elaborate simulations can

now be done, and as a consequence

researchers in that field attempt to

simulate macroscopic phenomena

starting with ab initio calculations at

the molecular level. Thus, contact

angles of liquids on plasma-deposited

fluorcarbon surfaces[16] were found to

be in good agreement with simula-

tions of ‘‘droplets’’ comprising some

1000watermolecules.[17] This simula-

tion is still far from being able to

describe a real droplet, but we hope to

obtain new insights about interac-

tions at solid–liquid interfaces via

molecular dynamics.

Finally, it should be stated that

phenomena become ever more com-

plex with regard to friction, hydro-



dynamics, and molecular kinetics, if

one considers the physics of moving

wetting lines. For such an approach,

the reader is referred to the paper by

Blake.[5]

Summarizing, numerous attempts

have been made since more than a

century to calculate the surface free

energy from measured data. At the

present time, all of these approaches

still coexist, and each one is used in

more or less restricted areas. Consid-

ering that the portion of surface

science that deals with surface free

energy is important in several very

different fields, a variety of scientific

disciplines are involved and come to

prominence depending on the parti-

cular focal issue: ‘‘Scientists tend to

think in terms of their most familiar

models. It is not accidental that the

earliest descriptions of the moving

wetting line and its associated con-

tact angle were in terms of displaced

equilibrium (chemists), friction (phy-

sicists), and viscous bending of the

liquid–vapor interface (engineers and

mathematicians).[5]’’ This sentenceby

Blake characterizes the contemporary

situation in the field of surface free

energy.

im

Summary

We agree with Strobel and Lyons that

comparing contact angles is often

much more reasonable than compar-

ing deduced data like surface energies,

because the latterarebasedonmodels,

in turn involving the influence and

knowledge of intermolecular forces at

the respective interfaces. Especially

regarding the interface between

liquids and solids, it is difficult to

consider and assess all of the forces

present, and to avoid complicating

effects like swelling, penetration, or

dissolution at the nanometer scale.

Papers which do not consider

advancingandrecedingcontactangles

are less informative, and they fall far

short of the potential offered by the

CAM methodology.

We also agree that no additional

information is provided if the surface

energy is calculated from both advan-

cingandrecedingangle. Specifying the

algorithm one uses and a single

calculation suffices. Moreover, if the

sample material and the treatment

procedure are given, the reader may

himself choose the algorithm he pre-

fers.

www.plasma-polymers.org 23

24

M. Muller, C. Oehr

According to published findings, we

would now like to place procedures for

measuring contact angles into an

hierarchical order: The lowest-priority

procedure comprises measuring an

arbitrary-taken contact angle from a

sessile droplet. Such data should be

avoided altogether. The second level of

priority will be the use of more than

one droplet for statistical purposes

(that is, the DIN procedure). On the

same level is the use of ‘‘test inks’’ for

surface tensionmeasurements (a ‘‘hor-

ror’’ for people who think in terms of

theoretical concepts). This approach is

tolerable forprocess control, but just to

indicate relative changes. The third

priority level is the use of advancing

and receding CAM (at different loca-

tions on the surface). On the fourth

level of priority, at least two or three

(also a subject for discussion) different

liquids are used to formulate an

impression about dispersive and polar

contributions to the surface free

energy. The fifth level comprises the

use of multiple liquids in order to

separate the surface tension into its

constituent parts, whereby more

information related to interaction of



the liquid with the solid becomes

available.

Finally, this branch of surface

science is still under development,

and it might be enhanced by enligh-

tened argumentation from different

sub-disciplines. Beside thermody-

namics, hydrodynamics (in case of

moving water contact lines), and

molecular kinetics can also shed some

additional light upon the observed

physical effects. Meanwhile, it may be

possible to carry out some new bot-

tom–up approaches.

Received: August 25, 2010; Revised:September 30, 2010; Accepted: October 1,2010; DOI: 10.1002/ppap.201000115

Keywords: advancing; calculation models;interaction forces; receding contact angle;surface free energy; wetting

[2] J. D. Andrade, R. N. King, D. E. Grego-

[1] J. Park, C. S. Lyons, M. Strobel, M. Ulsh,M. J. Kinsinger,M. J. Prokosch, J. Adhes.Sci. Technol. 2003, 17, 643.

nis, D. L. Coleman, J. Polym. Sci. Symp.1979, 66, 313.

im

[3] DIN 55660-2: Paints and varnishes –Wettability – Part 2: Determination ofthe free surface energy of solid sur-faces by measuring the contact angle;Draft version 2009-07-06.

[4] L. Gao, T. J. McCarthy, Langmuir 2009,25, 7249.

[5] T. D. Blake, J. Colloid Interface Sci.2006, 299, 1.

[6] R. Tadmor, Langmuir 2004, 20,7659.

[7] A. Einstein, Ann. Phys. 1901, 4, 513.[8] S. Sudgen, J. Chem. Soc. 1924, 125,

1177.[9] O. Exner, Collect. Czech. Chem. Com-

mun. 1966, 31, 3222.[10] S. N. Balasubrahmanyam, Curr. Sci.

2008, 94(12), 1650.[11] F. M. Fowkes, J. Phys. Chem. 1962, 66,

382.[12] [12a] D. K. Owens, R. C. Wendt, J. Appl.

Polym. Sci. 1969, 13, 1741; [12b]W. Rabel, Farbe und Lack 1971, 77,997; [12c] D. H. Kaelble, J. Adhes.1970, 2, 66.

[13] C. J. van Oss, R. J. Good, M. K. Chaudh-ury, Langmuir 1988, 4, 884.

[14] S. Wu, J. Polym. Sci. 1971, C34, 19.[15] J. K. Spelt, D. R. Absolom, A. W. Neu-

mann, Langmuir 1986, 2, 620.[16] J. Barz, M. Haupt, U. Vohrer, H. Hilgers,

C. Oehr, Surf. Coat. Technol. 2005, 200,453.

[17] J. M. Knaup, C. Kohler, M. Hoffmann,P. H. Konig, T. Frauenheim, Eur. Phys. J.Spec. Top. 2007, 149, 127.

DOI: 10.1002/ppap.201000115

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Comments on “An Essay on Contact Angle Measurements” by Strobel and Lyons