Advances in Contact Angle, Wettability and Adhesion, Volume 3

By K.L. Mittal

This is the third Volume in the series “Advances in Contact Angle, Wettability and Adhesion” initiated to consolidate information and provide commentary on certain recent research aspects dealing with this important topic. Its predecessor Volumes 1 and 2 were published in 2013 and 2015, respectively.
This new book comprising 15 research and review articles is divided into four parts: Part 1: Contact Angle Measurement and Analysis; Part 2: Wettability Behavior; Part 3: Hydrophobic/Superhydrophobic Surfaces; Part 4: Wettability, Surface Free Energy and Adhesion. The topics covered include:
 O Procedure to measure and analyse contact angles/drop shape behaviors.
 O Contact angle measurement considering spreading, evaporation and reactive substrate.
 O Measurement of contact angle of a liquid on a substrate of the same liquid.
 O Evolution of the axisymmetric droplet shape parameters.
 O Interfacial modulus of a solid surface.
 O Functionalization of textiles using UV-based techniques for surface modification—patterned wetting behavior.
 O Wettability behavior of oleophilic and oleophobic nanorough surfaces.
 O Wettability behavior of nanofluids.
 O Dielectrowetting for digital microfluidics.
 O Hydrophobicity and superhydrophobicity in fouling prevention.
 O Superhydrophobic/superhydrophilic hybrid surface.
 O Laser material processing for enhancing stem cell growth.
 O Wettability correlation for bioadhesion to different materials.
 O Determination of the surface free energy of solid surfaces: statistical consideration.
 O Determination of apparent surface free energy using hysteresis approach.

• Language: English
• Publisher: Wiley
• Release date: Feb 26, 2018
• ISBN: 9781119459958

Book preview

Preface

The present volume constitutes Volume 3 in the ongoing series Advances in Contact Angle, Wettability and Adhesion which was conceived with the intent to provide periodic updates on the research activity and salient developments in the fascinating arena of Contact Angle, Wettability and Adhesion.

The provenance and intriguing historical evolution of contact angle/wettability was provided in the Preface to the premier volume. Contact angle study has been accorded glamour as pointed out in the Preface to Volume 2 that 5 Nobel Laureates had evinced interest in the study of contact angle/ wettability phenomena directly or indirectly. Prof. Pierre-Gilles de Gennes, in particular, led quite a hefty research program in the broad realm of wettability and provided many fundamental insights apropos of wetting phenomena.

In the Preface to the current Volume, I would like to draw attention of the readers to three interesting topics. First, in the June 1, 2015 Issue of the Chemical & Engineering News (C&EN of the American Chemical Society) a very illuminating and thought-provoking write-up entitled ‘Sleeping Beauties’ Wake Up was published which referred to studies of 15 ‘sleeping beauties’ and seven of these were published in chemistry journals. Out of these 7 it is very pleasing to note that two were devoted to the study of wetting. The papers (which are very familiar today to every researcher in the field of wetting) by A.B.D. Cassie and S. Baxter (1944) and R.N. Wenzel(1936) had their Awakening only in 2002 and 2003, respectively. Today, these papers are routinely cited in articles dealing with wetting.

Second, I would like to point out that the contact angle has been dubbed as the poor man’s (or poor woman’s) surface analysis technique, as it does not require sumptuous equipment and contact angle measurements can be made using a modestly priced setup called goniometer with a small footprint. Carl Clegg (Rame-Hart Instruments Co.) in his December 2010 Newsletter has listed 50 things one can do with a goniometer. Here, the following eclectic list of interesting, technologically important and exotic applications of contact angle should suffice to underscore the ubiquity of contact angle: as a QC tool during contact lens fabrication; to optimize surface treatments that lead to better adhesion of solder; to check cleanliness of surfaces, which is vital to adhesion of coatings; to develop surface treatments to prevent ice build-up; to study the wetting effects of lubricants; to devise means to make self-cleaning surfaces; to study wetting properties of leaves and fruits to develop optimal pesticide products; to develop non-fouling surfaces; to improve biocompatibility of polymer-based medical devices; to control wetting behavior in MEMS/NEMS; to control fluid flow in micro- and nano-fluidics; to control wetting behavior of foods and pharmaceuticals to develop edible films and coatings; use for diamond and other gems to quantify their purity and detect counterfeits; and finally for authentication of rare coins, a delight for numismatists.

Third, I would like to mention that contact angle is not only scientifically and technologically important, but it has social implications also. In this vein, I would like to cite the paper entitled Dermatometry for coeds by my professor, Dr. Arthur W. Adamson, a doyen of surface science and a household name in the surface science community. Using lovingly called the Adamson plot one can check whether your drink has any alcohol in it, or the bartender is giving you plain water just by making a few contact angle measurements. Here I should also add that the wettability of human skins from various origins has been extensively investigated. I wonder if the contact angle value of human skin can be related to beauty (just kidding)! Apropos, one can easily check by contact angle measurement whether a person has taken shower.

Even a cursory look at the current literature will evince that since the recognition of the Lotus Leaf Effect there has been an explosive increase in research papers dealing with superhydrophobicity and all signals indicate that this high tempo of research will continue unabated. These days there is a tremendous interest in all kinds of phobicities (also philicities) and there is phenomenal interest in rendering surfaces omniphobic or panphobic, as materials with such surfaces offer many exciting applications, ranging from mundane to highly sophisticated.

Now coming to this volume which contains 16 articles written by active and renowned researchers. The book is divided into four parts: Part 1: Contact Angle Measurement and Analysis; Part 2: Wettability Behavior; Part 3: Superhydrophobic Surfaces; Part 4: Wettability, Surface Free Energy and Adhesion. The topics covered include: procedure to measure and analyse contact angle/drop shape behaviors; contact angle measurement considering spreading, evaporation and reactive substrate; measurement of contact angle of a liquid on a substrate of the same liquid; evolution of axisymmetric droplet shape parameters; interfacial modulus of a solid surface; functionalization of textiles using UV-based techniques for surface modification–patterned wetting behavior; wettability behavior of oleophilic and oleophobic nanorough surfaces; wettability behavior of nanofluids; dielectrowetting for digital microfluidics; hydrophobicity and superhydrophobicity in fouling prevention; superhydrophobic/superhydrophilic hybrid surface; determination of the surface free energy of solid surfaces: statistical considerations; determination of apparent surface free energy using hysteresis approach; wettability correlations for bioadhesion to different materials; laser material processing for enhancing stem cell adhesion and growth.

The articles included in this book are primarily based on presentations made at the Tenth International Symposium on Contact Angle, Wettability and Adhesion held at the Stevens Institute of Technology in Hoboken, NJ, July 13–15, 2016 under the auspices of MST Conferences. However, the authors were urged to make their manuscripts more general and review in nature. It should be recorded that all manuscripts were rigorously reviewed, revised (some twice or thrice) and properly edited before inclusion in this volume. So the material presented in this book is of archival value and meets the highest standard of publication.

Yours truly sincerely hopes that this current Volume 3 will be received as warmly as its predecessors. This book should be very valuable to anyone interested in staying abreast with the latest developments and perspectives in the domain of Contact angle, Wettability and Adhesion. Further, I hope the information consolidated in this volume will help spur further research and will serve as a catalyst in providing new research ideas.

Now comes the important, but fun, part of writing a Preface as it provides the opportunity to thank those who were instrumental in materializing this book. First and foremost, I am profusely thankful to the authors for their interest, enthusiasm, cooperation and for providing written accounts of their work, which were a desideratum to bring out this book. Also Martin Scrivener (publisher) should be thanked for his unflinching commitment and steadfast support for this book project and showing this book the light of day.

Kash Mittal

P.O. Box 1280

Hopewell Jct., NY 12533

E-mail: [e-mail]

January 14, 2018

Part 1

CONTACT ANGLE MEASUREMENT AND ANALYSIS

Chapter 1

A More Appropriate Procedure to Measure and Analyse Contact Angles/Drop Shape Behaviours

M. Schmitt1,2,3* and F. Heib3

1Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA; Mulhouse Cedex, France

2Institut for Coatings and Surface Chemistry, Niederrhein University of Applied Science, Krefeld, Germany

3Saarland University, Physical Chemistry, Saarbrücken, Germany

*Corresponding author: [e-mail]

Abstract

Surface science, which comprises the preparation, development and analysis of surfaces, is of utmost importance in both fundamental and applied sciences as well as in engineering and industrial research. During our research in the field of coatings/surfaces and coating materials, the analyses of wetting of coating materials and the coatings themselves led us to the field of dynamically performed drop shape analysis. We focussed our research efforts on the main problem of the surface science community, which is to determine the correct and valid definition and measurement of contact angles. So we developed the high-precision drop shape analysis (HPDSA) and three statistical contact angle determination procedures. HPDSA involves complex transformation of images from dynamic sessile drop experiments to x-y-coordinates and opens up the possibility of a physically meaningful calculation of curvature radii. This calculation of radii is the first step to an assumption-free link to the Laplace equation, which can deepen the understanding of the interface between the liquid and the vapour in relation to different properties and conditions (temperature, experimental technique, surface, etc.). The additional benefit of a tangent-free calculation of contact angles is presented in our 2014 and 2016 published papers. To fulfil the dire need for a reproducible contact angle determination/definition, we developed three procedures, namely, overall, global, and individual statistical analyses, which are based on, but not restricted to, HPDSA so that the procedures can be implemented to analyse contact angle data from commercial software. During the process of development we investigated wetting of ideal surfaces, contact angle hysteresis, dynamics of wetting, wetting of rough and chemically heterogeneous surfaces, the Wenzel and Cassie models, superhydrophobicity, superhydrophilicity and the rose petal effect, wetting transitions on rough surfaces, nonstick droplets, effects of ionic liquids (sticking droplets), etc..

Keywords: HPDSA, contact angle, advancing angle, receding angle, wetting, drop shape, hydrophobic, hydrophilic, superhydrophobic, statistical contact angle

1.1 Introduction

This chapter summarizes our drop-related research of the recent years until 2016 [1–13], Scheme 1.1, i.e. the drop shape analysis or, more specifically, the analysis of the dynamic drop shape behaviour, where the current focus is on contact angle (CA) measurements and analyses. Thus, this chapter deals with a stable and fast super-resolution image analysis in combination with a contour/radii calculation, and a suggested combination with one or all three statistical procedures presented to analyse contact angle data. This topic of our investigations can briefly be summarised by wetting. Wetting as an effect describing the contact between solid and liquid phases is important for life and nearly every application. For example, good wetting is a necessary but not a sufficient condition for the coating material to form a well-adhered coating (completely well-bonded coating without defects).

Scheme 1.1 Summary and history of the methods developed and their applications. Development of the super-resolution drop shape analysis is mainly focused in contribution [3], the application/development of the statistical CA analyses was finally developed/conceived in contribution [9]. The aims of statistical procedures are inter alia reproducible CA definitions (advancing/receding, up-/downhill) and motion behaviour analysis. Uncited applications are experimentally confirmed but not published, and applications and questions in dashed boxes are most likely possible but due to hardware or time aspects were not performed until now.

1.1.1 Brief Summary of the History of Modern Wetting

It is less known that Einstein, Schrödinger and Bohr devoted parts of their research activities to the capillarity and wettability. Several factors have revived an interest in wetting and wettability. The first of these was the discovery of the lotus effect (or superhydrophobicity) by Barthlott and Neinhuis in 1997 [14] (the famous paper, reporting the Lotus effect, entitled Purity of the sacred lotus, or escape from contamination in biological surfaces which has already been cited more than 4700 times!). The second factor was the rapid progress achieved in the field of wetting by the scientific school led by the Noble Prize winner P. G. de Gennes [15]. It is noteworthy that the main notions of the modern theory of wetting (such as disjoining pressure, superhydrophobicity, contact angle hysteresis, wetting transitions) are younger than the basic ideas of relativity and quantum mechanics. Hence, the field of wetting phenomena is a rapidly evolving field of modern science, full of exciting physical and chemical insights. (Quoting Prof. E. Bormashenko)

1.1.2 Vexing Question in Wettability

What is the most appropriate way to measure contact angle?

This is direct quotation from the preface to the book "Contact Angle, Wettability and Adhesion Vol. 6 from Dr. Kash Mittal[16] and the answer to this question is the first step to characterise the surface free energy of solids in a reliable/unquestionable manner, emphasizing the importance of contact angle determination for wetting and for actual wetting analysis. Human subjectivity and limitation (attention span, time required etc.) especially considering the progress in image recording (e.g. high speed cameras) is from our point of view an important part of this question which may be solved by the procedures presented. The understanding of wetting phenomena is extremely important from both fundamental and applied points of view. A broad diversity of biological, physical, and chemical effects involve wetting events so that this question is most important for the surface science community itself but especially for all of the multiple users of sessile drop experiments by optical observation of contact angle, OCA. The CA-research presented in this chapter is, therefore, focused on this fundamental question even through multiple different surfaces were investigated. Thus, the question was divided into measurement (Section 1.3) and analysis (Section 1.4) of contact angles. Measurements in terms of image analysis were done by a developed high-precision drop shape analysis (HPDSA) which is similar to localization[17] of Super-Resolution Microscopy leading to continuous"†1 data points. Analysis of the contact angles is possible by the three developed procedures, the overall, the global and the individual statistical analyses which can be a good starting point for answering the vexing question (i.e. different ideas concerning most appropriate contact angles) and not relying on trusting in oneself, students, known or unknown scientists/users or similar arguments. The following subsections contain specific background information on roughness, equilibrium CA and hysteresis which in the authors’ opinion is important to correctly perform and interpret contact angle measurements using the sessile drop technique.

1.1.3 Background

1.1.3.1 Force Balance and Roughness

From a current perspective the publications in Zeitschrift fur Physikalische Chemie [1, 2], Scheme 1.1, are preliminary works of the developed methods and procedures. Both contributions summarize the drop shape behaviour during dynamic (or more precisely dynamically performed) inclined plane and horizontal experiments with an immersed dispensing needle when monitored by the human eye in a clear and precise way. The dependence of the drop shape behaviour on surface roughness was investigated. For horizontal and inclined planes, different force distributions cause the triple line to advance or recede which affects the measured contact angles. Similar force-related studies were also performed in 2014 by Amirfazli and co-workers [18–20]. These publications [1, 2] identified the dependence of critical contact angle (critical motion) in addition to the usual jump contact angles on a rough surface for horizontal (volume of the liquid was varied) and inclined surfaces (inclination angle was increased). It was concluded [2] that the force balance causing the motion of the triple line (both on the uphill and downhill sides) depends not only on the surface roughness but also on the complex relationships among the curvature of the whole meniscus, the principal radii of curvature, the local and effective downhill forces, the drop volume and the gravity. The analysis of curvatures and principal radii of curvature during a dynamic sessile drop experiment has rarely been the subject of scientific research so far, especially during inclined plane experiments. Therefore, there is currently no appropriate image evaluation software available that allows determination of curvatures, principal radii of curvature and kinetic studies of sessile drops for non-axisymmetric drop shapes. So we decided to focus a part of our work on the development of a software routine that enables to evaluate drop shape parameters and thus to analyse contact angles and curvatures of sessile drops with high precision and sensitivity. We are convinced that the possibility to analyse the contact angle in more detail, especially the critical contact angle, will improve the method of contact angle measurement. This will result in an enhanced understanding of the force balance around the triple line. We are confident that appropriately recorded/logged and well performed contact angle determinations will lead to valid and reproducible results with respect to the wetting behaviour of solid surfaces [2]. Finally, the insufficiency of the SCA20 software of the OCA20 equipment, Subsection 1.3.4, led to the design of a unique routine. As stated above, the procedure presented has a solid physical background (the principal radius of curvature, Eqn. (1.1)) which is explained in the next subsection.

1.1.3.2 Selected Theoretical Aspects

Theoretical aspects of interfacial thermodynamics, such as the Gibbs dividing surface approximation (D-face) and the fundamental difference between surface free energy and surface tension are well known [21–23]. Our first publication [3] contains a short summary to help avoid misunderstandings and visualize future research directions. The two fundamental equations describing the two- and three-phase situations will be summarised in the following: The Laplace equation, Eqn. (1.1), and the generalised Laplace equation, Eqn. (1.2),

(1.1)

(1.2)

describe the relationship between the difference in the pressures Δp (i.e., the change in force ΔF per area) between the two phases with values of interfacial tensions γDα,β, principal radii of curvature Ri, curvature constants ci, excess adsorption ΓD, gravity acceleration g and angle relative to the gravitational field Φ. The Laplace equation, Eqn. (1.1), forms the fundamental basis for the ADSA theory [24–29], which is especially suitable for pendent drops. Following the Gibbs theory, the three-phase solid-liquid-vapour contact for a sessile drop corresponds to the intersection line of the undisturbed two-phase interfaces (D-faces) described by Eqn. (1.1) and Eqn. (1.2) of the solid and the liquid. This triple line represents the 2D-projection of the 3D-object, which is also known as the triple point†2.

Regarding this triple line, Thomas Young published in 1805 [30] a comprehensive work leading to one of the most well-known equations correlating the mechanical surface tension with the equilibrium contact angle θe in a thermodynamic and mechanical equilibrium, Eqn. (1.3),

(1.3)

(1.4)

Especially for solids with large mechanical moduli (stiffness of the solid material is high), additional parameters of the generalised Young equation [31], Eqn. (1.4), such as the line tension κs,l,v, are negligible. φs is the angle between the substrate surface and the local principal plane of the three-phase contact line [31]. To exclude the line-tension effects, large drops [25] with 0.05 mL volumes and approximately 8 mm radii ≈ R0 are used in most of our investigations. Unfortunately, the thermodynamic equilibrium contact angle θe (Young angle) cannot directly be measured. The contact angle hysteresis Δθ, which is equivalent to the difference between the advancing θα and the receding contact angle θr, introduced in the next subsection, is monitored for every solid. Well-written introduction to the problem may be found in the book Wetting of Real Surfaces [21], and in Handbook of Adhesion Technology [32, 33]. In a number of investigations only the roughness is considered to be the cause of this effect. Different models for considering the effects of roughness are summarized in Table 1.1. However, the contact angle hysteresis is measurable also for nearly ideal and flat surfaces, to which most of our recently analysed surfaces can be assigned. (A correlation exists with the force (and force distribution) inducing the movements [1, 2].) Hence, detailed studies are necessary for future research.

Table 1.1 Summary of the most well-known models for contact angles on real surfaces.

1.1.3.3 Contact Angle Analysis and Hysteresis

Contact angle measurements by sessile drop techniques are commonly performed to characterise solid surfaces in terms of wetting behaviour [41, 42], adhesion [43, 44], etc.. Theoretical and practical aspects of contact angle determination are described [3, 32, 33, 38, 45, 46], and the manufacturing techniques for surfaces are advanced to the point that fabricating specimens with well-defined, symmetrical and reproducible surface patterns [47–52] at both the microscopic and sub-microscopic levels is possible. However, there are almost no advantages in terms of data evaluation and local resolution to developing the measurement technique for surfaces. In fact, a huge number of publications ignore the existence of different contact angles, whereas some authors call attention to the experimental problems [1–3, 53]. The advancing θa and receding θr contact angles are essentially static characteristics of the wetting situation in solid/liquid pairs[54]. Some concepts like the most stable contact angle [55, 56] are convincing [57]. But certain assumptions and simplifications (huge droplets, vibrations) are not meaningful if specific surface effects (such as analysis of roughness/ heterogeneity distributions) are of interest. If the contact angle measurement is used as a probe for the surface, which is proposed in nearly all cases, the mentioned concepts are also unsuitable. Different procedures and definitions are used to obtain an advancing angle θa and a receding angle θr. These problems of surface science can be solved (or be much more comprehensible) by using the developed statistical or similar statistical procedures. The values of these specific angles on real surfaces are distinct from one another and result in the so-called contact angle hysteresis Δθ [47, 58]. In literature different reasons / influencing factors like activation energy, compensation of pinning, and dependence on initial conditions can be found. The problem of contact angle hysteresis is described in detail in multiple publications [21, 37, 59] and in the previous subsection. Commonly used procedures to identify only one advancing and only one receding angle by observation with the human eye are hardly comprehensible and are very subjective. Nowadays, even the conditions of experimental studies (involving measurements and data processing) are not transparent and, unfortunately, are often not described in literature, such as the wetting parameters (static or moving triple line, drop size, temperature, etc.) [21]. In addition, factors such as defining the angle before, during, or after the motion from the same or different images of the drop are rarely described. Only carefully performed static measurements under controlled experimental conditions[53], which, in the authors’ opinion, is similar to the slow-moving experimental procedure described in this chapter (quasi-static wetting situation), lead to reasonable data. Otherwise, only an angle that is between the advancing θa and the receding angle θr is obtained, and the claim to perform reproducible surface characterisation via this approach is not justified. Automatic or semiautomatic processing as presented in this chapter using high-precision drop shape analysis, HPDSA and the statistical techniques are much more suitable for the present storable optical data. These procedures can, for example, be fast (even implemented in c/c++ code [60]), consider every image, and are independent of the operator (attention span, know-how, etc.). Of course, further confirmations by additional investigations of different surfaces and liquids using different optical techniques are necessary, Scheme 1.1, to prove the general usefulness expected of these procedures. The unsolved aspects of the definition and measurement of specific angles (advancing/receding) present a major problem for every surface scientist not only in cases of inclined surfaces but also in cases of horizontal surfaces. According to some scientists, even the notion of the receding angle is very problematic for surface science [61]. When significant surface roughness and/or chemical modifications/heterogeneities are present, basic procedures, such as the definition of one (like immediately after application = as placed CA) or two angles (advancing and receding) by optical observation, are influenced to the point that no valid results can be obtained. Additionally, for flat, non-reactive, and homogeneous surfaces, minimal/slow drop movements are hardly observable by the human eye by watching the video, see Section 1.4.4. For example, the drops/triple lines on very flat homogeneous surfaces nearly immediately start to move with minimal velocity, but "the observed angle θ depends on the way the system was prepared [22]. Hence, the initial angle, even for some advancing angles, leads to unsatisfactory results. All the angles in situations with an inclined surface or volume variation are, by definition, dynamic ones, which are more or less affected by the velocity. From our point of view, the determination of specific angles, including static, dynamic, advancing and receding ones, from contact angle measurements depends on multiple experimental parameters; however, this determination must also be statistically analysed by an automatic data processing routine using meaningful analytical procedures that lead to values with minimal variance. In the work of Schmitt and Heib [3], we presented a technique based on fitting a Gompertzian function [62] to the contact angle data relative to the angle of inclination. This technique is useful for describing the data trends for an inclined plane experiment with a minimum number of parameters and results in characterisation of the average properties of the surface. In this chapter, three techniques that enable the determination of reproducible angles for both inclined plane and horizontal setups will be briefly described. The advancing and receding behaviours are analysed in a statistical manner based on the Gompertzian function and on the counting of statistical events, followed by dependent and independent statistical analyses. These analyses are called overall, global, and individual analyses. Detailed statistical analyses of the data are even possible for very flat and chemically homogeneous surfaces when a minimal" movement of the drop starts upon inclining the surface [6–8, 10] or varying the volume [9]. Using high-precision drop shape analysis (HPDSA) [3], non-axisymmetric droplets and contact angles of superhydrophobic surfaces can also be evaluated. For most of our published studies, the measured 0.05 mL water drops are non-axisymmetric ones obtained by inclining the sample surface or by varying the volume. The independent statistical (≡ global) analysis was first described in the publication on the analysis of a hydrophobic functionalized silicon wafer surface [6]. Because of the different motion behaviours, the authors defined the advancing angle as the downhill angle θd (at the front edge of the drop) and the receding angle as the uphill angle θu (at the back edge of the drop), as performed by several researchers. This procedure takes into account the difference in the force distributions affecting the triple line (by variation of parameters including the effective mass or centre of gravity if inclining a surface). This difference is important; examples are provided in a theoretical study published by Krasovitski and Marmur [63] and in experimental studies published by ourselves [1, 2]. The contact angles and the velocities of the triple points are obtained by HPDSA[3]. The statistical methods developed can also be used for data from commercial software (e.g. from the contact angle equipment), albeit generally with reduced sensitivity and precision. So, the approach is tested and optimised for an older OCA equipped with a slow frame rate of 25 Hz and a small number of pixels (768 × 574). Due to the slow frame rate and the vibrations of the motor, measurements of superhydrophobic surfaces are challenging for this equipment. This hardware limitation is also the reason that the main purpose of HPDSA which is the image transformation in principal radii of curvature for the whole drop, Scheme 1.1, is not pursued/published.

1.2 Experimental

Nearly all dynamic sessile drop experiments were performed with 0.05 mL of ultrapure water resulting in an initial diameter of the drops larger than 7.8 mm on defined locations of the sample surfaces, with controlled temperature, in a closed measuring chamber and after a sufficient delay time (> 2 h). These conditions ensured a constant and saturated vapour atmosphere (moist air). The volume and inclination angle variations are presented in the publications. Due to the optical magnification, one pixel corresponds to a length of about 0.02 mm. The video files are transferred to loss-free image files. The implemented fitting routines of the used OCA equipment, which are ellipse and tangent fittings, cannot independently analyse the uphill and downhill angles and do not lead to valid contact angles, especially for strongly inclined measurements [7]; therefore, the self-developed HPDSA-routine [3] was used. The camera angle relative to the horizontal might influence the obtained CA which was in all cases smaller than 3°. The experiments on horizontal surfaces have to be carefully performed as the influence of the immersed needle is especially huge for rough surfaces e.g. in the Wenzel-Derjaguin state or on sticky superhydrophilic surfaces. The Cassie-Baxter state often leads to difficulties in obtaining good raw data due to vibrations of the equipment or adhesion to the dosing needle (drop adheres to the