ABSTRACTS

The following is a list of the abstracts for papers which will be presented in FOURTH INTERNATIONAL SYMPOSIUM ON CONTACT ANGLE, WETTABILITY AND ADHESION. The listing is alphabetical by presenting author. This list is updated continually to add abstracts as they become available and make appropriate corrections. This list may be conveniently searched by using the editor provided with most popular browsers (e.g. Microsoft Explorer, Netscape, ... etc.)

Influence of the Surface Properties of Metal Leadframes on the Metal-Mold Compound Adhesion in Semiconductor Packaging

In order to prevent the integrated circuit chips from physical and environmental damage, semiconductor-packaging industry demands an excellent adhesion of epoxy-based mold compounds to metal leadframes that constitute these circuits. Optimization of surface properties of both the metal leadframes and the mold compound is deemed essential for maximum adhesion.

Metal-epoxy adhesion is affected by the surface energetics of metal leadframes. The work reports surface energies of Ni and Cu leadframes determined via contact angle method. Sequential thermal treatment and Ar-plasma cleaning, increases the surface energies of Ni metal leadframes but the adhesion to thermally treated frames is far better than the Ar-etched frames. ESCA studies of the Ni surfaces show that some Ni oxidation is essential for mold compound adhesion. On the other hand, thermally treated copper frames provide better adhesion up to a certain level. Although, further exposure to higher temperatures renders higher surface energies but adhesion is dropped to a significant extent. It seems likely that a "thin" layer of oxidized metal is beneficial to adhesion but a "thicker" layer leads to brittle and cohesive failure at the interface. It is suggested that the surface energy data alone may not be able to predict the adhesion performance and the surface chemistry data are needed for full understanding.

University of Alberta, 4-9 Mechanical Engineering Building, Edmonton, Alberta, T6G 2G8, CANADA

Contact Angle Measurement for Dispersed Microspheres Using Scanning Confocal Microscopy

Scanning confocal microscopy was used for contact angle measurement of individual microspheres. The measurements were carried out by using different laser-scanned layers of the particle floating on the air-water interface. The ratio of the diameter for the cross section of the protruded area of the particle at the air-water interface to the actual diameter of the particle is used for contact angle measurements. Two systems, i.e. glass and polystyrene microspheres with diameters of 3-10 and 6 micrometers, respectively, with water were used for this investigation (this size range of particles are most relevant to inhalation applications). Using the developed methodology, contact angles of 27º and 41º were measured (with water) for glass and polystyrene particles, respectively. The error in contact angle measurement for the developed methodology is determined to be generally about 1° with a maximum of 3° for contact angle of particles ranging from 2 to 24 micrometers in size. The contact angles of glass and polystyrene particles were compared to those obtained from pendent drop method and confirmed.

The Role of Surfactants in Hydrophobicity of Super Water-Repellent Surfaces

The effect of surfactants on wetting behavior of super-hydrophobic surfaces was investigated. Super-hydrophobic surfaces were prepared of alkylketene dimer (AKD). Aqueous solutions of sodium acetate (SA), sodium dodecyl sulphate (SDS), hexadecyltrimethylammonium bromide (HTAB) and n-decanoyl-n-methylglucamine (MEGA 10) were used to investigate the wetting of AKD surfaces. Advancing and receding contact angles for various concentrations of different surfactant solutions were measured. The contact angle results were compared to those of a number of pure liquids with surface tensions similar to surfactant solutions. We found that although the surface tensions of pure liquids and surfactant solutions at high concentrations are similar, the contact angles are very different. This difference results from different wetting mechanisms dominant for pure liquids and surfactant solutions on the AKD surface. Although surfactant solutions have surface tensions similar to those of pure liquids (with contact angles of below 90°), their contact angles are generally above 90° on AKD surfaces. Therefore, the usual behavior of super-hydrophobic surfaces against pure liquids is not observed for the surfactant solutions.

Deutsches Textilforschungszentrum Nord-West e.V., Adlerstr. 1, D-47798 Krefeld, GERMANY

Highly Hydrophobic Textile Surfaces Following Thin-layer Deposition

In the important field of technical textiles, highly hydrophobic and oleophobic finishes are increasingly demanded to create barrier coatings or to define self-cleaning properties. On the background of well-defined and durable finishes, physical processes and especially thin-layer deposition form an important research topic in the last years. The concept and some results from related studies on the application of photochemical and plasma processes shall be reported here.

A new approach to photochemical surface modification is given by the use of monochromatic UV excimer lamps. Fundamental studies have shown that it is possible to initialize grafting or even cross-linking of functional thin layers on the substrate surface by a treatment in reactive atmospheres. The general condition to achieve reactions like that is a marked difference of the absorbances of a low or non absorbing atmosphere and a strong absorbing substrate, which leads to radical processes initialized at the boundary between atmosphere and the activated substrate. Using perfluoro-4-methylpent-2-ene as an atmosphere the contact angle on PET-surfaces could be increased to 116°. Similarly, a 600 % increase of the drop penetration-time of water was found after the irradiation of a fabric made of p-aramide.

Well-known and widely applied in several industrial branches, surface modifications by gas discharge processes are based on the activation of the substrate mainly by electrons followed by chemical interactions between surface radicals and plasma 'particles'. Present research with regard to the treatment of textiles more and more favors atmospheric pressure plasma (DBD), which offers rather simple machinery and a continuous treatment. In order to establish functional thin-layers for highly hydrophobic surfaces, fluorocarbons can be used as process gases in 'closed', but nevertheless near atmospheric pressure reactors. The present state-of-the-art is a deposition of fluorinated thin-layers at approx. 0.5 nm/s with a total fluorine concentration on the surface up to more than 50 %. An oil repellence according to AATCC up to 7 to 8 was achieved.

1) Laboratoire Central des Ponts et Chaussées (LCPC), 58 bld Lefebvre, 75732 Paris Cedex 15 (FRANCE)

2) ITODYS, University Paris 7 and CNRS, 1 rue Guy de la Brosse, 75005 Paris (FRANCE)

Surface Energies of Hardened Cement Pastes and Their Mineral Hydrate Components

Concrete is the most common material in the fields of construction and civil engineering. Damaged concrete structures are often repaired by gluing stiff reinforcements on the damaged zones (such as steel or composite plates), using epoxy adhesives. A good wettability of the mineral surface by the epoxy resin is usually required in order to promote an accurate level of adhesion. Therefore, the surface energy of the cementitious substrate can be considered as an important parameter for the durability of the repaired structure.In a first part of this work, the surface energies of several hardened cement pastes, varying by their mineral compositions or by the water to cement ratio, were assessed using two experimental techniques:

- contact angle measurements of model wetting liquids, and calculations using Owens and Wendt method.

- inverse gas chromatography (IGC) analyses using a wide range of non-polar and Lewis acidic or basic probes.

Components of the surface energies were then determined using a modified van Oss-Good-Chaudhury (VOGC) method.A good agreement was found between the two methods, on the dispersive components of the surface energies Surprisingly, we observed no significant dependence of with the mineral composition of the cement paste or with the water to cement ratio. IGC characterization provided additional information and revealed the pronounced amphoteric character of the cement paste surface.

In a second part of this study, three model mineral hydrates representative of the actual components of cement paste, were investigated separately. These constituents are respectively calcium hydroxide Ca(OH)2, ettringite (3CaO.Al2O3.3CaSO₄.32H2O), and calcium silicate hydrate. These compounds appeared as fairly high surface energy materials, with dispersive contributions to the total surface energies ranging from 45.6 to 96.3 mJ.m-2. These surface properties were suspected to be dependent on the water content at the surface of the substrates, since they were significantly increased with temperature.

Self-assembled Mixed Adsorption Layers of Cationic And Nonionic Surfactants on Glass: Contact Angle and Autoradiography Data

Wetting of glass by aqueous binary mixed solutions was investigated. Mixtures of cationic (cetyltrimethylammonium bromide, CTAB and cetylpyridinium chloride, CPC) and nonionic (Triton X-100, TX-100) surfactants were used. The mole fraction of cationic surfactant (() was 0.2; 0.5 and 0.8; total surfactant concentration was (10-8 - 10-2) mol/l. The complete wetting of glass by single TX-100 solutions was observed. In contrast, the wetting isotherms of mixed solutions appeared to be similar to ones for cationic surfactants and had maximums for all systems investigated. The synergistic effect of wetting was founded: the maximum of contact angle isotherm's displacement to lower surfactant concentrations was observed. Effect was controlled by chemical structure of cationic component: the most dramatic maximum displacement (on three orders of concentration) for CPC/TX-100 was observed. Different wetting action of CTAB/TX-100 and CPC/TX-100 can be associated with various interaction forces between CTA+ and CP+ cations with TX-100 molecules in the mixed adsorption layer at the glass-solution interface as well as sterical factor upon the coordination of TX-100 ethylene oxide chain with cations having various configurations. This assumption was confirmed by the results of study the self- assembly of mixed layers by autoradiography method using tritium labeled mixture's components.

Quantitative "Structure - Activity" Relationships Based on IGC Data in Adhesion on Powders Surface

The main goal in theory of adhesion is to predict the adhesion work and their temperature and concentration variations using physico-chemical characteristics of the adhesive and substrate. Because the work is related to geometric mean of the surface free energies of the adhesive and substrate, the approaches based on combinations of their dispersive, acid and base components of surface free energies are commonly used to calculate it.

An approach based on theory of heterogeneous surface is used here to derive averages and variances of the dispersive component, surface donor and acceptor numbers for whole monolayer region of powder surface from measurements of inverse gas chromatography at finite concentration. Those characteristics are derived for fumed parent and mixed Si, Al and Ti oxides, silica gel, carbon black and carbonized silica gel.

Also, a model, connecting the adsorption free energy or enthalpy with molecular polarizability of the adhesives and their acid-base parameters in the Abraham's scale is proposed to predict the adhesion work.

Several relationships are derived to connect the quantum-chemical and electrotopological indexes for the adhesive molecules with the Abraham's parameters, the donor and acceptor numbers and with their dispersive, acid and base components of the surface free energies.

1) Ugine-Alz, Research Center, BP 15, 62330 Isbergues, FRANCE

2) Arcelor Flat Carbon, CED, 60761 Montataire, FRANCE

Favorable Effects of Acid-base Sites on Polymeric Coatings in the Kinetics of Oil Removal

Our aim is to assess the ease of cleaning for polymeric coatings currently used in the wall panels of the food industry. In the particular case of an apolar soiling (oil), the theoretical calculation predicts an easier cleaning by an aqueous phase on acid-base, and more particularly on monopolar basic surfaces.

We selected polyurethane (PU), polyethylene terephtalate (PET) and fluoro epoxy coatings tested in an as-received condition and after an accelerated ageing. Two surfaces with a similar composition (PU) and different topography were used. The surface free energy derived from contact angle measurements discriminates acid-base and monopolar basic surfaces (PU and PET) from an apolar surface (fluoro epoxy coating).

The germination of water droplets on polymeric coatings was observed by environmental scanning electron microscopy (ESEM). In similar experimental conditions, the shape and the number of water droplets depend on the surface chemistry of the polymeric coatings. On PU coatings, there are many small water droplets whereas on apolar coating, the droplets preferentially germinate on surface defects.

Thus, we adapted the two liquid phase system in order to study the displacement kinetics of the oil-coating interface by an aqueous phase. The alkane phase was here replaced by oil whereas the aqueous one contains non-ionic and anionic surfactants. The kinetics is ten times lower on the apolar coating than on PU and PET. We also evaluated the cleaning kinetics of polymeric coatings by an empiric test. It is based on the surface soiling by oil and cleaning in a laminar flow cell. We also observed a higher cleaning kinetics on PU and PET surfaces than on the fluoro epoxy coating. In our case of study, the physical chemistry of surface expressed by the monopolar basic character plays a relevant role in the cleaning whereas the topography has a minor effect.

Dept. of Engineering Materials and Industrial Technologies, University of Trento, Via Mesiano 77, 38050 Trento, ITALY

Martino Negri and Nadia Gaeti; CNR IVALSA, S. Michele all'Adige (Trento)

ITALY

The Application of Wilhelmy Experiment to Porous Materials: the Case Of Stone and Wood

From eighties the computer controlled Wilhelmy experiment is a common method to measure the contact angle; the typical samples have parallelepiped shapes and are relatively flat and homogeneous; the liquids are not viscous. Some literature results have been presented for more complex and asymmetric shapes or for more complex experimental situations. However, one of the commonest problems during such experiments using porous samples is the absoption of the liquid. In the case of porous stones and wood the results can be doubtful for the significant variation of the sample weight. In the present paper a new evaluation procedure is presented for the correction of the Wilhemy experiments made on porous samples; the method corresponds to the modelling of the Washburn-like mechanism of absorption and produces coeherent results. Some experiments are presented and analysed for calcareous stones and different kinds of woods and some comparisons are made between compact and porous samples. Comments and experimental comparisons are also made about the actual possibility of using the Washburn equation for the evaluation of the contact angles.

1) Biocolloid and Fluid Physics Group, Department of Applied Physics, Faculty of Sciences, University of Granada. E-18071 Granada, Spain.

2) Research Group of Emulsions, Probisa-EUROVIA, Pinto (Madrid), Spain

Effect of the Heterogeneity Pattern on the Shape of Sessile Drops

The assumption of ideal solid surface is usually used in the study of the wetting phenomena. Unfortunately, this model is insufficient when the drops lack axial symmetry. This effect is due to the non-ideal characteristics of real surfaces, such as the roughness or the chemical heterogeneity.

The aim of this work is to determinate the influence of the surface patterns on the shape and the contact angle of sessile drops.

A numerical approach based on Surface Evolver has been applied to generate sessile drops on solid surfaces with different heterogeneity patterns (like stripes, chessboard...). The equilibrium shape of drops on these surfaces was attained by means of the minimization of their free energy. Local contact angles along the drop contact line are compared to the homogeneous case according to the chosen parameters (energy amplitude and characteristic lengths).

The mean contact angle on heterogeneous surfaces underestimates the effective contact angle proposed by Cassie. This observation is remarkable when the characteristic length of heterogeneity is greater than the drop size.

Our method is a powerful tool to predict the wetting behaviour of sessile drops on real solid surfaces.

How Laplacian Are the Interfaces with Nanofluids?

Nanofluids are described as conventional liquids with a certain load of solid mesoscopic particles. The equation of Young-Laplace, ÄP = ãLF( 1/R1 + 1/R2 ), establishes the shape of a liquid-fluid interface for pure liquids. In this equation, ÄP refers to the difference in the pressure between the liquid and fluid phases, ãLF to the liquid-fluid interfacial tension and R1 y R2 to the principal radii of curvature of the surface defined by the interface. However, is this equation equally valid to the nanofluid-fluid interfaces?.

In this work, we demonstrate experimentally that these shapes are not completely laplacian. We set a contour detection device to acquire the profiles of axisymmetric sessile drops. This device uses the ADSA-P technique to obtain the liquid-fluid interfacial tension, ãLF, and the contact angle, è. Using a confocal microscope as a goniometer, we can get the contact angle of these drops and compare it with the one given by ADSA-P. We present the difference between these contact angles as a new estimator that shows how laplacian are the shapes of the nanofluid-fluid interfaces. We have obtained this estimator for different concentrations of monodisperse suspensions of nanoscopic latex particles in water over a glass surface, and we have observed how the shape of these surfaces moves away from a laplacian shape as the concentration of particles increases.

1) Biocolloid and Fluid Physics Group, Department of Applied Physics, Faculty of Sciences, University of Granada. E-18071 Granada, SPAIN

2) Research Group of Emulsions, Probisa-EUROVIA, Pinto (Madrid), SPAIN

Development of a High Performance Goniometer Based on Confocal Microscopy

Contact angle is a fundamental quantity to determine the adhesion work of a liquid on a solid surface. Real solid surfaces exhibit diverse levels of chemical heterogeneity and morphologic anisotropy. This evidence involves the existence of a meaningful range of values of contact angle instead of an only value as happens for ideal solid surfaces.

Confocal Scanning Light Microscopy is an excellent tool to provide topographies with high precision. In this sense, different roughness parameters were compared to choose the most suitable in wetting studies. However, we propose in this work to use this device as a goniometer for hydrophilic solids. This approach requires two scannings of the fixed solid surface at different times: before and immediately after the deposit of the sessile drop. The superposition of both relief allows as well to discern the drop contact line as to measure the apparent contact angle and the true contact angle in each point of the detected contact line. Confocal Microscope demands certain time to achieve a full scanning of drop section. Consequently, the drop volume must be relatively small. However, the evaporation of a sessile drop is faster as the volume decreases. Hence we optimised the relation between scanning time and drop volume.

The enhancement of Confocal Microscope as goniometer was carried out with measurements of contact angle on glass slides and titanium pieces using small water drops.

1) Grupo de Medios Porosos, Facultad de Ingeniería, Universidad de Buenos Aires, ARGENTINA.

2) Lab FAST, Universite Paris Sud, Orsay, FRANCE

Motion of the Triple Contact Line: a Hydrodynamical or a Molecular Issue?

An experimental study of the constant velocity displacement of various water- glycerol solutions by air in PVC capillary tubes is reported. This topic is of particular interest in connexion with dewetting processes for surfaces covered by a liquid film. Variations of the dynamical contact angle with velocity and their relation to the physicochemical properties of the systems studied are more specifically investigated.

These results as well as an extensive set of those reported by many other authors are analyzed in the framework of the two main current theoretical approaches of the moving contact line problem (the Cox [1] (hydrodynamical) and the Blake [2] (molecular) models).

These comparisons indicate that no single model can account for all experimental observations. Explanations for apparent discrepancies in previous observations [3] of the dewetting of films of water-glycerol solutions on PVC are suggested

[1] R. G. Cox, "The dynamic of the spreading of liquids on a solid surface. Part 1. Viscous flow", J. Fluid Mech., 168, 169-194, 1986.

[2] T. D. Blake, "Dynamic contact angle and wetting kinetics", Wettability, edited by J. C. Berg (Dekker, New York, 1993), 251-309.

[3] G. Callegari, A. Calvo, J. P. Hulin, Colloid and Surfaces A, 206, 167-177, 2002.

1) Unité de Chime des Interfaces, Université Catholique de Louvain, Croix du Sud 2/18, 1348 Louvain-la-Neuve, BELGIUM

2) Ugine & Alz, Centre de Recherche d'Isbergues, BP 15, 62330 Isbergues, FRANCE

Smart Surface Conditioning Systems Involving Amphiphilic Diblock Copolymers

In order to prevent fouling and to enhance cleanability of materials, surfaces with switchable hydrophobicity properties, « smart surfaces », offers promising perspectives. The aim of this research is to develop surface conditioning systems which respond to the polarity of the environment.

Two model substrata (glass, gold) were conditioned by poly (styrene)-b-poly (acrylic acid) (PS₁₆₅₀₀-PAA₄₅₀₀ , PS₅₁₀₀-PAA₃₉₀₀ , PS₄₃₀₀-PAA₁₉₅₀₀) using different procedures (dip-coating, adsorption, spin-coating). The conditioning layers were characterized by XPS, AFM and dynamic wetting.

Both substrata dip-coated with PS₄₃₀₀-PAA₁₉₅₀₀ showed a very high hydrophobicity (_water > 120°). The layer formed on gold was resistant to extensive washing with water. This is attributed to stronger London - van der Waals interactions on gold as further illustrated by the amount of organic contamination found on a gold surface exposed to laboratory atmosphere.

Both substrata spin-coated with PS₄₃₀₀-PAA₁₉₅₀₀ showed a switching of surface hydrophobicity upon a change of the environment (air, aqueous solution) ; this property was accompanied by an enhanced cleanability after fouling with a carbon black suspension.

Study of Surface Charge Properties of Mineral and Surface Modified Substrates by Wettability Measurements

In this work, we have developed simple thermodynamic analyses and experimental methods to determine the point of zero charge (pzc) of oxides, metal surfaces, glass, and of chemically modified glass surfaces. The experimental method consists of measuring the water contact angle in air, or in presence of another immiscible liquid, as a function of the pH of the water drop. Our study concerns mainly bare glass and glass substrates modified with functional silanes.

In particular, we have determined that the glass considered has a pzc around 3 and that a glass coated with aminopropyltriethoxysilane has a pzc around 7, due to the contribution of amine and free silanol groups. This value is well corroborated with data obtained from electrophoretic mobility and pH-potentiometric titration performed on aminated silica (aerosil). In addition, the theoretical function relating the cosine of the water contact angle to pH is in very good agreement with our experimental data. Therefore, the surface charge behavior of these substrates is perfectly characterized.

From the surface charge characterization of aminated glass, it was possible to correlate the ionic interactions between this substrate and deoxyribonucleic acid (DNA) molecules. The binding or retention of DNA molecules on aminated glass is improved when the substrate is positively charged and DNA is negatively charged. The charge behavior of DNA results mainly from the presence and contribution of phosphate groups.

It will be also shown that the determination of the pzc of other substrates (silver oxide, chromium-plated steel) from water contact angle measurements at various pH is possible from our thermodynamic analysis. The wetting measurements on a non-charged modified glass substrate will be also discussed.

DROPPED OUT

Ramon Cerro; University of Alabama in Huntsville, Chemical and Materials Engineering, 321 Sparkman Dr., Huntsville, AL 35899

The Effect of Molecular Forces on Slip, Dynamic Contact Angles and Flow Patterns at a Moving Contact Line

(Abstract not yet available)

1) ITODYS, University Paris 7 and CNRS, 1 rue Guy de la Brosse, 75005 Paris (France)

2) Laboratoire d'Electrochimie Moléculaire, University Paris 7 and CNRS, 2 place Jussieu, 75251 Paris cedex 05 (France)

The Surface Energy of Iron Surfaces Modified by the Elctrochemical Reduction of Aryl Diazonium Salts

The surface modification of metal or carbon surfaces by the electrochemical reduction of aryl diazonium salts is well documented [A. Adenier et al., J. Am. Chem. Soc. 2001]. This versatile method permits to modify conducting surfaces by a variety of functionally inert (alkyl or perfluorinated) or reactive (-OH) groups and this is for several purposes.

In this work, iron surfaces were modified by a range of functional groups X (X= COOH, CH₂CH₂OH, CH₂OH, C₁₂H₂₅, OH). The surface morphology of the modified iron surfaces was investigated by AFM which shows a uniform deposition of the aryl diazonium groups with an average roughness below 5 nm. The surface composition has been characterized by XPS. In some circumstances, high resolution XPS can bring evidence for the grafting of the aryl groups onto iron via a covalent C-metal bond [K. Boukerma et al., Langmuir 2003].

The surface energy of the modified planar iron surfaces was assessed by contact angle measurements of a range of classical wetting liquids such as water, CH₂I2, DMSO, ethylene glycol, and formamide. The surface energy components were determined by the method of van Oss-Good-Chaudhury (VOGC) on the one hand , and for comparison by the classical method of Owens and Wendt (OW), on the other hand. The two methods agree very well on the total surface energy (g). The C₁₂H₂₅-treated surface appears as the less energetic (g=31.8 mJ/m²) whilst COOH- and CH₂OH-terminated surfaces appears as the most energetic (g ~ 41 mJ/m²). The most acidic surface is CH₂OH-modified iron and not the carboxylated one, perhaps due to the very strong association of adjacent carboxylic groups confined to the surface of COOH-treated iron. Surprisingly the COOH-treated surface has the highest g_S^- value (16.5 mJ/m²), due to the Lewis amphoteric behaviour of this group. In the OW approach, the COOH-treated surface exhibits the highest degree of polarity (g_S^p = 9.0 mJ/m²).

It is concluded that the properties of the molecules attached to the surface are well expressed by contact angle measurements, and this is important for possible applications of the process.

1) ITODYS, University Paris 7 and CNRS, 1 rue Guy de la Brosse, 75005 Paris (FRANCE)

2) CBMM, Polish Academy of Sciences, Sienkiewicza 112, 90363 Lodz (POLAND)

3) present address : LISE, FUNDP, 61 rue de Bruxelles, B-5000 Namur (BELGIUM)

Hydrophobic Protein-conducting Polypyrrole Interactions. A Study by Contact Angle Measurements

Adsorption of human serum albumin (HSA) onto conducting polypyrrole powders; doped with chloride (PPyCl), dodecyl sulfonate (PPyDS) and tosylate (PPyTS); has been monitored in buffered saline solution at pH 7.4. The decreasing trend of adsorption was : PPyTS > PPyDS > PPyCl. Electrochemically synthesized PPyCl, PPyDS and PPyTS films were used as model surfaces for contact angle measurements in order to determine the different interactions between the protein and the polypyrrole substrates. Static, advancing and receding water contact angle (q_W) qualitatively suggested that the PPyTS is the most hydrophobic conducting polymer among the three under test. The van Oss-Good-Chaudhury (VOGC) method was further used to determine the dispersive, acidic and basic components of the surface free energy (g_S^d, g_S⁺ and g_S^-, respectively) of the conducting polypyrroles. These components show that polypyrrole generally behaves as a strong Lewis acid. The surface free energy components were subsequently used to assess the absolute hydrophobicity of the substrates (DG_1W1), that is the PPy-PPy interaction in water. More importantly, the VOGC theory permitted to determine DG_1W2, the free energy of protein-PPy interaction in water. The decreasing trend of the DG_1W2 absolute values was found to be : PPyTS > PPyDS > PPyCl, which correlates well with the absolute hydrophobicity of the substrates, thus emphasizing the important role of hydrophobic interactions at the protein-PPy interface. In the case of PPyTS and PPyDS, acid-base interactions were the dominant contribution to DG_1W2.

Protein-coated PPyTS (the most hydrophobic) and PPyCl (the hydrophilic) substrates were further characterized by water contact angle measurements as a function of the HSA coverage. q_W-vs-coverage plots showed three domains (from low to high coverage) : a sharp decrease from 80 to 61°, a minimum value around 50°, and then an increase up to a plateau of 63° corresponding to pure HSA [C. J. van Oss, 1996]. These results were interpreted in terms of protein conformation and orientations caused by the type of its interactions with polypyrrole.

1) Dept. of Electrical Engineering, Princeton University, Princeton, NJ

2) Dept. of Chemical Eng., Princeton University, Princeton, NJ

Thermocapillary Droplet Propulsion on Chemically Uniform and Chemically Striated Surfaces

Recent experiments have demonstrated the usefulness of thermocapillary actuation for droplet propulsion, mixing and handling in open format microfluidic applications [1-3]. A better understanding of what mechanisms control the speed and threshold values for droplet mobilization is required, as well as the development of mixed surface coatings for high quality, reproducible motion and effective liquid confinement. We present results of thermocapillary droplet migration generated by a constant shear stress on both chemically uniform and chemically striated surfaces. As previously reported for homogeneous surfaces [4], droplet mobilization only occurs above a threshold thermal gradient or threshold droplet radius. We characterize the motion instead in terms of a threshold depinning force per unit length, which successfully describes all the liquids tested. Above the depinning transition, the droplet speed, which is controlled by thermocapillary, capillary and viscous forces, increases monotonically with this reduced force parameterization. These results compare favorably to numerical predictions of a generalized Ford and Nadim model [5] subject to two fitting parameters, namely the slip coefficient and the magnitude of contact angle hysteresis. The generalized model contains additional terms representative of the effects of laterally confining moving droplets by chemical patterning, which leads to overall faster speeds for the same droplet volume.

[1] A. A. Darhuber, J. P. Valentino, S. M. Troian and S. Wagner, J. Microelectromech. Sys. 12, 873 (2003).

[2] A. A. Darhuber et al., Appl. Phys. Lett. 82, 657 (2003).

[3] A.A. Darhuber, J. Z. Chen, J. M. Davis and S. M. Troian in Transport and Mixing at the Microscale, J. M. Ottino and S. Wiggins, Eds., Phil. Trans. Royal Society: Series A (2004).

[4] J. B. Brzoska, F. Brochard-Wyart and F. Rondelez, Langmuir 9, 2220 (1993).

[5] M. L. Ford and A. Nadim, Phys. Fluids 6, 3183 (1994).

* Corresponding author

1) Dept. of Electrical Engineering, Princeton University, Princeton, NJ

2) Dept. of Chemical Eng., Princeton University, Princeton, NJ

Capacitive Sensing of Droplets for Microfluidic Devices Based on Thermocapillary Actuation

Electronically addressable thermocapillary flow has successfully been used to propel droplets of polar or apolar liquids along chemically patterned surfaces of glass and silicon [1,2]. Automation for microfluidic applications, however, requires integrated sensors for monitoring droplet position, volume, composition and mass loss. Because of their high sensitivity to either conducting or insulating liquids, capacitive sensors are particularly useful in this respect. For this purpose, we have designed [3] a two-inverter RC relaxation circuit connected in parallel to an interdigitated array of coplanar electrodes. These compact dual-use electrodes, which are insulated from the overlying liquid by a PECVD SiO₂ layer, also provide the Joule heating for thermocapillary actuation. The resolution and maximum response time of our sensing array is about 0.07 pF and 10 ms at 370 kHz. A tapered interdigitated electrode design helps maintain high spatial resolution and sensitivity even in the presence of liquid samples with non-uniform surfaces. This design is based on optimization of the effective electrode width and field penetration depth presented by a co-planar arrangement. Comparison of experimental results with model predictions validates the usefulness of these concepts. In addition, we present experimental results demonstrating use of the sensing array for detecting droplet volume, position, composition and mass loss due to evaporation.

[1] A. A. Darhuber, J. P. Valentino, S. M. Troian and S. Wagner, J. Microelectromech. Sys. 12, 873 (2003).

[2] A. A. Darhuber et al., Appl. Phys. Lett. 82, 657 (2003).

[3] J. Z. Chen, A. A. Darhuber, S. M. Troian and S. Wagner, submitted to Lab on a Chip.

Determination of the Apparent Surface Free Energy of Highly Hydrophobic Solids from Contact Angle Hysteresis

(Abstract not yet available)

Determination of Contact Angles on Microporous Particles Using the Thin-layer Wicking Technique

The properties of particle-stabilised emulsions, especially with regard to phase inversion, are very dependent on the contact angle that the particles experience at the oil-water interface. For the very small particles used for such emulsions it is impossible to measure this contact angle directly. Its value could be calculated if it were possible to determine the components of the solid surface free energy.

We have investigated the imbibition of five probe liquids into a porous bed of silicagel (commercial TLC plates) using the thin-layer wicking technique. For all liquids, the difference between wicking rate for bare plates and for those pre-contacted with the vapours is large but it is not due to an advancing angle effect on bare plates. Our analysis shows that it is due to the diversion of flowing liquid into blind pores which are already filled in the pre-contacted case. Thus a new model is proposed describing wicking in a porous medium with very small blind pores by introducing a parameter into the Washburn equation that corrects for this capillary condensation effect. The parameter needed is determined independently using gravimetric adsorption measurements. When this modified Washburn equation is used, the difference between advancing and receding contact angle is actually quite small. When the averages are used as the Young's contact angles, values for the surface energy components of silica are obtained that are completely consistent between the five liquids and compare well with literature values.

1) Departamento de Química, Universidade da Madeira, 9000-390 Funchal, PORTUGAL

2) Centro de Estudos da Macaronésia, Universidade da Madeira, 9000-390 Funchal, PORTUGAL

3) Departamento de Química, Universidade do Minho, 4700-320 Braga, PORTUGAL

4) Ecole Française de Papeterie et des Industries Graphiques, BP65, 38402 Saint Martin d'Hères, FRANCE

Surface Modification of Banana-Based Lignocellulose Fibres

Lignocellulose raw materials from Musa acuninata colla were isolated from two different parts of this annual plant and characterised as such and after chemical modification. In fact, the rachis and the pseudo-steam of banana plant were tested: Firstly, the rachis was submitted to different mechanical treatments leading to the obtention two different morphology, i.e., fibres and hole material powder. Then after, the pseudo-stem of banana plant was ground to powderous material. These three morphologies were characterised, before and after different chemical grafting, in terms of their morphology by optical and Scanning Electron Microscopy (SEM), as well as their structural changes by FTIR spectroscopy and wettability using contact angle (CA) measurements and Inverse Gas Chromatography (IGC).

It was shown that the surface energy of these virgin materials corresponds closely to those of other common lignocelluloses, except the fact that their polar contribution to the surface energy was found slightly lower that of bleached pure cellulose. The chemical grafting of these materials was carried calling upon the following coupling agents: phenyl isocyanate, maleic anhydride, alkyl ketene dimer and alkenyl succenic anhydride. The extent and efficiency of this modification was examined by SEM, which showed the apparition of smooth surfaces. FTIR spectra displayed the presence of peaks characteristics to the attached groups. These findings were confirmed by wettability tests.

Dynamics of Contact Angles on Soft Surfaces

(Abstract not yet available)

and Tenzin Wangdu; Dept. of Engineering Materials and Industrial Technologies, University of Trento, Via Mesiano 77, 38050 Trento, ITALY

About the Possibility of Experimentally Measuring an Equilibrium Contact Angle and its Theoretical and Practical Consequences

The measure of the contact angle has been for years considered as a "comedy of errors"; in recent years the introduction of more sophisticated techniques, of computer controlled devices and the general better understanding of surface structures has lead to a greater precision and accuracy of measurements. However it is common to neglect the difference between the advancing contact angles and the Young angle or to underestimate the role and meaning of receding values. In previous papers it has been developed an experimental procedure, called Vibration Induced Equilibrium Contact Angle (VIECA) applied to Wilhelmy experiment, which appeared able to provide a really stable and equilibrium-like value; that procedure was based on previous rare literature trials. VIECA results seemed as well connected with the advancing and receding values through simple but approximated relations. Moreover it appears independent on the roughness and heterogeneity of the analysed surfaces in the majority of cases. In the present paper VIECA method is extended to the sessile technique and some comparison are made with the common advancing or "static" evaluations of contact angle. Some theoretical modelling of the physical situation induced by the application of mechanical vibrations to the meniscus or to the drop is proposed. The main consequence of this resultsis that the contact angles of common surfaces in common liquids is overestimated; a more subtle consequence is the effect on the evaluation of the surface free energy through the most common semiempirical models.

Solid Surface Tension : Contradiction Between Theoretical, Calorimetric and Contact Angle Data

(Abstract not yet available)

Determination of Solid Surface Tension at the Nano-scale Using Atomic Force Microscopy: Recent Advances Made at Michigan Tech

Because engineered surfaces such as thin inorganic or organic films, self-assembled organic monolayers, and chemically-modified polymeric surfaces cannot be melted, dissolved, or fractured, their surface/interfacial tension cannot be determined through any conventional surface tension measurement techniques. Therefore, new surface tension characterization methods need to be developed. Atomic force microscopy (AFM) is capable of solid surface characterization at the microscopic and sub-microscopic scales. As demonstrated in several laboratories in recent years, it can also be used for the determination of surface tension of solids from pull-off force measurements.

In this presentation, measurements of pull-off forces carried out at Michigan Tech for several model AFM tips of engineered surface chemical functionality will be presented. The pull-off forces were obtained at controlled loads and contact times, and measured for substrates coated with gold films modified with self-assembled thiol monolayers of varying functionality as well as for polymeric colloidal probes. The results have been used to calculate the solid surface tension based on particle-surface contact mechanics models. The success of this approach relies on determination of the geometry of interacting surfaces.

Additionally, the surface tension values determined from the AFM pull-off forces will be compared to those calculated based on contact angle measurements and using the Lewis acid-base interactions theory. Discrepancies between results from two different approaches will be discussed in view of surface roughness effects that can influence the magnitude of measured pull-off forces.

1) Laboratoire de Tribologie et Dynamique des Systèmes, Ecole Centrale de Lyon, Ecully, FRANCE

2) Groupe Arcelor: Ledepp, Metz, FRANCE

Understanding Paint Flow Behavior for Automotive Applications by Dynamic Contact Angle Measurements

The aspect of a vehicle represents a major factor in the judgement of a motorist as well as its mechanical performances or its duration in time. The painting surface is thus a significant criterion for a car. The quality of this aspect is determined by the tension of painting (Orange peel effect) and the clearness of the image reflected by the surface (Distinctness Of Image). The surface of an autobody is constituted by a superposition of several painting layers (4) requires a design of high technology. However, it is possible to note that coating and treatment brought on surface can show certain defects. In comparison with the multi-layer complexity of the structure, it was shown that a work on the roughness of sheet allows an optimization of spreading out and flow of painting, therefore a better surface quality. In this work, we carried out an design framework linking a scale of surface topography and a dynamic wettability measurement to understand the painting flow mechanism on sheet painted or not. The multi scale analysis by continuous wavelet transform constitutes the principal mode of analysis of surfaces and allowed us to find correlation between the rough surface quality and the wettability parameters.

A Molecular Interphase Model for Wetting of Organic Solids

Recent experiments have shown the existence of a non-polar interphase at the boundary between water and 1-octanol [ Nature 424, 296 (2003)]. Also, it is well known that when acid-base liquids are placed in contact with functional polymers, surface reconstruction can occur to form an interphase unique to the specific liquid-polymer pair. Starting with the premise that the chemical potential of this interphase will affect the wetting thermodynamics, our analysis of interphase acid-base interactions, structure and density led to the hypothesis that (when normalized by values of a "dispersion force only" reference solid) the dispersion force contribution to the work of adhesion for a given organic surface is inversely proportional to that solid's work of adhesion with hexadecane. To test this conjecture, the dispersion and acid/base components of the work of adhesion were calculated from measured contact angles of hexadecane, diiodomethane, water, glycerol and formamide on two series or organic surfaces: commercial polymers and self assembled monolayers (SAMS). Inverse proportionality between the dispersion and acid-base components is observed and the slopes are identical for contact angles of the three acid-base probes on the polymer series. On the SAMS, however, there is a different slope for each probe, all of which are significantly different from those for the polymer series. Calculated from these plots, differences from the mean are thermodynamic parameters that characterize each acid-base interphase. When the acid-base components for water (amphoteric) are plotted against the average of the acid-base components of glycerol (acid) and formamide (base), excellent linear relationships are found. The differences in the slopes in both types of characteristic plots are explained in terms of the spatial distribution of interacting functional groups, new electronic structure and decreased density in the interphase. Our results imply that methods to determine solid "surface energies" from contact angle data are fundamentally incorrect.

Influence of the Hydrophobic and Hydrophilic Characteristics of Sliding and Slider Surfaces on Friction Coefficient : in Vivo Human Skin Friction Comparison

Introduction : The objective of this study was to investigate whether hydrophilic/hydrophobic balance (Hi/Ho) of the surface strongly modified the friction coefficient (m). The Hi/Ho balance is determined using the relationship between the critical surface tension gc (Zisman : which delimits the wetting capacity ) and the surface tension of water gH2O (water : reference element of Hi/Ho balance).

Method : Critical surface tension gc is determined (according to Zisman's principle) through the measurement of advancing contact angle q of a serie of ethanol/water dilution. Friction coefficient depends on several parameters: types of probe motions (rotational vs. linear, surface roughness and physicochemical parameters of surfaces in contact). In this study, the wettability parameters for six surfaces (human skin forearm, Teflon, silflo*, elastomer E**, steel, and glass) are measured and their influences are compared to friction coefficient m.

Results : This study shows that when the hydrophobia tendency of the surfaces is high coefficient is low. The use of three sliding materials (Teflon, steel, and glass) , the friction of different Hi/Ho balance confirm the importance of these physicochemical parameters in m. Teflon with high hydrophobia has a low m. The friction coefficient increases when hydrophobia of sliding and slider surfaces decreases.

Conclusion : Friction coefficient value depends on the nature of slider surface and its physicochemical properties. In vivo, the friction coefficient may quantify the influence of lubrificant/emolients/moisturizers. For example the friction coefficient of hydrated skin (through the action of moisturizing products) is higher than the friction coefficient of dry skin. The relation between the friction coefficient and the Hi/Ho balance can be reversed in the presence of water and sebum on forehead (e.g).

Wetting and Adhesion of Gelatin Films in Contact with Media Used to Fill Soft Gelatin Capsules

It is known that composition of the media used to fill soft gelatin capsules affects the ability to seal the capsule. Particularly, experience has shown that media containing high levels of surfactants prevent adequate sealing of capsules. In this investigation, factors affecting the adhesion of gelatin films are explored. The wetting properties of relevant materials on gelatin films have been investigated by measuring contact angles. Surfactant spreading and its effect on adhesion is also explored. The influence of media on the mechanical properties of the films is also explored.

Surface Free Energy of Solids: A Comparison of Models

Frequently the contact angles of several probe liquids on a given solid are used for calculation of solid surface free energies. Models by Fowkes, Kwok and Neumann, van Oss, Chaudhury and Good as well a by Chang and Chen have been used for such calculations. Each of the above models has been championed in the literature. It been noted by the present author and others that the use of different models may lead to different qualitative results. A disinterested comparison of the various models has not been made. In the present paper, a comparison of the calculations is undertaken in order to determine the limitations of each model. Particular attention to the assumptions required for contact angle data to be used for surface free energy calculations is given. The effect of the degree to which the experimental contact angle data meet the required assumptions on the calculated surface free energy are addressed.

Contact Angles and Hysteresis on Surfaces with a Single Heterogeneous Island

Wetting behavior was studied on surfaces with a single, circular heterogeneous island. Lyophobic is-lands were created on lyophilic Si wafers using polystyrene (PS). Alternately, lyophobic perfluoro-alkoxy (PFA) fluoropolymer film was etched to make lyophilic domains. Contact angles and hystere-sis were measured with water and hexadecane. Small sessile drops were deposited on the center of an island and liquid was sequentially added, eventually forcing the contact line to advance beyond the is-land perimeter onto the surrounding area. Even though the underlying contact area contained a mix-ture of lyophilic and lyophobic domains, the contact angles, both advancing and receding, were equal to the angles exhibited by the homogeneous periphery. Or in other words, if the heterogeneity was completely contained with the contact area and did not intersect the contact line, then no area averag-ing of the contact angles occurred. These findings suggest that interactions at the contact line, not the contact area, control wetting of heterogeneous surfaces.

A Criterion for Ultralyophobic Surfaces

Very rough surfaces can suspend small water drops, prevent wetting, and cause contact angles to ap-proach 180°. A criterion -- based on contact line density -- is proposed for predicting the conditions that produce ultralyophobic surfaces, where above a critical value, drops are suspended by asperities. Critical values of the contact line density can be calculated from contact angles, asperity shape, and information about the contact liquid, such as density and surface tension. The criterion was found to correctly predict suspension for several model surfaces prepared by lithography techniques. These findings agree with the observations of a number of investigators that have suggested that asperity height may be less important than asperity shape in determining wetting.

Contact Angles on Rough and Ultralyophobic Surfaces

In most cases, if placed on a rough surface, a drop will collapse over the asperities, completely engulf-ing them. Interaction of a collapsed liquid with asperities can lead to large values of hysteresis and tenacious drop retention. Less frequently, for very rough surfaces, drops can be suspended atop asper-ities, leaving air (or vapor) between them. This suspension produces very large apparent contact an-gles, characteristic of ultralyophobicity. Suspended drops usually roll off these super repellent sur-faces with the slightest perturbation. In this study, apparent contact angles from suspended and col-lapsed drops were modeled by accounting for rough edges and employing a linear average of contact angle along the perimeter of the drop, rather than an area average of cosine. This linear model suggests that for suspended drops, both advancing and receding angles should increase relative to the corre-sponding smooth surface. Alternatively, for drops that have collapsed over surface asperities, advanc-ing contact angles should increase, while receding angles should decrease. These findings are compared to experimental data from a number of model surfaces.

The Effect of Acid Species on Wettability Alteration of Calcite Surfaces as Studied by Contact Angle Measurement

Generally outcrop calcite surfaces are water-wet, but in the most observed cases, calcite oil reservoirs are oil-wet and recoveries from such reservoirs are low. The results from performed experiments show that long chain carboxylic fatty acids in the presence of water film are most likely the reason of such alteration in wetting behaviour. The applied approach using contact angle illustrates that this adsorption is strong at low pH 7, while becomes week with increasing the pH. In addition, the strength of adsorption on calcite is significantly depends on the salinity. Presence of Mg⁺² and SO₄^-2 decrease the strength of the adsorption of carboxylic group on the surface at low pH, however it increases with increasing the pH.

Monitoring the Surface Tension of Reactive Epoxy-Amine Systems to Control Wetting in Various Applications

Epoxy resin formulations are widely used in various applications such as adhesives, coatings, as matrix materials in fibre-reinforced composites and in the microelectronic industry. Since the surface tension of resins can be regarded as the "driving force" of wetting and levelling it is of great practical importance to control and to adjust this parameter.

By employing a new strategy ADSA-P (axisymmetric drop shape analysis-profile) was used to monitor simultaneously the surface tension and the density of these reactive mixtures from sessile drops. The kinetics of the bulk reactions was quantified by Fourier Transform Infrared Spectroscopy (FTIR) and the changes in the molecular composition of the surface region were studied by X-ray photoelectron spectroscopy (XPS).

Department of Agricultural Engineering and Household Technology, P. O. Box 27, FIN-00014 University of Helsinki, FINLAND

Can Contact Angle Measurements Be Used for Predicting Soiling and Cleaning Tendency of Plastic Flooring Materials?

Plastic flooring materials are used widely in public, office and residential buildings. The growing interest in easy-to-clean surfaces has immensely increased during recent years and some applications have been made by manufactures. The possibility to predict the cleaning behaviour of materials during their lifecycle as early as in the design phase of a building would make it possible to choose the most appropriate material for each purpose. A first step in order to achieve this aim is to identify the surface properties of flooring materials that are the most prominent in preventing soiling or making them easy to clean.

Determining surface energy components from contact angle measurements requires measurement of the contact angle of droplets of several liquids with studied materials. Determining the wetting or the surface energy of flooring materials from contact angle data is not straightforward. For commercial flooring materials, errors may arise from surface roughness and from chemical heterogeneity.

The aim of this study was to test the feasibility of contact angle measurement in studying and evaluating the soil adhesion and cleanability of 11 different commercial flooring materials. The influence of surface properties such as wetting and surface free energy on soil adhesion and release were evaluated by contact angle measurements and by topographic descriptions using 3D-profilometer.

The surface properties were examined using both static and expanding contact angle technique and the cleanability was evaluated colorimetrically. The results indicate the potential of contact angle measurements to evaluate cleanability of flooring materials, although the relationship is rather complex and depends both on soil types and on cleaning solutions.

Surface properties of PET Fabrics after Ammonia Plasma-Simulating Treatments

Ammonia plasma treatment of fabrics is known to give the fabric surface interesting properties, including high positive charge in combination with a hydrophobic character. Most other common plasma treatments, using oxygen, air or nitrogen as process gas, normally give the fabric surface a negative charge and greatly improved wetting properties.

However, one important drawback of the ammonia plasma treatment is that it can not be used in large scale industrial plasma processes without special precautions made, due to environmental and safety reasons. It is therefore of great interest to find a plasma process that can be run at large scale, giving the fabric surface the same properties as does ammonia plasma treatment. The objective of this study has been to investigate whether ammonia plasma treatment of fabrics can be imitated with respect to surface charge, chemical surface composition and wettability using other gases or gas mixtures. Such gases are e.g. ammonia diluted with nitrogen and hydrogen mixed with nitrogen and these two approaches have been evaluated in this study.

Interaction of Liquid Sprays with Fibrous Substrates

Liquid sprays are used extensively in consumer products as well as in the production of textile and related goods. The distribution of these materials in the fibrous substrates is important for their efficient function. The distribution of these sprays depends on the nature of their interaction with fiber surface. In this presentation we present data and visual information on the interaction of hair sprays on human hair and sprays of model finishes on nonwoven fabrics.

We have discussed the stability of liquid films on fiber surfaces and discussed the conditions of film breakup and the formation of droplets, their movement along the fiber, and how it leads to uneven deposits. We have also examined the deposition of sprays on crossed fibers and the role of fiber surface energy in the final location of the liquid drop.

Finally, we have examined the role of Laplace pressure in the distribution of the liquid spray in a thermally bonded bimodal nonwoven fabric. The results indicate that it may be possible to deposit different substances on the fibrous and the thermally bonded regions.

Experimental Observation of the Electrical Properties of Electrowetting on Silicon Single-crystal Substrates

An aqueous drop and silicon single-crystal substrate interface shows electrowetting behavior while using an electrical potential applied between the drop and the substrate. We experimentally investigated the electrical properties of electrowetting on silicon single-crystal substrates. Experiments were conducted using three electrodes. The potential of the silicon is controlled versus a reference electrode immersed in a liquid drop on the surface. This method is usually used in electrochemical experiments. Electrowetting on silicon substrates occurs with a positive potential beyond a threshold of a few volts and does not occur at voltages below the threshold. (Electrowetting is also observed using a negative potential. However, only a positive potential is described in this presentation.) When a substrate is kept at a constant potential, electrowetting persists over a few tens of seconds.The period of electrowetting depends on the applied potential. After electrowetting stops, the current decreases to zero. Electrowetting behavior on silicon substrates depends on the potential sweep speed. Electrowetting is not observed with a lower sweep speed than 0.01V/s in our experiments. Intermittent electrowetting occurs with a 0.01-0.1V/s sweep speed, and continuous electrowetting is observed with a sweep speed above 0.1V/s. After electrowetting, the silicon surface changes to a hydrophilic surface. The surface change is believed to be due to ionic adsorption and reaction on the silicon-liquid interface occurring during electrowetting. In regard to electrowetting on silicon substrates our experimental results reveal the necessity of exploring not only the applied potential sweep but the transfer rate of ions contained in the drop, the charge rate of the electrical double layer, and the adsorption and reaction rate on the interface.

1) Saarland University, Geb. 22-6, POB 151150, D-66041 Saarbruecken, GERMANY

The Static Contact Angle Hysteresis, Young`s Angle of Equilibrium and the Calculation of Solid Surface Energies

Since many decades, wetting experiments are performed in order to derive the surface energy of solids in the following way. For the given solid, a series of test liquids provides, at least on technical surfaces, a set of advancing and receding contact angles q_a, q_r which are coupled to the static contact angle hysteresis Dq=q_a-q_r. Here, the key problem arises of how to derive the Young's contact angle q_e from the measured q_a, q_r. q_e is a thermodynamic excess quantity that describes the thermodynamic equilibrium at the triple line of contact between solid, liquid and the common vapour phase. This q_e is the input into all empirical and theoretical models for the calculation of the solid surface energy from wetting. Hence, the common practice of using q_a instead of q_e is questionable.

The paper shows how q_e is obtained experimentally for pure polymer solids by variation of roughness /1-4/ or with the help of dedicated Scanning Force Microscopy /5/. The crucial experimental points are discussed. With these q_e-data, the common models are utilised for the calculation of polymer surface energy. The results are compared to the surface energy data obtained with the q_a-data. Finally, these surface energy data are discussed briefly.

1 .R.-D. Schulze, W. Possart, H. Kamusewitz, C. Bischof: Young`s equilibrium contact angle on rough solid surfaces Part. I An empirical determination; Journal of Adhesion Science and Technology (1989) 3(1) pp. 39-48

2. W. Possart and H. Kamusewitz: The Thermodynamic and Wetting of Real Surfaces and their Relationship to Adhesion International Journal of Adhesion and Adhesives (1993) 13(2) pp. 77-83

3. H. Kamusewitz, W. Possart and D. Paul: Measurements of thermodynamic quantities In: Polymer Surfaces and Interfaces, by K.L. Mittal and K.-W. Lee (eds.) VSP (1997) pp. 125-143

4. H. Kamusewitz, W. Possart and D. Paul: The relation between Young's equilibrium contact angle and the hysteresis on rough paraffin wax surfaces. In: Colloids and Surfaces A: Physicochemical and Engineering Aspects. Vol. 156 (1999) 271-279

5. H. Kamusewitz, W. Possart: Wetting and scanning force microscopy on rough polymer surface: Wenzel`s roughness factor and the thermodynamic contact angle. Appl. Phys. A 76 (2003) 899-902

1) Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, THE NETHERLANDS

2) Department of Applied Physics, Faculty of Science, University of Granada. 18071 Granada, SPAIN

3) Department of Physics, Faculty of Sciences, Abdelmalek Essaadi University, 93000 Tetouan, MOROCCO

Surface Properties of Floured Montmorillonite Particles. Interactions Through Aqueous Medium

Rassoul is a clay extracted from Atlas Mountains in North Africa, characterized by its high capacity of adsorption and ionic exchange. To characterize its chemical nature, the technique of atomic absorption was used for the determination of the bulk composition. We also measured the chemical surface composition of the rassoul particles by X-Ray Photoelectron Spectroscopy (XPS). The most interesting result corresponds to the abundant amount of Fluorine existing on the surface. The crystalline structure of the clay was studied by X-Ray diffraction. Chemical and structural results have demonstrated that rassoul is a mineral clay that belongs to smectite group. In particular, the structure of rassoul is the same as sodium montmorillonite clay, but a partial substitution of OH- by F- ionsexists in the octahedral sheet of the clay. The specific surface of rassoul particles was also measured. It was determined by adsorption of N2 using the B.E.T. multipoint method. The very high specific surface area of the particles explains the great ionic exchange capacity of the rassoul particles. Finally, the size and shape of the particles were analyzed by electron microscopy technique (SEM) and demonstrated the laminar conformation of the particles.

In the second part, we have proceeded to the measurement of the contact angles of water, diiodomethane and formamide on the surface of cleaned and homoionized rassoul and natural rassoul in form of pellets and of covered glass. These experiments allow to estimate the rassoul surface free energy and its components for the two kinds of this clay, as well as to study the effect of cleaning and homogenizing on the surface free energy of rassoul. The obtained values were compared with those found by Wicking technique. Using both results, we calculated the free energy of interaction of rassoul particles through aqueous medium. In all cases, this free energy was positive indicating repulsive interactions between the studied colloidal particles.

1) Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, THE NETHERLANDS

2) Department of Applied Physics, Faculty of Science, University of Granada. 18071 Granada, SPAIN

3) Department of Physics, Faculty of Sciences, Abdelmalek Essaadi University, 93000 Tetouan, MOROCCO

Influence of Heavy Metals Adsorption on The Surface Energetic

Properties of Fluorinated Montmorillonite Clays "Rassoul"

The presence of heavy metals in natural waters can affect seriously the aquatic environment because of its strong toxicity. The transport of polluants, like heavy metals, in natural waters are facilitated by the adsorption on colloidal clay particles. In fact, the adsorption of such substances influences the surface energetic properties of the clay. The aim of this work was to study the above mentioned adsorption using the Contact Angle technique for the estimation of the free ener! gy of the fluorinated montmorillonite clay "rassoul". The results shows that the LW component is not really affected by the presence of heavy metal but the electron donor parameter is the most affected. This variation in the surface free energy properties constitutes a first insight on the adsorption of all this substances on the rassoul surface. This conclusion will be verified by determination of the surface free energy of interaction between particles in aqueous medium. The component LW is not very influenced by the presence of all these substances, but the component AB is very affected specially in the case of the mercury, the surface free energy of interaction between particles becomes negative. In the case of Cadmium, Pb and Sodium triphofate, the component acid base is decreased but it remain positive.

Electrowetting: A Versatile Tool for Microfluidics

Electrowetting is a versatile tool to reduce the apparent contact angle of partially wetting conductive liquids by several tens of degrees via an externally applied voltage. We studied various fundamental and applied aspects of equilibrium liquid surface morphologies both theoretically and experimentally.

Our theoretical analysis showed that surface profiles on homogeneous surfaces display a diverging curvature in the vicinity of the three phase contact line. The asymptotic contact angle at the contact line is equal to Young´s angle, independent of the applied voltage. With respect to the morphology of the liquid surface, contact angle variations achieved by electrowetting are equivalent to those achieved by varying the chemical nature of the substrates, except for electric field-induced distortions in a region close to the contact line.

Experimentally, we studied the (global) morphology of liquid microstructures substrates with stripe-shaped electrodes. As the local contact angle is reduced by increasing the applied voltage, liquid droplets elongate along the stripe axis as expected. For droplets on a single surface with stripe electrode, there is a first order wetting morphological phase transition where elongated droplets transform into translationally invariant cylinder segments with the contact line pinned along the stripe edge and vice versa. If the liquid is confined between two parallel surfaces with parallel stripe electrodes, the elongation is continuous. Experimental results are compared to analytical and numerical models.

Furthermore, we observed a self-excited oscillatory regime in which the drop periodically detaches from and reattaches to the wire that provides the applied voltage. By balancing surface tension and electrostatic forces, we constructed a phase diagram, which explains the occurrence of droplet oscillations above some threshold voltage. Self-excited droplet oscillations are shown to speed up mixing in microfluidic applications by approximately two orders of magnitude, compared to pure diffusion.

Spontaneous Absorption of Polymer and Biopolymer Solutions by Capillaries: Meniscus Behavior

Spontaneous absorption of wetting fluids by thin capillaries is characterized by high velocities of the meniscus propagation, which varied in our experiments from 10 to 100 cm/s. To visualize such fast flows we employ a CMOS camera combined with a specially designed delivery system. Experiments are conducted on aqueous solutions of polyethylene oxide, polyacrylamide, l-DNA, and other biopolymers. It is shown that the meniscus remains nearly flat during its travel over almost ten capillary radii. This indicates the absence of appreciable velocity gradients across the capillary. Our findings contradict to a conventional assumption that the meniscus shape equilibrates almost immediately. The dynamics of the contact angle relaxation and the meniscus equilibration are studied in detail. We employ special tracers to reveal the evolution of flow patterns at the meniscus. The observed phenomena are discussed focusing on the effects of viscoelasticity.

University of Alberta, Edmonton, Alberta T6G 2G8, CANADA

Calculations of Interfacial Tensions and Contact Angles from Intermolecular Potentials

(Abstract not yet available)

Contact Angles and Surface Energetics: History and Perspective

(Abstract not yet available)

The Usefulness of the Lifshitz-van Der Waals/acid-base Approach for Interfacial Tensions

(Abstract not yet available)

Effect of adhesion on electrokinetic Flow in Micro- and Nano-channels: A continuum and a generalized lattice Boltzmann Model to Microfuidics

(Abstract not yet available)

Adsorption-Mediated Self-Propelled Droplets

(Abstract not yet available)

Investigation of Chemical Hydrophobisation Treatment of Surfaces of Solid Particles in a Fluidized Bed

The hydrophobicity of solid particles plays an important role in a number of technological, environmental, and biological phenomena. For example, in the mineral flotation process, the hydrophobicity of minerals affects their floatability. In several cases, it is necessary to decrease the wettability of the particles in order to make them hydrophobic. Chemical treatments by molecular grafting consist in attaching a molecular monolayer of hydrophobic groups over particle's surface by means of a chemical covalent bond.

In this study, we try to substitute the hydrophobisation process, prepared in an organic solvent by a new process in a dry phase taking less time and consuming fewer amounts of products. It consists in treating the hydrophilic particles (silica microballs) while mixing them, in a fluidized bed fed by hot air with an adequate quantity of porous particles previously soaked in a grafting reagent. The quantity of reagent so trapped is progressively released by evaporation coming into contact with the available hydrophilic functions of the target solid particles. Different hydrophobic reagents (having various chain lengths, reactivities, etc.) and different kinetics parameters are studied to optimize the grafting yield. The hydrophobicity of treated particles is evaluated using the contact angle measurement from Washburn, Dynamic Vapour Sorption and Environmental Scanning Electron Microscopy tests.

Wetting on Heterogeneous Surfaces

We present a study of the effect of chemical heterogeneities on wetting. The system considered is a micron / mm size droplet impacting on a surface at a velocity of a zero to a few meters per second. The surfaces studied are gold coated silica wafers patterned by microcontact printing.

It is shown that the evolution of the static macroscopic shape of the droplet can be understood when the size of the chemical defects increases. We find that for chemical defects more that 10 times smaller that the diameter of the droplet the shape of the latter remains essentially spherical. The only observable phenomenon is a small corrugation of the contact line at high magnification. When the size of the defect is of the same order as the droplets, a whole spectrum of shapes appears. Quantitative modeling using Lattice Boltzmann solution of the Navier-Stokes equations allows interpretation of the fine structures of the contact line observed experimentally. We show that the various faceted configurations can be achieved by controlling the inter-defect distance, the impact velocity and the wettability contrast.

In addition, we show that the dynamics of spreading on the same kind of substrates as above. This time the size of the defect is chosen not to affect the shape of the droplet, in order to be able to accurately measure the evolution of the radius as a function of time. The wetting kinetics on chemically hetero- and homogeneous surfaces will be compared where the contact angles are the same.

Two New Models of Intermolecular Hydrogen Bonds

(Abstract not yet available)

Determination of Wetting in Porous Medium

(Abstract not yet available)

Vibrational Spectra of 2.6-lutidine in Solutions, Liquid Phase and Orientationally - Arranged Wall - Adjacent Layers

(Abstract not yet available)

Wetting Phenomena at Structured Surfaces

Substrate surfaces are considered which are chemically and/or topographically structured. The size of the corresponding surface domains or grooves can be varied over a wide range from the millimeter down to the nanometer scale. Depending on the pattern and wettability of these domains and/or grooves, the surfaces exhibit a variety of different wetting morphologies. In addition, these systems undergo morphological transitions or shape bifurcations at which the wetting morphology changes in a characteristic and typically abrupt manner. Two examples, which will be discussed, are provided by (i) striped surface domains and (ii) parallel surface grooves. The talk will also address our current understanding of line tension effects.

Surface Free Energy and Adhesion of Proteins, Cells and Bacteria onto Irradiated Polymer Surfaces

Ion irradiation in the keV range is a powerful method to engineer the surface composition of polymers and their surface physico-chemical properties. In particular, it has been demonstrated that it is possible to enhance or reduce the interaction of various biological systems with ion irradiated polymer surfaces, achieving specific and selective adsorption/adhesion processes.

In this paper we will report recent experimental results on the irradiation-stimulated selective adsorption of aminoacids, proteins and cells on carbon- and silicon-based polymer surfaces. The results will be discussed in view of the modification of the structure-property relationships, with a specific reference to the Surface Free Energy (SFE), for various surfaces including polystyrene (PS), polyhydroxymethylsiloxane (PHMS), polyethyleneterephtalate (PET) and polycaprolactone (PCL), SiO₂ (quartz and pyrolytic oxides), silicon-carbon alloys (a-Si:C,H).

The adsorption/organization processes of selected oligopeptides and proteins for surfaces of different SFE have been studied by means of in situ technique, as the Quartz Crystal Microbalance with Dissipation monitoring and near Field Microscopy, as well as ex situ techniques as Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Small Spot X-Ray Photoelectron Spectroscopy (XPS).

The paper reports also evidence of the different cytocompatibility towards ion irradiated polymer surfaces of different SFE. In particular, we studied the cell attachment, adhesion and spreading of Normal Human Dermal Fibroblast cells onto poly(hydroxymethylsiloxane) and poly(ethyleneterephthalate) surfaces modified by modified by 50 keV Ar⁺ beams. Again, the phenomenological connection between cell response and the radiation-induced changes of the polymers surface chemical structure and related Surface Free Energy is discussed in view of the results of X-Ray Photoelectron Spectroscopy and Static Contact Angle measurements.

The biological response is interpreted in terms of the different modification trends of the Surface Free Energy components with ion irradiation parameters. The results points towards a critical role of the electron-donor character of the surfaces, which seems able to trigger specific response of the biological systems

Strategies for Superhydrophobic Surface Design

Many plant leaves, notably of the Lotus flower, use a self-cleaning mechanism based on super-hydrophobicity of the leave surface. Sharks use skin super-hydrophobicity to reduce drag. The principles and design strategies of super-hydrophobicity are presented and discussed in terms of surface features that lead to high contact angles and low roll-off angles.

1) Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systèmes, Bat H-10, 36 Avenue Guy de Collongue, BP 163, 69134 Ecully cedex, FRANCE

2) Ecole Nationale d'Ingénieurs de Saint-Etienne58, rue Jean Parot, 42100 Saint-Etienne, FRANCE

Analysis of Adhesive Contact of Human Skin in Vivo

Recent work of the literature based on the wettability method, could shown the hydrophobe and hydrophyle aspect of the surface of the human skin according to the quantity of lipids or sebum of the skin surface areas.

In this work, we compare the Dupre's energy of adhesion of the human skin, determined by the wettability method and a contact mechanics method which we developed.

The contact mechanics method is based on a micro indentation test with a weak load (20 - 30 mN). This approach allows to measure a continuous normal load versus a displacement of a spherical steel and glass indentors. The adhesive aspect of skin surface is assessed by the measurement of the negative efforts at the end of the unloading cycle. The Dupre's adhesion energy can be determined by the theories of adhesive elastic contact. Moreover we analyse the evolution of adhesion according to various experimental parameters such as the normal load, contact time, indentation speed.

The results of measurement obtained by our approach remain comparable with those obtained by the method of wettability, which enabled us to follow the kinetics of human skin surface absorption, by controlling the force of adhesion according to time.

The Wetting and Drying of Pore Networks

(Abstract not yet available)

Contact Angle Dynamics of Droplets Impacting on Flat Substrates

An experimental study is presented on contact angle dynamics of water droplets striking orthogonally smooth surfaces with widely disparate wetting characteristics (wetting to non-wetting). Fundamental information related to the relation between apparent (macroscopic) contact angle q and contact line speed V_CL is presented. The impact conditions correspond to Re = O(100) - O(1000), We = O(10), Ca = O(0.001) - O(0.01), Oh = O(0.001) and Bo = O(0.1). In this parameter regime, inertial, viscous, and capillary phenomena act simultaneously to influence contact line motion and arrest. A combined molecular-hydrodynamic theory of the dependence of contact angle on contact line speed was found to fit the experimental q vs. V_CL data, yielding physically reasonable molecular-kinetic parameters for wettable and partially wettable surfaces. Impact on non-wettable surfaces is followed by rebound, and reveals that both advancing and receding contact angles do not change with contact line speed. The combined molecular-hydrodynamic theory could not predict the experimental data for non-wettable surfaces. Contact angle hysteresis was detected on each surface, however it was found to be minimum on the non-wettable surface. It is concluded that droplet impact at moderately high Weber numbers on surfaces of varying wetting characteristics does not conform to a simple relation between apparent contact angle and contact-line velocity.

1) Department of Materials Science and Engineering and A. J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, PA 19104, USA

2) Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA

Wetting Behavior of Aqueous Fluids Contained in Multiwall Carbon Nanotubes

Wetting of carbon surfaces by water is highly variable and depends on the spatial arrangement of the carbon atoms. The present study examines the wetting behavior of multiphase aqueous fluids trapped in multiwall carbon nanotubes made hydrothermally [1]. These tubes have length of 1-10mm and diameter of 50-100nm. Transmission electron microscopy (TEM) examination reveals that the carbon walls have a conical scroll structure [2] (lattice fringes oriented at angles 8-16° with respect to the tube axis) and are highly wettable by the water phase. Electron energy loss spectroscopy (EELS) measurements suggest functionalization of inner and outer tube surfaces by carboxyl or hydroxyl groups, which are believed to attach to exposed graphene edges during the nanotube synthesis. The presence of the above groups on the tube walls can account for the hydrophilic character of the hydrothermal carbon nanotubes, as reported in [3, 4].

1. J. Libera and Y. Gogotsi, Carbon 39, 1307 (2001).

2. H. Ye et al., Nanotechnology 15, 1 (2004).

3. C. M. Megaridis et al., Phys. Fluids 14, L5 (2002).

4. Y. Gogotsi et al., Appl. Phys. Lett. 79, 1021 (2001).

1) Institute of Chemical and Environmental Research (IIQAB-CSIC), 08034 Barcelona, SPAIN

2) Applied Physics and Optics Department, Physics Faculty, University of Barcelona, 08034 Barcelona, SPAIN

Contact Angle on Modified Keratin Fibers

Keratin fibers are hydrophobic in nature due to the presence of an outermost monolayer of fatty acids covalently bonded to the protein matrix of the epicuticle membrane of the fiber [1]. This characteristic exerts a considerable influence on the shrinkage of wool fabrics submitted to aqueous washing process, on the adhesion to polymers and on the dye diffusion into the fiber bulk. It is known that the hydrogen peroxide at alkaline pH or low temperature plasma (LTP) treatments provide hydrophilic properties to the fiber surface and consequently the shrinkage is reduced and the polymer adhesion and dye diffusion are improved [2,3].

This communication is focusing on surface characterization of the chemical modification undergone by the surface of the keratin fibers treated with hydrogen peroxide or with an oxidative LTP. For this purpose, wetting force measurements on single fibers according to the Wilhelmy procedure and XPS analysis were carried out. The formation of oxygen-based functional groups from carbon and sulfur atoms on the fiber surface and the partial or total removal of fatty acids promote a decrease of the advancing contact angle of treated fibers. The XPS analysis provides a correlation between contact angle value and the oxygen atomic concentration on surface.

1. A.P. Negri, H.J. Cornell and D.E. Rivett, Textile Res. J. 63, 109-115 (1993).

2. R. Molina, P. Jovani, F. Comelles, E. Bertran and P. Erra, J. Adhesion Sci. Technol. 16, 1469-1485 (2002).

• R. Molina, P. Jovani, D. Joci, E. Bertran and P. Erra, Surf. Interface Anal. 35, 128-135 (2003).

Electrowetting: a Versatile Tool for Microfluidics

(Abstract not yet available)

Laser Induced Photochemical Substitution of Hydrophilic and Hydrophobic Groups onto PMMA Surface for Fibrin-Free Intraocular Lens

Minute patterned hydrophilic and hydrophobic groups were photo-chemically substituted on the PMMA surface with Xe2 excimer lamp and ArF excimer laser irradiation. By this selective photochemical surface modification, hydrophilic and hydrophobic groups arrayed alternately have been formed on the sample surface. The modified sample was soaked in the fibrin water solution, and the fibrin adsorption density was measured with amid band of infrared spectrum. The results show that the fibrin adsorption rate of the sample substituted with hydrophilic groups increased while that of the sample substituted with hydrophobic groups decreased. Moreover, the fibrin adsorbed rate of the sample with the micro domain structure of hydrophilic and hydrophobic groups was reduced to one-fifth that of the non-treatment sample. By this selective photochemical surface modification, the fibrin free intraocular lens has been developed for blocking after-cataract.

A Simple Geometrical Model to Predict Evaporative Behavior of Spherical Sessile Droplets on Impermeable Surfaces

A simple model to predict evaporative behavior of spherical sessile droplets on impermeable surfaces is presented. The model is capable of predicting changes in shape and volume of spherical droplets because of evaporation from the droplet's cap area. It can be demonstrated that at any moment all geometrical parameters of a spherical droplet on a surface (volume, contact angle, contact radius and area, cap radius and area, droplet height) may be easily calculated from basic geometrical relations if any two of them are known (e.g. initial droplet volume and initial contact angle, or droplet cap radius and height). Droplet dynamic behavior because of evaporation is further determined using the known value of the evaporation rate from the unit area of the droplet cap (or evaporation flux), which in most cases can be assumed constant for stable climate conditions. The bulk evaporation rate from the droplet cap decreases proportionally to the shrinking cap area. More complex droplet behavior may be simulated if a known receding contact angle value is given. This model was used to simulate experiments performed by the authors as well as reported by other researchers. It is demonstrated that the simple geometrical relations actually account for many features of the dynamic sessile droplet behavior reported in the literature. It is also demonstrated that the often reported bulk evaporation rate, not adjusted for changing droplet cap area should not be used as a meaningful indicator of droplet dynamics.

Effect of Sessile Droplet Geometry on the Intensity of Evaporation in Non-saturated Environments

One of the most important factors of sessile droplet behavior observed in non-saturated environments is evaporation of the probe liquid from the droplet's cap area. The intensity of evaporation over the cap area is not uniform. In case of flat droplets the intensity of evaporation at the droplet edge is reported to be higher than at the center, while in case of larger contact angles the higher degree of vapor saturation in the semi-confined spaces around the droplet edge is thought to suppress evaporation. For modeling purposes, this phenomenon may be quantified as the effect of contact angle or more generally of droplet geometry on the average intensity of evaporation from the unit cap area (or the evaporation flux).

In this study, the effect of droplet geometry on the intensity of evaporation from the cap area is investigated experimentally. The average evaporation flux for water sessile droplets of about 4 microliter on a range of impermeable polymer surfaces (characterized by various surface energies) was measured in a climate-controlled chamber at 23º C and 50% relative humidity. Changes in droplet shape, volume and contact angle were recorded by a digital camera with microscopic lenses at specific intervals of time and quantified by means of a digital image analysis technique.

The results and conclusions will be discussed during the presentation.

Wetting of Nanocapillaries

I will present an overview of new molecular simulation methods developed to study capillary condensation and evaporation in nanocapillaries, formation of liquid bridges, and cavitation. Simulation results are compared with high-resolution experiments on mesoporous molecular sieves.

Some Studies of Excimer Laser Treatments of Ultra High Molecular Weight Polyethylene Fibers

Ultra high molecular weight polyethylene (UHMWPE) fibers have high specific strength and excellent resistance to chemicals. However, these fibers have found limited use in advanced composites because of their poor adhesion to most resins. This paper will discuss the results of KrF (248 nm) and XeCl (308) excimer laser treatments to improve their surface topography and chemistry to improve their adhesion to epoxy resins. Both laser treatments can be carried out in gas as well as liquid environments and are successful in increasing the fiber surface polarity by incorporating oxygen and nitrogen containing groups. The acid-base component of the fiber surface energy increased significantly as a function of the total laser energy (fluence and number pulses). The treatments also show an increase in surface roughness with the total laser energy. The paper will discuss the correlation between the acid-base interaction and the fiber/resin interfacial shear strength (IFSS) as well as between the surface roughness and the IFSS. As expected, KrF laser shows significantly higher effect than XeCl laser.

Vibrated Sessile Drops : Contact Line Oscillations and Instabilities

We study the effects of vertical vibrations on large water drops of

centimetric radius deposited on hysteretic substrate. By varying the

frequency and the acceleration amplitude we observe different types of

oscillations. i) At low amplitudes, the contact line is fixed, the drop

present surface waves. ii) At higher amplitude, we observe a depinning of

the contact line and a shift of the resonance frequencies. We study the

dynamics of the contact line. iii) By still increasing the excitation

amplitude, we observed triplons : non-axisymmetric contact line

fluctuations which are parametrically excited by the drop radius

oscillations. We study the birth and the growth of these modes.

Functional Groups and Materials Generated by Plasma Treatment of Polyolefin Films

Contact angle with water increases and adhesion properties are considerably improved generally by surface treatment such as corona or plasma discharge of polyolefin films. When the treated film is exposed to air or vacuum, all the properties related to adhesion decline with time. This phenomenon is not clarified yet. Some one insists that polymer chain having functional groups moves into the inside of the film with time. To clarify this phenomenon more concretely, the surface analysis and component analysis of solvent soluble materials were carried out by using IR, XPS, and GC/MS. It was found that volatile materials i.e., low molecular weight compounds, were generated by the treatment and vaporized with time.

Determination of Surface Energy and Contact Angle Between Wood and Model Solutions by Meniscus Shape Analysis

The adhesion phenomena have an important role in several technological and natural processes. Surface free energy measurement is one of the main methods for adhesion investigation.

For determination of adhesion characteristics was used an experimental method, based on advanced immersed plate method, allowing to determine the surface energy parameters for porous and soaking substrates as wood. According to this method substrate specimen (wood) is immersed into exploratory liquid and formed meniscus shape is measured by using video measuring system.

In the current study the model solutions with special emphasis on adhesion parameters were investigated. The experiments were performed in the isothermal conditions at 21.5 °C by using three-component solutions (ethylene glycol, resorcinol and water) and birch wood.

Inertial Motions of Interfaces

In most interfacial processes (wetting, capillary rise), confinement of the flow makes viscous force dominate inertia, which leads to the classical laws of spreading or impregnation. However, the motion at short time scale should result from a balance between a driving interfacial force and inertia (which opposes the motion), which leads to very different dynamics. We also show that these short time scales correspond in many cases to macroscopic distances, making these inertial corrections useful in many practical situations.

The Multiple Roles of Interfacial Tension in Fluid Phase Equilibria and Fluid-Solid Interactions

The tension at the interfaces separating the three phases of matter is a unique property in that it can reveal to us a great deal of information about the phases in contact including the direction and extent of mass transfer of components, their proximity to equilibrium, the nature of fluids distribution relative to one another, the contact angle and the spreading and adhesion behavior of liquids on solid surfaces.

In this paper we examine, with supporting experimental data, the multitude of roles played by interfacial tension in establishing (1) the phase behavior characteristics of solubility, miscibility, and the associated mass transfer mechanisms in multicomponent fluid systems, (2) the nature of fluids distribution in gas-oil-water three-phase systems, and (3) the spreading and adhesion characteristics in solid-liquid-liquid systems through dynamic contact angles.

Max-Planck-Institut for Colloid- und Interface Science, D-14476 Potsdam/Golm, GERMANY

Wetting Properties, Interfacial Mobility and Aggregation Behaviour of Long Chain Alkanes at Solid/vapor Interfaces

In the vicinity of the solid/liquid phase transition molecularly thin surface coverages of long chain alkanes (typical 30 C-atoms) at SiO2/air interfaces show an amazingly rich variety of different morphologies and topologies. This can be attributed to the interplay between 1.) wetting/dewetting processes, 2.) the liquid/solid phase transition and. 3.) the influence of the interface. The interface, for instance, leads to a solidification of the interfacial alkanes ("surface freezing"), which causes a (de)wetting transition of the liquid alkane (autophobic behaviour). On the other hand, it also facilitates the mobility/diffusion of "solid" alkanes at temperatures below bulk melting, which leads to a wetting (spreading) behaviour of solid alkane films. Recent experimental results are presented and analyzed.

Contact Angle Measurements of Sessile Drops Deformed by a Dc Electric Field

Sessile water drop geometries on various surfaces have been studied with and without exposure to electric fields with the aim to understand the phenomena involved in the generation of tonal emissions from wet high voltage transmission lines. It has been demonstrated that an approach to noise reduction is the application of particular coatings to the high voltage conductors. Various coatings/treatments need an evaluation under a wide range of conditions including discharge levels with respect to contact angle measurements. In a small-size arrangement, water drops of well-controlled volume have been placed on horizontal stainless steel and aluminium surfaces which had undergone a variety of treatments, singly and in combination: sandblasting, glass bead blasting, hydrophobic and hydrophilic coatings. The surfaces mentioned formed the lower electrodes of a 10 mm parallel plate discharge gap. Contact angles have been determined by first producing an electronic image of the drop which is then assessed by means of software developed here. It has been seen that the voltage at which instability, represented by the growing axial elongation of the drop with increasing electric field, is reached depends strongly on the shape of the water drop and thus on the surface contact angle - which in turn is controlled by the surface properties. In fact, from a contact angle of about 90° (hydrophobic surface) to about 5° (strongly hydrophilic) the instability voltage roughly doubles for a 100 µl droplet in a 10 mm gap. Obviously, there is less deformation with a droplet of smaller contact angle.

In Situ Study of Liquid Transport in and Rupture of Carbon Nanotubes

The wetting and fluidic properties of various types of carbon nanotubes (CNT) are currently being studied for diverse applications, including their use as nanofluidic one-dimensional probes for the controlled transport of fluids to well-defined locations. In this research, the field emission Environmental Scanning Electron Microscope (ESEM) has proved to be a powerful tool in the in situ analysis of the wetting of CNT with different liquids at high magnification. Additionally, the ability of the ESEM to condense and evaporate liquids within the chamber has enabled the dynamic study of CNT/liquid interactions [1].

CNT have been fabricated by chemical vapor deposition (CVD) in an Al₂O₃ membrane [2]. These CNT are straight, have disordered walls, and appear transparent under the electron beam of the ESEM. When studying the interaction of these CNT with water, it was possible to see liquid menisci inside the CNT [3]. From the measured contact angles, it is clear that these CNT are hydrophilic.

In addition, rupture of these thin-walled CNT was observed inside the ESEM chamber as a result of rapid changes in pressure at constant temperatures. This has never been observed in the absence of liquid and is not the result of electron beam damage. If the CNT are on a hydrophobic substrate, condensation of water occurs preferentially inside the CNT. As the pressure is decreased and evaporation of water is induced, breakage of the CNT walls occurs.

1. Rossi, M.P.; Gogotsi, Y.. Microscopy and Analysis, 2004, In Press.

2. Bradley, J.C.; Babu, S.; Ndungu, P.; Nikitin, A.; Gogotsi, Y.. CPS:030302, 2003.

3. Rossi, M.P.; Gogotsi, Y.. Nano Letters, 2004, In Submission.

Spreading of Molten Metals and Oxides

Because of its technological importance, a large body of empirical knowledge about the high-temperature spreading of liquid metals and oxides has been accumulated, but the results are complex, ambiguous and even inconsistent. Reported spreading velocities for high-temperature systems are typically several orders of magnitude slower that expected from theoretical predictions and from comparisons with velocities for organic liquids with similar viscosities. In addition, after high-temperature spreading, reaction products have frequently been found at or near the interface. The physicochemical mechanisms that control the wetting and spreading in such reactive systems are still subject of controversy.

In this work high-speed photography combined with a drop transfer setup was used to study the spreading kinetics of a variety of liquid metals and ceramics. This setup provides a unique opportunity to systematically analyze isothermal spreading. The studies were performed in systems with different degrees of reactivity: from non-reactive couples such as molten oxides spreading on refractory metals like molybdenum to highly reactive systems where chemical reactions or different degrees of interdiffusion are expected (e.g. lead-free solders spreading on different metallic substrates). The results are compared with existing fluid flow and atomistic models. The effects of temperature, atmosphere and the interfacial reactions are also examined.

Wetting Versus Confinement-induced Fluid Phase Transitions

On the basis of Monte Carlo simulations in the grand canonical ensemble and mean-field lattice density functional calculations phase transitions in fluids interacting with solid substrates are discussed. The impact of nonplanarity as well of the solid substrate as well as its decoration with chemical patterns on the wetting behavior of simple fluids will be discussed. If the distance between the solid surfaces becomes comparable with the range of fluid-fluid interactions wetting phenomena compete with confinement-induced phase transitions like capillary condensation. The degree of confinement is crucial for the type and location of the phase transition as will be demonstrated in particular for binary mixtures.

Thin Film Coating of Textile Materials with Special Biopolymers and Enzymes

Surface chemistry and structure determines most uses of textiles (adhesion, skin contact, handle...) whereas the fiber core has to give strength. Thus, new strategies have to be developed for imparting permanent functionality to textile surfaces. This aim can be reached by using e.g. biocompatible polymers like carrageenans, chitosans from the carbohydrate family or complexing molecules like cyclodextrins or some of their derivatives for permanent finish.

Some of these polymers show special properties which may be used for textile surface finishing: Thus, possibly leading to new functional textiles, i.e. for:

• •Fabrics for wound care,

• •Textiles for sensitive skins like neurodermitics,

• •Textiles with slow-release function for medical agents for transdermal therapy.

Especially the potential swelling/deswelling behaviour of hydrogel-like carbohydrates bears the chance to regulate the micro-climate between textile and human skin. Research work is be done on the degree of derivatization of the biopolymers in order to obtain a balance between sufficient chemical linkage points onto the base fabric but simultaneously retaining almost all bulk properties.

Besides carbohydrates even proteins (e.g. enzymes) can be bonded to textile surfaces. The covalent fixation succeeds in using a carrier specific activation, which only slightly impairs the enzyme activity. By an additional cross-linking the enzyme load onto the carrier increases and the fabrics are usable for several times. Using a photochemical activation the enzymes can be fixed even on polyester fabrics (PET). This new technique is also usable for other organic catalysts.

An Investigation into the Deviations from Ideal Behaviour During Electrowetting

Electrowetting refers to the decrease of the contact angle between a conducting liquid (aqueous KCl) and an insulated electrode when external voltage is applied across the solid/liquid interface. Whilst the effect has been known previously, the topic has enjoyed an increased popularity in recent time, particularly with respect to possible applications in microoptical and micro-fluidic devices. We report our recent findings on some fundamental aspects of electrowetting. In the ideal approximation electrowetting is entirely ascribed to the polarisation of the insulating layer, i.e. the system has a constant capacitance. We have experimentally demonstrated that this is not always the case - the electrical double layer at the solid/liquid interface (whose influence was thought to be negligible) can contribute significantly to deviations from the ideal parabolic shape of the electrowetting curve. The specific adsorption of anions, OH- in particular, may create a pronounced deformation of the electrowetting curve at positive potentials. A critical review of the saturation phenomenon (the fact that the contact angle does not decrease beyond a certain threshold voltage) is presented. Previously defined mechanisms are only partially valid under certain experimental conditions. We propose a new predictive model for saturation, based on simple thermodynamic considerations. Our model provides a good description of our own results and is in plausible agreement with many of the published data.

Combellas²

1) Ecole des Mines d'Alès,6 avenue de Clavières, 30319 ALES CEDEX, FRANCE

2) Ecole Supérieure de Physique et Chimie Industrielle de Paris

The Detailed Structure of a Perturbed Wetting Triple Line

The essential form of an initially straight, wetting triple line, perturbed by the presence of a (higher surface free energy) "defect" on the solid surface, has been recognised for a long time, and corresponds to a logarithmically decaying form. However, less attention has been paid to the behaviour of the triple line within the domain of the defect. This was actually studied a few years ago, from a theoetical standpoint, leading to the prediction of an inversion of curvature. Recent experimental work hes been undertaken on the electrochemical treatment of PTFE, leading to small treated areas of higher wettability of typical dimensions of ca. 200 microns. Wetting experiments have been undertaken with such solids and results confirm the general conclusion of inverted curvature of the triple line in the treated zone. However, the "excess wettability" ot the treated zones, as evaluated experimentally, was greater than predicted theoretically. Possible causes are discussed.

Dept. Materials Engineering and Production Technologies, University of Trento, Trento, ITALY

Molecular Connectivity Methods for the Characterization of Surface

Energetics of Polymers

The design of new materials for specific applications is more and more assisted by the use of prediction models providing an a priori indication of material properties. Whenever a theoretical knowledge of the structure-property relationship is not available, quantitative informations can be obtained by semiempirical statistical models which relate the available experimental data to an appropriate set of materials parameters. The topological description of the molecular structure, based on graph theory formalism, has been particularly useful in the characterization of organic compounds. Starting from a graph representative of the molecule, some numerical indicators (topological parameters) can be calculated and reliably used to express structure-property relationships. In the first part of this work the ability of topological models to predict the surface tension of liquids is checked, mainly hydrocarbons, ammines, carboxylic acids and alcohols. The second part discusses the applicability of topological methods to the prediction of surface free energy and Good-van Oss-Chaudhury acid-base components of polymer surfaces. In particular, we investigate the possible advantages of using prediction models entirely based on topological parameters, in comparison with both the classical additive-group approach and the more recent developments, which propose hybrid correlations with group contributions and a limited number of topological indices.

Preventing the Bacterial Colonisation of Surfaces: the "Ultra-low Surface Energy" Approach

An alternative to mechanical abrasion and to chemical attack on established bacterial colonies is to coat surfaces with non-toxic materials onto which these colonizers will not adhere. Essentially, to inhibit settlement by manipulating the forces generated by the targeted surface such that they become incompatible with those of bacterial colonizers. For a polymeric material, these are the van der Waals' forces and sometimes also acid/base interactions.

We have recently investigated the use of "ultra-low surface energy" fluoropolymer coatings as a possible means of preventing the bacterial colonization of surfaces: methods for the fabrication of smooth film structures will be described and their determined physical characteristics will be presented; detailed observations of their resistance to bacteria and other fouling species will be considered on the basis of their surface energy characteristics.

Drop Shape, Contact Angle, Solubility and the Determination of Zero Time Dynamic Interfacial Tensions

(Abstract not yet available)

A Traube Rule for Protein Adsorption to the Liquid-Vapor Interface

(Abstract not yet available)

Anisotropic Wettability of Macroscopic Form I Paracetamol Crystals

The wetting behavior of probe liquids on a complex organic pharmaceutical solid was investigated by several techniques. These include liquid probe techniques, sessile drop contact angles on powder compacts and capillary rise into powder bed and a vapor probe Inverse Gas Chromatography (IGC) technique. In addition, contact angles of a sessile drop were measured directly on a single macroscopic paracetamol crystals. Crystals provides flat, smooth and highly ordered rigid surface, ideal for contact angle measurements by sessile drop method. Advancing contact angles were measured with several probe liquids on facets {201}, {001}, {011} and {110} of the macroscopic crystal and on cleaved facets {010}, representing the dominant facet of a milled sample. Dispersive surface energy calculated from contact angles were found to be marginally different while the polar components varied significantly for the various facets. Powder characterization techniques generated surface energy data which corresponded to the surface properties of the cleaved facet, indicating that the lowest attachment energy facet is postulated to be the dominant facet in milled samples. This study shows that the wettability of form I paracetamol crystals is anisotropic and these differences may be attributed to variations in local surface chemistry on each facet.