The following is a list of the abstracts for papers which will be presented in SECOND 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.)
European R&D, Kodak Limited, Headstone Drive, Harrow, Middlesex, HA1 4TY, UK
The Influence of Electrostatic Fields on Static and Dynamic Contact Angles
Although classical electrowetting has a long history stretching back to the time of Lippman (1875), it has tended to remain something of a laboratory curiosity. However, for the past 20 years or so, electrostatic fields have been widely used in liquid coating processes to achieve high coating speeds on dielectric materials. Electrowetting also has a number of potential applications in areas where switchable wetting could be beneficial. As a result there has been a small but significant resurgence of interest in the underlying phenomena. Nevertheless, until very recently, almost nothing had been published on how electrostatic fields might affect dynamic wetting and the dynamic contact angle.
This talk will describe recent work showing how static and dynamic contact angles respond to AC and DC fields. The experimental data will be compared with a model in which simple equations for the electrostatic effect are combined with the molecular-kinetic treatment of dynamic wetting. Although the model is successful, uncertainties remain concerning both our understanding and application of electrowetting, especially in the presence of other wetting phenomena such as contact angle hysterisis.
Numerical Simulation for Development of Boundary Conditions for Moving Contact Line Problems
(Abstract not yet available)
A Pragmatic View on Testing Using Static and Dynamic Contact Angles
Contact angles as a testing method has over the years moved outside the solid surface applications for which they originally were defined.. Already fifty years ago contact angles were introduced for paper applications with little success. Today the contact angle approach has been suggested for applications such as tissues and powders absorbing a liquid droplet in fractions of a second. Most testing will produce numbers, but can these numbers always be compared and how can I validate my test procedure?
The characterization of the surface hysteresis (wetting/de-wetting) of a solid substrate is often referred to as a "dynamic contact angle" described by the advancing and receding contact angles. The concept of "dynamic contact angle" has also been used to describe the interaction between a liquid and a surface as a function of time. This paper explains the practical usage of contact angles and the relationship between wetting (contact angle), absorption (volume) and adsorption (spreading). Finally a robust approach for trouble-shooting of surface related problems will be presented with applications from Inkjet, NCR and copy paper as well as adhesion and surface modification of laminates by corona treatment.
Role of Lifschitz-van der Waals and Acid-Base Interactions in Yeast Adhesion on Solids
(Abstract not yet available)
Alain Cagna; IT Concept, Parc de Chancolan, Fr-69770 Longessaigne, FRANCE
Maurice Bourrel; Total-Fina-Elf, Groupement de Recherches de Lacq, BP 34, Fr-64170 Lacq, FRANCE
Evaluation of the Surface Energy of Porous Bodies and Powders by Capillary Rise: Discussion on the Existence of a Prewetting Film, and Proposal for a New Method of Evaluation
The theory of the capillary rise, its practical use for evaluating surface energy of divided or porous solids and the supposed existence of a prewetting film in this dynamic system are examined. A general equation describing the variation of mass due to the progression of the liquid is established, this equation being reduced to the traditional form of Washburn when the mass is low. Even in its simplified form, the equation shows two unknown factors GAMMA-sol and DELTA-gamma accounting respectively for the structure of the porous network that the liquid invades, and for the capillary forces. The determination of these two unknown factors is not a commonplace. Many authors use a completely wetting liquid (null contact angle) to evaluate GAMMA-sol, alleging that in this case DELTA-gamma is reduced to the surface energy of the concerned liquid gamma.
Contrary to these authors, we suggest here that the fast forward movement of the liquid front can prevent the establishment of a prewetting film even when the coefficient of spreading is higher than zero. Revisiting the data published by Grundke and Coll. (Progr Colloid Polym. Sci, 1996, 101, 58 and van Oss & Coll. (J. Adhesion Sci. Technol., 1992, 6, 413, we show that the assumption DELTA-gamma = gamma in the case of the considered wetting liquids is probably erroneous. We propose moreover a new method to treat the experimental data resulting from measurements taken with homologous series of van der Waals liquids.
Measuring Surface Forces and Contact Angles of Individual Microspheres with the Particle Interaction Apparatus (Pia)
An experimental set-up, PIA, was build with which interparticle and surface forces can be measured. The set-up is based on the principles of atomic force microscopy. Therefore particles were attached to atomic force microscope cantilevers which served as force sensors. The distance was adjusted with a calibrated piezo translator. With PIA advancing and receding contact angles on individual microspheres at the air-water and liquid-liquid, namely hexadecane-water, inter-faces were determined. For this purpose spherical silanated silica particles (R=2.35 m) and polystyrene particles of different radii (R=1.80-4.38 m) were attached to the atomic force microscope (AFM) cantilevers. Polysterene particles were sintered onto AFM cantilevers to provide stability in the organic solvent (see figure). Then the equilibrium position of the microsphere at the air-water interface of a drop or an air bubble was measured. From the equilibrium position the contact angles were calculated. Advancing and receding contact angles determined with silanated silica particles (a=97° and r=81°) agreed roughly with contact angles measured on similarly prepared planar surfaces (a=95° and r=83°). The apparent contact angles measured on polystyrene particles decreased with increasing particle size. This can be interpreted by assuming a negative line tension.
Uncured Silicone Sealant Surface Tension : a New Quantitative Tool for the Measurement of the Sealant Ease of Use
The behavior of silicone sealant during its use, from the extrusion of the cartridge to the completion of the joint, has been identified as being the key feature for professional applicators. This feature called "ease of use" is very complex and includes many different criteria such as the ease of extrusion, of joint smoothing, the surface finish, the action of the " tooling aid" (aqueous solution of surfactant ) used during the smoothing operation...Several of these criteria seem directly linked to surface properties of the uncured sealant. In order to try to translate these subjective properties into quantitative measurements and understand the controlling parameters, surface tensions of uncured sealants were measured using the solid-liquid contact angles technique. These surface tensions were correlated with ratings collected from professional applicators for the smoothing criteria. A correlation was also built between the contact angle of different "tooling aid" solutions at sealant surface and the ease of smoothing obtained by using these " tooling aids". On a more fundamental point of view, the evolution of the contact angle of a water droplet at the surface of uncured sealant within time helped to understand the migration or reorientation of polar entities from bulk to sealant surface.
Wetting and Dewetting Kinetics of Shear-Thinning Liquids
The spreading of liquids on solid surfaces is of considerable interest and importance in many fields of industrial activity. The dynamic aspects of spreading are particularly relevant in several practical applications, which involve adhesion phenomena, such as coating, adhesive bonding, printing and composite manufacturing.
The spreading of Newtonian liquids has been largely described in literature from the hydrodynamic theory. The essential of this theory will be reviewed and illustrated with examples (PDMS oils). We have recently extended the hydrodynamic theory of liquid spreading to non-Newtonian fluids (PDMS filled with silica particles, typographic inks), in wetting and dewetting modes. The very common case of the shear-thinning behavior will be considered in this study.
The spreading kinetics of a liquid drop on a rigid substrate is governed by viscous dissipation in the liquid, the capillary driving force being compensated by the braking force resulting from viscous shearing in the liquid. In the case where the liquid is not Newtonian but shear-thinning, a deviation from the classical hydrodynamic theory for wetting is observed.
The slightly non-spherical shape, of shear-thinning liquid drops having a size smaller than the capillary length may be also simply interpreted by calculating that the actual viscosity increases from the edge to the center of drops in the wedge shape formed during wetting near the advancing liquid front.
No drastic changes are observed in the dewetting mode as compared with the general behavior of Newtonian liquids. The rate of growth of dry zones nucleated in an unstable liquid film stays constant, as for Newtonian liquids, at least at the early stages of the growth of dry patches.
Wettability: the Role of the Molecular Scale
The wetting properties of a liquid on a solid surface are usually expressed in terms of macroscopic quantities, like spreading parameters. However, structural changes occurring at the mesoscopic and molecular scale may play a leading role in the observed behaviors.
At equilibrium, the processes induced by the adsorption of a monolayer are well known for surfactant molecules. We present here examples of wettability changes related to the presence of nematic orser in liquid crystals. The isotropic phase of n-cyanobiphenyls does not wet silica, while the nematic phase does. Close to the transition, time dependent effects are observed, correlated with the ability of the system to build up on the solid a thin organized structure composed of three molecular layers. Ellipsometric profiles and contact angle measurements are used for characterization
Dynamic wetting, and specifically dissipative processes are to a large extent controlled by thr first molecular layers on the surface. The presence of adsorbed water may be critical. We present data on the spreading of thin layers of polydimethylsiloxanes on silica at various relative humidities. In the dry case, the dissipation is controlled by the mesoscopic scale and the relevant parameter is the bulk viscosity of the polymer. In the moderately wet case, the dissipation is controlled by the friction of the first molecular layer on the surface and the molecules proceed mainly through reptation processes. Closer to saturation, a free diffusion of molecules takes place.
Surface Energetic Parameters of Some Organic Liquids
(Abstract not yet available)
Is Contact Angle Hysteresis Due to a Film Present Behind the Drop -
5 Years Later
Recently receding contact angles increasingly attract attention in studies of wetting phenomena. The difference between the advancing and receding contact angle of the same liquid on the same solid surface is termed 'contact angle hysteresis'. The hysteresis is being ascribed as due to the solid surface roughness and/or chemical heterogeneity of the surface. These possible mechanisms of the hysteresis appearance do not exclude another interpretation of the receding contact angles, what was presented in 1994 during the 10th Symposium on Surfactants in Solution (Caracas, Venezuela) (in Surfactants in Solutions, A.K. Chattopadhyay and K.L. Mittal, Eds., V.64, pp.31-53, Marcel Dekker, 1996). In this approach the liquid film presence behind the drop is considered to be the cause of the observed hysteresis, of course excluding the rough solid surfaces. In the present contribution new experimental results will be presented from the literature (published after our previous presentation) to verify the suggested interpretation. Especially interesting seems to be interpretation of the receding contact angles published by Drelich et al. (J. Colloid Interface. Sci.178, 720 (1996); 202, 462 (1998), Langmuir, 11, 3491(1995); 12, 1913 (1996); 13, 1395 (1997)), which were obtained on several mineral surfaces precovered with different long-chain fatty acids.
Wetting Transitions on Structured Substrates
The contact between fluids and microstructured surfaces opens a wide range of interesting applications. The detailed knowledge of the strongly inhomogeneous density distribution of simple liquids on geometrically or chemically structured surfaces is a prerequisite for the specific design of man-made soft structures. By using density functional theory, Landau theory, and simulations the equilibrium fluid structures near the three-phase contact line between a solid, liquid and vapor, in a wedge, across a chemical stripe, and on curved substrates, are studied with a particular emphasis on wetting properties. These examples serve as elementary paradigms for more complicated structures.
1) Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931, USA
2) Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112, USA
3)Tomah Products, Westhollow Technology Center, P.O. Box 1380, Houston, TX 77251-1380, USA
Contact Angle Relaxation for Ethoxylated Alcohol Solutions on Hydrophobic Surfaces
In de-inking flotation of wastepaper pulp, hydrophobic ink particles are collected by air bubbles and transported to a froth layer. Surface active chemicals control the surface tension of water and froth stability. Ethoxylated alcohols are among the popular surfactants selected for this application in US paper recycling plants. The frothing potential of ethoxylated alcohol surfactants is well known, however, the effect of these chemicals on the properties of hydrophobic ink has not been examined. In this regard, the systematic measurements of contact angles for ethoxylated alcohol solutions on hydrophobic toner and polyethylene surfaces were carried out using the sessile-drop technique. The drops of C12H23(C2H4O)nOH (n=4 to 7) solutions, with a concentration ranging from .001mM to 1mM, were placed on the toner and polyethylene substrates. The contact angle relaxation was recorded over a period of 30 minutes. The results of this study show that ethoxylated alcohols adsorb on toner and polyethylene surfaces and reduce their hydrophobicity. Time-dependent changes at the solid-surfactant solution interface reinforce the contact angle instability. The contact angle relaxed over several minutes and the kinetics of contact angle relaxation was dependent on the surfactant concentration. Specifically, the kinetics of contact angle relaxation increased from about 0.02 to 0.1 degrees per minute with an increase in the concentration of ethoxylated alcohol from .001mM to 0.05mM. The kinetics of contact angle relaxation increased three to seven times more for solutions in which the ethoxylated alcohol concentration was higher than the critical micelle concentration.
Dynamic Adsorption as the Determinant of Forces between Solids
Adsorption of surface-active material changes the forces between solids, so it is common to relate forces or adhesion to the extent and nature of adsorption. Forces in adhesion are often described by theories that use material properties for isolated surfaces. However, the forces acting on the solid surfaces actually change the properties of the surfaces, and in particular, they change adsorption to the surfaces. Hall, (J. Chem. Soc. Faraday Trans. 2, 1972, 68, 2169-2182 and Ash et al. (J. Chem. Soc. Faraday Trans. 2, 1973, 69, 1256-1277.) derived a simple quantitative expression relating changes in equilibrium adsorption as a function of separation between the solids to changes in surface force as a function of chemical potential of the adsorbate. This expression shows that when addition of a surface-active agent succeeds in changing the force between solids, the extent of adsorption must change with separation. Our goal is to quantify the extent of adsorption and to understand its impact on surface forces and adhesion. In this presentation we describe experimental measurements of the force acting between silica surfaces as a function of both separation and the chemical potential of surfactant in solution. Force-Distance data was obtained by Atomic Force Microscopy. This data was used to determine the extent of changes in surfactant adsorption as a function of separation using Hall's method. Three surfactants have been investigated: dodecyltrimethylammonium bromide (C12TABr), C16TABr, and C16TACl.
The Effect of the Acid-base Chemistry of Lactose on its Adhesion to Gelatin Capsules: Conclusions from Contact Angles and Other Surface Chemical Techniques.
The performance of dry powder inhalation formulations depends upon the adhesion properties of each component of the formulation. Here we explore the adhesion of lactose to gelatin capsule materials. The surface characterization of each of these materials has been performed using are variety of techniques. Some of the experimental techniques used include contact angles, IGC, FMC, and XPS as well as SPM. Each of the pharmaceutical grade materials used in the study was found to have varying degrees of surface contamination. Because lactose is a powder and capsules are made of gelatin films not all experimental techniques or theoretical surface chemical models are suitable for application to both materials. From our experimental data it is clear that acid-base contributions are a significant contribution to the adhesion of these materials. It is also clear that characterization of a given surface as an acid or base not yet possible because results using different methods and/or theoretical models can give seemingly contradictory results. Preliminary assessment of component compatibility based upon acid/base characterization by different techniques should be approached with considerable caution.
2) CEA-CEREM, 17 rue des Martyrs, 38054 Grenoble CEDEX 9, FRANCE
Specific Characteristics of Wettability at High Temperatures
Wetting studies in "high temperature" systems (molten metals on solid metals or ceramics, 11M- a glasses on solid metals, etc.) are motivated by the importance of wetting properties in the fabrication of metal-matrix composites by liquid routes, the joining of metals and ceramics by soldering, brazing or glazing and the fabrication of bard multi-materials by liquid-phase sintering.
Although many basic concepts established for chemically inert ambient temperature systems are also valid for high temperature systems, these latter systems possess specific characteristics that exert a major influence on their wetting properties. First, the surface properties of nearly all metals and many ceramics, and thus wetting and adhesion, are extremely sensitive to impurities, mainly oxygen, that are invariably contained in furnace atmospheres. Second, many liquid/solid couples are far from chemical equilibrium and the resultant interfacial reactions can affect both spreading kinetics and the ultimate degree A wetting. Finally, the high atomic mobility at high temperatures, combined with small particle sizes (a few micrometers or less), allow solid particles to reach their equilibrium shape on solid substrates in reasonable times ("solid wetting").
The influence of oxygen on wetting, the role of interfacial. reactions on spreading and the "solid wetting" will be illustrated by results obtained by the sessile drop (or particle) method during the last decade.
Contact angles and hysteresis of polyamides
Aliphatic polyamides (PA's) represent interesting substrates for examining the affects of polarity on wetting, as they are heterogeneous at the molecular level, containing a mixture of nonpolar methylene groups and polar amide groups. While the methylene groups can be expected to interact through weaker van der Waals forces, amides groups should interact much more strongly via hydrogen bonding.
PA's with varying amide content and polyethylene (PE) were molded against glass to produce surfaces with similar roughness. After cleaning, chemical composition of the surfaces was verified with x-ray photoelectron spectroscopy. Advancing and receding contact angles were measured from small sessile water drops. Contact angles decreased with amide content and hysteresis increased. Free energies of hysteresis were calculated from contact angles.
For PE, which is composed entirely of nonpolar methylene groups, free energies were equivalent to the strength of dispersive van der Waals bonds. For PA's, free energies corresponded to partial contributions from the dispersive methylene groups and polar amide groups. If extrapolated to 100% amide content, the free energy of hysteresis was equal to the strength of the hydrogen bond between water and amide groups.
Dynamic Contact Angle Effects on the Hysteretic Saturation-Pressure
Relationships During High Capillary Number Imbibition and Drainage
Process in Porous Media
During imbibition and drainage processes occurring at high flow rates the capillary pressure-saturation degree relationship may depend on the macroscopic liquid flux.
One reason for this is the dependence of the solid-liquid-gas contact angle at the moving liquid-gaseous interface on the flow velocities, as found in systems such as long cylindrical capillaries. In a previous theoretical study (J. Adhesion Sci. Technol., 13:1495-1518, 1999) a conjecture was made that at a prescribed capillary pressure the criterion for the liquid phase to invade an empty pore is defined by the Young-Laplace equation, but with the expected dynamic contact angle used instead of the static one. That study applied a constraint that the sequence of pore filling during imbibition is from the smallest to the largest (and vice versa for a drainage process). In the analysis reported in the present lecture this constraint is being relieved by introducing a simple pore-scale hysteretic mechanism, which allows entrapment of the non-wetting gaseous phase during imbibition and delaying the withdrawal of the wetting liquid phase from pores surrounded by smaller ones during drainage. An iterative procedure enables a quantitative estimation of the hysteretic and dynamic capillary pressure-saturation degree relationships. For a given capillary pressure during wetting the degree of liquid saturation decreases with increasing liquid flow velocity, and vice versa for drainage. This effect of the liquid flux is more pronounced as the width of pore-size distribution increases.
Microscopic Origins of Macroscopic Wetting Phenomena
Wetting phenomena on a macroscopic scale are controlled by the fluid structure and dynamics in a microscopic region near the contact line. In this confined, microscopic region the properties of the fluids and interfaces are not the same as they are away from the contact line. Thus, the origins of Young's equation or apparent dynamic contact angles - concepts often used to describe macroscopic wetting - lie a region of space which is rarely directly accessible to experiments. To predict macroscopic wetting phenomena, we are forced to infer the microscopic behavior from macroscopic measurements. We will discuss two investigations of this microscopic to macroscopic connection: We will examine the need for precise measurement techniques and hydrodynamic descriptions to probe the unique hydrodynamics near moving contact lines which control the spreading of simple liquids. Next, we will describe the self-assembly near contact lines of spreading of a surfactant solutions. This study focuses on the origins of the forces which control the rich spreading behavior of these fluids and highlights the need to understand Young's equation as describing interfacial properties near the contact line.
Department of Physic, Extremadura University, 06071 Badajoz, Spain
Physical Features of Experimental Setups in Imbibition Experiments
A tool for the determination of the contact angle and other thermodynamics parameters of powdered solids is the imbibition of liquid in such media. The experimental setups commonly used for this purpose allow the measure either the distance advanced by the liquid or the increase of weight of the porous body against the time. In this work we summarize some relevant tips we found for the accurate analysis of the experimental results by Washburn's equation. So, with regard to the first, it was found that the discordance between the experimental behavior and that predicted by Washburn's equation was due to the selection of the reference system, disappearing when this equation was used in its polynomial expression. In relation to the second experimental method, our analysis proves that together with the imbibition, the formation of the meniscus on the base of the porous solid also contributes to the increase of weight, being necessary to rescale the experimental data to remove this last effect, which leads to the need of using a parabolic form of Washburn's equation. Finally, the application of these expressions of Washburn's equation proves both experimental techniques were equivalents to carry out the imbibition experiments.
1) Department of Physics, Faculty of Sciences Extremadura University, Av. Elvas s/n 06071 Badajoz, Spain
2) Department of Microbiology, Faculty of Medicine, Extremadura University, Avenida Elvas s/n 06071 Badajoz, Spain
Effects on the Physico-chemical Surface Properties of Two Enterococci of the Growing Medium and Experimental Environment
It has been developed a comparative study of the physicochemical surface properties of two different strains Enterococcus faecalis ATCC and Enterococcus faecalis 72 by measurements of contact angle of probe liquids, zeta potential and MATH. The propose of this work has been to study the influence on the surface properties of the growing medium of both strains (TSB, TSB supplemented with 10% serum and TSB supplemented with 50% urine) as well as the effect of the temperature (22ºC and 37ºC) and the buffers employed (KPi and PBS). From the results obtained, it has been predicted the adhesion behavior on glass and silicone rubber based on the DLVO theory.
1) Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931, USA
2) Department of Mining and Mineral Process Engineering, University of British Columbia, Vancouver V6T 1Z4, Canada
Contact Angles on Heterogeneous Coal Surface: The Effect of Mineral Matter
It is well known that coal separation and reconstitution processes in coal preparation plants largely depend on the wettability of coal particles. Several researchers observed that the wettability of coal decreases significantly with an increasing amount of mineral matter in the coal samples. However, detailed studies on the effect of mineral inclusions on both advancing and receding contact angles have never been reported. For the first time, the influence of the discrete nature of inorganic impurities of the coal surface on contact angles is examined with advanced surface analysis instrumentation. Both advancing and receding contact angles are measured with an automatic image analysis system using captive-bubble and sessile-drop techniques. The distribution of inorganic inclusions on the coal surface is examined by scanning electron microscopy. The amount of mineral matter is determined by micro-slicing the coal sample and low-temperature plasma de-ashing. Finally, the size of mineral particles is measured using the Elzone particle size analyzer. The results of this study demonstrate the influence of the mineral matter and its discrete nature on measured contact angles. For example, a significant scatter of contact angles is found for specimens of the same coal. Nevertheless, it is possible to deduce a correlation between the amount and size of inorganic inclusions and contact angles. Both advancing and receding contact angles decrease with increasing amount of mineral matter and increasing size of individual mineral particles.
Characterization of Polymer Surfaces Using Axisymmetric Drop Shape Analysis (ADSA): Structure Property Relationships
Contact angles of liquids on polymer surfaces are widely used to predict wetting and adhesion properties of polymers by calculating their solid surface tension. The theory is based on the equilibrium of an axisymmetric sessile drop on a flat, horizontal, smooth, homogeneous, isotropic, rigid axed insoluble solid surface. In practice, difficulties arise, since the basic assumptions implied in Young's equation (i.e., the presence of a single, unique contact angle and constancy of the interfacial tensions) are often violated. Recently, it has been shown that axisymmetric drop shape analysis (ADSA) is a suitable experimental technique to reveal contact angle complexities which prevent use of the measurements for energetic calculations. Experimental procedures and criteria for measuring and interpreting meaningful, contact angles have been. developed. We have applied this technique to study structure property relationships of polymers (polymethacrylates, maleimide copolymers) possessing the same (polar) polymer backbone, but varying length and size of (non-polar) alkyl side chains. The question arises whether the surface tension of these polymers reflects the systematic change in the bulk structure. The data are compared to previous results obtained for polystyrenes with allcyl substituents. The relationship between molecular structure and exposure of molecular groups at the surface and the effect on the surface tension of the polymers will be discussed.
Manipulating the Wettability of Fluoropolymer Surfaces
In studies of wetting kinetics , where the dependence of contact angle on wetting line velocity has been studied, solid fluoropolymer surfaces have been found to be more ideal than surfaces hydrophobised by other methods (eg. silanation). This is due, at least in part, to their extremely low energy and low contact angle hysteresis, the latter as a consequence of the chemical and physical homogeneity of fluoropolymer surfaces. Fluoropolymers have also been utilised in electrowetting studies  where the energy of the solid/electrolyte interface, and therefore the liquid, is manipulated by an externally applied electrical field. Electrowetting offers a number of exciting applications eg. optical filters, microfluidics. However a number of challenges remain, for example saturation of the electrowetting effect and switching lifetime, which raise questions about the material properties and behaviour of electrowetting surfaces. In this presentation recent progress in our laboratories reconciling electrowetting performance with material properties will be presented.
1. Email: email@example.com
2. Schneemilch, M., Hayes, R.A., Petrov, J.G and Ralston, J. Langmuir 1998, 14, 7047
3. Welters, W.J.J and Fokkink, L.G.J. Langmuir 1998, 14, 1535.
*I.F.T. of Iasi
Surface Energy after Plasma Treatment for Polymer as Substrates for Gold Deposition
The technology of thin layers deposits has continued to progress as long as the thin layers formation's mechanism was better understood.
To create substrates for gold deposits, the polymers have been analyzed and compared. These polymers have been polypropylene (PP), polyethylene tereftalate (PET), polyvinyl carbonate (PVC) and Teflon (tetrafluorethylene).
The cleaning using ions bombardments has been done using a high voltage source (3kV - 100mA) and the light discharge in oxygen atmosphere of 5.10-2 torr.
The follow-up of the surface quality has been done measuring the contact angle between the double distilled water drops, formamyde and polymer thin sheets.
The crystal state of the polymers from the discharge treated thin sheets as well as those non-treated has been followed by X-ray diffraction.
Following a powerful corrosion (0.9kV; 30mA; 5*10-3torr; 20min.) over polymer thin sheets PET, PP, PVC and Teflon (tetrafluorethylene) is proven that formaldehyde wets completely the surface, which means that q=0, and cosq=1.
From these information is easily seen that following corrosion the polarization component of surface tension is strongly modified.
This value of the adhesion energy is a couple of times smaller than the coesion energy of metals and justifies the superficial mobility of the gold adatoms on the polymer.
The surface energy used in Young's equation will determine q=119,7o.
Following the corrosion process, a partial re-crystallization process takes place, at least in the superficial layer of the samples. The dimension of the crystallites is increased following the corrosion with 10-15%.
The Teflon (tetrafluorethylene) samples presents crystallites having enlarged dimensions, while the polypropylene sample (PP) suffer the most important variation of the average size of crystallites.
X. Chen, A. Neimark and Y. Kamath; TRI/Princeton,Princeton, NJ 08542
Stability of Liquid Films on Fiber Surfaces-Role of Fiber-Liquid Adhesion
(Abstract not yet available)
1) Research Associate, Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, JAPAN
2) Department of Precision Machinery Systems, Interdisciplinary Graduate School of Science and Engeneerig, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, JAPAN
Electrowetting on Single-Crystal Substrates of Silicon
An electrochemical wetting and dewetting on silicon substrates are found in this study. These phenomena are investigated about relationships among voltage, current, contact angle, and wetting area experimentally. A contact angle of a pure water drop (the wetting area is about 1mm in diameter) is about 75 degrees on a silicon surface. We observe here that a direct voltage of not more than 10V impressed between the silicon substrate and the pure water drop causes the expansion of the wetting and the contact angle becomes nearly zero degrees. The voltages causing a electrochemical wetting vary with the crystal orientation of a surface, the type of impurity in a substrate and the resistance of a substrate. However, This phenomenon is irreversible with the pure water. Next, experiments with some electrolytic solutions instead of the pure water are conducted. The drops of the solutions spread by lower voltage than the drops of the pure water. When a substrate is set to touch a cathode and an anode is immersed in a drop of sodium sulfate solution (0.01mol/l), the expansion of the wetting takes place at 2.5V. Then the slight shrinkage is observed when the reverse voltage is impressed.
1) Dept. of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
2) Department of Mechanical Eng., University of Toronto, Toronto ,Ontario M5S IA4, CANADA
Lifshitz van der Waals/ Acid-Base Approach Revisited
(Abstract not yet available)
1) Dept. of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
2) Department of Mechanical Eng., University of Toronto, Toronto ,Ontario M5S IA4, CANADA
Contact Angle Interpretation = Combining Rule for Intermolecular Potential
(Abstract not yet available)
Direct Movement of Liquid Drops on Patterned Surfaces Using a Reversible Self-Assembly Process
Surface energy changes effected by molecular adsorption can cause drops to move on surfaces spontaneously. The requisite surface energy gradients can be created by applying dilute solutions of an alkylamine in a nonpolar liquid onto gold surfaces that have been derivatized with CO2H-terminated alkanethiols to expose a surface of densely packed CO2H groups. The drops can be directed to move along specified paths using patterned surfaces that contain inner tracks exposing CO2H groups and exterior domains of oleophobic CH3 groups. Drop velocities of approximately 1 cm/s can beachieved. The adsorption process allows the sequential transport of two drops on a common track and also the regeneration of the initial high energy surface for reuse. The effects of factors including the chain length of the alkylamine and its concentration on the motion of liquid drops will be discussed in this presentation as well as the relationships between the change in surface energy that powers the drop and the kinetic energy of the macroscopically moving drop. Kinetic and thermodynamic models for describing the dependence of the drop velocity on adsorbate concentration will be presented and contrasted using experimental data from this system and a related study that employed organosilanes as adsorbates.
Surface Rearrangements Of Polymeric Coatings Containing Fluorinated Chains as Studied by Contact Angle and other Methods
Polymeric films were prepared from different polyester copolymers containing perfluoropolyether chains. Inspections of the films by contact angle and X-ray photoelectron spectroscopy showed a strong difference between the surface and the bulk composition, with a highly hydrophobic and liophobic outermost layers made by the alignement of the perfluoroalkoxy groups. Between the two sides of the films different surface characteristics were found depending on the substrate employed. A dynamic surface rearrangement was observed when the films were exposed to different environments: depending on the chemical composition and on the structure of the films the restructuring could be reversible to the original state.
1) College of Pharmacy, Division of Pharmaceutics, University of Iowa, Iowa City, IA 52242
2) Yamanouchi Shaklee Pharma Research Center, 1050 Arastradero Road, Palo Alto, CA 94304
Surface Free Energies of Cellulose Ether Films
The surface properties of a series of cellulose ethers polymers cast as films on glass substrates were determined using sessile drop and advancing drop contact angle goniometry. The polymers studied were hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), hydroxypropylcellulose (HPC) and hydroxyethylcellulose (HEC) and ethylcellulose (EC). The Lifshitz-van der Waals (LW) and acid-base surface tension components and electron donor/acceptor parameters were calculated from the contact angles data according to the method of van Oss, Chaudhury and Good. The total surface free energy of these cellulose ethers ranged from 30 mJ/ m2 for EC to 50 mJ/m2 for HEC. The contribution of the acid-base component of surface free energy to the total surface free energy of the polymers ranged from 1-12%, which was considerably lower than that for cellulose. The polymers demonstrated near monopolarity and had dominant electron donor (Lewis-base) character. Trends in values obtained for the free energy of hydration and interfacial free energy of interaction (in water) of the polymers using the LW component and electron donor/acceptor parameters were in good agreement with their relative bulk solubility and hydration behavior. Estimates of the acid-base character of the polymers from work of adhesion terms calculated using the liquid wetting data were consistent with those obtained using the Lifshitz-van der Waals/acid-base approach. The Lifshitz-van der Waals/acid-base approach provided a reasonably consistent method for relative ranking of the surface properties of cellulose ethers and was useful in evaluating factors affecting film adhesion to pharmaceutical substrates.
Surface Energy of Fluorine-Containing Liquid Crystalline Polymers
Applying our novel thin film polymerization technique, we have prepared thin co-polymerized films through polycondensation reactions from 3 monomers, e.g., acetoxynaphthoic acid (ANA), acetoxy acetanilide (AAA) and 4,4'-(hexafluoroisopropylidene)bis(benzoic acid) (6F acid). The polycondensation reactions were conducted on a hot plate at 300C and were characterized by FTIR. The thin films were examined by polarized microscopy for their liquid crystalline texture, and also used to determine surface energy components by the Lifshitz-van der Waal-acid-base theory (van Oss-Chaudhury-Good method).
Experimental data suggests that these fluorine-containing thin copolymerized films exhibited smaller surface energy than those main-chain liquid crystalline polymers. With increase in fluorine molar concentrations from 5 to 10, surface energy decreased from 37 mJ/m2 to 34 mJ/m2, however, with further increase in fluorine molar concentrations to 20 and 30%, no obvious decreases in surface energy were found. The X-ray photoelectron spectroscopy (XPS) results revealed similar trends, which might be due to fully occupation by the fluorine atoms at the surfaces.
1) Institute of Polymer Research, Hohe Strasse 6, D-01069 Dresden, Germany
2) Institute of Physical Chemistry, Leninsky Prospect 31, 117915 Moscow, Russia
Some Insights into the Relationship Between Surface Chemistry, Wettability, and Energetics of Glass Fibres and Adhesion Strength of Composites
The structure and properties of fibre - polymer matrix interface play a significant role in the development of final composite performance. The "quality" of this interface is strongly dependent on the surface characteristics of a fibre.
Polyolefines as matrix materials are important in the field of composites because of their excellent corrosion and wear resistance, whereas due to absence of polar groups the adhesion to reinforcing glass fibres is very weak and has to be improved by able modification of both fibre and matrix.
In our work, glass fibre surfaces were sized with silane coupling agents and different film formers. The characteristics of differently surface treated fibres were characterised using two different surface-sensitive methods - wetting (Wilhelmy technique) and inverse gas chromatography. For comparison wettability tests were also performed using model glass samples (plates) surface treated in the same manner.
The main goal of this work is to analyse existing approaches for determination of surface energetics and polarity parameters of the variously treated glass surfaces and to establish correlation between these quantities and adhesion strength of model composites (single glass fibre - maleic anhydride grafted polypropylene matrix).
Some difficulties of evaluation of energetic parameters from wetting and inverse gas chromatography with respect to interfacial specific interactions will be discussed.
Superspreading Surfactant Systems
Microgravity technologies often require aqueous phases to spread over nonwetting hydrophobic solid surfaces. At a hydrophobic surface, the air/solid tension is low, and the solid/aqueous tension is high, and therefore a large contact angle forms when an aqueous drop is placed on a hydrophobic surface. Instead of spreading, the drop is held in a meniscus by the large angle. On earth gravity can force the liquid gathered into the meniscus to topple over and at least partially spread; but in microgravity these menisci are held rigidly in place. Surfactants facilitate the wetting of water on hydrophobic surfaces by adsorbing on the water/air and hydrophobic solid/water interfaces, lowering the surface tensions of these interfaces and thereby reducing the contact angle. Hydrocarbon surfactants (i.e. amphiphiles with a hydrophobic moiety consisting of an extended chain of methylene -CH2- groups attached to a large polar group to give aqueous solubility) are capable of reducing the contact angles on surfaces which are not very hydrophobic, but do not reduce significantly the contact angles of the very hydrophobic surfaces such as parafilm and polyethylene. Trisiloxane surfactants (amphiphiles with a hydrophobe consisting of methyl groups linked to a trisiloxane backbone in the form of a disk ((CH3)3-Si-O-Si-O-Si(CH3)3) and an extended ethoxylate (-(OCH2CH2)n-) polar group) can significantly reduce the contact angle of water on a very hydrophobic surface and cause rapid and complete (or nearly complete) spreading (termed superspreading).
Our overall goal is to develop and verify a theory for how trisiloxanes cause superspreading, and then use this knowledge as a guide to developing more general hydrocarbon based surfactant systems which superspread. We propose that the trisiloxane surfactants superspread because their structure allows them to strongly lower the high hydrophobic solid/aqueous tension when they adsorb to the solid surface. When the siloxane adsorbs, the hydrophobic disk parts of the molecule adsorb onto the surface removing the surface water. Since the cross sectional area of the disk is larger than that of the extended ethoxylate chain, the disks can form a space filling mat on the surface which removes a significant amount of the surface water. The water adjacent to the hydrophobic solid surface is of high energy due to incomplete hydrogen bonding; its removal significantly lowers the tension and reduces the contact angle. When a hydrocarbon surfactant adsorbs onto a hydrophobic surface, because the polar groups are usually much larger than the cross section of the chain, the chains aligned perpendicular to the surface cannot cohere in a way to significantly remove the surface water.
To verify this hypothesis, we studied, using Fourier Transform Infrared Spectroscopy (FTIR), the O-H stretching vibration of the surface water when a siloxane and a soluble hydrocarbon polyethoxylated surfactant (C12E6 (CH3(CH2)11(OCH2CH2)6OH)) adsorb from an aqueous phase onto model hydrophobic surfaces fabriacted from self-assembled monolayers.. Because the hydrogen bonding of water near a hydrophobic surface is incomplete, non hydrogen bonded OH groups exist near the surface. The O-H stretching vibration of these hydroxyls adsorbs infrared radiation at longer wavenumebers than the hydrogen bonded hydroxyl. IR spectra as the siloxanes adsorb demonstrate a reduction in the adsorbance in the wavenumber range in which the -OH stretching adsorbs, particularly at the far end which corresponds to the free -OH. The hydrocarbon surfactant did not exhibit as marked reduction in the stretching vibration, and there was almost no change in the free -OH adsorbance.
To develop a hydrocarbon based superspreading susyem, we studied a binary surfactant systems consisting of a very soluble polyethylene oxide and a long chain alcohol dodecanol. By mixing the alcohol with a soluble surfactant with a large polar group, we have found that the contact angle of the mixed system is very low. We hypothesize that the alcohol fills in the gaps between adjacent adsorbed chains of the large polar group surfactant. This "filling in" removes the surface water and affects the decrease in contact angle. We confirm this hypothesis by demonstrating that at the air/water interface the mixed layer forms condensed phases while the soluble large polar group surfactant by itself does not. We present drop impact experiments which demonstrate that the dodecanol/C12E6 mixture is effective in causing impacting drops to spread on the very hydrophobic model OTS surfaces.
Wetting on Real Surfaces
Some fundamental aspects of contact angle theory and measurement are discussed. It is shown that line tension cannot be constant, and a new equation, which accounts for the dependence of line tension on the contact angle itself, is presented. One of the consequences of this equation is that line tension is important also in cylindrical systems. The effects of roughness and heterogeneity are discussed with reference to the Young equation and the line tension concept. The Wenzel and Cassie equations are rigorously analyzed and theoretically shown to be true only for a drop which is very large compared to the scale of roughness/heterogeneity. Numerical simulations demonstrate and support this conclusion.
Contact angle hysteresis is discussed in terms of necessary and sufficient conditions. The necessary condition, as has been known for many years, is the multiplicity of metastable equilibrium states. The sufficient condition, which has been recently revealed, is the dependence of these equilibrium states on the volume of the drop. Results of calculations are shown for two-dimensional and three-dimensional systems. Of special interest is the predicted difference between the drop and captive bubble techniques.
Contact angle measurement using the Capillary Bridge System (CBS) is explained. The main advantages of this approach are the ability to calculate an average contact angle for the whole contact line, and the use of force measurements rather than direct optical measurements.
The Wetting Properties of the Human Skin
One of the important function of the human skin is to minimize the water loss and the penetration of foreign substances. This barrier property is exerted by its outermost layer, the stratum corneum, a layer made up horny cells stuck together by intercellular lipids. In spite of extensive topical applications of pharmaceutic or cosmetic formulations, the skin surface wetting properties have been poorly investigated. The first works were carried out in 1950s by the measurement of contact areas and contact angles of water and lipids droplets. The objective of the following works was to determine the skin surface free energy as compared to polymeric surface. Our contribution was related to the changes of the skin surface properties in presence of skin surface lipids or consecutively to a washing treatment. The skin roughness contribution to the wetting has been investigated through dynamic contact angle measurements. The significance of these findings were discussed in relationship to skin surface roughness parameters.
Contact Angle Characterisation of Surface Polymer Modifications after Radiation Treatment
Nowadays, radiation treatment such as UV, alpha and plasma... are used to improve the adhesion of paint film to polymers. Many surface analyses techniques exist. The contact angle measurement seems to be an appropriate and easy-to-use technique for evaluating the surface modifications produced by these treatments.
This paper gives the results obtained using the drop-angle measurement (angle measurement or calculation of the surface energy) on polymers versus the treatment conditions (type of radiation (alpha, UV), duration, atmosphere...). These results have been compared to those obtained by other techniques, notably using infrared spectroscopy (FT IR). In this paper, a strong correlation is shown between the apparition of certain functional groups, the evolution of the surface's wettability and the film adhesion. However, in certain cases, the contact angle measurement no longer appears as a surface state characteristic of the material due to its degradation (cracking, porosity).
Consequently, it is necessary to use complementary methods such as, for example, SEM imagery to characterise the surfaces of materials that have been greatly modified.
1) Department of Chemistry and Physics, The Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK.
2) Unilever Research Laboratory, Quarry Road East, Bebington, Merseyside L63 3JW, UK.
Measurement of Particle and Oil Deposition on Fibers
A technique for the quantitative investigation of wetting dynamics and droplet or particle deposition/removal on fibres is discussed. The method presented uses light from a laser diode, which is guided down a textile fibre. Transmission losses are dependent on the type and extent of droplet or particle coverage of the fibre surface. Changes in light transmission can be induced by deposition of droplets of the dispersed phase of an emulsion, by the removal of particles or droplets from the fibre surface, or from changes in the refractive index of the fluid in which the fibre is immersed. These changes in transmission intensity are monitored in real-time using a lock-in amplifier principle. Experiments for polyester fibres of diameters 80 m and 800 m immersed in a range of oils, for the evaporation of droplets of water from the fibers and for fibers in an emulsion system are presented. The results are discussed in terms of the droplet shape, droplet kinetics and the extent of fiber coverage by droplets.
A Phase Transition in thin films of an n-Alkane
Thin films of n-octadecane adsorbed on mica surfaces from vapour close to the bulk melting point (Tm) have been studied in a Surface Force Apparatus. Using data on the growth rate of capillary condensates between the mica surfaces in contact and film thickness measurements we have identified a transition in the structure of the adsorbed films a few degrees above Tm. As T decreases the alkane layers appear to undergo a transition to a more ordered structure, akin to the postulated surface freezing of long-chain liquid n-alkanes. Analogous results have been obtained with n-hexadecane and n-heptadecane adsorbed on mica.
We have also studied the surface tension of n-octadecane in the vicinity of the bulk melting point using both the maximum bubble pressure and Wilhelmy plate methods. The onset of surface freezing is indicated by a sharp drop in surface tension at a constant temperature. This transition is accompanied by an increased film stability resulting in longer bubble lifetimes at the liquid surface. Variations in bubble lifetime reflect changes in the mechanical properties of the interfacial film from liquid-like to solid-like at the lower temperatures. Our results show that the bubble surface may be supercooled below the surface freezing temperature.
The temperature hysteresis observed both at the alkane-vapour interface and with the alkane films adsorbed on mica show that the phase transitions are first order.
The Greening of Contact Angles: Measurements of Living Cells and Tissues
Over the past 50 years, the use of contact angle measurements to discern interfacial properties of synthetic materials has had strong proponents in both theoretical and technology fields. Applications-driven research led by Zisman and others, for instance, produced significant advances in coatings and low-friction polymers. Theoretical developments by Fowkes, Good, Kaelble, Neumann, van Oss, Wu, and others generate vigorous debate, as different approaches to dissection of surface "polarity" are evaluated. Requirements for equilibrium at the triple interface, absence of liquid/solid solubility or swelling, and surface homogeneity (e.g. chemical, textural) have been well-respected by all of these pioneers. Within the past two decades, however, there has been a "greening" of contact angle measurement, in the sense that newer practitioners of the science have chosen to use contact angle data to attempt to understand interfacial properties of more challenging materials such as living cells and tissues. In doing so, some have cast off the demands of prevailing theory, at considerable risk to physical soundness of the conclusions drawn. The extension of contact angle measurements to the estimation of surface energies and critical surface tensions of bacteria and mammalian tissues (e.g. teeth, blood vessels) has led to exciting new concepts in biomedical and environmental engineering. Data taken under nonequilibrium conditions between diagnostic fluids and surfaces, surprisingly, often have been more informative than results developed from only equilibrium contact angle data. The next generation of scientists and engineers introduced to contact angle methodologies must come to understand the power of theoretical restrictions and benefits of "greening" flexibilities, both.
Contact Angle and Force Measurements on Carbohydrate Coated Surfaces
Water-soluble carbohydrates and glycosaminoglycans (GAGs) covalently bonded to substrate surfaces are of great interest in several different fields of the biomedical devices industry. In aqueous environments, they yield highly hydrated interfaces and can engage specific interactions with biomolecules.
From the point of view of contact angle measurement, this kind of surfaces are very interesting both for practical and theoretical reasons. In particular, the role of hydration in the properties of these surfaces is well acknowledged. It must be underlined that, when probed by water contact angle measurement, these surfaces do not fulfill all the requirements of Young surfaces. Thus, it is of interest to evaluate what kind of information can be obtained from methods based on contact angle measurements.
In this communication, we present our results on the characterization by contact angle and Atomic Force Microscopy (AFM) of surfaces bearing covalently linked alginic acid and GAGs overlayers. X-ray Photoelectron Spectroscopy (XPS) and cell (fibroblasts) adhesion testing were used to confirm the surface chemistry and to evaluate the cell-resistant properties of these surfaces.
AFM force-distance curves, obtained in 1-10 mM NaCl solutions, show that, in every case, carbohydrate and GAGs linking results in the formation of a soft, compressible aqueous/solid interface. All tested surfaces can generically be defined "hydrophilic", with advancing water contact angles below 30 deg., yet both the adhesional interfacial forces measured by the AFM and cell-adhesion tests show that they have different interfacial properties.
Contact angle measurements with a set of liquids with different Lewis acid/base properties (ethylene glycol, low molecular weight polyethylene glycol, dispersive liquids) and that yield Young interfaces with the tested samples were performed. These measurements allow to highlight the different contributions to the surface field of forces of the tested surfaces, even if a complete description of their cell-resistant properties by contact angle measurement alone was not possible.
Effect of Solvent Treatment on Wetting and Adhesion Properties of Ultra-high Strength Polyethylene (UHSPE) Fibers
The effects of 1,2,3,4-Tetrahydronaphthalene (tetralin) solvent treatments on surface characteristics and adhesion of ultra-high-strength polyethylene (UHSPE) fibers to epoxy resin were studied. The paper will discuss the tetralin treatment of SpectraTM 1000 (UHSPE) at various temperatures for different lengths of times and its effect on fiber characteristics. The changes in the fiber surface topography were investigated using scanning electron microscopy (SEM). Surface chemistry changes were characterized by FTIR spectroscopy in the attenuated total reflectance mode. Dynamic wettability measurements were made using Wilhelmy technique. The fiber/epoxy resin interfacial shear strength (IFSS) was evaluated by the single fiber pull-out test. The fiber surfaces after the pull-out test were also characterized by SEM. The SEM photomicrographs and wettability data show that the surface roughness of the fibers increases after the tetralin treatment. However, no significant changes were seen in their surface energies or acid-base characteristcs. The IFSS results indicated that the Tetralin treatment improves the adhesion strength of UHSPE fibers with epoxy resin. Surface topography of the pulled out fibers indicated an adhesive mode of failure.
Experimental Studies and Modeling of Stability of Liquid Coatings on Fibers.
We present the results of comprehensive studies on stability of liquid films on fibers. Dynamics of coating rupture, formation of microdroplets, their evolution and coalescence were explored with a computerized visualization system. A series of short movies reveals subsequent stages of this one-dimensional Ostwald ripening. The thermodynamic conditions were established for critical regimes of film stability for different liquid-solid pairs and the characteristic time of the instability development was determined. The film-fiber adhesion is accounted for in terms of the disjoining pressure which includes the long-range and short-range components. A hydrodynamic model for droplet coalescence was elaborated. Examples include low-viscosity and high-viscosity liquid coatings on wettable and non-wettable substrates. The role of hydrodynamic conditions in stability of coatings on moving fibers is analyzed with applications to spin finishing. Special attention is paid to the factor of fiber curvature. It is shown that as the fiber diameter decreases to the nanoscale, a transition from wetting to non-wetting may occurs. This effect may be critical for nanocomposite fabrication.
Initial Contact Angle Measurements on Variously Treated Dental/Medical Titanium Materials
The attachment of cells onto titanium surfaces is clearly an important phenomenon in the area of clinical implant dentistry. A major consideration in designing implants has been to produce surfaces that promote desirable responses in the cells and tissues contacting the implants. In this study, three titanium materials (commercially pure ,Ti-6Al-4V and TiNi) are treated mechanically, chemically and mechano-chemically to produc different surface concave roughness. Using four media (distilled water, 1% NaCl, suspension of human neutrophils, suspension of osteosarcoma cells), the initial surface contact angles were measured. It was found that (1) the contact angle increases linearly with average roughness when the angles are higher than 45 degrees, while (2) it decreases linealy with roughness when the angle is less than 45 degrees. It was suggested that surface crystallography may influence these findings.
Adhesion of Bilayer-Forming Lipids onto Polymeric Films
The adsorption of bilayer-forming lipids from vesicles onto polymeric films obtained by spin-coating was investigated by measuring contact angles (water droplets method) and ellipsometry. Dioctadecyldimethylammonium bromide (DODAB) vesicles (0.3 mM) and spin-coated polystyrene sulfate (PSS) films interacted in water at 25 oC for 3 h. Under air, the mean average thickness (D) of the dried adsorbed layer is (2.3 0.2) nm. The advancing (A) and receding angles (R) are (75 4)o and (69 4)o, respectively. D values and the low wettability indicate the formation of a DODAB monolayer with polar groups oriented to the PSS surface and the hydrocarbon chains oriented to the air (adsorption being driven by ion-pairing between cationic lipid polar heads and the anionic surface).
Polystyrene amidine (PSA) films spin-coated on bare silicon or silanized wafers interacted with phosphatydilcholine (PC) vesicles (0.02 -0.5 mM) in water at 25 OC. D for the dried PC adsorbed layer on PSA or PSA/silanized wafers are (2.5 0.2) nm, i. e. a monolayer, or (5.4 0.5) nm, i. e. two monolayers, respectively. The wafer silanization orients the polystyrene chains to the SiO2 surface and terminal amidines to the air. In water, PC adsorbs onto PSA as a bilayer driven by complexation between amidine and phosphatidyl moieties. In air, a second monolayer deposition occurs due to van der Waals attraction to the first one. The A contact angle for PSA surface without silanization is 90 o (terminal amidines oriented to the wafer and polystyrene chains to the air); PC adsorbing due to the hydrophobic interaction between lipid hydrocarbon chains and PS. The lipid assembly at the polymer surface (monolayers, bilayers or two monolayers) depends on nature of interaction forces between vesicles and surfaces.
(1) MPI fuer Kolloid- und Grenzflaechenforschung, 14424 Potsdam, Germany
(2) Universitaet Ulm, Abteilung Angewandte Physik, Albert-Einstein-Allee
11, 89069 Ulm, Germany
Three-Phase Contact Line Energetics from Nanoscale Liquid Surface Topographies
The contact line tension of various three-phase systems (solid-liquid-vapour) are determined from the liquid surface topography data obtained with TappingMode Scanning Force Microscopy (SFM). Due to the nanometer resolution of the SFM technique, the liquid structures can be analysed in two completely complementary ways, one of which is based on the modified Young equation the other on the effective interface potential derived from the profile of the vapour-liquid interface in the three-phase region. Both approaches agree quite well for the systems investigated. Contact line tensions are in the range of 10e-11 J/m to10e-10 J/m which is consistent with theory. Futhermore, the measured effective interface potentials can be explained by standard molecular interaction models.
Surface Energy and Surface Roughness Changes Produced by Irradiating Polymers with Ultraviolet-Ozone
The surface energies of PMMA [Poly (Methylmethacrylate)] and Polycarbonate which have been exposed to UV radiation in an ambient ozone-air atmosphere have been elucidated from surface tension and drop contact angle data using the test liquids ethylene glycol, formamide, glycerol, methylene iodide and water. Comparisons of surface energy values obtained using Kaelble's two liquid method and Good's three-liquid method are reported and it is tentatively suggested that atmospheric moisture may play a role in producing discordant values since the test liquids ethylene glycol, formamide and glycerol are highly hygroscopic in nature,
It has been demonstrated that UV/ozone irradiation produces changes in surface roughness. NVII shows three distinct regions, firstly where at low irradiation times the surface roughness is enhanced, Following this, the roughness lessens before increasing finally to a terminal value. The behavior is somewhat similar for Polycarbonate although the dramatic increase in roughness exhibited by WIN is absent. The roughness characteristics are quite different for PMMA where a large change iii roughness is observed at one specific radiation time. Thus presently it is not possible to predict surface roughness changes for a particular polymer and more studies on the morphological changes occurring at the surface are being made.
*(National Center for Microgravity Research)
Fluid Film Pulling in the Presence of Evaporation
Volatile PDMS oils form films on SiO2 substrates when the substrates are pulled out of the bulk fluid. The films have constant length after steady state has been reached. The behavior of the evaporative film is fundamentally different from that of the non-evaporative film. The latter has a infinite length as the substrate is pulled out of the bulk fluid and the shape near the bulk meniscus is described by BrethertonÕs model. While the former has a finite length in steady state and has a different shape from that described by existing models for nonvolatile fluids. We investigated the effect of evaporation rate, fluid viscosity and withdrawing velocity on the length and volume of the film. We found that the length and volume each has a power law dependence on the substrate velocity. We also found that the shape of the film is compatible with a simple model of a film of evaporating fluid. Moreover, we found that the static contact angle and the Marongoni effect play important roles in the stability of the film.
Surface Driven Spreading on Hydrophobic Surfaces
Because the initial spreading coefficient is highly unfavorable, spreading of water over hydrophobic substrates is not possible unless surfactants are present in the water drop. We study the spreading of aqueous surfactant-laden droplets on liquid alkanes (dodecane, tetradecane, and hexadecane) and on mineral oil. Only at concentrations above the critical spreading concentration where the aqueous solution contains a dispersion of vesicles and liposome aggregates does spreading commence. Using videomicroscopy, we then follow the spreading dynamics as a function of surfactant type and concentration, drop volume, subphase type, and environment humidity. We find that no precursor film forms ahead of the main droplet, and that droplets eventually cease spreading to form equilibrium lenses.
We argue that the rate of surfactant arrival at the stretching air-water and oil-water interfaces determines the droplet spreading kinetics. As the drop expands, the interfacial areas increase, in turn demanding more surfactant adsorption. We hypothesize that spreading takes place at nearly a constant adsorption amount corresponding to a spreading coefficient that is positive but close to zero. We quantify this new surfactant-driven spreading mechanism by solving the convective-diffusion equation for a model expanding cylindrical-disk lens. All experimental observations are explained by the proposed surfactant-driven spreading mechanism including quantitative agreement with the measured spreading rates. New studies on surfactant-laden aqueous drops spreading on hydrophobic solids surfaces confirm similar surfactant rate-limited spreading kinetics
Wetting Topology and Molecular Ordering of Alkanes at Sio2/air Interfaces: Rolling Hills, Surface Freezing, and Eggs, Sunny-side up
Long-chain alkanes (16 to 50 C-atoms) show interesting wetting topologies and molecular orderings at SiO2/air interfaces. Well above the bulk melting temperature one observes a (nearly) complete wetting film of liquid alkane with a "rolling hill" topology. Droplets with very small contact angles coexist with a film of liquid alkane of about 16nm thickness. In a temperature range between bulk melting and several degrees above, droplets of liquid alkane coexist with a condensed alkane monolayer at the interface ("surface freezing"). In the monolayer the molecules are oriented upright and densely packed. If the amount of alkane which is spread on the surface is not sufficient for one complete monolayer, the alkanes are still oriented upright. They aggregate into dendritic and/or seaweed-like domains. Below the bulk melting temperature the alkane droplets freeze into amorphous/polycrystalline bits. If the temperature is kept for some time close to, but below the bulk melting temperature, these frozen droplets can be annealed. They form mesa-shaped islands consisting of stacks of alkane lamella oriented parallel to the interface. Often one finds the rest of the original frozen droplet on top ín the center of the islands ("eggs, sunny-side up"). Alkanes as simple molecules are destined for fundamental wetting studies. We are investigating how their wetting topologies and wetting kinetics are correlated to the molecular ordering and what forces influence them.
C. Merkl, T. Pfohl, and H. Riegler (1997), Phys. Rev. Lett. 79(2): 4625
A. Holzwarth, S. Leporatti, and H. Riegler (1999), submitted
Simulations of Contact Line Motion on Disordered Surfaces
The equations of motion for contact lines on disordered surfaces were solved numerically. The amount of contact-angle hysteresis, morphology of the contact line, and contact-line velocity were studied as a function of the strength and distribution of the disorder. Near the onset of motion, the contact-line is a self-affine fractal characterized by a roughness exponent of 0.4. Critical exponents describing the onset of motion, were determined using finite-size scaling. The exponents are universal and will be compared to renormalization group calculations and experiment.
Influence of Silanes on the Surface Charge and Wetting of Industrial E-glass Fibers
Industrial E-glass fibers have been treated with commercial mono- and diaminosilanes in order to modify the wetting behavior and the surface charge. The silanes were adsorbed from both organic and aqueous solutions in order to investigate the influence of the solvent. The properties of the solvents have been related to the dispersivity (hydrophobicity) the acid-base properties and the solubility parameters in order to characterize the adsorption on a common scale. In addition to the recording of the contact angle and the -potential of the fibers the change in surface composition was investigated with ESCA (XPS) spectroscopy and the binding with FTIR spectroscopy.
It was found that silane added in a small amount was firmly bound to the surface influencing all the properties investigated. Thus, in acidic solutions an enhanced cationicity is observed due to a preferential adsorption of the hydrolyzed silanol groups. In alkaline solutions the silane seems to be preferentially adsorbed with the amine groups to the glass fiber surface thus producing an enhanced anionicity due to ionization of silanol groups.
The degree of ionization is dependent on the pH of the solutions with dissolved silane used to treat the glass fibers. However, added in excess a phase separation occurs forming a partly condensated silane skin mechanically bound to the surface. Then the surface properties are determined by the bulk properties of the phase separated siloxane polymer formed. It has been found that in glass fiber reinforced polymer composites the siloxane slin is mixed to an interfacial layer between the polymer and the glass fibers anchored only to a fraction of surface sites. Ureidosilane is divergent from the other aminosilanes in that it behaves as a non-charged polymer surface due to a resonance structure formed between the carbonyle and the amine groups fully neutralizing the cationicity.
1) Laboratoire de Chimiométrie, Equipe de Recherche Technologique
Université Claude Bernard Lyon1 & Ecole Supérieure de Chimie Physique Electronique, 43 Bvd du 11 Novembre 1918, Bât 308 - 69622 Villeurbanne, FRANCE
2) Laboratoire de Tribologie et Dynamique des Systèmes, UMR 5513 -Ecole Centrale de Lyon, 36, Ave Guy de Collongue - BP 163 - 69131 Ecully, FRANCE
Hydrophobic Mechanochemical Treatmentof Metallic Surfaces
The mechanochemical treatment is a new coating technology to produce the characteristics needed for optimal surface function. This simple industrial method has been investigated and tested to obtain hydrophobic, anti-corrosion thin films onto industrial metallic surfaces. This process allows chemisorption of specific functional molecules and simultaneous generates a suitable topography. Wetting techniques were investigated as a potential tool to assess surface properties. It was shown that this cheap, easy and time economical technique could be used to control the homogeneity of the mechanochemical treatment. Subsequently, a design framework correlating topography and wettability with the mechanochemical treatment is needed to understand the mechanisms controlling this treatment and further implement it on an industrial scale. The novel design method relates process and performance based on concurrent engineering. It also embodies three design domains - process, physical, functional - and shows their interrelations. Linear, logarithmic and exponential regressions are the most commonly used statistical methods in surface engineering. No correlation has been established to date between a surface parameter and performance. However, the Partial Least Squares regression remains a promising method for correlating topography to wetting properties. Such a correlation would permit wettability prediction from topography and other processing parameters.
1) UFR Mathématiques, Analyse Numérique, UMR CNRS 5585, Bât. 101
Université Claude Bernard Lyon 1, 43 Bvd du 11 Novembre 1918, Bât 308 - 69622 Villeurbanne, FRANCE
2) Laboratoire de Chimiométrie; Equipe de Recherche Technologique, Université Claude Bernard Lyon1 & Ecole Supérieure de Chimie Physique Electronique, 43 Bvd du 11 Novembre 1918, Bât 308 - 69622 Villeurbanne, FRANCE
On the Effective Equations Describing Spreading of a Small Viscous Drop over a Rough Surface
We consider the spreading of a small viscous drop over a rough surface containing asperities of the characteristic size being smaller than the characteristic size of the drop. The distribution of the roughness is supposed to be statistically homogeneous and ergodic. Under these assumptions we derive a non axisymmetric form of the thin film equation, with a matrix coefficient M giving the effective influence of the roughness.
Determination of the matrix M requires solving the Stokes system with a jump of the shear stress at z=0 and with the no-slip condition for the velocity at the random rough surface. The elements of M are the volume limits of the space averages of the corresponding viscous energy. Furthermore, we generalize the Weissberg-Prager and Rubinstein-Torquato bounds for the random porous media to the case of viscous flows over a surface with random asperities. The obtained bounds for M are compared with experiments.
Surface Hydrophobicity, Protein Adsorption and Mammalian Cell Adhesion
While substratum hydrophobicity has only a moderate influence on the adsorption of collagen from pure solutions, it affects drastically the adsorption of collagen in the presence of competing compounds (Pluronic F68, a PEO-PPO-PEO copolymer surfactant; albumin; constituents of fetal calf serum). The reduction of the adsorption of collagen in the presence of Pluronic F68 is weaker as the oxygen concentration at the polystyrene (PS) surface increases. The reduction is not appreciable on surface-oxidized poly(ethylene terephtalate) (PET) but is very strong on native PET, and on poly(vinylidene difluoride) (PVdF) and bis-phenol A polycarbonate (PC), either native or surface-oxidized.
When polystyrene-based substrata are exposed simultaneously to extracellular matrix (ECM) proteins (collagen, laminin, fibronectin) and Pluronic F68, a weaker substratum hydrophobicity favors adsorption of the proteins and subsequent adhesion of human hepatoblastoma cells derived from the HepG2 line. This occurs both when the substrata are pre-conditioned by a solution of proteins and Pluronic F68, and when the proteins are produced by the cells themselves in the presence of Pluronic F68. On the other hand, when ECM proteins are pre-adsorbed, they are not displaced by Pluronic F68 and cell adhesion is not influenced by substratum hydrophobicity. The influence of surface hydrophobicity is further illustrated by the behavior of WI38 fibroblasts cultured in a serum containing medium: they attach and spread on a silicon wafer; they do not spread (within 24h) on poly(methyl methacrylate) (PMMA), unless the latter is pre-conditioned with a solution of collagen.
The selectivity of the adsorption of ECM proteins (collagen, fibronectin) on substrata of low hydrophobicity, in competition with Pluronic F68, has been used to obtain a selective adhesion of mammalian cells (MSC 80 mouse schwannoma, PC 12 rat adrenal pheochromocytoma, rat hepatocytes) on tracks made by photolithography at the surface of polystyrene.
Several data indicate that the water contact angle is not the only surface property controling protein adsorption selectivity and mammalian cell adhesion. Combined XPS and AFM studies indicated that both surface nanoroughness and hydrophobicity influence the organization of the adsorbed film of collagen, in water or after drying. On the other hand it was observed that rat hepatocytes cultured in a serum-free medium adhered poorly on PMMA, better on PMMA covered by a continuous film of adsorbed collagen, and even better on PMMA covered by a collagen net with meshes of about 1 m. This points to the influence of the organization of the adsorbed protein film on mammalian cell adhesion.
Detection of a Hydrogel-like Layer at the Surface of Oxidized Polyethylene by Dynamic Wetting Measurements
Polyethylene oxidized with sulfo-chromic acid was analysed by XPS and dynamic wetting in water, using the Wilhelmy plate method. Therefore the wetting records were corrected for buoyancy. The contact angle was plotted as a function of the position of the three-phases contact line rather than the position of the water surface, by accounting for the change of meniscus shape.
An oxidation treatment as short as 10 s provoked the appearance of different oxygen bearing functions (ratio O/C 0.15) and produced a more hydrophilic surface. Treated samples showed a variation of the advancing contact angle as a function of position for wetting cycles other than the first one. The influence of cycling rate and surrounding humidity showed that variations were due to a progressive evaporation of water and were indicative of a water retention capacity or a drying time. The effect observed increased with duration of the oxidation treatment.
As the oxidized sample was stored in air for periods of the order of days, hydrophibicity was recovered progressively, as indicated by the advancing angle of the first wetting cycle, and the effect of an evaporation process became less apparent. This trend was reversed when a sample stored in air was subsequently stored in water for periods of the order of days. These variations are explained by the presence of a hydrogel-like layer at the surface of oxidized polyethylene and its swelling and dehydration properties. This interpretation is supported by the variation of the wetting hysteresis loop when water is replaced by an alcaline solution : while the advancing contact angle of the first wetting cycle does not change, the hysteresis loop indicates a more important swelling in presence of the alcaline solution as compared with pure water.
The Use of CCl4 Saline Contact Angle on Contact Lens Surfaces to Study Adsorption from Ophthalmic Solutions
(Abstract not yet available)
des Matériaux, P. M. Fourt, B. P. 87, 91003 Evry Cedex, FRANCE
Wetting Behavior on a Contaminated Surface
In many wetting problems, of a static or dynamic nature, it is generally considered that each phase involved has specific values (or ranges of values) pertaining to its surface free energy and the various contributions thereof. These values are assumed to remain constant with time and thus so do the specific interactions between two given phases in contact. However, many real systems, biological or industrial, involve the presence of a third component (often in small quantities) at, or near, the interface between the two bulk phases (e.g. the sizing on fibres, the presence of oil on metal to be adhesively bonded, contamination on a surface to be painted).
We consider a simplified model of such a system. In particular, we assume a solid surface, S, ideal in all respects apart from its being covered by a very thin, continuous film of liquid 2, on which a drop of liquid 1 is deposited. The initial effective interfacial free energy, solid/drop, is similar to that of the interface liquid 1/liquid 2, yet as time passes, liquid 2 diffuses into the bulk of liquid 1, thus increasing (by hypothesis) the solid/drop interaction whilst spreading occurs. The concomitant effects of diffusion and wetting lead to modification of the latter. Not only is the spreading process accelerated, but the wetting front may extend beyond its equilibrium position before starting to recede to its final configuration. It is suggested that this (naive) model could be exploited to improve the efficiency of surface coverage in some applications.
Instability and Rupture of Thin Liquid Films by Contact Angle Gradients on Chemically Heterogeneous Surfaces
It is now well understood that the characteristic length and time scales of the spinodal instability of thin liquid films on nonwettable homogeneous surfaces depend on the equilibrium contact angle. However, most naturally occurring or deliberately patterned substrates are chemically heterogeneous, and show a spatial gradient of the contact angle. We propose (Phys. Rev. Lett. 84, 931, 2000), and simulate a new heterogeneous mechanism of thin film instability and rupture engendered by the wettability contrasts on mesoscales. The length and time scales of this instability depend on the spatial variation of the contact angle, rather than the contact angle itself. Thus, films on completely wettable surfaces are destabilized by the presence of small nonwettable patches, and even more interestingly, the lifetimes of films on nonwettable surface are reduced by orders of magnitude by the inclusion of small wettable patches. Simulations reveal that the following morphological features contain an unambiguous signature of the presence of heterogeneities: (a) absence of spinodal surface undulations prior to the birth of a hole, and (b) formation of complex and locally ordered morphologies, e.g., a "castle-moat" pattern, concentric "ripples", and "flowers". These features are frequently witnessed in experiments, but cannot be reconciled to the theory of spinodal dewetting (Phys. Rev. Lett. 81, 3463, 1998) on homogeneous surfaces.
Mukul M. Sharma; Departments of Petroleum, Geosystems and Chemical Engineering, University of Texas at Austin, Austin, TX 78712
Measurement of Critical Disjoining Pressure for Dewetting of Solid Substrates
This paper presents a new technique for measuring the capillary pressure required to rupture thin aqueous films on solid substrates. The method involves the direct measurement of surface forces between a probe (glass sphere or oil droplet) and a solid substrate immersed in a fluid phase using an Atomic Force Microscope. The data can be directly used to measure the critical disjoining pressure at which metastable thin films are destabilized. The magnitude of the critical disjoining pressure pressure is a quantitative measure of the stability of the thin film.
Results are presented for the dewetting of mica and glass substrates immersed in brine. The critical disjoining pressure required to rupture a brine film between mica and two crude oils (Moutray and Prudhoe Bay) are reported for different salinities and pH. It is found that while the pH dependence is consistent with DLVO theory predictions the salinity dependence is not. Experiments conducted with polar and non-polar components of the crude oil show that critical values for the polar fractions agree with DLVO theory while those for the non-polar fraction do not. These observations may be attributed to hydrophobic interactions between the oil and the mica surfaces.
It is shown that the critical capillary pressure provides a direct indication of the height above the water table at which mixed-wet conditions may be expected in the oil reservoir. For example, the wettability alteration predicted for Prudhoe Bay agrees with the wettability measurements reported by ARCO as a funtion of height above the water table.
Determination of Wettability, Tack and Initial Adhesion of Hot Melts and Pressure Sensitive Adhesives from Viscosity Units
All measured hotmelts and pressure sensitive adhesives show more or less thixotropic relationship and a flowlimit. Generally the flowlimit characterizes the limit between the solid an the liquid phase. The flowlimit can be measured by decreasing shear stress and evaluated with the following formula:
D= (tau - f)/(eta0) + ((tau - f)exp(-k/(tau-f)))/etad (1)
Hypothetical bals of glue have to be fluid being able to wet substrates, therefore they have to surmount their own flowlimit. The resulting models of calotte or spherical segment have a plane base. The shear stress of the weight of the ball and the area of plane base i in balance identical with the flowlimit. This is the condition for calculating the contact angle (beta) and also the wettability of glue 1/2(1+cos(beta)).
Formula 1 gives also the viscosities of first and second Newtonian section eta0 and etainfinity the last with the formula:
(1/etainfinity) = (1/eta0) + (1/etsd) (2)
Tack can be calculated with the following formula:
Tack = fx(½)(1 + cos(beta))xetainfinityx(1/eta0) (3)
Tack and initial adhesion are similar if you have an ideal substrate. For non-ideal substrates you have to calculate in formula 3 with the real contact angle to get initial adhesion.
The development of the formulas and their use will be shown in a compact manner as well as the resulting conclusions.
Characterization of Non-uniform Wettability on Flame-treated
Flame treatment of polypropylene (PP) film involves the use of impinging, conical flames to oxidize the surface of the PP. The inhomogeneous environment of the conical flames creates non-uniformities across the surface of the film. The wettability of a flame-treated surface can be difficult to measure because there are alternating "lanes" of more-or-less oxidized polymer. The Wilhelmy plate technique of measuring the contact angle of water in air yields all of the information needed to quantify these non-uniformities. The flame-treated substrate used for the contact-angle measurements is carefully prepared so that the lanes are parallel to the surface of the water as the sample is being advanced and retracted from the liquid. The microbalance measures the forces acting on the surface of the substrate as a function of the position of the PP in the liquid. These force measurements can then be used to calculate contact angles at particular positions on the substrate. The force measurements can also be manipulated to provide a quantitative indication of the uniformity of flame treatment. Use of this method enables determination of the optimal flame-to-film separation to obtain uniform surface treatment.
Wetting of Solids by Binary Surfactant Solutions: Synergistic Effect
The traditional techniques of wetting control are limited in many respects. However, a new approach has been suggested recently, which utilizes synergistic effects between surfactants. The target of this research was the wetting of hydrophilic (glass) and hydrophobic (polystyrene) substrates with binary surfactant aqueous solutions. The surfactants were cationic - cetyltrymethylammonium bromide (CTAB) and non-ionic Triton X-100. The most variable parameters were the mole fraction of CTAB; its values were limited 0; 0.2, 0.5, 0.8 and 1.0 only CTAB. The total surfactant concentration C varies in the 10-8 - 10-2 M range. Contact angles were measured by sessile drop (advancing) and captive bubble (receding) techniques. Experimental data were compared with contact angle calculations, which were fulfilled for " ideal behavior" model. It supposes the absence of synergetic effects for surfactants mixture. The observed contact angles on the glass surfaces were lower then the calculated ones. The largest divergency occurs in the concentration range of 10-4 - 10-3 M. Corresponding to micellazation in these mixtures. For polystyrene the dependence of contact angles versus mole fracture CTAB correlates well with the surface tension isotherm. The large hysteresis between advancing and receding contact angles was observed for C = (10-8 - 10-5) M, and probably is the result of nonuniform structures of surfactant layers adsorbed on solid surfaces.
Wetting in Protein Systems
This report presents a review of the basic regularities and peculiarities of the wetting of protein substrates and contact angle of protein aqueous solutions on varoius solid surfaces (hydrophilic and hydrophobic). Three proteins were used: gelatin, serum bovine albumine, and - chemotrypsine. Three types of protein substrates were studied:
1. Gels with different structures and mechanical properties ("soft" and "hard"),
2. Protein adsorption layers previously formed at the different interfaces (aqueous solution/air or organic liquid) and transferred later on various solid substrates (glass, metals, polymers),
3. Protein adsorption layers which were formed in the aqueous solution directly.
The main results:
The strong effect of "soft" gel surface deformations on contact angles was observed systematically. These data are corrected sufficiently with A.I. Rusanov theory for deformable surface wetting.
Conformations of protein macromolecules (helix or globule) strongly influence contact angles. Especially sharp effects were observed for transferred adsorption layers. Clean correlation was established between contact angles and proportions of hydrophilic/hydrophobic groups in the protein macromolecules.
Wetting in protein systems is sensitive to temperature and pH changes. Also some additives (e.x. carbamide) effectively influence the contact angles, especially on the protein gels wetting ability.
Contact Angle Hysteresis and Surfactant Adsorption in the Vicinity of the Three Phase Line.
The correlation between contact angle hysteresis and the surfactant adsorption in the vicinity of three phase line (TLP) was investigated using sessile drop and autoradiography techniques.
Advancing and receding contact angles were measured in glass capillaries for aqueous solutions of cetyltrimethylammonium bromide (CTAB) in a wide concentration range 10-9 and 10-2 M. The sessile drop technique was used for contact angles on glass, quartz, paraffin and PTFE substrates while the structure of the adsorption layers was studied by autoradiography using tritium labeled surfactant. As a rule, the adsorption layers were not continuous. In fact, quite the opposite of the island surfaces was observed; the solid surface in the vicinity of TPL was heterogeneous. The number of islands and their dimensions were especially large near the TPL and increased with an increase in surfactant concentration. Simultaneously, the contact angle exhibited the tendency to hysteresis continuously increase, as the adsorption layer near TPL became more non-uniform.
Apparently the adsorption layer structure at large distances from TPL does not influence on the contact angle hysterasis in such extent as in the vicinity of TPL.
*(National Center for Microgravity Research)
Hydrodynamics Controlling Contact Angle Relaxation
Contact angles describe how a fluid wets a solid surface. For steadily moving contact lines, dynamic contact angles have been examined for a variety of material systems. However, in nature and in technological applications, unsteady contact line frequently occurs. We have studied the relaxation of contact line motion and dynamic contact angle after an abrupt change in the relative velocity of the solid surface and the fluid body. In our experiment, unsteady effects are small enough that, for most times, contact line motion can be described as StokeÕs flow and the contact angle relaxation can be described with a quasi-steady state model. A relaxation time for the contact line motion can be measured and modeled. However, at early times after the surface velocity change, inertial effects can be detected.
Controlling Mechanisms and Spreading Transitions in Reactive Wetting
Reactive wetting, in which a chemically active element is added to promote wetting of noble metals on nonmetallic materials, is a very important technological phenomena in many processes like brazing, glass/metal bonding, infiltration, liquid phase sintering and many others. From a fundamental point of view, the main goal is to identify, measure and model the atomic mechanisms that control reactive wetting. From a technological perspective the main goal is to sort out the material properties that dominate reactive wetting on a macroscopic scale.
In this work, the energetics and kinetics of necessary steps involved in spreading are outlined for comparison to those for formation of the compounds that typically accompany reactive wetting. These include: fluid flow, triple line ridging and active metal adsorption. Under certain conditions all of these can be faster than compound nucleation. Thus, situations of liquids spreading ahead of the reaction, with energetics dictated by a metastable interface with very low associated interfacial energy are regarded as highly likely; this may be more typical than believed, and perhaps the most effective regime. In this case spreading kinetics will be dictated by the adsorption kinetics or the movement of the triple line ridge. From the analysis, a rational or variability and hysteresis in wetting also emerges.
Determination of Contact Angles in Different Size Pores of a Material
Two methods for contact angle determination were developed based on the liquid porosimetry (LP) data. The LP measures advancing and receding capillary pressures of different size pores in a porous structure. Any liquid that wets a sample (water, organic liquids, solutions) including liquids in practical applications can be used. The automated TRI/Autoporosimeter is used for measurements. The LP experiment starts in the advancing mode and continues into the receding mode.
The first method uses two liquids. One liquid has a known contact angle with the sample. The second is a liquid of interest. The comparison of the capillary pressures for two liquids provides the contact angle data for different size pores.
The second method uses one liquid. The LP experiment starts in the advancing mode with a dry sample, continues into the receding mode, then continues into a second advancing mode with a sample that is already wet, and finishes in the second receding mode. The comparison of the capillary pressures of different size pores for the dry sample with the wet sample provides the contact angle data for different size pores in the sample.
The contact angles of different size pores of tested samples are presented.
Influence of Plasma Treatment on the Wettability of Composite Membranes
(Abstract not yet available)
1) Department of Microbiology, State University of New York at Buffalo, Buffalo, New York 14214-3000
2) Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14214-3000
3) Department of Geology, State University of New York at Buffalo, Buffalo, New York 14214-3000
Influence of The Water-Air Interface on The Apparent Surface Tension of Aqueous Solutions of Sugars, Polysaccharides, Proteins and Surfactants
Sugars such as sucrose and glucose, when dissolved in water, increase its measured surface tension. Polymers of sucrose and glucose, on the other hand, cause a decrease in the surface tension of their aqueous solutions. From the surface tension properties of solid layers of dried sucrose and glucose, and from the aqueous solubilities of these two sugars, their surface tension components and parameters in the dissolved state could be determined. It follows from these that the surface tension of these sugars in the dissolved state is about twice as high as that of water. From the surface tension data of both sugars it also follows that their molecules are repelled by the water-air interface, which leaves a thin zone inside the water-air interface that is depleted of sugar molecules, which gives rise K a measured surface tension for these
aqueous sugar solutions that is only a few mi/m 2 higher than that of water, instead of more than 70 mJ/m2 higher (for 100% sugar). The cause of .the high sur-
face tension of sugars in the dissolved (liquid) state lies in the elevated free energy of cohesion between the electron-acceptor and the electron-donor sites of
molecularly dissolved sugar molecules, as manifested ' by a very elevated Lewis
acid-base (AB) surface tension component, gamma(AB) , of about double that of water. In contrast with these sugars, with their polymers the strong electron-donor/
electron-acceptor interaction energies ' no longer occur which lends ficoll
and dextrin a gamma(AB) value of only about 40% of that of water. These polymer molecules are also repelled by the water-air interface, so that the decrease in the measured surface tensions of their aqueous solutions is again-rather modest.
Partly hydrophilic/partly hydrophobic solutes such as proteins and surfactants are attracted to the water-air interface. Their bulk solution comprises a (rather dilute) concentration of solute, but the solute concentration at the water-air interface tends to be much higher than the solubility limit, with the hydrophobic moieties of these molecules protruding into the air. This causes a drastic apparent decrease in the measured surface tension of the aqueous solution, generally already quite noticeable at very low solute concentrations.
The high bulk liquid surface tension (i.e., the surface tension on the aqueous side of the depletion zone) of aqueous sugar solutions enhances their solvophobic capacity, so that for example, a 55% (w/w) sucrose solution can flocculate suspensions of mineral particles (e.g., silica, clay particles), rich are stable in pure water. Many inorganic salts, in aqueous solution, have the same property.
Thus, whether solutes are repelled by the water-air interface (e.g., sugars, polysaccharidesi inorganic salts), or attracted to that interface (e.g., proteins, surfactants), the measured apparent surface tension of any such an aqueous solution is not proportional to the free energy of cohesion of the bulk liquid. Therefore, in all cases contact angle measurements done with non-homogeneous liquids give misleading results; only pure, single, liquids may be used for contact angle measurements, to obtain meaningful surface tension data.
A Study of Contact Angles in Optoelectronic Materials
Contact angle measurements between were made to allow us to study the effect of wavelength of illumination on the surfaces of hydrogenated amorphous silicon thin films, p-type crystalline silicon, and diamond thin films. Advancing and receding contact angles of polar liquids on diamond, and on etched and non-etched silicon surfaces were measured by the Sessile drop and the Wilhelmy plate methods at three wavelengths: red (640.4 nm), green (536.2 nm), and violet (402.8 nm). The results indicate that the observed contact angles of pure water strongly depend on both the wavelength of the illumination used and the condition of the sample surface. The type "p" or "n" of each material surface was also determined as a function of the species present on the samples. These surface phenomena, related to the surface photovoltage effect and the electric double layer, were seen to be completely reversible. The Lippmann-Young equations were used to explain these interfacial effects and may be useful for the study of the behavior of optoelectronic material surfaces.
R. Vera-Graziano, S. Muhl, and F. Rivera-Torres, J. Coll. Interface Sci., 155, 360-368 (1993).
R. Vera-Graziano, S. Muhl, and F. Rivera-Torres, J. Coll. Interface Sci., 170, 591-597 (1995).
R. Vera-Graziano, F. Rivera-Torres, and A. Ordoñez-Medrano, Revista Mexicana de Física, 45, S-1, 174-178, 1999.
Evaluation of Wetting Properties of Powders from Gas Adsorption Experiments
It is generally assumed that spreading pressure can be neglected when deriving surface free energy of hydrophobic samples from contact angles measurement. In the case of powders, water adsorption isotherms reveal that non negligible amounts can be adsorbed, even on low energy surfaces. It is for instance the case of talc. Although this mineral is considered as a hydrophobic clay mineral, with a static water contact angle arround 85° on basal faces, the features of water vapor adsorption isotherms correspond rather to a hydrophilic surface. Water vapor adsorption isotherms then appear as not suitable for obtaining univocal information about the hydrophobicity of powder surfaces.
The fact that hydrophobic powders can adsorb large amounts of water vapor means that adsorption sites exist on their surfaces. The distribution and energy properties of these adsorption sites can be analyzed using argon and nitrogen low pressure quasi-equilibrium adsorption volumetry. Such experiments were carried out for powders of low and high surface energy such as hydrophilic and hydrophobic clay minerals, calcium carbonates and silica samples. The surface energy distributions calculated from argon and nitrogen isotherms were compared with water vapor adsorption isotherms to derive information on surface wetting properties.
A Critical Analysis of the "Equation of State" Theory
The so-called "equation of state" is one of the theories on the subject of surface energetics. Along the last years the debate among researchers supporting this point of view and the defenders of the most common "multicomponent approach" has often been very intense. The debate among researchers with different points of view can be useful if it is done without prejudices, but only considering the actual advantages and difficulties of the different theories. On this conceptual basis, in the present paper we have tried to analyse the "equation of state" theory from a thermodynamic and numerical point of view to test its internal coherence and some of its experimental and practical conclusions. Some remarks are developed on the comparison with the acid-base theory.
Water: How "Dispersive" it Is and Other Stories
Among the supporters of acid-base theory of surfaces there is a experimental datum which appears as a "taboo": the evaluation by Fowkes of the dispersive component of water, made in a famous paper about 40 years ago. The dispersive component of water was evaluated as 21.8 mN/m with a very low standard deviation. In the present paper we show as this estimate is probably wrong because Fowkes neglected the spreading pressure of hydrocarbon on water surfaces. The introduction of this evaluation allows us to calculate a larger value of the water dispersive component and this fact has important consequences, because water and its properties represent the most common reference scale for the acid-base properties of solid surfaces. Some general comments on the use of the spreading pressure in acid-base theory are also developed, commenting on recent papers on the subject.
1) Wood Technology and Processing, KTH - Royal Institute of Technology,
SE - 100 44, Stockholm, Sweden. E-mail: firstname.lastname@example.org
2) University of Maine, Advanced Engineered Wood Composites Center,
Orono, ME 04469, USA
Factors Influencing the Determination of Wood Wettability:
- Extractives Contamination, Wicking, and Bulk Sorption Effects
Based on wetting studies by column wicking and the Wilhelmy plate method, the following three factors have been determined to influence the interaction between wood and probe liquids:
The first factor concerns the risk of dissolution of extractives in the probe liquids which in most cases distinctly decreases their surface tension. The second factor concerns capillary transport of the probe liquids into void spaces of wood and along the surface structure in the fiber direction which result in a pronounced non-equilibrium state. The Wilhelmy wetting force measurements indicate that an initial inertia effect may be present that is mainly dependent on the density of the liquid, and distinctly increases the initial force acting on a specimen just when the wood specimen contacts the liquid. The third factor concerns the observation of bulk sorption during wetting analyses of wood particles by column wicking. This adsorption process and resulting swelling of the wood substance is indicated as a distinct release of heat inside the columns. It is also suggested that vapor is moving in advance of the visible liquid front causing 'pre-wetting' of the wood particles.
Colloidal Fluids Wetting Solid Surfaces
The wetting of self-organized structures, such as, colloidal particles and surfactant micelles, assume ever increasing importance in current and emerging technologies. The film thickness stability and the microstructure of these liquid films containing colloidal particles are important for understanding the liquid wetting, spreading and adhesion phenomena on a solid surface and have implications for building nanostructured materials on a solid surface. Recently, we have used reflected light video-microscopy to directly observe the colloidal particle structuring phenomena in the liquid film-meniscus region. The wedge film was found by blowing from an aqueous suspension of monodispersed and charged latex spheres against an optically smooth glass plate. Particle radial distribution functions were measured, which showed the coexistence of two-dimensional hexagonal and cubic packing structures in the wedge film. We also monitored particle oscillations due to Brownian motion and noted that the particle mean square displacement for the ordered and disordered structures inside the film-meniscus region. We calculated the particle-particle interaction potential from the measured radial distribution function and found that, due to the particle crystallization in the wedge film, the particle interaction potential becomes oscillatory and, therefore, the film disjoining pressure versus film thickness has an oscillatory decay. The impact of structural disjoining pressure due to particle ordering in the confined geometry of the three-phase contact region on improving wetting characteristics of colloidal fluids on solid surfaces will be discussed.
Surface Properties of Alkyl Amines as Revealed by Contact Angle
And Interaction Force Measurements
Fatty amines are widely used as collectors in flotation of non-sulfide minerals. It is believed that at higher pH and/or higher concentration precipitation of the collector colloid as free amine occurs at the surface of floated mineral. Unfortunately, the surface properties of these colloids have not been thoroughly studied. In the present work we studied the wetting characteristics of the amine surface in aqueous solutions at different pH values using the sessile drop and captive bubble techniques. These results are compared with interaction force and adhesion measurements by atomic force microscopy using the hydrophobic polyethylene colloidal probe procedure. Interaction forces were described using extended DLVO theory and the work of adhesion between the polyethylene sphere and the amine surface was calculated and compared with the work of adhesion between the amine surface and an air bubble.
Interpretation of the Dynamic Contact Line with Disjoining Pressure, Large Capillary Numbers, Large Angles, and Pre-wetted or Entrained Films
The issue of the stress singularity at the dynamic contact line has been a topic of debate for some time. The recent contributions of Elyousfi, et al. (1998) and Ishimi, et al. (1998) assume that the angle of the interface approaches the equilibrium contact angle at a thickness corresponding to a molecular dimension or the adsorbed film thickness. This assumption implies that the angle changes discontinuously from that of the equilibrium angle to zero at the contact line and thus the curvature is singular. Telezke, et al (1988) overcame this singularity by including the disjoining pressure for small film thickness. However, their model was based on the lubrication approximations and thus is limited to small angles. Combining both of these approaches results in a description of the contact line region without a stress singularity and is valid for large capillary numbers, large angles, and pre-wetted or entrained films. The "state variables" of the system are the curvature, angle, and thickness. These variables are continuous from the meniscus to the flat film. There has been reluctance to use the disjoining pressure for dynamic contact lines because of the lack of validity of the Hammaker theory below a thickness corresponding to a monolayer. We show that the disjoining pressure isotherm below a monolayer thickness for a van der Waals fluid can be described by a two dimensional van der Waals equation of state or the de Boer isotherm. This isotherm describes the adsorption of alkanes on alumina and carbon. Calculations with this isotherm for nonwetting fluids can be used to estimate the equilibrium contact angle for van der Waals fluids.
Contact Angle and Hydrophobic Force
(Abstract not yet available)
Double Layer -Charge Displacement by Adhesion and Spreading of Living Cells
The potentiostatic control of surface charge density and interfacial tension of an electrode aqueous interface offers a possibility for direct studies of non-specific interactions in cell adhesion and development of adhesion based sensors for environmental particles. Unicellular marine alga, Dunaliella tertiolecta (Chlorophyceae) of micrometer size and flexible cell membrane was used as a model and 0.1 M NaCl, pH=8, as supporting electrolyte. The dropping mercury electrode acts as an in situ adhesion sensor with adhesion and spreading rates being enhanced by the hydrodynamic regime of electrode's growing fluid interface. The adhesion of single cells causes a displacement of counter-ions from the electrical double layer. The instant flow of compensating current reflects the dynamics of adhesive contact formation and spreading of a cell.. The amperometric technique allows precise recording of adhesion signals of individual cells and measurement of cell contact area, the distance of the closest approach and calculation of adhesion forces.
Effect of Potential on Adhesion, Spreading and Detachment of Organic Droplets at Aqueous Metallic Interface
The phenomena of electrowetting and recent application of electrical potential to shape organic microdroplets at solid substrates into arrays of lenses, with focal length that can be reversibly changed, point out to the importance of a general understanding of dynamics and equilibria of wetting processes at electrodes. We shall use simple video recordings to demonstrate how spreading and shape of organic droplets at the mercury/aqueous electrolyte interface depend on the interfacial forces that are controlled by the applied potential. We indicate a method to measure interfacial forces in the system organic droplet-mercury electrode/aqueous electrolyte solution when adhesion counteracts buoyancy of a droplet. The wetting phenomena identified in this system apply more generally for the droplets and insoluble films on solid electrodes and the findings can be extrapolated to complex electrochemical systems of technological importance.