The following is a list of the abstracts for papers which will be presented in THIRD 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.)
Heterogeneous Chemical Reactions in Multiphase Flow: Influence of the Induced Wettability changes on the Relative Permeabilities
(Abstract not yet available)
Dynamic Evolution of Contact Angle on Wettable Surface During Evaporation
(Abstract not yet available)
Surface Properties of Soy-based Polysoaps
Vegetable oils are obtained from abundantly available and renewable agricultural products. They are also biodegradable and safe to use. These properties make vegetable oils the preferred raw materials over petroleum-based oils for the manufacture of various consumer and industrial products. Vegetable oils are widely applied in the development of lubricants, printing inks, cosmetics, soaps and surfactants. In this work, soybean oil was used to synthesize a series of polysoaps. The surface properties of these polysoaps were investigated as a function of their chemical structure. The surface properties that were investigated include: dynamic and equilibrium surface tension of water, and oil/water interfacial tension. The effects of molecular weight and counter ion on these surface properties are discussed.
Macroscopic Length Scale and Flow Effects on Interface Profiles Around Moving Contact Lines
(Abstract not yet available)
1) Usinor R&D, Research Center of Isbergues, BP 15, 62 330 Isbergues, FRANCE
2) Usinor R&D, Centre d'Etudes et de Développement, 60 Montataire, FRANCE
Effect of the Physical Chemistry of Organic Coating Surfaces on Fouling and Cleanability with a Particular Reference to the Food Industry
The physical chemistry of various organic coatings is investigated with respect to fouling and cleaning ability. The polymers selected in this study are polyester, PVDF, PVC and PET.
We define two types of solids : (i) high energy surfaces with a polar component such as PET and PE ; (ii) surfaces with a lower solid surface free energy such as PVC and PVDF. After ageing, only the PVC surface displays some modifications in the solid surface free energy.
Surface soiling is realized by spraying oil on the surfaces. The number of droplets and their shape strongly depend on the surface physical chemistry of materials. The soiled surfaces are then cleaned in a laminar flow cell with an alkaline detergent. It is possible to model the cleaning kinetics. All the surface cleanings obey a primary order. In our experimental conditions, PET and PE surfaces are easier to clean than PVDF et PVC ones.
The difference in clean ability observed on the various polymers is explained by the physico chemical properties of the surface and more particularly, by the components of the solid surface free energy. It should be noted that the surface topography is not relevant in this mechanism.
1) Institut Curie - P.C.C - UMR 168, 11, Rue Pierre et Marie Curie - 75231 Paris cedex 05 - FRANCE
2) Rhône Poulenc - 85, Avenue des Frères Perret, BP 62 - 69192 St Fons - FRANCE
Shocks in Inertial Dewetting
We present the first observation of a shock in the fast dewetting of a water film (thickness e) from hydrophobic glass. The film dewets by the nucleation and growth of a dry patch (velocity V), surrounded by a rim which collects the liquid. The velocity of dewetting V(e) follows the Culik law for the bursting of soap films, with a driving force including both capillarity and gravity. The profile of the rim is measured by the deflection of a laser beam. The shock is observed when the rim surfs on the immobile film at velocities V*>(ge)1/2, the velocity of gravity waves (g = gravitational acceleration) in shallow water. Ripples emitted in front of the rim gives us a signature of the shock.
Contact Angle Measurement on Real Surfaces
Contact angle measurement and interpretation on real surfaces is complicated: they always exhibit -at some scale and to some extent- roughness and heterogeneity. Both lead to irregular drop shapes and promote contact angle hysteresis. Furthermore, many real surfaces -like wood and some stones- present anisotropy of their superficial properties, giving raise to elliptical rather than circular contact lines. In addition to these difficulties, biological surfaces in their native state as well as rocks during its use are always hydrated, yielding very low contact angles, which are difficult to measure. Axisymmetric Drop Shape Analysis-Contact Diameter (ADSA-CD) was developed to measure contact angles on non-ideal and/or highly hydrophilic surfaces. The ADSA-CD methodology computes mean contact angles from the maximum contour (usually measured from a picture of the drop looking from above) and the volume of sessile drops with a known surface tension. This technique returns a single value of contact angle when is applied to irregular drops (a mean maximum diameter). In these cases different contact angles can be defined on surfaces with a clear anisotropy. ADSA-CD has been used until now only for static contact angle measurements, i.e. at constant drop volumes.
We present an upgrade of the method:
- it permits to control very precisely the drop volume, and to capture simultaneously drop images for contact angle determination, permitting this way to use ADSA-CD for hysteresis measurements, allowing this way interpretation of low rate advancing and receding contact angles in terms of surface energetics.
- the maximum contour of the drop can be fitted to an ellipse by means of a semi-automatic procedure of image processing, yielding two contact angles and therefore taking into account inherent anisotropy.- it is also capable to measure captive bubble low rate dynamic contact angles, permitting this way to measure precisely low angles on biological or otherwise hydrated substrates. We performed an experimental study with model surfaces, which permits to identify the advancing bubble angles with the receding drop angles. We were therefore able to measure hysteresis on biological tissue, which has a too low receding drop angle to be measured by ADSA-CD. Finally, we have improved ADSA-CD technique resolving the general Young-Laplace equation using first-order perturbations. Non-Axisymmetric Drop Shape Analysis-Contact Diameter (NADSA-CD) needs the same input variables of ADSA-CD and the recognition of an appropriate roughness model describing the surface. The knowledge of the surface topography is experimental accessible using different techniques like Atomic Forces Microscopy, Scanning Electronic Microscopy and/or measurements of roughness. The complete laplacian interface and the local contact angle are computed. The experimental set-up is the same that for ADSA-CD although NADSA-CD requires the complete radial component of the three-phase line.
Viscous Dissipation and Rheological Behavior near the Solid/liquid/vapor Triple Line. Application to the Spreading of Silicone Oils
One basic problem in liquid spreading is the hydrodynamic description of the viscous braking force near the moving contact line. With a sharp wedge profile, the viscous energy dissipation becomes infinite, which would mean that a drop is not able to spread on a solid surface (infinite braking force). The problem is currently solved by introducing a "cut-off" length, for example by considering that the drop profile is curved near the solid/liquid/vapor triple line by Van der Waals long range forces.
We propose a new approach to solve the problem of divergence at the solid/liquid/vapor triple line. It consists of observing that the rheological behavior of a Newtonian liquid may be modified near the triple line due to high shear rate. Above a critical value of the shear rate, near the triple line and near the solid surface, the liquid becomes shear-thinning, so that the apparent viscosity of the liquid decreases as the shear rate increases. As a result, there is no divergence of the viscous energy dissipation and of the braking force.
This new description of the viscous braking phenomenon in liquid spreading is well supported by spreading experiments of silicone oils on glass substrates. The silicone oils are Newtonian below a critical value of the shear rate. Above this critical value, the viscosity of the liquid decreases according to a power law of the shear rate.
Wettability of Particles in Solid-stabilised Emulsions
A theoretical treatment is developed for the wetting of a solid spherical particle at an oil-water interface in terms of the components of the surface free energies of all three phases. The approach uses the Owens and Wendt method for calculating an interfacial tension from the components for the appropriate phases. Calculated oil-water contact angles for a solid of given hydrophobicity with a range of oils of different polarity show good agreement with experimental data. For a hydrophobically modified silica surface, oils of increasing polarity give higher oil-water contact angles measured through water. The results are in line with experimental observations of the effect of oil type on Pickering emulsion type where more polar oils preferentially give water-in-oil emulsions. The approach is also used to predict the type of solid particles that would be required to stabilise emulsions of two immiscible oils. For hydrocarbon-fluorocarbon pairs, it is predicted that perfluorinated particles would stabilise fluorocarbon-external emulsions whilst silica particles with a high coverage of hydrocarbon groups should stabilise hydrocarbon-external emulsions.
Wetting and Capillary Condensation in the n-Hexane/n-Perfluorohexane System
We have used a modified surface force apparatus (SFA) to study adsorption on mica surfaces from binary mixtures of n-hexane and n-perfluorohexane, in vapour and liquid. Films adsorbed from vapours (p/p0 > 0.9) of the pure liquids range in thickness from 1 to 4 nm, in crude agreement with the predictions of non-retarded Lifshitz theory. The observed deviations from theory show a qualitative difference between the two liquids, which may reflect differences in the significance of structural effects. Adsorption from vapours over (one-phase) liquid mixtures above the bulk critical temperature of 23 ° C gives rise to films which are significantly thicker than those adsorbed from the pure vapours, with a weak maximum in thickness observed near the critical composition of the bulk liquid mixture.
Capillary condensation of liquid from near-saturated vapour occurs in the gap between the two mica surfaces. The measured compositions and mean meniscus radii of curvature of the condensates formed in mixed vapours are in broad agreement with the predictions of the binary Kelvin equations. n-Hexane is preferentially adsorbed at the mica/liquid interface of the condensates, with the extent of adsorption depending on the position in the liquid-liquid phase diagram. Liquid-liquid phase separation is observed in condensates formed from vapours over n-perfluorohexane-rich mixtures at, or very close to phase-coexistence. This results in novel two-phase or 'double' condensates, with an inner condensate of n-hexane-rich phase within an outer condensate of n-perfluorohexane-rich phase. Capillary condensation from vapours coexisting with phase-separated bulk mixtures gives rise to condensates of the n-hexane-rich phase. It appears that the composition of a mixed condensate may show a significant time-dependence in the initial stages of condensation for some mole fractions.
On Molecular and Continuum Boundary Conditions for a Miscible Binary Fluid
We show that molecular dynamics simulations can furnish useful boundary conditions at a solid surface bounding a two-component fluid. In contrast to some previous reports, convective-diffusive flow is consistent with continuum equations down to atomic scales. However, concentration gradients can produce flow without viscous dissipation that is inconsistent with the commonly used Navier slip condition. Also, differential wetting of the two components coupled to a concentration gradient can drive convective flows that could be used to make nano-pumps or motors.
A New Thermodynamic Formula for the Study and Interpretation of Wettability in Capillarity Phenomena
This fundamental thermodynamical formula treats the wettability phenomena in its more total energy aspect. It clearly allows to follow and interpret the wettability phenomenon in the tubes whatever the nature of the surface. It explained the theoretical and experimental physical relation which exists between the variation of the curve of the meniscus in one way and the intensity of the liquid solid interaction and the liquid liquid interaction in other way.
Moreover,this formula allows to explain and to measure with an attracting reproducibility the curve of the meniscus. This curve can be explained by the attenuation of intensity of solid liquid interaction according to its range along the liquid layer which is in the capillary. In other way this attenuation can be explained by the competition between the liquid/liquid and solid/solid interactions in the capillary. This formula makes it easily to measure and to interprete this attenuation according to:
liquid/liquid Interaction only ; ·
solid/liquid Interaction only ; ·
That can allow us to conclure that the exploitation of this approach makes the practical determination of the surface tension of the liquids without support effect and the gravitational fields is possible under ordinary conditions by simple combinations of measurements!
That permit us finally to isolate the behavior of the liquid in opposite to the solid liquid interaction in the capillary and measures it!
The problem of roughness and porosity for the determination of wettability can be solved whatever the nature of the considered surface in a very broad material range.
Contact Angles on Heterogeneous Surfaces of Regular and Random Microscopic Patterns
Fabrication of micro-devices and control of surface properties at microscopic, submicroscopic, and molecular levels poses a new challenge for interfacial engineering. Solid surfaces of defined topography and heterogeneity are artificially fabricated in research and development laboratories. Fluid behavior on such surfaces is not clearly understood yet and has been studied in recent years. In this presentation, the sessile-drop contact angles measured on model two-component surfaces consisting of a micron-sized regular pattern made of self-assembled monolayers of thiols of different functionality will be discussed. Both the experimental results and theoretical predictions indicate that the shape of the three-phase contact line influences the value of the measured apparent contact angle. For example, two different contact angles were measured on model heterogeneous surfaces composed of parallel hydrophobic and hydrophilic strips. Contact angles were lower when measured with the strips normal to the three-phase contact line than those measured with the strips tangential to the three-phase contact line. This anisotropy in contact angle value is not observed for substrates having a random heterogeneity pattern such as coal specimens with inorganic inclusions.
Determination of the Solid Surface Free Energy from Pull-Off Force Measurements Using Atomic Force Microscopy
There is no simple analytical technique used for direct measurements of the surface free energy of solids having engineered surfaces. In recent years, the surface free energy of solids has been determined indirectly through contact angle measurements using a series of probe liquids of varying polarity and next making calculations based on the Lewis acid-base interactions theory. However, this contact angle measurement-based approach relies on large, homogeneous, and inert substrates. Atomic force microscopy (AFM) is capable of characterization of solid surfaces at the microscopic and submicroscopic scales. It can also be used for the determination of surface free energy of solids from pull-off force measurements. In this presentation, the application of the AFM instrument for the surface free energy determination will be critically reviewed. Also, experimental data on the AFM pull-off force measurements between self-assembled monolayers of thiols will be presented. The surface free energy values calculated from AFM pull-off force measurements will be compared with the values calculated from the Lewis acid-base interactions theory based on the contact angle measurement results.
Contact Angle Hysteresis on Fibers and Powders
In the absence of roughness, advancing and receding contact angles reflect the low-energy and the high-energy patches, respectively, and the difference(hysteresis) is related to heterogeneity. A simple apparatus was constructed to measure the pressure required to initiate movement of probe liquids out of a column packed with fibers or powders. Bartell's analysis of these data provides an estimate of receding contact angles. The advancing contact angles on the same samples are first determined with Washburn's analysis of probe liquid wicking rates. Although those analyses are strictly correct only for one capillary diameter, the results provide technologically useful information. In practical applications, data from three or four probes (plus silicone reference) are reduced to "surface energy components" using theories that introduce further scientific uncertainties. Examples are from characterization studies of surface treatments and sizings on continuous and short fibers. Independent information on surface chemical heterogeneity is obtained by applying the Thomason-Dwight model to XPS and EDXS data. Assessment of the thickness and coverage of sizings with SEM is enhanced using backscattered electron images.
Characterization of Surface Free Energies and Surface Chemistry of Solids
The surface chemical and surface energetic nature of materials used in the formulation of commercial products or used in the manufacture of these products is often important to the final quality of the product. Despite the importance of surface chemistry to the ultimate performance of the product, not as much recognition as is deserved, has been given to the characterization of surface chemistry and the effects of its variation on product performance. Difficulties with both theoretical descriptions of interfaces and the measurement of surface chemical characteristics make the incorporation of material surface chemical specifications for the manufacture of many products challenging. Both theoretical developments related to surface characterization and experimental methods used to characterize the surface chemical properties of materials are discussed in this paper. This paper reviews the methods that have been used to characterize surfaces and discusses the merits and limitations of the approaches that have used to characterize materials in order to better understand the mechanisms behind contact angle, wettability and adhesion.
A Thermodynamic Model for Wetting Free Energies from Contact Angles
It is widely acknowledged that liquids spread on solid surfaces due to molecular interactions between the two phases. The stronger the interaction, the greater the wetting. In this regard, contact angles are a measure of liquid-solid adhesion and are often used to estimate surface energies. To gain insight into underlying molecular interactions, surface energies are partitioned into a non-polar and a polar fraction or an acid portion and a base portion. While this approach has proven useful in practice for estimating adhesion strength, it does not lend itself to direct comparison of the strength of molecular interactions measured with other experimental techniques.
A simple thermodynamic model for determining the molar free energy of wetting for a small, sessile drop on a solid surface has been derived by assuming that spreading can be treated as adsorption. According to the model, for large contact angles, molar free energies are small. As contact angles tend toward zero, free energies increase exponentially. Free energies calculated using contact angles taken from the literature for various liquid- solid pairs agreed reasonably well with the bond strengths measured by other experimental techniques.
Roughness-Induced Transition from Partial Wetting to Complete Dewetting
A correlation between the roughness of a solid surface and the contact angle Theta of a liquid has been often reported but rarely quantitatively characterized. Chemically homogeneous surfaces are prepared by dip-coating glass slides with an amorphous fluoropolymer (Teflon AF1600). The roughness is altered by incorporating various amounts of l jAm silica spheres into the polymer coating and quantified by AFM. Contact angles of several pure liquids are determined by the Wilhelmy technique. At low surface coverage (less than a monolayer of particles) cosh increases (when Theta0 < 90°, i.e. if Theta on a smooth surface is obtuse) or decreases (when Theta0 > 90°) linearly as predicted by Wenzel equation, but the slope is quite different from the theoretical prediction (cosThetao). At higher surface concentration of particles 0 becomes virtually independent of the roughness ratio r (= actual area/geometric area), The influence of surface roughness on static wetting cannot be uniquely described by r. In both cases a sharp transition. from partial wetting to (almost) complete dewetting occurs in the vicinity of Theta0 = 90°.
SPT division, Kapeldreef 75, B3001 Heverlee, BELGIUM
Influence of Transient pH on Particle Redeposition During Rinse
(Abstract not yet available)
and Peter Pershan1,2
1) Division of Applied Science, Harvard University, Cambridge MA 02138
2) Physics Department, Harvard University,.Cambridge, MA 02138
3) Department of Physics, BNL, Upton, New York 11973
4) IBM Watson Research Center, Yorktown Heights, NY 10598
Wetting on Nano-structured Surface: Microscopic View.*
The fundamental physics associated with wetting on nano-scales is of wide current interest. In addition this phenomena is important for a broad range practical applications such as the emerging soft condensed matter micro-technology. Theoretically structured surfaces have recently been predicted to exhibit a crossover between capillary condensation and wetting phenomenon for which the wetting exponent is directly influenced by the pattern of the structured surface.
We studied the wetting of nano-patterned silicon surfaces with near hexagonal packing of 20 nm diameter holes with mean separation of 43 nm The thickness of the wetting layer (1-30 nm) of cyclohexane was controlled to angstrom-level accuracy. We will report x-ray scattering results for the evolution of the microscopic structure of the wetting layer. Observed effects include changes in the structure of the wetting layer with increasing thickness and strong deviations from the traditional 1/3 power law dependence of the thickness of the wetting layer on chemical potential offset from coexistence. From analysis of the x-ray results it is possible to determine the detailed shape and volume of the nano-holes. The observed dependence of the film thickness on chemical potential is in good agreement with recent theoretical predictions.
*Work supported by NSF-DMR-98-72817, NSF-DMR-01-124936 and the Rothschild Foundation.
Switching Wettability of Polymer Surfaces Induced by Roughness and Chemical Modification
There is a renewed interest in the effect of surface roughness and heterogeneity on wettability. In particular, rough solid surfaces possessing a low surface free energy are very promising for super water-repellent synthetic polymer materials with reduced adhesion properties. A combination of water-repellent properties and wettability in the same material could be used to regulate adhesion and would extend the range of applications of these synthetic materials. Our aim was to prepare structured surfaces capable for reversible switching between ultrahydrophobic and hydrophilic states upon external stimuli. The "grafting to" approach was used to synthesize switchable mixed polymer brushes on the surface of radio-frequency plasma etched poly(tetrafluoroethylene) (PTFE) film.
It was the aim of this study to characterize quantitatively correlations between the morphology (roughness) and chemical composition of these surfaces and contact angle hysteresis. The roughness was altered by the treatment time of radio-frequency oxygen plasma etching and quantified by scanning force microscopy (SFM). The chemical composition of the surfaces was characterized by x-ray photoelectron spectroscopy (XPS). Advancing and receding contact angles of water were determined using axisymmetric drop shape analysis (ADSA) and a conventional goniometer sessile drop technique.
Pattern Formation in Drying Nano-colloidal Dispersion Drops
The formation of complex aggregation patterns in drying colloidal dispersion drops still remains, in many regards, an intringuing and incompletely understood interface phenomenon. And this is especially true for nanosize and charge stabilized colloidal dispersions which, in other respects, are of potential interest in the development of functional nanosize materials and devices. From a more fundamental viewpoint, surface structures resulting from drying nano-colloid dispersions are in many respects similar to those formed in biodispersions from the aggregation and self-organization of viruses or proteins, which both are nanosize objects. Therefore, understanding and identifying the underlying mechanisms and critical parameters which govern the emergence and topological features of these complex surface patterns is of both fundamental and practical importance. We here present some of the basic aspects of this problem through the drying of nanocolloidal dispersion drops at model molecular surfaces, and the formation of complex surface structures. The results show that large-scale fractal patterns are formed through a sequential process, involving the growth of bulk seeds, which, in the late stage of the drying, are structured by the hydrodynamic flow at the substrate. The strong coupling between the surface topography and the hydrodynamic shear was shown to produce both symmetric and asymmetric fractal patterns, usually observed in the axial and radial flow modes in the standard Hele-Shaw cell. A simple phenomenological model is proposed to account for both these results and their analogy with shear instabilities and structure formation in purposely designed Hele-Shaw cells.
Dynamics of Wetting in Disordered and Granular Media
We have studied wetting dynamics in various inhomogeneous model systems, both two and three dimensional. Spontaneous imbibition fronts generated bycapillary rise between two roughened glass plates clearly shows a crossover from a roughness exponent around 0.8 at small length scales to an exponent of 1/2 at larger scale, which corresponds to Kardar-Parisi-Zhang behaviour. Based on the characteristic scaling we found for the crossover length, we believe this to be the first clear identification of crossover from nonlocal to local wetting dynamics in imbibition. We report on similar effects in 3D imbibition of a liquid in piles of glass sheres. Furthermore, the effect of invading liquid upon a granular system of this kind has been studied by molecular dynamics simulations. We find a characteristic critical yield stress which is solely due to the liquid bridges between the grains. Close to the yield stress, characteristic scaling laws are found for the diffusivity of the grains as well as for a dynamical order parameter describing the size of the avalanches in the system at which the wet material yield as by Our results may explain why previous experimentsconducted with Hele-Shaw cells yielded roughness exponents which were scattered significantly, and were much larger than predicted by the KPZ theory.
An Interferometric Investigation of Moving Contact Line Dynamic in Spreading Polymer Liquids
A non-invasive optical technique is used to investigate the characteristics of dynamic contact line of viscous polymeric liquid drops on solid substrates. Three important parameters (dynamic contact angle, thickness of precursor layer, and length of precursor layer) have been measured by PSLFI (Phase-shifting laser feedback interferometry) and it is shown that each one of these parameters is depend on the relative magnitude of Capillary number, Ca=U /. We also study the effect of angle of inclination of the substrate on these parameters by repeating the experiment by varying the angle of inclination. PSLFI system has the lateral resolution of 0.5 m and vertical resolution of 10 nm, which enable us to measure these nano-scaled features close to the contact line precisely without disturbing the spreading process.
1) Bayer AG, Kaiser-Wilhelm-Allee, D-51368 Leverkusen, GERMANY
2) SuNyx GmbH, Stolberger Str. 370, D-50933 Köln, GERMANY
3) COSMOlogic GmbH&CoKG, Burscheider Str. 515, D-51381 Leverkusen, GERMANY
Roughness and Topology of Ultra-hydrophobic Surfaces
Ultra-hydrophobic surfaces have considerable technological potential for various applications due to their extreme water-repellent properties. When water contacts such surfaces, contact angles approaching 180° may be obtained. These surfaces owe their unique wetting properties to a subtle interplay between surface chemistry and mesoscopic topological structure. In this paper, we discuss the requirements regarding surface roughness for ultra-hydrophobicity. We demonstrate by numerical calculations of contact angles on idealized surfaces that the topological nature of the surface roughness has a major influence on the water-repellency of hydrophobic surfaces. In order to validate this theory, we present an electrochemical method to experimentally determine important model parameters, characterizing the solid/liquid interface. Furthermore, we substantiate our theoretical findings with experimental data on contact angles on hydrophobic surfaces with well-characterized surface roughness and topology.
1) Department of Mechanical Engineering, University of Alberta, Edmonton, T6G 2G8, CANADA.
2) Department of Mechanical and Industrial Engineering, University of Toronto, Toronto,M5S 3G8, CANADA.
Contact Angle Interpretation: Combining Rule for Intermolecular Potential
(Abstract not yet available)
Effects of Surface Nanostructures on Wetting
We show here low-rate dynamic contact angle results by an automated contact angle system: axisym-metric drop shape analysis (ADSA). Our aim is to study systematically the effects of nanostructures and SAMs' orientation on surface energetics and wetting. Our results suggest that surface preparation of gold substrates for monolayer self-assembly affects the observed wetting angles. For example, the contact angles of water on C18SH/annealed gold substrates and C18SH/non-annealed ones vary considerably. Fourier Transform Infrared Spectroscopy (FT-IR), Variable Angle Spectroscopic Ellipsometry (VASE), Atomic Force Microscopy (AFM) were used to obtain further information on SAMs' orientation and packing den-sity and correlate them with the observed wetting phenomena. Our contact angle results were interpreted in terms of surface nanostructures and surface energetics.
Laibinis2 and Daniel Y. Kwok1;
1) Department of Mechanical Engineering, University of Alberta, Edmonton, T6G 2G8, CANADA
2) Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.
Chemical Influences on Adsorption-Mediated Self-Propelled Drop Movement
We examined reactive wetting on chemically patterned surfaces and employed decahydronaphthalene DHN) droplets that contained various amounts of an n-alkylamine to reactively wet and move about surfaces that expose a dense packing of carboxylic acid functionalities. The amine compounds adsorb onto this surface and produce one with a lower energy that exposes methyl groups, thereby causing a local surface energy gradient that is sufficient to induce a self-propelled movement of the contacting droplets on the surface. We present here self-assembling and reversible systems for droplet movement. Equations to predict droplet velocity that is based on the Langmuir adsorption isotherm and first-order kinetics will be discussed.
Wetting of Particles: Partition of Micro- And Nano-particles
Partition of particles between a liquid phase and a fluid phase can be achieved based on their wetting properties. The partition coefficient depends on the interfacial tensions of the three phases, on the particle shape, and on line tension. In addition, contact angle hysteresis may play an important role. Calculations of the effects of these parameters on the partition coefficient will be shown.
Wetting in Porous Media
Wetting in porous media is discussed from three different perspectives: (a) penetration criterion and equilibrium, with emphasis on systems with limited size; (b) kinetics of penetration and the redistribution effect; and (c) wettability characterization of porous media.
1) Environmental and Fluid Modelling Group, University of Plymouth, U.K.
2) Omya AG, Oftringen, SWITZERLAND.
Indirect Measurement and Modelling of the Shrinkage Forces Acting During the Drying of a Paper Coating Layer
Coated paper consists of a mineral coating (e.g. clay or calcium carbonate) fixed to the fibrous paper substrate with a binder (natural such as starch or proteins, or synthetic such as latex). The coating is applied to the substrate as a slurry, which then dries and shrinks. The forces acting during such shrinkage have been measured indirectly, by observing the curling of strips of a synthetic substrate coated with calcium carbonate and different binders. The surface force acting to cause the deflections were calculated using standard theory for the deflection of beams. The porous structures of the dried coating layers were studied by electron microscopy and mercury porosimetry. The porosimetry data was interpreted using Pore-Cor, a software package able to generate modelled network structures and simulate the behaviour of fluids within them . Both the water distribution during drying, and dynamic wetting, have been studied in Pore-Cor's virtual reality environment . Insights have been gained into the capillary forces acting during the drying process which are responsible, together with the film-forming of the polymeric binder, for the shrinkage of the coating layer.
1. Schoelkopf, J., Ridgway, C. J., Gane, P. A. C., Matthews, G. P. and Spielmann, D. C. 2000. Measurement and network modelling of liquid permeation into compacted mineral blocks. Journal of Colloid and Interface Science, 227, 119-131.
2. Schoelkopf, J., Gane, P. A. C., Ridgway, C. J. and Matthews, G. P. 2001. Influence of Inertia on Liquid Absorption into Paper Coating Structures. Nordic Pulp and Paper Research Journal, 15, 422-430.
1) Department of Mechanical Engineering, University of Illinois at Chicago,
Chicago, IL 60607, USA
2) Department of Materials Engineering, Drexel University, Philadelphia, PA 19104, USA
Fluid Interface Dynamics Induced Thermally Within Individual Capped Multiwall Carbon Nanotubes
Experimental measurements on multiphase fluid transport at sub-micron length scales are scarce. In this study, thermal experiments are performed using electron irradiation or resistive heating as a means of inducing motion/phase-change of the fluid contents of individual carbon nanotubes in the high vacuum of a transmission electron microscope (TEM). TEM is shown to resolve fluid interface motion within the nanotubes with spatial resolution near 1nm. The nanotube samples are prepared by a hydrothermal method of catalytic nanotube synthesis, which produces multiwall, capped carbon nanotubes with inner diameters in the range 30-100 nm and lengths up to a few microns. These tubes are hollow and contain a high-pressure encapsulated aqueous multicomponent fluid, which displays clearly segregated liquid and gas by means of well-defined menisci. Wetting and dewetting of the inner carbon walls by the water-based fluid are clearly demonstrated in the recorded TEM image sequences. Experimental evidence is presented of nanometer-scale liquid films rapidly moving fluid along the nanochannel walls with velocities of 1mm/s or higher. The present experiments demonstrate that hydrothermal nanotubes open new horizons for the pursuit of challenging fluid problems at length scales approaching molecular dimensions.
Electrostatic Stabilization of Fluid Microstructures
The equilibrium shapes of fluid mircostructures are surfaces of constant mean curvature. This requirement poses serious constraints on the design of microfluidic devices involving complex geometries. Electrostatic forces can be used to overcome these limitations, as we exemplify in electrowetting experiments. At high voltage, we observed stable fluid structures consisting of a macroscopic liquid drop (R=O(1mm)), mircoscopic droplets (r=O(.01mm)), and thin liquid channels which connect the micro droplets permanently to the macroscopic drop. Despite a huge gradient in Laplace pressure, these complex fluid microstructures are stable on a time scale of seconds. In a simple model we show that the stability stems from a negative electrostatic pressure in the micro droplets, which balances the Laplace pressure. Using patterned electrodes, we also demonstrate the use of this concept for microfluidic applications.
Line Tension and the Surface Profile of Liquid Droplets on Chemically
We performed numerical simulations to study the equilibrium shape of the liquid vapor interface for liquid droplets on substrates with alternating hydrophilic and hydrophobic stripes. The wettability pattern leads to a modulation of the three phase contact line and the adjacent parts of the droplet surface. For finite values of the line tension Gamma, the surface profile is determined by both the wettability contrast and the line tension. For Gamma > 0, the modulation amplitude decreases and eventually vanishes for Gamma > 1e-8 J/m. From the simulation data, we extracted the local contact angle along the contact line and its local curvature. Within well defined sections of the contact line Gamma can be determined using a local interpretation of the modified Young equation. The accuracy of this procedure critically depends on the sharpness of the transition between the hydrophilic and the hydrophobic parts. The results validate the data analysis procedure and the results from recent AFM experiments  and support the conclusion that Gamma = O(1e-10 J/m).
 T. Pompe and S. Herminghaus, Phys. Rev. Lett. 85, 1930 (2000).
Amino Group Substitution on PET Film and Wetting Behavior of Collagen Cell Adhesion for Artificial Ligament
The PET has been widely used for medical materials such as an artificial ligament because of its strength and antibacterial action. However, when transplanted in human bodies, its compatibility is not good enough to adapt to the collagen that grows from living body tissues. Then we substituted amino group, which has a high affinity for collagen, on the PET surface by ArF laser. PET is highly hydrophobic and does not dissolve well in aqueous solutions. To avoid this reaction we make a thin ammonium fluoride solution layer on the PET surface with capillary phenomenon. Then an ArF laser beam was irradiated vertically onto the sample. The result of this treatment showsthat an untreated sample having the contact angle of 80 $B!k (B with water and the bonding strength of only 1.0 kg/cm 2 with collagen was improved to have the contact angle of 22 $B!k (B and the bonding strength to be 12 kg/cm 2. When the treated sample had been implanted into the subcutaneous tissue of a rabbit $B!G (Bs regions dossals, existence of leukocyte colonies that are indicators of human histocompatibility was confirmed on the hydrophilic parts of the sample.
Application of Droplet Dynamics Analysis for Assessment of Water Penetration Resistance of Coatings
The objective of the present work is to develop an experimental method based on droplet dynamics analysis for the assessment of water penetration resistance of coating systems on wood or wood composites. Water droplets of about 4 cmm were deposited on coated specimens placed in a climate-controlled chamber at 23º C and 55% relative humidity. Changes in shape, volume and contact angle of the droplets were recorded by a digital camera at specific intervals of time and quantified by means of a digital image analysis technique. Axial symmetry of the droplet was assumed and hence the method could be used only for isotropic surfaces. Reference tests were performed on impermeable surfaces (Plexi Glass) to determine the droplet evaporationrate, so that effects of evaporation, penetration into the coating and surface dynamics could be separated. Comparison of dynamic characteristics of the water volume penetrating into the surface through unit area was used for assessment of water penetration resistance of different coatings. Additionally, the dynamics of changes in contact angle over time (about 45 minutes until complete evaporation of the droplet) was examined for information on hysteresis effects and surface energy dynamics of the coatings. Robustness of the method as well as results obtained for a series of different coating systems on medium-density fiberboard will be further discussed.
1) University of Alberta, 4-9 Mechanical Engineering, Edmonton, Alberta T6G 2G8, CANADA
Contact Angles: Measurement and Interpretation
(Abstract not yet available)
1) Center for Modeling and Characterization of Nanoporous Materials, TRI/Princeton, P.O. Box 625, Princeton, New Jersey 08542
2) Institute for Problems in Mechanics RAS, 101(1) Prospect Vernadskogo, Moscow 117526, RUSSIA
Absorption of Droplets by Porous Substrates: New Experimental and Modeling Studies
We developed a new technique to monitor spontaneous absorption of droplets by porous substrates based on an optical electronic measuring system and a model to describe fluid flow in a pore during penetration. The method is efficient in studies of viscous and visco-elastic complex fluids.
1) Department of Physics, Indian Institute of Technology Roorkee, Roorkee - 247667 Uttaranchal, INDIA
2) Department of Metallurgy & Material Science Engineering, Indian Institute of Technology Roorkee, Roorkee - 247667, Uttaranchal, INDIA
Dynamic Evolution of Contact Angle on Wettable Surface During
The interfacial forces which determine the interaction between a liquid (fluid) and solid surface have been investigated under dynamic conditions of evaporation. The evaporation characteristics of probe liquids and its influence on the droplet of the liquid on a solid substrate have been observed. The changes in mass, contact angle, solid-liquid contact radius during evaporation of small droplets (15mg-90mg) of water on the substrate of glass and polycarbonate, and the droplets of methyl alcohol on polycarbonate and polyethylene have been noted. In all the systems under investigation, the droplets wet the solid surface of the substrates. The evolution of contact angle and contact radius with the progress of evaporation has been investigated for each droplet-substrate system in order to identify the common trend. Although in the systems of water on polycarbonate and glass, the contact radius remained constant with the progress of evaporation but the same behavior has not been observed in methyl alcohol on polycarbonate and polyethylene. The change in free energy with the changes of contact angle and/or contact radius has been examined theoretically to find thermodynamic justification for the observed behavior of the contact angle and the contact radius.
1) Division of Applied Science, Harvard University, Cambridge MA 02138
2) Physics Department, Harvard University,Cambridge, MA 02138
Microscopic Structure of the Thin Wetting Film on Liquid Surfaces: Hydrocarbon/Fuorocarbon Study.*
The liquid surface provides a unique opportunity to study wetting on a structureless substrate. Wetting of a liquid by another liquid also allows for study the liquid-liquid interface as one of the liquids evolves from a thin 2D wetting layer to bulk.
We report an X-ray scattering study of the microscopic structure of wetting films of perfluoromethylcyclohexane (PFMC), C6F11CF3 on the surface of CH3(CH2)18CH3 (C20) just above the melting point of the molten alkane.
The thickness of the wetting layer (0.7-30 nm) was varied by control of the chemical potential of fluorocarbon vapor. The evolution of the microscopic structure of the fluorocarbon layer along the surface normal was measured in-situ using x-ray reflectivity and diffuse scattering. Observed effects include suppression of the surface frozen phase of the alkane by the thinnest absorbed fluorocarbon layer (d ~0.7nm) as well as complete wetting by the PFMC. The thickness of the wetting layer follows the expected 1/3 power law dependence on the chemical potential difference. The evolution of microscopic structure of with increasing film thickness, and will be discussed in detail.
*Work supported by NSF-DMR-98-72817, NSF-DMR-01-124936 and the Rothschild Foundation.
The Leidenfrost phenomenon occurs when a liquid is placed on a very hot
plate. Then, it is observed that it forms drops which float above the plate,
because of the vapour layer which comes in between the solid and the liquid.
We describe different aspects of this familiar phenomenon, in particular the
drop shape and the characteristics of the vapour layer.
Recent Developments in Oil-in-water Electrowetting
Electrowetting is a recent technique which allows to modify the wettability of a surface through the bringing in of free electrical charges. Fundamentals and recent advances, including applications, will be recalled. The focus will be put on oil-in-water electrowetting - its specifity and recent experimental and theoretical results. More particularly, contact angle saturation features will be presented, wetting defects obtained by electrowetting, and feasibility of a "electrostatic surface force apparatus".
The Effect of Rock Surface Characteristics on Reservoir Wettability
Significant strides have been made in recent years in gaining better understanding of the role of fluid compositions on reservoir wettability. However, our knowledge of the effects that the solid surface characteristics have on establishing and altering wettability is quite limited. This study examines the effect of rock mineralogy and surface roughness on wettability in rock-brine-hydrocarbon systems. The wettability is characterized by using two different techniques. The Wilhelmy plate technique has been used to obtain dynamic (advancing and receding) contact angles averaged over the surface area of the solid substrate used. These results are compared with point-values of dynamic contact angles measured using the dual-drop dual-crystal (DDDC) technique for both smooth and rough solid surfaces of different mineralogy and roughness. The surface roughness of the samples used has been characterized by using an optical Profilometer.
The results indicate close agreement of water-advancing contact angles measured by the Wilhelmy and DDDC techniques thereby enhancing the confidence in the repeatable measurements that have been made at reservoir conditions using the DDDC technique. The preliminary indication from the work on surface roughness effects is that the advancing angle, and hence wettability, are affected by roughness to a certain extent beyond which roughness ceases to impact measured dynamic contact angles.
The Concept, Characterization, Concerns and Consequences of
Contact Angles in Solid-Liquid-Liquid Systems
The production of petroleum fluids from underground reservoirs involves the movement of fluid-fluid interfaces through the tortuous paths within the porous rocks. The movement of oil, brine and gas phases relative to one another depends on the nature of distribution of these fluids in the rock pores as well as on the strength of interaction between the fluids and the rock surface. While the distribution of the phases is largely governed by the spreading coefficient, which involves the interfacial tensions at the three fluid-fluid interfaces, the strength of rock-fluids interactions depends on the dynamic (advancing and receding) contact angles subtended by the moving fluid-fluid interface with the rock surface. These interfacial phenomena of fluid-fluid spreading and the rock-fluids interactions have generally been lumped into one parameter called wettability in petroleum engineering literature. Although yielding the advantage of a single global term for all the interactions, the term wettability has masked our understanding of the surface and interfacial phenomena of adhesion and spreading and their influence on the dynamics of fluids in petroleum reservoirs. The situation is further complicated by the fact that the conventional techniques used to measure dynamic contact angles in solid-liquid-vapor (S-L-V) systems have failed to yield meaningful results when applied to solid-liquid-liquid (S-L-L) systems such as the rock-oil-brine system of interest in petroleum engineering. This has resulted in concerns and skepticism surrounding the applicability of the contact angle concept to S-L-L systems to characterize their wetting tendency. The presence of a thin wetting film of an immiscible liquid on the solid surface and the parameters governing the stability of the wetting film are factors that control the preferential wetting behavior in S-L-L systems. In addition to addressing these concerns, this paper highlights the recent developments related to techniques of making meaningful measurements of fluid-fluid interfacial tensions and dynamic contact angles in crude oil - brine - rock systems at actual reservoir conditions of pressure and temperature as well as the influence of surfactants and rock surface characteristics on rock-fluids interactions.
Photo-induced Reversible Wetting Behavior on Structured Surfaces
Surface substrates such as gold and quartz can be chemically modified with self-assembling pyrimidine molecules, attached to long alkyl chains terminated by an appropriate functional group such as thiol. When irradiated with discrete wavelengths of light these surfaces undergo substantial reversible wetting changes over a number of irradiation cycles. The novel aspect of this system is that neither the liquid phase nor the solid phase needs to be altered in order to effect a large change in contact angle. The pyrimidine molecules utilized in the system are C-5 modified uracils that are able to dimerise when irradiated with light of a specific wavelength. The wetting behavior is brought about by a change in the surface charge that, in turn, is highly dependent on the functionality of the pyrimidine groups. Surfaces patterned with rnonolayers incorporating these dynamic molecules exhibit different wetting properties depending upon the wavelength of light the surface is exposed to. The significance of this work is discussed with respect to molecular structure and possible applications.
Wetting Properties and Interfacial Molecular Ordering of Long Chain Alkanes at Planar Solid/Vapor Interfaces
We present and analyze wetting and pattern formation phenomena of long chain alkanes at solid/vapor interfaces which result from the peculiar interfacial molecular ordering (e.g., surface freezing) of these molecules.
1) H. Schollmeyer, B. Ocko, and H. Riegler, Langmuir, in press (2002)
2) A. Holzwarth, S. Leporatti, and H. Riegler, Europhysics Letters 52(6): 653 (2000)
3) C. Merkl, T. Pfohl, and H. Riegler, Phys. Rev. Lett. 79(2): 4625 (1997)
Wetting of a Substrate by Nanocrystallites of Iron or Platinum
Transmission electron microscopy experiments have been carried out to investigatet the behavior of nanocrystallites of iron and platinum on an alumina substrate during heating in oxygen and hydrogen atmospheres. In an oxygen atmosphere, both the small and the large iron crystallites emited patches of multilayer thin films. These patches coalesced to generate a continguous film among a large number of crystallites. During subsequent heating in hydrogen, the films were reduced and ruptured, generating patches surrounding the existing particles, or independent patches that contracted generating particles in locations initially free of particles.
During the experiments involving platinum on alumina, the crystallites appeared to be smaller during the heating in oxygen and almost regained their initial size during the subsequent heating in hydrogen. In reality, they extended as an undetectable film during the heating in oxygen and contracted to their initial size during the subsequent heating in hydrogen. Some thermodynamic explanations of the phenomena observed will be provided.
Two-Component Surface Energy Characterization as a Predictor of Wettability for Surface Modified Carbon Blacks in Polymer Matrices
Conductive composites are made by dispersing conductive particles or powders into polymer matrices. The most widely used conductive additive is carbon black. The conductivity of the product composite strongly depends on how well the carbon black is dispersed within the molten polymer. However, untreated carbon black is extremely hydrophobic, and does not wet or disperse well in many molten polymers. It is therefore important to surface treat the carbon black, and to find an optimal level of surface-modification for carbon black for each polymer one wishes to make conductive, in order to best balance the performance-to-cost ratio. The objective of this work was to find a means of predicting the level of surface-modification necessary for carbon black to disperse in any given polymer, prior to running expensive trial extrusions. Our approach was to use a two-component surface energy model to determine surface energy values for the carbon blacks and surface tension values for polymer melts, from contact angle results. By combining the surface energy results with simple wetting thermodynamics, we then established a predictive theory, into which the surface tension properties (polar and dispersive) for a polymer can be input, and the requirements for a carbon black that will effectively disperse (overall surface energy and surface polarity) results. The relevance of this research is that it is extendable to any dispersion problem, to include, especially, pigments in coatings.
Effect of Vapor on Contact Angle
The Young equation presents solid surface free energy, solid-liquid interfacial free energy, and liquid surface free energy as the variables determining the contact angle of a liquid at equilibrium on a smooth nondeformable solid surface. In 1937 Bangham and Razouk (B/R) point out that the solid surface free energy may be changed by adsorption of the vapor of the liquid. This change, they point out, is equal to the decrease in free energy of the solid surface (designated "pie"), on equilibrating it with the vapor of the liquid. The surface chemical community has since interpreted the B/R modification as indicating that adsorption of the liquid vapor always decreases the solid surface free energy and always increases the contact angle. In this overview of work done on this subject during the past decade, it is shown that neither of these interpretations are correct.
Centre des Matériaux P.M.Fourt, B.P.87, 91003 EVRY CEDEX, FRANCE.
Condensation Transport in Triple Line Motion
Triple line motion during wetting or dewetting involves, as yet, incompletely understood phenomena occurring very near the "line" itself. A range of models has been proposed in the literature with two principal bases corresponding to (a) hydrodynamic resistance (Poiseuille shear) balancing capillary force, and (b) liquid molecular transfer obeying a kinetics approach. We postulate the existence of an additional type of transfer due to liquid evaporation/condensation.
The Kelvin equation predicts the ratio of equilibrium vapor over a curved liquid meniscus to that over a flat surface, but this is modified for a thin film on a solid substrate. If contact angle is altered by triple line motion, local meniscus curvature is modified from its equilibrium configuration.
This in turn leads to local condensation from the vapor phase for an advancing contact line and potentially, although this is less obvious, local evaporation at a receding triple line. Thus, an already moving contact line has its motion enhanced by vapor transport. The exact problem is complex mathematically, but we endeavor to obtain a first-order perturbation solution. Resulting equations suggest an alternative explanation for some (but not all!) data previously attributed to a molecular kinetics interpretation.
Higashitani; Department of Chemical Engineering, Kyoto University, Kyoto, 606-8501 JAPAN
Wetting-induced Interaction Between Solid Surfaces
The interaction force between a rigid nanosphere and a flat plate in a vapor of non-polar spherical molecules is explored using a grand canonical Monte Carlo (GCMC) technique. Our findings are as follows. (i) The force between the sphere and plate becomes attractive at the surface distances where capillary condensation takes place in the gap between the surfaces. (ii) The onset of the attractive force becomes further as the relative vapor pressure increases. (iii) The curve of the pull-off force (or the adhesion force) as a function of the relative vapor pressure has a peak. (iv) As the attractive interaction of the surfaces with a fluid molecule decreases, the peak position is shifted to the higher relative pressure and the peak height becomes smaller. (v) At the relative vapor pressure where the pull-off force becomes maximum, the surface coverage of the surface by fluid molecules is about 0.45 regardless of the strength of the surface-fluid attraction; in addition, the radius of curvature of the meniscus is almost equal to the diameter of the fluid molecules. Our simulation results are compared in details with those of force measurements by the atomic force microscope (AFM) and the surface force apparatus (SFA).
1) School of Textile and Fiber Engineering, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta GA
2) Dept of Chemical Eng., McGill University, Montreal, CANADA
Forced Spreading of Complex Fluids on Cylindrical Substrates
In many industrial applications it is necessary to coat a solid substrate with a fluid. This is usually accomplished by dragging the solid object through the fluid of interest at various velocities. At zero velocity the film thickness is zero and at infinite velocity it will be zero as well, since the fluid does not have enough time to form a coating. Thus a maximum wetting speed naturally enters the problem of forced wetting, as has been discussed in the literature. In this talk we are interested in addressing the issue of forced wetting of fluids where the fluids are complex fluids. In particular, we confine our attention to the special case of the solid substrate having a cylindrical geometry (fibers). We consider the case of a nematic fluid and a polymer solution whose concentration is above the overlap concentration being coated onto a polypropylene fiber. In both cases the initial thickness of the fluid film coated on the fiber is proportional to the capillary number, Ca (defined as the ratio of the viscous forces to those due to surface tension), to the first power. We present a model to account for the observed thickness dependence on the capillary number.
Peculiarities of two dimensional wetting
A two dimensional wetting system is investigated. It is realized by a langmuir film of methyl octadecanoate on a pure air/water interface in three phase coexistence (liquid expanded, liquid condensed, gas). The liquid condensed phase is used as the substrate. Droplets consisting of gas phase resp. liquid expanded phase partial wetting the liquid condensed phase are observed.
In a two dimensional wetting system there is no equivalent to the tension of the wetting line. Instead of a wetting line there exists only a point. Additionally, substrate inhomogenities in a two dimensional system can exist only in one direction. Therefore, a two dimensional wetting system is in some respects simpler than a three dimensional system and there is hope to separate effects which are unavoidably mixed in three dimensional systems. The system is complicated by the surface potential of the monolayer phases which causes long range dipolar interactions. The main effects of these interactions are the dependency of the contact angle from the droplet size and the dipole density of the different phases.
The line tensions between the liquid expanded phase and the gas phase resp. liquid condensed phase are investigated. The surface potentials of the different phases are investigated. The contact angle depending on size and dipolar density is compared with a theoretical model. They are in qualitative agreement.
The (surface) shear viscosities of the liquid expanded and liquid condensed phases are investigated. The results are 4*10-7 Ns/m for the liquid condensed phase and an upper limit of 2*10-9 Ns/m for the liquid expanded phase. Fluctuations of the wetting point (the two dimensional analog of the wetting line) are observed.
Rensselaer Polytechnic Institute, Troy, NY 12180-3590
Adsorption, Evaporation, Condensation, and Fluid Flow in the Triple Line Region
Intermolecular interactions in the three phase contact line region, where a liquid-vapor interface intersects a solid substrate, have been extensively studied because of their importance to many equilibrium and non-equilibrium phenomena such as contact angle, adsorption, spreading, evaporation, condensation, boiling, wetting and stability. Since the chemical potential is a function of both temperature and pressure, a Kelvin-Clapeyron model for the triple interline region which includes both these effects on the local vapor pressure gives an enhanced understanding of both equilibrium and non-equilibrium processes. Kinetic theory connects the variation of the local vapor pressure to interfacial mass transfer. The relative importance of interfacial temperature jump to the interfacial pressure jump is evaluated. Theoretical and experimental results are discussed.
For example, combining an isothermal change in the interfacial pressure jump (Kelvin effect) with an isobaric change in the interfacial temperature jump (Clapeyron effect) gives a general equation for the equilibrium film thickness profile at the contact line. Adding a chemical potential gradient, the importance of evaporation followed by multi-layer adsorption can be evaluated and compared to surface diffusion. The infinite stress at the contact line can be easily relieved by evaporation-adsorption in many systems and related to spreading on a heated surface.
An Experimantal Study of Contact Line Pinning by Capillary Rise and Fall
It has been proposed that the pinning of contact line may be described as a kind of dyanmical phase transition in which the speed of the system tends to zero algebraically as the driving force is reduced towards a threshold. We explored this idea by visualizing contact line movement in the rise and fall of a water column inside a capillary tube. While we were able to observe the predicted stick-slip behavior and algebraic slowing near the pinning threshold, we found that the results were not universal. The macroscopic dynamics appear to be ultimately determined by the microscopic state of the wetting film on the surface.
Adhesion Signals of Environmental Particles
We investigate adhesion of soft particles at electrified interfaces as a novel principle for a direct analysis of organic microparticles in aquatic environments. The electrochemical techniques employing microelectrodes and high sensitivity time-resolved recording of amperometric signals allow the detection of single adhesion events in real time. The adhesion and spreading of a single particle causes measurable displacement of surface charge of the electrode at a fraction of electrode surface in a range of positive and negative surface charge densities. The flow of compensating current reflects the dynamics of the adhesive contact formation and the distance of closest approach.
This project is aimed to provide a framework for the development of a new class of adhesion based sensors and electrochemical instrumentation for the research and monitoring of reactive micro and nano particles in aquatic environments. We hypothesize that the interfacial properties of organic particles measured electrochemically are those that govern interfacial interactions (adhesion and aggregation) in nature.
V. Zutic, V. Svetlicic, Interfacial Processes, In: Handbook of Environmental Chemistry, Marine Chemistry ( P.Wangersky, ed.) Vol.5 Part D, pp. 150-165, Springer-Verlag, 2000.V. Svetlicic, N. Ivosevic, S. Kovac, V. Zutic, Charge displacement by adhesion and spreading of a cell: Amperometric signals of living cells. Langmuir 2000, 16, 8217.
V. Svetlicic, A. Hozic, Probing cell surface charge by scanning electrode potential. Electrophoresis (in press)
V. Svetlicic, E. Balnois, V.Zutic, J. Chevalet, N. Vdovic, V. Turk, Gel microparticles in seawater, Langmuir (in press).