Showing papers on "Wetting transition published in 1984"

A model for contact angle hysteresis

Abstract: We discuss the behavior of a liquid partially wetting a solid surface, when the contact angle at equilibrium θ0 is small, but finite. The solid is assumed to be either flat, but chemically heterogeneous (this in turn modulating the interfacial tensions), or rough. For weak heterogeneities, we expect no hysteresis, but the contact line becomes wiggly. For stronger heterogeneities, we first discuss the behavior of the contact line in the presence of a single, localized defect, and show that there may exist two stable positions for the line, obtained by a simple graphic construction. Hysteresis shows up when the strength of the defect is above a certain threshold. Extending this to a dilute system of defects, we obtain formulas for the ‘‘advancing’’ and ‘‘receding’’ contact angles θa, θr, in terms of the distribution of defect strength and defect sharpness. These formulas might be tested by controlled contamination of a solid surface.

Complete and incomplete wetting by adsorbed solids

Abstract: We carry out two calculations which show that a substrate which attracts a solid adsorbate too strongly prevents that adsorbate from wetting it. In the first, the adsorbed film is modeled microscopically as a set of layers in registry with one another. The appropriate thermodynamic potential is minimized at zero temperature with respect to the common lateral spacing and the individual layer heights for systems of up to 20 layers. For Xe, Kr, and Ar adsorbed on graphite, we find that the film thickness should exceed 20 layers, in agreement with experiment. A similar result is found for Ne, in disagreement with experiment. For atoms attracted very strongly, we find a thickness less than 20 layers, and therefore incomplete wetting behavior. In the second calculation, the film is modeled as an elastic continuum. We show that strain caused by the substrate potential induces a long-range force which, in general, prevents complete wetting of a solid film.

Wetting and Growth Behaviors in Adsorbed Systems with Long-Range Forces

Abstract: The growth and possible wetting behaviors of an adsorbed film are studied employing a solid-on-solid model in the presence of a hard wall and external potential $V(h)$ which is of long range. The model is analyzed with the use of position-space renormalization-group methods within the Migdal approximation. The existence of wetting transitions and their nature depends on the asymptotic behavior of $V(h)$ at large distances. We find that critical wetting cannot take place in this model. From what is known of $V(h)$, we conclude that wetting can be observed only along the gas-liquid phase boundary; however, first-order transitions between thin and thick films, which may be experimentally difficult to distinguish from wetting, can be observed along any phase boundary. The nature of the global phase diagram depends on the form of $V(h)$ and several general behaviors are presented. In particular, in the layering subregime we find that the limit of layering critical points is indeed the bulk roughening temperature as had been suggested by de Oliveira and Griffiths. The scaling of these layering critical points is given explicitly.

Statistical Mechanics of a Simple Model of the Nematic Liquid Crystal-Wall Interface II

Abstract: A simple model of the nematic liquid crystal-wall interface introduced in our previous paper1 has been extended. The effect of an impenetrable wall has been taken into account by making a gradient expansion with respect to the orientational order parameter profile, resulting in an additional term in the effective surface potential as a function of the order parameter at the wall. Either a first-order or a second-order surface phase transition is possible with the modified surface potential. In addition to the wetting transition found in our previous paper, another wetting transition, in which a layer of the paranematic intrudes between the nematic and the wall, is also possible.

Wetting Transition in Solid Films: Reflection-High-Energy-Electron-Diffraction Study of Multilayers of C F 4 Adsorbed on Graphite

Abstract: The stability of multilayer formation versus clustering of C${\mathrm{F}}_{4}$ films adsorbed on graphite has been studied by reflection high-energy electron diffraction. It is found that an incomplete-to-complete wetting transition occurs at ${T}_{w}=37$ K, a temperature far below the bulk triple point (89.5 K). The presence of a hysteresis indicates that the transition is first order.

Wetting phenomena with long‐range forces

Abstract: We study the effect of long‐range, power‐law forces on a critical continuous wetting transition within classical mean‐field theory. Repulsive forces destroy the transition; attractive, long‐range forces drive the transition first order and we locate the resulting prewetting line and associated prewetting critical point.

Thin Films, Contact Angles, Wetting

Abstract: Flotation, being the technique of removing selected solids from aqueous suspension by attachment to a second fluid phase, presents the physical chemist with a complex web of problems. No fewer than three interfaces are involved in interplay with thin liquid films, and the time scale of the process virtually ensures that conditions are far from equilibrium.

Marangoni effect in the water wetting of surfactant coated human hair fibers

Abstract: Wetting force curves have been obtained, using a technique based on the Wilhelmy principle, for individual human hair fibers treated with a cationic surfactant (stearyldimethylbenzylammonium chloride). The characteristic stick-slip nature of these curves has been attributed to desorption of the surfactant from the fiber surface into the wetting liquid at the contact line. Differences in surfactant concentration between the vicinity of the contact line and the bulk liquid give rise to surface tension gradient related flows in the film near the contact line, which result in apparent changes in contact angle and wetting force. Such surface flows originating from surface tension gradients are known as Marangoni effects. An apparent increase in contact angle and reduction in wetting force in the dynamic Wilhelmy measurement seem to result when the surface tension of the liquid at the contact line is lower than that of the bulk liquid, giving rise to a surface tension gradient. An instantaneous decrease in contact angle and increase in wetting force seems to be due to the jumping of the meniscus when the surface tension gradient is reduced by the diffusion of the surfactant into the bulk liquid. The amplitude and frequency of the stick-slip depends on the distribution of the surfactant and the nature of its binding to the fiber surface, respectively.

A comment on the order of the wetting transition at a solid–fluid interface

Abstract: We have repeated calculations of Teletzke et al. which describe the wetting transition at some models of a solid–gas interface. Our numerical results differ significantly from theirs in several cases. Contrary to Teletzke et al. we find that this transition is always first order for a Lennard‐Jones 9‐3 substrate potential when the van der Waals integral theory is employed.

Reply to ‘‘A Comment on the order of the wetting transition at a solid–fluid interface’’

Abstract: Evans and Tarazona have raised a disagreement with the results of Teletzke et al. concerning wetting transitions predicted by van der Waals’ integral theory with exponential fluid potential and two different fluid‐solid potentials. We reexamined these results and in agreement with Evans and Tarazona we conclude that the wetting transition is first order when a Lennard‐Jones 9‐3 fluid–solid potential is chosen. The results are supported by the predictions of a simple model, a discontinuous thick film.

Wetting of the container wall as a critical-point phenomenon. III. Wetting by contacting conjugate solutions of ternary systems

Abstract: In the ternary systems benzene-ethanol-water and ethyl acetate-ethanol-water, the contact angles of the meniscus between two immiscible liquid phases (i.e., conjugate solutions) and a solid substrate tend toward 90° as compositions approach the consolute point at a constant temperature; just as, in a binary system, the contact angles between the corresponding three phases have been shown (in the previous paper of this series) to tend toward 90° as the temperature approaches the critical point. The quantitative expression for the variation of the contact angle with the concentration of the cosolvent in the vicinity of the consolute point in a ternary system also bears a formal mathematical resemblance to that found for the corresponding variation of the contact angle with temperature in a binary system.

Nonaxisymmetric equilibrium shapes of a drop on a plane

Abstract: A nonuniform temperature distribution, the presence of surface-active substances and impurities, and also other factors lead to a change in the wetting angle along a plane A study is made of the influence of a small perturbation of the equilibrium contact angle α on the shape of the free surface of the liquid Two cases are considered: a surface of small slope in a gravity field and a nearly spherical shape under conditions of weightlessness The equilibrium shapes of a liquid drop on an inclined plane under conditions of hysteresis of the wetting are also obtained

Influence of a magnetic field on the contact wetting angle in bubble boiling

Abstract: It is experimentally shown that an external magnetic field applied to a bubble boiling region improves wetting of the surface heating the boiling liquid.

Effect of Gravity on Contact Angle Hysteresis

Abstract: The effect of gravity on the stability of equilibrium of a fluid interface in contact with a rough and heterogeneous solid surface is studied for a system with cylindrical symmetry. The equilibrium contact angles measured in relation to the mean solid surface are then evaluated and contact angle hysteresis is discussed. The general effect of gravity is to decrease contact angle hysteresis. The effect depends on the size (width) of the fluid interface. As the size of the fluid interface decreases, it may happen that the advancing contact angle becomes smaller than the receding angle (contact angle inversion). There is also an effect of inclination of the solid surface on hysteresis. The hysteresis behaviour is predicted in detail in limiting cases and by considering a solid surface with sinusoidal grooves.

The behaviour of contact angles of wetting for liquid crystalline drops on glass near the phase transition

Abstract: The experimental values of contact angles of wetting for pazoxyanisole - glass in the nematic and isotropic phase have been obtained. At the point the phase transition, a change in the value of contact angle of wetting is observed. The values of adhesion energy, have been calculated for different orientation of molecules with respect to the glass support. It is well-known /I/that contact angles of drops on solid supports enable an information on interfacial energy γSL to be obtained. A change in the structure of liquid results in alteretion of the contact angle as it a has been shown recently for polymer solutions /2/. It seem to be quite possible that orientational ordering of liquid crystals has to affect the value of contact angles of wetting. Nevertheless, this value was not found to very upon phase transition from the isotropic to liquid crystalline state /3/. We studied the behaviour of contact angle of wetting for a drop of a classical liquid crystalline compound p-azoxyanisole (PAA) in it...

Anisotropy of the Work of Adhesion for a Nemiatic Crystal on a Shaped Support

Abstract: Under dynamic conditions, experimental data on contact angles of wetting for p-azoxyanizole on glass have been obtained. The contact angle of wetting is found to depend on support relief and the values of the contact angle are observed to change at the temperature of the liquid crystal phase transition. The value of energy required for turning a molecule of a nematic crystal by 90° on a shaped surface has been calculated.