Showing papers on "Wetting transition published in 1991"

Contribution to the study of reactive wetting in the CuTi/Al2O3 system

Abstract: The wetting (kinetics of spreading and stationary contact angles) of CuTi alloys on monocrystalline alumina under high vacuum, at a temperature of 1373 K, by the sessile drop technique was investigated. The morphological and chemical characteristics of the metal-ceramic interface were determined by scanning electron microscopy and microprobe analysis. When the results are analysed, three distinct effects of the Ti solute on wetting can be identified and evaluated semi-quantitatively: (a) a reduction in the solid-liquid interfacial tension by adsorption into the liquid side of the interface; (b) a reduction in this tension by formation of a TiO metallic-like oxide layer in the solid side of the interface; (c) a contribution to the wetting driving force due to the free energy released at the interface by the reaction between Ti and Al2O3.

Complete Wetting of a Rough Surface: An X-Ray Study

Abstract: The evolution of the surface structure of a wetting film on a rough surface as a function of the film thickness has been studied by x-ray specular reflection and surface diffusion scattering. For thin films (\ensuremath{\lesssim}60 \AA{}) the liquid surface is characterized by static undulations induced by the roughness of the substrate; however, with increasing film thickness the structure is dominated by thermally induced capillary waves. The data are quantitatively described by a model with exclusively van der Waals liquid-substrate interactions.

Penetration of tin and zinc along tilt grain boundaries 43° [100] in Fe-5 at.% Si alloy: Premelting phase transition?

Abstract: Tin and zinc penetration along the tilt grain boundary 43° [100] in b.c.c. Fe-5 at.% Si alloy is studied in the temperature range from 652 to 975°C. Wetting transition of grain boundary by the tin-rich melt at Tw = 810 ± 5°C is observed. About Tw there is a thin wetting film at grain boundary. With zinc penetration along the grain boundary a wetting film has been observed at all temperatures studied. Behind that film there is a region with an unusually high diffusivity of zinc, and below that region there is a region of “ordinary” grain boundary diffusivity. Such a phenomenon may be explained in terms of the phase transition “grain boundary-thin wetting film on the boundary”, which is commonly known as a premelting phase transition. A model is proposed which explains the form of the temperature dependence of the concentration cBt, at which such transition occurs, and, in particular, the influence of the “paramagnet-ferromagnet” transition in the bulk on the premelting transition. The influence of the temperature dependence of the volume solubility limit, c0, on the cBt(T) dependence is also discussed. In critical region below Curie point Tc critical exponents d of magnetic part of activation free energy of bulk and grain boundary diffusion are calculated. Critical index d for grain boundary diffusion by premelting layer, as well as activation energy in paramagnetic region, lies in the interval between bulk values of d and estimation of d for truly two-dimensional grain boundary diffusion.

Dynamics of surface enrichment : a theory based on the Kawasaki spin-exchange model in the presence of a wall

Abstract: A mean-field theory is developed for the description of the dynamics of surface enrichment in binary mixtures, where one component is favored by an impenetrable wall. Assuming a direct exchange (Kawasaki-type) model of interdiffusion, a layerwise molecular-field approximation is formulated in the framework of a lattice model. Also the corresponding continuum theory is considered, paying particular attention to the proper derivation of boundary conditions for the differential equation at the hard wall. As an application, we consider the explicit solutions of the derived equations in the case where nonlinear effects can be neglected, studying the approach of an initially flat (homogeneous) concentration profile, where the surface concentration is the same as in the bulk, towards equilibrium, where some surface enrichment of one component is present. It is shown that the concentration profile shows a transient minimum (the distance of this minimum from the wall diverges as √ as the timet in this “quenching experiment” goes to infinity). Both the lattice theory and the continuum theory are shown to yield equivalent results. Finally, an outlook to possible extensions (dynamics of wetting transition etc.) and further applications is given.

Wetting and premelting phase transitions in 38° [100] tilt grain boundary in (Fe-12 at.% Si)-Zn alloy in the vicinity of the A2-B2 bulk ordering in Fe-12 at.% Si alloy

Abstract: Zinc penetration along grain boundary tilt of 38° [100] in bicrystals of the Fe-12 at.% Si alloy is studied by means of an electron microprobe analysis. At Tord = 770°C the alloy undergoes an A2-B2 ordering transition. At Tw = 749°C a wetting transition occurs in grain boundaries of the alloy. At temperature above Tw, the diffusion permeability of the grain boundaries changes abruptly at Cbt. This change may be explained, assuming a premelting transition to occur in grain boundaries. The Cbt (T) line on the phase diagram terminates by critical point at Tcrit = 807 ± 2°C. A protrusion can be seen on the Cbt (T) line and on the solvus line near Tord. The maximum on the temperature dependence of the diffusion permeability of grain boundaries corresponds to approximately the same temperature. The hypothesis of a premelting transition on grain boundaries provides possibility of explaining all the observed phenomena, in particular, the disappearance of wetting and premelting below the ordering temperature in the bulk.

Effect of Confinement on Wetting and Drying Between Opposing Boundaries

Abstract: We study quasi-wetting transitions in confined systems in which capillary condensation is suppressed. In particular, we are concerned with adsorbates between opposing walls (one wall favours wetting, the other drying). We employ an Ising model and calculate the global phase diagram for a slab of width L and boundaries with opposite surface fields, in Landau theory. We find novel first-order, critical, and tricritical quasi-wetting transitions, which converge smoothly, for L → ∞, to the familiar wetting transitions. We question the recently proposed novel mechanism for critical-point shifts in films.

Relaxation modes of the contact line of a liquid spreading on a surface

Abstract: THE dynamics of spreading of liquids on solid surfaces are important in several practical and industrial processes, such as painting, lubrication and oil recovery in a porous medium. In all these situations, the motion of the 'contact line' at the edge of the advancing fluid is perturbed by the roughness and chemical contamination of the solid, which distort the line and give rise to poorly understood hysteresis effects. To elucidate these effects, one must begin by studying simplified processes, for example the time response to a perturbation of the line on an ideal surface. Here we describe a study of the dynamics of a contact line under controlled conditions of partial wetting, in which we determine the relationship between the characteristic decay time and the wavelength of a perturbation imposed on the line. When relaxation is controlled by the fluid viscosity, there is good agreement with theoretical predictions, but we observe deviations at long wavelengths which we ascribe to gravitational effects that weaken the contact line's elastic response.

Microemulsions: Structures, surfactant layer properties and wetting transitions

Abstract: We review briefly the basic known features of microemulsion structures, emphasizing the importance of the surfactant layer bending elasticity. The results for water-alkane-nonionic-surfactant systems, confirming the close relationship between the maximum characteristic size in the microemulsion and the persistence length of the surfactant layer, are presented. We show that microemulsions are formed when the surfactant layer bending moduli are in a well defined range: if the bending modulus is too large, ordered lamellar phases are obtained, while if it is too small, the surfactant film cannot form, and the medium is a structureless molecular mixture. The evolution between microemulsions and molecular mixtures is continuous; its relationship with the wetting transition between the microemulsion and the two excess phases is discussed.

Partial internal wetting of catalyst particles: Hysteresis effects

Abstract: Condensation-evaporation hysteresis associated with the internal wetting of catalyst particles is examined here by the Kelvin and Cohan equations. It is shown that in the absence of information on the thermal history, it is difficult to predict the extent of internal wetting and the associated reaction rate, for states with partial internal wetting, with the actual rate lying anywhere between the limits specified by the Kelvin and Cohan equations. Hysteresis effects arising from thermal cycling are discussed, and the predictions are shown to agree with the experimental data of Kim and Kim (1981a). Experimentally observed transitions between states with markedly different extents of internal wetting and with associated sudden evaporation or condensation are also rationalized by the theory.

Determination of the location and order of the drying transition with a molecular-dynamics simulation

Abstract: The wetting transition is mostly found to be first order, but the nature of the drying transition in realistic systems is debated. We have simulated the wetting and drying of a wall and, by a careful inspection of the variation of the contact angle on the approach of the transition, find strong evidence for a continuous drying transition. Moreover, the drying point is located an order of magnitude more accurately with respect to previous attempts.

Wetting transitions at a solid-fluid interface

Abstract: Nonlocal density functional free energy theories are used to investigate the nature of the wetting transition of a Lennard‐Jones fluid near a Lennard‐Jones wall. In particular, the wetting behavior is investigated for argon films on a solid carbon dioxide substrate for which the Lennard‐Jones parameters are known. In accordance with previous predictions of local density functional theories, the transition is found to be of first order. The locus of the prewetting line of the Ar–CO2 system is calculated. Density profiles obtained from the Tarazona and generalized hard rod models are both in qualitative agreement with Monte Carlo simulations of Finn and Monson, although Tarazona’s model performs better quantitatively.

Model for line tension in three-phase equilibrium

Abstract: We propose a model free-energy functional of two order parameters with which to calculate the interfacial and line tensions in three-phase equilibrium. The Euler-Lagrange equations for the free-energy minimum are solved exactly, yielding the spatial variation of the order parameters analytically. In terms of a parameter b2 in the model the three interfacial tensions, in dimensionless form, are 12 (1+b2), 12 (1+b2) and 2. When b2=3 the three phases play symmetrical roles and the line tension, again in the appropriate units, is calculated to be −6/π + 2√3 = −0.755…. A wetting transition, where the sum of two of the interfacial tensions becomes equal to the third, occurs as b2 → 1+. A quantity that approximates the line tension is found to vanish proportionally to the first power of the vanishing contact angle as the wetting transition is approached.

Polymer adsorption in a poor solvent

Abstract: We discuss polymer adsorption in a poor solvent. For a single chain we find a wetting transition on an attractive wall when the thermal correlation lenght ξ is of the order of the adsorption lenght D. If D is larger than ξ, for weak adsorption, the polymer chain partially wets the solid surface in the same way as a small liquid drop ; if D is smaller than ξ, strong adsorption, the chain forms a flat pancake of thickness D. The adsorption of a polymer solution in a poor solvent is considered only when the chains wet the surface. At very low bulk concentration the adsorbed chains form a two dimensional dilute polymer solution which is either in a poor solvent or in a good solvent depending on the value of the attractive excluded volume. When this two dimensional solution is in a poor solvent it undergoes a first order demixing transition analogous to the so-called prewetting transition. At a higher bulk concentration the surface layer becomes continuous, its thickness grows and diverges when the bulk concentration reaches the coexistence curve. We also study the wetting transition of the polymer solution. These results can be applied to diblock copolymer adsorption in a selective solvent. The prewetting transition can also exist in this case if the soluble block is not too large.

Critical Droplets in First-Order Wetting Transitions

Abstract: It is shown that in the metastable region of a first-order wetting transition critical droplets occur which sit on the wall of the system. Under general conditions the critical droplet has a single unstable growth mode but its shape is stable with respect to all other fluctuations. These properties can be derived analytically for a simple model system with a one-dimensional wall.

Short-distance expansion for the local susceptibility and pair correlation function at continuous wetting transitions

Abstract: A scaling ansatz is proposed for the density profile rho (z) (with z the distance normal from the substrate) for a fluid undergoing a continuous wetting transition at a wall. The ansatz predicts that, in the fluctuation dominated regime, the local susceptibility ( delta rho (z)/ delta mu )T, with mu the chemical potential, has a simple power law position dependence for short distances z<< zeta perpendicular to , the perpendicular correlation length. The power law is determined by the wetting critical exponents, which is different for complete and critical wetting. The scaling predictions are confirmed in dimension d=2 by explicit analysis for an interfacial Hamiltonian model of the wetting transition. For critical wetting a scaling ansatz for the form of the transverse moments of the two-point correlation function G is postulated which now yields the z1, z2 dependence of G in terms of critical exponents. Calculations again confirm the scaling theory for d=2. The analysis highlights the different qualitative and quantitative features of the response functions for critical and complete wetting for fluids with short-ranged forces.

A search for prewetting in a molecular dynamics simulation

Abstract: A first-order wetting transition should be accompanied by a prewetting transition. Although many such wetting transitions have been reported, clear observations of prewetting are rare. We report on a search for prewetting in a wall-fluid system that has previously been shown to exhibit a first-order wetting transition. The fluid consists of a vapour phase whose density is increased towards the coexistence density while the coverage of the wall-vapour interface is monitored. This approach to coexistence is studied as a function of the interaction strength between the wall and the fluid. Although we do recognize the presence of a wetting transition in the adsorption curves, the more subtle effects associated with prewetting cannot be observed unambiguously.

Wetting and adsorption at substrates and grain boundaries

Abstract: Wetting near grain boundaries is a fairly new topic in comparison with wetting at substrates (or walls). A Landau theory is reviewed that describes the wetting and interfacial unbinding transitions near grain boundaries (or defect planes). The results are compared with those for wetting at substrates. Restricting attention to short-range substrate-adsorbate forces, critical-point wetting (as conjectured by Cahn) applies for substrates, but not for grain boundaries, unless the grain boundary remains ordered at the bulk critical point. It is argued that, quite generally, surface order at the bulk critical point implies critical-point wetting. In this article I review some key results of our studies of adsorption and wetting near grain boundaries [1, 2]. At the same time I present new arguments and conjectures, which clarify and update the framework of understanding that is emerging.

Formation of Concave-Convex Interface Shape during Oriented Eutectic Growth†

Abstract: The concave-convex interface shape formation of oriented eutectic has been analysed in microscopic scale. The proposed model has considered some physical phenomenona of eutectic growth, in particular preceding of the wetting phase by the leading one. The mechanism of curvature formation of the leading phase is different from that of the wetting one. The worked out model has been refered to the microscopic observations of the Pb–Cd eutectic interface shape. The sinusoidal approximation to the interface has been used to divide the lead distance into the leading phase protrusion and the wetting phase retraction. The behaviour of the contact angle of the wetting phase as a resultant of different physical factors' interaction during eutectic growth has been underlined.

The wetting transition in a random surface model

Abstract: We continue our analysis of the phase diagram of a discrete random surface, with no “downward fingers,” lying above a flat two-dimensional substrate. The surface is closely related to the 2D Ising model and its free energy is exactly solvable in much (but not all) of the phase diagram. There is a transition at temperatureT w from a high-T infinite height or wet phase to a low-T finite height or partially wet phase. Previously it was shown that when a parameterb, related to the contact interaction, is positive,T w is independent ofb and there is a logarithmic specific heat divergence asT w is approached fromeither side. Here we show that forb<0,T w does depend onb and there isno thermodynamic singularity from the wet phase. The partially wet phases forb⩽0 andb>0 differ in the absence or presence of a monolayer covering the entire substrate; this results in a first-order transition across the lineb=0,T

Theory of interfacial phase transitions in surfactant systems

Abstract: The spin-1 Ising model, which is equivalent to the three-component lattice gas model, is used to study wetting transitions in three-component surfactant systems consisting of an oil, water, and a nonionic surfactant. Phase equilibria, interfacial profiles, and interfacial tensions for three-phase equilibrium are determined in mean field approximation, for a wide range of temperature and interaction parameters. Surfactant interaction parameters are found to strongly influence interfacial tensions, reducing them in some cases to ultralow values. Interfacial tensions are used to determine whether the middle phase, rich in surfactant, wets or does not wet the interface between the oil-rich and water-rich phases. By varying temperature and interaction parameters, a wetting transition is located and found to be of the first order. Comparison is made with recent experimental results on wetting transitions in ternary surfactant systems.

How universal is critical-point wetting?

Abstract: The phase transition from partial to complete wetting and its relation to bulk and surfacc critical phenomena are examined for various categories of wetting problems. Landau theory calculations are combined with scaling arguments. Complete wetting near the bulk critical point ("critical-point wetting" as proposed by Cahn, and developed systematically by Nakanishi and Fisher) appears associated with the presence of spontaneous or imposed surface order at the bulk critical point, "/~.. Conversely, partial wetting near 7"~ appears associated with a vanishing surface order parameter at T c. Thus, partial and complete wetting appear associated with distinct universality classes of surface critical phenomena.

A technique for measuring contact angles at particles

Abstract: The behavior of particles at the phase transition between a gas or liquid and another liquid phase may be characterized by the so-called wetting (or contact) angle. In contrast to plane surfaces, which allow such measurements to be conducted relatively easily, the determination of the wetting angle between a liquid and the surfaces of fine particles demands extensive preparative measures. The method adopted by Heertjes and Kossen [12] for example, involves compressing a tablet from the powder in question, from which the wetting angle may then be derived. The compression process naturally modifies the superficial structure of the particles, and hence their wetting characteristics. This paper describes a preparation technique with which the wetting angle between a liquid and fine particles can be derived without modifying the surface texture. This involves coating and adhering a monolayer of the powder on a carrier plate and measuring the composite wetting angle of the particles and the adhesive film. The specification of the adhesive's wetting angle and areal porosity of the carrier medium allows the wetting angle of the given powder subsequently to be evaluated.

Fundamentals of wetting and spreading with emphasis on soldering

Abstract: Soldering is often referred to as a mature technology whose fundamentals were established long ago. Yet a multitude of soldering problems persist, not the least of which are related to the wetting and spreading of solder. The Buff-Goodrich approach to thermodynamics of capillarity is utilized in a review of basic wetting principles. These thermodynamics allow a very compact formulation of capillary phenomena which is used to calculate various meniscus shapes and wetting forces. These shapes and forces lend themselves to experimental techniques, such as the sessile drop and the Wilhelmy plate, for measuring useful surface and interfacial energies. The familiar equations of Young, Wilhelmy, and Neumann are all derived with this approach. The force-energy duality of surface energy is discussed and the force method is developed and used to derive the Herring relations for anisotropic surfaces. The importance of contact angle hysteresis which results from surface roughness and chemical inhomogeneity is presented and Young's equation is modified to reflect these ever present effects. Finally, an analysis of wetting with simultaneous metallurigical reaction is given and used to discuss solder wetting phenomena. 60 refs., 13 figs.

Does the longer-range wall potential in generalized Sullivan models favour a first-order wetting transition?

Abstract: The discussion based on the generalized Sullivan models for the wetting transition has unit now argued that the longer-range wall potential tends to favour a first-order wetting transition. The present paper, however, proposes an analytically derived counter-example.

Effects of temperature and conditioning on contact lens wetting angles.

Abstract: Because wettability is not always examined under standard conditions, we investigated the temperature dependence of saline wettability on unconditioned and conditioned polymethylmethacrylate (PMMA), cellulose acetate butyrate (CAB), and three silicone acrylate lens materials. Sessile drop contact angles were measured in a humidity chamber at 23 degrees C and 34 degrees C using laser-assisted contact angle goniometry. In separate experiments, saline-stored and preconditioned lenses were examined either with or without rinsing. Sessile drop contact angles at 34 degrees C were within 2 degrees to 5 degrees of the room temperature values for both conditioned and unconditioned lenses, demonstrating a negligible temperature dependence. At both temperatures, the conditioned PMMA, CAB, silafocon A, and pasifocon C lenses wet slightly better, by 1 degree to 12 degrees, than unconditioned lenses. However, this increase was only significant with PMMA and silafocon A (P less than 0.05) and reversed when the preconditioned lenses were rinsed repeatedly in saline and reexamined. The results suggest that for these materials: 1) in vitro saline contact angles do not approach those seen on the eye, and this discrepancy can not be explained by temperature or conditioning; and 2) conditioning does not increase material wettability but merely forms a temporary hydrophilic interface that is more wettable than the lens material.

The Contact Angle of Liquids in Porous Media

Abstract: Summary We developed a method to determine the contact angle of a liquid penetrating a porous medium by a non-linear fit of the continuously measured liquid uptake over a period of some minutes. This method makes it possible to measure the contact angle without any additional assumptions on completely wetting reference liquids. The method was applied to porous glass frits with water, toluene, and different silicone oils. The measured contact angles ranging from 60° to 80° are much higher than the equilibrium contact angles on glass.