Showing papers on "Wetting transition published in 1995"

Wetting effects on the spreading of a liquid droplet colliding with a flat surface: Experiment and modeling

Abstract: In this paper an experimental and theoretical study of the deformation of a spherical liquid droplet colliding with a flat surface is presented. The theoretical model accounts for the presence of inertia, viscous, gravitation, surface tension, and wetting effects, including the phenomenon of contact‐angle hysteresis. Experiments with impingement surfaces of different wettability were performed. The study showed that the maximum splat radius decreased as the value of the advancing contact angle increased. The effect of impact velocity on droplet spreading was more pronounced when the wetting was limited. The experimental results were compared to the numerical predictions in terms of droplet deformation, splat radius, and splat height. The theoretical model predicted well the deformation of the impacting droplet, not only in the spreading phase, but also during recoiling and oscillation. The wettability of the substrate upon which the droplet impinges was found to affect significantly all phases of the spre...

Viscoelastic Dissipation in Wetting and Adhesion Phenomena

Abstract: When a liquid drop is placed on a smooth, rigid, solid substrate, it spreads until the final thermodgnamic equilibrium is attained. The kinetics of spreading of the drop are controlled by conversion of capillary potential energy into viscous dissipation within the liquid. However, if the solid is sufficiently soft, a local deformation, or »wetting ridge«, may form near the wetting front and the motion of the latter may lead to viscoelastic dissipation. We describe cases in which viscoelastic dissipation dominates and thus where spreading speed depends on bulk properties of the solid, rather than on liquid viscosity. This behavior in wetting can be considered to be analogous to dissipation phenomena in the adhesion of elastomers. Therefore, a comparison is made between wetting and adhesion dynamics. It appears clearly that a parallel can be drawn between the wetting of rubber and rubber adhesion to glass when both phenomena involve the same viscoelastic material. Kinetics of formation and breaking of corresponding interfaces is controlled by the damping properties of the soft solid substrate. In this paper, the viscoelastic properties of elastomers are described by two parameters, n and U o ; n is the usual speed power factor (n0.5-0.6) and U o a characteristic speed below which a fraction of the elastic strain energy in the wetting ridge is dissipated

Wetting behaviour in the Al-Si/SiC system: interface reactions and solubility effects

Abstract: The sessile drop technique was used to study the wetting behavior of Al-Si alloys on SiC sintered ceramic substrates under vacuum in the 700--1,100 C temperature interval. Al-Si alloys with Si concentrations up to 50% were tested. An expected non-wetting/wetting transition was observed at 900--1,000 C due to the presence of an alumina film surrounding the molten alloy. At higher temperatures wetting was observed and the Si concentration of the alloy has a marked effect on the measured contact angles, {theta}. At 1,100 C {theta} decreases from 55 to 25{degree} when instead of pure Al an Al12.3%Si or an Al16.6%Si alloy is used. The suppression of the formation of a continuous Al{sub 4}C{sub 3} layer at the interface and a process of dissolution and reconstruction of the SiC surface, due to the increased Si concentration of the Al-Si alloys, are the key factors to explain the observed behavior.

Influence of surface-roughness on the wetting angle

Abstract: A this paper the influence of surface roughness on contact angles in the system of liquid Al wetting solid surfaces of Al2O3 has been studied. It was observed that contact angles of liquid Al vary significantly on different rough surfaces of Al2O3. A model is proposed to correlate contact angles with conventional roughness measurements and wavelengths by assuming a cosine profile of rough grooves with a Gaussian distribution of amplitudes. In comparison with the experimental results, the model provides a good estimate for describing the influence of surface roughness on contact angles of liquid Al on Al2O3.

Wetting transition on grain boundaries in Al contacting with a Sn-rich melt

Abstract: The contact angle θ at the intersection of a grain boundary in Al bicrystals with the solid Al/liquid Al−Sn interphase boundary has been measured for two symmetric tilt {001} grain boundaries with tilt angles ϕ of 32° and 38.5°. The temperature dependencies θ(T) present the evidence of the grain boundary wetting phase transition at Tw. The observed hysteresis is consistent with the assumption that the wetting transition is of first order. The determined discontinuity in the temperature derivative of the grain boundary energy is−5.6 μJ/m2K (Tw1=617°C) for the boundary with a low energy (ϕ=38.5°) and −17 μJ/m2K (Tw2=604°C) for the grain boundary with a high energy (ϕ=32°).

Extensive wetting due to roughness

Abstract: Typically, a small mass of eutectic SnPb solder wets a copper surface and flows radially outward to form a hemispherical shape with a contact angle of approx. 15–20 deg. When a similar mass of solder wets a thick electroless copper coated substrate, rapid radial flow commences and surprising new effects occur. Thick coats of electroless copper have a nodular surface structure and spreading on it does not subside until all solder is consumed. When the nodular structure is wetted by solder a “coastline” with many nearby “islands” are defined. Photos of regions at the wetting front were taken in the backscatter imaging mode of an SEM. These images show that solder wets the valleys between the surface nodules forming a delicate, lacy arrangement. The geometry of this “coastal” solder structure is described as fractal-like having a dimension D = 1.38 making it similar to drying fronts and cloud configurations. The importance of surface roughness in wetting phenomena is discussed in the light of an extensive history on the subject. It is shown that for spontaneous flow, assisted by roughness, the surface geometry must consist of local angles that are larger than the equilibrium contact angle. Kinetics of the wetting process are demonstrated by image analysis of wetted area taken from videotaped experiments. These experimental kinetics are shown to be similar in form to flow in open channel capillaries.

Fluid wetting on molecularly rough surfaces

Abstract: We examine the effects of structural changes of solid surfaces on fluid wetting by performing molecular dynamics simulations on a model system. The system consists of a Lennard‐Jones fluid confined between two identical solid surfaces under conditions that induce the formation of both a liquid and vapor region. The smooth surface is three layers of solid particles arranged in a perfect fcc lattice. We represent a molecularly rough surface by adding to the smooth surface a fourth layer consisting of less than a full monolayer of solid particles. This model corresponds to some physical situations where there is surface irregularity, such as surface defects due to vacancies or surface structures caused by adsorption of foreign particles onto a perfect surface. The fluid density profiles near the rough surfaces are different from those near the smooth surface and reflect the detailed molecular structure of the surfaces. More importantly, the changes in surface structure alter significantly the contact angle b...

The liquid-vapour phase transition in fluid mercury

Abstract: The paper discusses recent experimental results in the liquid-vapour critical region of metals which show that the existence of the metal-non-metal transition noticeably influences the electronic, thermodynamic, structural and interfacial features of the fluid. The main emphasis is on surface-induced phenomena. Reflectivity experiments on mercury against an optically transparent sapphire window close to the vapour-liquid coexistence curve reveal clearly the existence of a prewetting transition of mercury on the sapphire substrate. The transition line, which terminates at high temperatures at a prewetting critical temperature T pwc lying below the bulk critical temperature T c and at low temperatures at the wetting transition temperature T w lies close to the bulk vapour-liquid coexistence curve.

Nematic wetting at the free surface of 4‐cyano‐4′‐n‐alkyl‐biphenyls

Abstract: The free surface of a homologous series of low molecular weight liquid crystals 4‐cyano‐4′‐n‐alkyl‐biphenyls (nCB, n=5,6,7,8) is studied by reflection ellipsometry. On approaching the bulk isotropic–nematic transition temperature TNI from the isotropic phase, the growth of a nematic layer at the surface is observed. The director orientation in the layer is identified as homeotropic. The ellipsometric data are analyzed in terms of a perturbation calculation. It is shown that for 7CB and 8CB very close to TNI, where the layer has a thickness d≥50 A, both d and the order parameter S0 at the surface can be determined. In all other cases only one parameter, which corresponds to the nematic coverage, can be extracted from the data. Results are discussed in terms of a wetting process. Partial wetting by the nematic phase is observed for n=6,7,8. On approaching TNI from lower temperatures where the bulk is in the nematic phase a small increase of excess surface order is measured which is interpreted as a homeotropic nematic wetting layer possessing higher order than the nematic bulk.

Maximum overheating and partial wetting of nonmelting solid surfaces.

Abstract: Surfaces which do not exhibit surface melting below the melting point (nonmelting surfaces) have been recently observed to sustain a very large amount of overheating. We present a theory which identifies a maximum overheating temperature, and relates it to other thermodynamical properties of the surface, in particular to geometrical properties more readily accessible to experiment. These are the angle of partial wetting, and the nmelting-induced faceting angle. We also present molecular dynamics simulations of a liquid droplet deposited on Al(111), showing lack of spreading and partial wetting in good agreement with the theory. PACS numbers: 68.10.Cr, 68.45.Gd, 61.50.Jr

Nucleation at first-order wetting transitions

Abstract: A brief review is given of the status of both theory and experiments of nucleation phenomena at first-order wetting transitions, focusing on the early stages of the decay of metastable thin and thick wetting films. Generally, the decay of metastable films is initiated by the formation of critical nuclei, which for a metastable thin film are droplets on the wall of the system, and holes in the wetting layer for a metastable thick film. The shapes of critical droplets and critical holes are characterized in the various regions of the wetting phase diagram. The nucleation rate of critical nuclei is determined, which is a measure of the lifetime of the metastable states. The connection of the theoretical results with recent experiments on nucleated wetting layers is discussed. An outlook on open problems concludes the review.

Thermodynamic Analysis of Solute Effects on Contact Angles. Equilibrium Adsorption of Cationic Surfactants at Silica-Vapor and Silica-Water Interfaces

Abstract: It is shown that the equilibrium wetting tension, defined thermodynamically as the difference between the solid-vapor and solid-liquid interfacial tensions, can be measured to the same high degree of certainty and accuracy as the surface tension of liquids. Just as for surface tension isotherms the wetting tension isotherms can be analyzed through the Gibbs adsorption equation. This allows for unambiguous evaluation of the adsorption difference at the two interfaces. As an example we have investigated the wetting of smooth and homogeneous silica glass surfaces by solutions of cetyltrimethylammonium bromide (CTAB). At small concentrations (below the point of zero charge) equilibrium monolayer adsorption is larger at the solid-vapor interface than at the solid-liquid interface. At higher concentrations (near the critical micelle concentration) the adsorption difference changes sign due to formation of bilayer at a solid-liquid interface. Adsorption reequilibration within the hydrophobic monolayer occurs when a three-phase contact line is displaced. This reequilibration is slow and explains large contact angle hysteresis typically observed when measurements are done under nonequilibrium conditions.

Review of Some of the Experimental Evidence for the Novel Wetting of Helium on Alkali Metals

Abstract: We review some of the experimental evidence for the novel wetting and the behavior of helium on alkali metal substrates. We begin with a brief summary of the essential theoretical predictions, describe the initial confirmation experiments and then discuss some of the more detailed recent measurements which document the behavior of helium on these weak binding substrates.

Surface-induced alignment at model nematic interfaces

Abstract: We have applied the generalized van der Waals theory to a model liquid crystal which includes, explicitly, all of the second-order terms in the spherical harmonic expansion of the anisotropic intermolecular potential. We have investigated the orientational order induced by each one of these terms, as well as the order resulting from the competition of various terms included in the potential. It was shown that, for appropriate choices of the relative strengths of the spherical harmonic coefficients, the theory is capable of accounting qualitatively for all the orientational effects observed at nematic interfaces, including tilted orientations. In particular, different molecular alignments at the nematic-vapor and nematic-isotropic interfaces of a given nematogen, were described as the result of competing terms in the anisotropic interactions. Additionally, we have shown that temperature-driven orientational transitions may occur, in systems characterized by this type of interaction. For a given choice of parameters, we have also found an orientational transition, which is related to a wetting transition at the nematic-vapor interface. Finally, it was shown that complete wetting of the isotropic liquid-vapor interface by the nematic phase may be destroyed as a result of the competition between different terms in the potential. Similarly, a reentrant wetting transition at the nematic-vapor interface by the isotropic phase was found, as a result of this competition, at the nematic-vapor interface.

Interfacial wetting in a liquid binary alloy

Abstract: We report the first study of interfacial wetting at the liquid-vapour surface of metallic gallium-bismuth alloys approaching the miscibility gap. Ellipsometry has been used to probe the interface continuously with increasing temperature along the coexistence curve. Below the monotectic (eutectic) temperature Tmono a liquid Ga-rich phase is in equilibrium with solid bismuth and the real part epsilon 1 of the complex dielectric function measured at the liquid surface clearly reflects Ga-like behaviour. Above Tmono the second liquid Bi-rich phase becomes stable and epsilon 1 changes from Ga-like to Bi-like behaviour. We conclude that the Bi-rich phase, which has the higher mass density, now covers the low-density phase. This can be consistently interpreted as complete wetting of the interface between the vapour and the Ga-rich phase by a coexisting Bi-rich layer.

Evaporation Preempts Complete Wetting

Abstract: A volatile liquid wetting a substrate forms a thick, uniform film in equilibrium with a saturated vapor phase. We show that under evaporating conditions such a liquid is effectively non-wetting, i.e. stable as a droplet on top of a thin film. This is in agreement with recent observations. Here we calculate the contact angle of the droplet as a function of the liquid-vapor surface tension and the partial pressure. This effect will be useful as a means of probing the microscopic interfacial forces involved in wetting via macroscopic measurements.

Complete wetting in three dimensions I. Connection between correlation functions and generalized effective Hamiltonian theory

Abstract: We study the pair correlation function for an inhomogeneous fluid or Ising-type spin system near a wall with particular attention to the complete wetting phase transition. We show that one can unify a generalized interfacial Hamiltonian theory with a mean-field treatment of correlations provided we follow a systematic scheme for reconstructing order-parameter fluctuations. Near a complete wetting transition it is necessary to use a model effective Hamiltonian H I (2) [ l 1 , l 2 ] which is a functional of two collective coordinates in order to properly describe the coupling between fluctuations near the wall and the depinning fluid (αβ) interface. This gives an accurate description of the Ornstein-Zernike-like fluctuations of particles located near the αβ interface and the non-Ornstein-Zernike behavior of correlations near the wall. We show that the off-diagonal elements of the stiffness matrix characterizing H I (2) [ l 1 , l 2 ] are related to singular behaviour of the free-energy.

Wetting kinetics of liquid aluminum on an al2o3 surface

Abstract: The wetting process of a liquid aluminium drop on a solid Al2O3 surface has been investigated. Two different models have been proposed: a hydrodynamic model based on the effect of viscosity as well as a diffusion model which considers diffusional effects. The prediction from the models was compared with experiments. It was found that the wetting process of a liquid aluminium drop on an Al2O3 surface can be represented by the diffusion model rather than by the hydrodynamic model. In contrast, the hydrodynamic model could be employed to describe the wetting process of a liquid polymer drop on a solid.

Line and boundary tensions at the wetting transition: Two fluid phases on a substrate

Abstract: We develop and analyze a mean‐field model free energy that describes two fluid phases on a substrate in order to calculate the (numerically) exact line and boundary tensions, on approach to the first‐order wetting transition. A theory based on the van der Waals theory of gas–liquid interfaces is used. We implement a multigrid algorithm to determine the two‐dimensional spatial variation of the density across the three‐phase and boundary regions, and hence, the line and boundary tensions. As the wetting transition is approached, the tensions approach the same, finite, positive limit with diverging slopes. We compare our results with those of recent related work.

Wetting, prewetting and surface transitions in type-i superconductors

Abstract: Within the Ginzburg-Landau theory, which is quantitatively correct for classical superconductors, it is shown that a type-I superconductor can display an interface delocalization or “wetting” transition, in which a macroscopically thick superconducting layer intrudes from the surface into the bulk normal phase. The condition for this transition to occur is that the superconducting order parameter |ψ|2 is enhanced at the surface. This corresponds to a negative surface extrapolation length b. The wetting transition takes place at bulk two-phase coexistence of normal and superconducting phases, at a temperature TD below the critical temperature Tc, and at magnetic field HD = Hc(TD). The field is applied parallel to the surface. Surprisingly, the order of the wetting transition is controlled by a bulk material constant, the Ginzburg-Landau parameter κ. This is very unusual, since in other systems (fluids, Ising magnets,…) the order of the wetting transition depends on surface parameters that are difficult to determine or control. For superconductors, first-order wetting is predicted for 0 ≤ κ < 0.374, and critical wetting for 0.374 < κ < 1/√2. In the case of first-order wetting, the prewetting extension is also found. Unlike in standard wetting problems, the prewetting line does not terminate at a critical point but changes from first to second order at a tricritical point. Twinning-plane superconductivity (TPS) is reinterpreted as a prewetting phenomenon. The possibility of critical wetting in superconductors is especially interesting because this phenomenon has largely eluded experimental verification in any system until now. Furthermore, superconductors provide a realization of wetting in systems with short-range (exponentially decaying) interactions. This is very different from the usual long-range (algebraically decaying) interactions, such as van der Waals forces, and has important consequences for the wetting characteristics.

Complete wetting in three dimensions II. Renormalization of the capillary parameter

Abstract: We present a renormalization group (RG) analysis of critical effects at the complete wetting transition for three-dimensional systems with short-ranged forces based on the two-field Hamiltonian H I (2) [ l 1 , l 2 ] introduced in paper (I). We show that the effective value of the capillary parameter determining critical amplitudes is changed under RG flow due to coupling between spin fluctuations near the wall and the depinning fluid interface. We compare our predictions with data from recent Ising model Monte Carlo studies which probe finite-size effects at complete wetting presenting what we believe to be the correct interpretation of the simulation results. We find good quantitative agreement and emphasize that this could not be achieved using a standard effective Hamiltonian approach.

Equilibrium contact angle and intrinsic wetting hysteresis

Abstract: Wetting and adhesion should be treated as inherently irreversible processes, because, in the process of joining and separating two surfaces, available energy is dissipated. In a system that undergoes inherently irreversible processes, the second law of thermodynamics is always satisfied and can offer no information regarding the system’s equilibrium state. Therefore, to determine the equilibrium wetting angle, the law of energy conservation should be used. As a result, the advancing contact angle is found to be intrinsically higher than the receding one, regardless of the surface heterogeneity, and the extent of hysteresis is dependent on the amount of energy dissipated in the process of wetting and dewetting.

Influence of the "Wetting Ridge" in Dry Patch Formation

Abstract: Spreading of a liquid drop to equilibrium on a soft, viscoelastic substrate can be much slower than on an equivalent hard material due to energy dissipation in the deformed solid near the wetting front : the liquid surface tension creates a wetting ridge which moves with the triple line. When a film ofnonwetting liquid is applied to a solid surface, dry patches may nucleate and grow. In the case of a viscoelastic solid, it is shown that the speed of dry patch enlargement is also largely moderated by energy dissipation brought about by the strain cycle of the solid in the triple-line region. The speed of triple-line recession is shown, both theoretically and experimentally, to be constant, in agreement with the capillary regime of the same process on a hard solid, but the viscoelastically controlled spreading rate is about an order of magnitude smaller.