Wetting Phase Transition Research Articles

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

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°).

Tie lines of the grain boundary wetting phase transition in the Al-Sn System

Temperature dependencies of contact angles θ at the intersection of grain boundaries in Al bicrystals with solid Al/liquid Al-Sn interphase boundaries were studied. For this purpose, two Al bicrystals were grown with tilt {001} grain boundaries having misorientation angles 9 = 38.5‡ (near 19 coincidence misorientation) and ϕp = 32‡. These boundaries possess different energies. The tempera-turesTw of the grain boundary wetting phase transition for these two boundaries were determined, and the corresponding conodes in the two-phase area of the Al-Sn phase diagram were constructed. AboveTw, the contact angle θ = 0 and a layer of the liquid phase completely wets the grain boundary. The temperature of wetting transitionTw1= 6047± 1 ‡C, for the grain boundary with high energy (ϕp = 32‡) is lower thanTw2 = 6177 ± 1 ‡C for the grain boundary with low energy (ϕ = 38.5‡). Above the temperature interval where all the grain boundaries become wetted, the solid phase may exist only as isolated single crystalline “islands≓ in the “sea≓ of melted phase.

Absence of a stiffness instability for a model critical-wetting transition in three dimensions

We have tested the generality of the stiffness instability mechanism recently proposed by Fisher and Jin (1993) for the critical wetting phase transition in three-dimensional systems with short-ranged forces. We extend the analysis of Fisher and Jin to a class of Aukrust-Hauge type models and find that the stiffness instability is specific to the case where the unrenormalized transition is precisely second order (i.e where the mean-field specific heat critical exponent is precisely zero). A linear functional renormalization-group analysis of this Aukrust-Hauge class of models yields exactly the same dramatic fluctuation-induced non-universal critical exponents that have previously been predicted. We conclude by considering possible implications of this result on future Ising model simulations and on the basis of this propose a numerical test for the validity of existing renormalization-group analyses of continuum effective interfacial Hamiltonians.

LINE TENSION AT WETTING

A review is presented of recent theoretical advances on a fundamental problem in statistical mechanics that concerns the three-phase contact line ℒ and its tension τ near a wetting phase transition. In addition to answering the intriguing question whether or not ℒ and τ vanish at wetting, recent work has also revealed that τ displays universal singular behavior, reflecting critical phenomena associated with the wetting transition. Three factors are crucial for determining the fate of ℒ and τ at wetting: the order of the wetting transition, the range of the intermolecular forces, and the upper critical dimension du, above which mean-field theory holds and below which fluctuations dominate. For most systems studied experimentally, du < 3, so that the mean-field predictions should be correct in d = 3. In the thermal fluctuation regime, for d < du, hyperscaling predicts the value 2(d – 2)/(d – 1) for the critical exponent of τ(θ), in the limit that the contact angle θ approaches 0.

Line tension at wetting: finite-size effects and scaling functions

We study the effect of finite system size on the tension 7 of a three-phase contact line. This is especially important in systems with forces that are sufficiently long-ranged for 7 to diverge in the thermodynamic limit. Near wetting phase transitions we propose an anisotropic finite-size scaling hypothesis, which takes into account the effect of thermal capillary-wave fluc- tuations. The singular behaviour of T near a first-order wetting transition is analyzed. Scaling functions are calculated explicitly in the mean-field regime, which includes the physically most common systems with van der Wads forces in dimension d = 3.

Line tension at wetting: interface displacement model beyond the gradient-squared approximation

We study the transition zone or contact line between a thin film and bulk liquid, and calculate the line tension τ, employing an interface displacement model equivalent to Derjaguin's and de Gennes' approach. We investigate the behaviour of τ in the limit that the contact angle ϑ tends to zero, approaching a wetting phase transition. Previous results for wetting and prewetting, derived in the gradient-squared approximation of the model, remain valid when the gradient is included to all orders. The interesting singular behaviour of τ at wetting is universal, due to the critical phenomena that have recently been found to underly first-order as well as continuous wetting transitions. Finally we critically review two contact-line instabilities that have been associated with contact-angle hysteresis, and propose an alternative physical interpretation.

Fluctuations, correlation functions and interfacial Hamiltonians for the complete wetting phase transition

Exact statistical mechanical sum rules are combined with thermodynamic scaling arguments to determine the leading-order singular contributions to the transverse moments of the density-density correlation function G of a simple fluid undergoing a complete wetting phase transition, from off-bulk coexistence, at a structureless substrate (wall). Contrary to our earlier suggestion, capillary-wave-like fluctuations do manifest themselves throughout the wetting film and lead to a transverse correlation length ξ‖(a), for both particles near the wall, which diverges with the same critical exponent as ξ‖, the correlation length appropriate for both particles in the depinning liquid-gas interface. Explicit results from a mean-field (density functional) calculation are consistent with our general predictions. In the weak fluctuation regime (d < 3), scaling methods are used to derive the short distance expansions of the density profile and moments of G for short ranged forces. These expansions, which describe singula...

Effect of long-range surface interactions on wetting transitions of polymer mixtures.

We examine the effect of long-range surface interactions on surface wetting phase transitions in polymer mixtures. Within a perturbation scheme, we have found an analytic solution for the volume-fraction profile near the surface which can be compared with that obtained from a model with short-range surface interactions only. We find that the long-range interactions should not be ignored in the interpretation of recent experimental results.

Derivation of Landau theories and lattice mean-field theories for surface and wetting phenomena, from semi-infinite Ising models

Abstract The phenomenological aspects of surface and interfacial phenomena such as wetting phase transitions are commonly studied using the classification scheme provided by the Landau theory. Although this approach is very meaningful, it is also important to establish a firm connection between the phenomenology and the microscopic models of statistical mechanics. This is true, especially in view of the remarkable sensitivity of wetting phenomena to the details of the substrate-adsorbate interactions. We study such connections and derive a variety of lattice mean-field theories, which make a bridge between the Landau theory and the semi-infinite Ising model with a surface. We discuss standard mean-field approximations (MFA) and improvements thereof, renormalization group techniques, reaction-field approximations, and cluster variation methods. We pay special attention to the derivation of the newly introduced triplet surface field h 3 in the Landau theory for wetting.

Critical wetting and surface-induced continuous phase transition on Cu3Au(001)

In the bulk Cu3Au is known to undergo adiscontinuous order-disorder phase transition atT 0=663 K. Spin polarized low-energy-electron diffraction (SPLEED) reveals acontinuous surface-induced order-disorder phase transition-equivalent to a critical wetting transition—on the surface of Cu3Au. The results indicate that nearT 0 the observed singularity is related to the surface order parameter which can be described by a functional dependence of the formt β1 with β1=0.77±0.06.