Grain Boundary Phase Transitions and their Influence on Properties of Polycrystals

Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The experimental data on GB wetting phase transitions in numerous systems are analysed. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity obtained in various systems both in polycrystals and bicrystals are analysed. In case if two solid phases are in equilibrium, the GB “solid state wetting” can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such GB phase transition occurs if the energy of two interphase boundaries is lower than the GB energy in the phase 1.     

Long-range critical wetting: Experimental phase diagram

We present ellipsometry and interferrometry experiments which allow us to observe the transition point between the standard first-order wetting and the long-range critical wetting. Moreover we provide a direct measurement of the free-energy singularities in the sequential wetting scenario of alkanes on water.     

Wetting behaviour of a model symmetric binary mixture with partially miscible components from a density functional approach

We investigate wetting transitions in a binary fluid at a solid surface by means of a density functional approach. For this purpose we use the symmetric binary mixture model, which exhibits a demixing in a bulk phase. We concentrate on the evaluation of the phase diagrams in the case of adsorption from a gas phase at a fixed composition. Our calculations have revealed different scenarios, leading to the change of wettability of the surface. In the case of adsorption from an equimolar bulk gas the wetting transition may be of the first or of the second order. In the case of non-equimolar bulk composition we observe either a transition from partial to complete wetting, or a first-order transition between two partial wetting states.     

Wetting of a plane with a narrow solvophobic stripe

ABSTRACT

We present a numerical study of a simple density functional theory model of fluid adsorption occurring on a planar wall decorated with a narrow deep stripe of a weaker adsorbing (relatively solvophobic) material, where wall-fluid and fluid-fluid intermolecular forces are considered to be dispersive. Both the stripe and outer substrate exhibit first-order wetting transitions with the wetting temperature of the stripe lying above that of the outer material. This geometry leads to a rich phase diagram due to the interplay between the pre-wetting transition of the outer substrate and an unbending transition corresponding to the local evaporation of liquid near the stripe. Depending on the width of the stripe, the line of unbending transitions merges with the pre-wetting line inducing a two-dimensional wetting transition occurring across the substrate. In turn, this leads to the continuous pre-drying of the thick pre-wetting film as the pre-wetting line is approached from above. Interestingly we find that the merging of the unbending and pre-wetting lines occurs even for the widest stripes considered. This contrasts markedly with the scenario where the outer material has the higher wetting temperature, for which the merging of the unbending and pre-wetting lines only occurs for very narrow stripes.     

Random anisotropy nematic model: Connection with experimental systems

We study theoretically the phase behavior of the continuum Random Anisotropy Nematic model. A domain-type pattern is assumed to appear in a distorted nematic liquid crystal (LC) phase. We map the model parameters to physical quantities characterizing LCs confined to Controlled-Pore Glasses and LC-aerosil dispersions. The domain size dependence on the disorder strength is obtained in accordance with the Imry-Ma prediction. The model estimates for temperature shifts of the paranematic-nematic phase transition and for the critical point, where this transition ceases to exist, are compared to the available experimental results.Received: 28 March 2004, Published online: 29 June 2004PACS: 61.30.-v Liquid crystals - 61.30.Dk Continuum models and theories of liquid crystal structure - 61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions     

Corner wetting in a far-from-equilibrium magnetic growth model

The irreversible growth of magnetic films is studied in three-dimensional confined geometries of size L×L×M, where M≫L is the growing direction. Competing surface magnetic fields, applied to opposite corners of the growing system, lead to the observation of a localization-delocalization (weakly rounded) transition of the interface between domains of up and down spins on the planes transverse to the growing direction. This effective transition is the precursor of a true far-from-equilibrium corner wetting transition that takes place in the thermodynamic limit. The phenomenon is characterized quantitatively by drawing a magnetic field-temperature phase diagram, firstly for a confined sample of finite size, and then by extrapolating results, obtained with samples of different size, to the thermodynamic limit. The results of this work are a nonequilibrium realization of analogous phenomena recently investigated in equilibrium systems, such as corner wetting transitions in the Ising model.     

Long-range critical wetting: observation of a critical end point

Alkanes deposited on aqueous substrates exhibit two different types of wetting behavior: alternatively to the usual first-order wetting transition, a sequential-wetting scenario of a long-range critical wetting transition preceded by a first-order thin-thick transition may be observed. Here, we present the first successful experimental attempt to locate the transition point between the standard first-order wetting and the long-range critical wetting: a critical end point, observed in a mixture of pentane and hexane which is deposited on an aqueous solution of glucose. Furthermore, we present the first direct measurement of the contact angle in the intermediate wetting state (frustrated-complete wetting) in the sequential-wetting scenario of hexane on brine and compare to theoretical predictions.     

Reduced linewidth enhancement factor due to excited state transition of quantum dot lasers

The carrier induced refractive index change and linewidth enhancement factor α due to ground-state (GS) and excited-state (ES) transitions have been compared by measuring the optical gain spectra from an InAs/GaAs quantum dot (QD) laser structure. It is shown that the ES transition exhibits a reduced α-factor compared to the value due to the GS transition. This result can be explained by the α-factor due to the ES transition having a smaller increase from the non-resonant carriers in the combined state of the wetting layer and InGaAs strain reducing layer than the α-factor increase due to the GS transition, since the relaxation time for carriers from the combined state of the wetting layer and InGaAs strain reducing layer to the ES is shorter than to the GS. The result reported here shows another advantage of using ES QD lasers for optical communication, in addition to their higher modulation speed.     

Wetting properties of rare gases on weak substrates

Since the original prediction that liquid He does not wet Cs at low temperatures and the soon after experimental observation of a wetting transition on this system, noble gases on alkalis have become model systems for the study of wetting transitions and of their accompanying line of prewetting transitions off coexistence. Here we review very briefly the theory of wetting and prewetting and discuss some results on the properties of rare gases adsorbed on alkali surfaces obtained with the use of the density functional theory and of accurate adsorbate-substrate potentials.     

Phase transitions of adsorbed fluids computed from multiple-histogram reweighting

This paper demonstrates the effectiveness of using multiple-histogram reweighting (MHR) to study phase transitions in confined fluids by examining capillary condensation, prewetting, and layering transitions for different systems. A comparison is made with previously published simulations, where available, to establish the accuracy of MHR as applied to inhomogeneous systems. Overlap between adjacent state points is assessed through single-histogram reweighting. Capillary condensation for methane adsorption in slit-like graphite pores exhibits 2D behaviour. Crossover of the effective exponent for the width of the coexistence curve from 2D Ising-like (1/8) further away from the critical point to mean-field (1/2) near the critical point is observed. The reduced critical temperature, the density and the effective value of the exponent for the model system are 0.77, 0.482, and 0.119, respectively, based on a fit to the simulation data. Prewetting transitions are observed for adsorption of Ar on solid CO₂ using model potentials. The wetting temperature is estimated based on the intersection of the pre-wetting and bulk vapour-liquid lines, and also by extrapolation to zero of the difference between the saturation and prewetting chemical potentials. The reduced wetting temperature is estimated to be around 0.69. The reduced prewetting critical temperature, calculated from the disappearance of the two peaks in the density probability distribution, is estimated to be 0.92. The monolayer to bilayer (1-2) transition for propane on graphite is computed over a range of temperatures. Results for the 1–2 layering transition computed from MHR from a small system are in good agreement with grand canonical Monte Carlo simulations for a much larger system.     

Phase transitions, interfacial fluctuations and hidden symmetries for fluids near structured walls

Fluids adsorbed at micro-patterned and geometrically structured substrates can exhibit novel phase transitions and interfacial fluctuation effects distinct from those characteristic of wetting at planar, homogeneous walls. We review recent theoretical progress in this area paying particular attention to filling transitions pertinent to fluid adsorption near wedges, which have highlighted a deep connection between geometrical and contact angles. We show that filling transitions are not only characterized by large scale interfacial fluctuations leading to universal critical singularities but also reveal hidden symmetries with short-ranged critical wetting transitions and properties of dimensional reduction. We propose a non-local interfacial model which fulfills all these properties and throws light on long-standing problems regarding the order of the 3D short-range critical wetting transition.     

GaAs pyramidal quantum dot coupled to wetting layer in an AlGaAs matrix: A strain-free system

In this contribution, the electronic and linear and nonlinear optical properties of pyramid-shaped GaAs quantum dots (QDs) coupled to wetting layer (WL) in an Al_0.3Ga_0.7As matrix have been investigated. This nanostructure is relaxed from strain effects due to very small lattice-mismatching. Three transitions of P-to-S, WL-to-P, and WL-to-S were considered and the corresponding transition dipole moments, oscillator strengths, and linear and nonlinear optical properties regarding to these transitions were investigated as a function of the QD height. The results showed that for P-to-S transition, which is a purely in-plane-polarized transition, the dependence of electronic and optical properties on the size is moderate and can be neglected. But for WL-to-P and WL-to-S transitions, which are in-plane- and z-polarized transitions, respectively, the electronic as well as optical properties are strongly size-dependent. Furthermore, a competition between WL-to-S and WL-to-P transitions was observed when the QD size changed.     

Threshold criterion for wetting at the triple point

Grand canonical simulations are used to calculate adsorption isotherms of various classical gases on alkali metal and Mg surfaces. Ab initio adsorption potentials and Lennard-Jones gas-gas interactions are used. Depending on the system, the resulting behavior can be nonwetting for all temperatures studied, complete wetting, or (in the intermediate case) exhibit a wetting transition. An unusual variety of wetting transitions at the triple point is found in the case of a specific adsorption potential of intermediate strength. The general threshold for wetting near the triple point is found to be close to that predicted with a heuristic model of Cheng et al. This same conclusion was drawn in a recent experimental and simulation study of Ar on CO2 by Mistura et al. These results imply that a dimensionless wetting parameter w is useful for predicting whether wetting behavior is present at and above the triple temperature. The nonwetting/wetting crossover value found here is w approximately 3.3.     

Wetting phenomena in the (3 × 1) phase of a model for H on Fe(110)

Interfaces between the three physically distinct, but equivalent domains in the (3 × 1) phase of a lattice gas model for the adsorbate system H/Fe(110) and its Ising analog are studied. In the ground state two types of wetting transitions are found where a light or a heavy domain wall decomposes into two heavy or two light walls separated by the third domain. These transitions give rise to wetting lines in the phase diagram which are located using Monte Carlo techniques.     

Extraordinary wetting phase diagram for mixtures of Bose-Einstein condensates

The possibility of wetting phase transitions in Bose-Einstein condensed gases is predicted on the basis of Gross-Pitaevskii theory. The surface of a binary mixture of Bose-Einstein condensates can undergo a first-order wetting phase transition upon varying the interparticle interactions, using, e.g., Feshbach resonances. Interesting ultra-low-temperature effects shape the wetting phase diagram. The prewetting transition is, contrary to general expectations, not of first order but critical, and the prewetting line does not meet the bulk phase coexistence line tangentially. Experimental verification of these extraordinary results is called for, especially now that it has become possible, using optical methods, to realize a planar "hard wall" boundary for the condensates.     

具有不同作用势的两平行墙间流体的浸润相变

用解析方法讨论了当两平行墙对流体施加不同的作用势时流体的浸润相变。系统的状态很敏感地依赖于两墙的作用势和两墙间距离的大小。即使两墙差别很小,在一定条件下仍可发生浸润一墙而不浸润另一墙的现象。系统中发生的浸润相变可能是一级的,也可能是二级的。 关键词：     

Wetting phase transitions and critical phenomena in condensed matter

Equilibrium wetting phase transitions and critical phenomena are discussed from a phenomenological point of view. The ubiquitous character of the wetting phase transition is illustrated through its occurrence in a variety of condensed matter systems, ranging from classical fluids to superconductors and Bose-Einstein condensates. The intriguing behaviour of the three-phase contact line and its line tension, at wetting, is an example of a fundamental problem in this field on which much progress has been made.     

Nonequilibrium wetting transition in a nonthermal 2D Ising model

Nonequilibrium wetting transitions are observed in Monte Carlo simulations of a kinetic spin system in the absence of a detailed balance condition with respect to an energy functional. A nonthermal model is proposed starting from a two-dimensional Ising spin lattice at zero temperature with two boundaries subject to opposing surface fields. Local spin excitations are only allowed by absorbing an energy quantum (photon) below a cutoff energy E _c . Local spin relaxation takes place by emitting a photon which leaves the lattice. Using Monte Carlo simulation nonequilibrium critical wetting transitions are observed as well as nonequilibrium first-order wetting phenomena, respectively in the absence or presence of absorbing states of the spin system. The transitions are identified from the behavior of the probability distribution of a suitably chosen order parameter that was proven useful for studying wetting in the (thermal) Ising model.     

Wetting mode transition of nanoliter scale water droplets during evaporation on superhydrophobic surfaces with random roughness structure

During evaporation, shape changes of nanoliter-scale (80-100 nL) water droplets were evaluated on two superhydrophobic surfaces with different random roughness (nm-coating, μm-coating). The square of the contact radius and the square of the droplet height decreased linearly with evaporation time. However, trend changes were observed at around 170 s (nm-coating) and around 150 s (μm-coating) suggesting a wetting mode transition. The calculated droplet radii for the wetting mode transition from the average roughness distance and the average roughness height of these surface structures were approximately equal to the experimental values at these trend changes. A certain level of correlation between the roughness size and droplet radius at the wetting mode transition was confirmed on surfaces with random roughness.     

Theory of interfacial phase transitions in surfactant systems

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.This paper is dedicated to J. K. Percus in honor of his 65th birthday.