Heterogeneous diffuse interfaces: a new mechanism for arrested coarsening in binary mixtures. Heterogeneous diffuse interfaces

We discuss the dynamics of binary fluid mixtures in which surface tension density is allowed to become locally negative within the interface, while still preserving positivity of the overall surface tension (heterogeneous diffuse interface). Numerical simulations of two-dimensional Ginzburg-Landau phase field equations implementing such mechanism and including hydrodynamic motion, show evidence of dynamically arrested domain coarsening. Under specific conditions on the functional form of the surface tension density, dynamical arrest can be interpreted in terms of the collective dynamics of metastable, non-linear excitations of the density field, named compactons, as they are localized to finite-size regions of configuration space and strictly zero elsewhere. Aside from compactons, the heterogeneous diffuse interface scenario appears to provide a robust mechanism for the interpretation of many aspects of soft-glassy behaviour in binary fluid mixtures.     

Phase-field model of long-time glasslike relaxation in binary fluid mixtures

We present a new phase-field model for binary fluids, exhibiting typical signatures of soft-glassy behavior, such as long-time relaxation, aging, and long-term dynamical arrest. The present model allows the cost of building an interface to vanish locally within the interface, while preserving positivity of the overall surface tension. A crucial consequence of this property, which we prove analytically, is the emergence of free-energy minimizing density configurations, hereafter named "compactons," to denote their property of being localized to a finite-size region of space and strictly zero elsewhere (no tails). Thanks to compactness, any arbitrary superposition of compactons still is a free-energy minimizer, which provides a direct link between the complexity of the free-energy landscape and the morphological complexity of configurational space.     

浓度边界层中成长汽泡的界面特性

对于双组分池内核态沸腾，热边界层中生成的汽泡，同时也处于浓度边界层中．本文建立了边界展中汽泡表面张力模型，对温度和浓度引起的表面张力变化进行分析．结果表明，表面张力从汽泡顶部到底部是递增的，从而形成液体沿界面自上而下的Marangoni流．通过汽泡底部微层相界面方程的求解，探讨了微层变形特征，由此分析汽泡脱离机理．微层变形与文献[1]中指出的接触线的变化从本质上是一致的．     

Interfacial roughness in a near-critical binary fluid mixture: X-ray reflectivity and near-specular diffuse scattering

Measurements are presented of the X-ray specular reflectivity and near-specular diffuse scattering of the interface in a near-critical mixture of hexane and perfluorohexane. A lineshape analysis of the scattered intensity at each temperature yields values for the interfacial tension and interfacial width. The temperature variation of the tension and width so-obtained are consistent with current understanding of this interface, which holds that there is, firstly, an intrinsic width over which the fluid density varies smoothly from one coexistence composition to the other, and, secondly, that the interface acquires an additional and larger statistical interfacial width as a result of capillary fluctuations. Received 1 April 1998     

Theory of the liquid-vapour interface of a binary mixture of Lennard-Jones fluids

We present results of calculations of the equilibrium surface tension and density profiles for the liquid-vapour interface of a binary mixture of Lennard-Jones 12-6 fluids. The calculations are based on a density-functional theory for the Helmholtz free energy of the inhomogeneous mixture. This is a ‘microscopic’ generalization of the van der Waals-Cahn-Hilliard theory for the interface of a binary mixture.

Our calculations cover the full range of liquid-vapour coexistence and the whole range of concentration. We find a correlation between the excess surface tension of the mixture and the surface segregation (adsorption) of the species with the lower surface tension. The ways in which segregation and excess surface tension depend on the Lennard-Jones parameters of the pure components are briefly discussed. Our results for the excess surface tension of mixtures of Ar and N₂ and Ar and CH₄ are compared with experiment; the agreement is reasonable.     

Influence of density stratification on stability of a two-layer binary-fluid system with a diffuse interface

A system of two layers separated by a diffuse interface which is created due to a phase separation in a binary liquid in the gravity field, is considered. The influence of the density stratification and the gravity on the stabilization of the solution is studied. The stability of two-layer base solutions with respect to long-wave disturbances in the framework of the linear stability analysis is investigated in the case of small density ratio and large Galileo number. It is found that the action of gravity can stabilize the equilibrium state.     

Simulation of phase equilibria and interfacial properties of binary mixtures on the liquid-vapour interface using lattice sums

Molecular dynamics simulations of Lennard-Jones binary mixtures were performed to obtain phase equilibria and thermodynamic properties for the liquid—vapour interface. The dispersion interactions were handled using the lattice sum method where the full interaction is obtained and there is no requirement for any long range correction to the properties. The application of the method using the Lorentz—Berthelot combining rule for unlike interactions is discussed. The coexisting densities, adsorption of molecules at the interface and surface tension are the main results of this work. Coexisting properties were compared with Gibbs ensemble Monte Carlo results and with those of the grand canonical Monte Carlo method combined with the histogram reweighting technique, and good agreement was found. The lattice sum method results were compared with those of the spherically truncated and shifted potential to analyse the truncation effect. The adsorption of molecules at the interface and surface tension increase with interaction.     

Surface Tension of Multi-phase Flow with Multiple Junctions Governed by the Variational Principle

We explore a computational model of an incompressible fluid with a multi-phase field in three-dimensional Euclidean space. By investigating an incompressible fluid with a two-phase field geometrically, we reformulate the expression of the surface tension for the two-phase field found by Lafaurie et al. (J Comput Phys 113:134–147, 1994) as a variational problem related to an infinite dimensional Lie group, the volume-preserving diffeomorphism. The variational principle to the action integral with the surface energy reproduces their Euler equation of the two-phase field with the surface tension. Since the surface energy of multiple interfaces even with singularities is not difficult to be evaluated in general and the variational formulation works for every action integral, the new formulation enables us to extend their expression to that of a multi-phase (N-phase, N\geqslant2N\geqslant2) flow and to obtain a novel Euler equation with the surface tension of the multi-phase field. The obtained Euler equation governs the equation for motion of the multi-phase field with different surface tension coefficients without any difficulties for the singularities at multiple junctions. In other words, we unify the theory of multi-phase fields which express low dimensional interface geometry and the theory of the incompressible fluid dynamics on the infinite dimensional geometry as a variational problem. We apply the equation to the contact angle problems at triple junctions. We computed the fluid dynamics for a two-phase field with a wall numerically and show the numerical computational results that for given surface tension coefficients, the contact angles are generated by the surface tension as results of balances of the kinematic energy and the surface energy.     

A front-tracking/ghost-fluid method for fluid interfaces in compressible flows

A front-tracking/ghost-fluid method is introduced for simulations of fluid interfaces in compressible flows. The new method captures fluid interfaces using explicit front-tracking and defines interface conditions with the ghost-fluid method. Several examples of multiphase flow simulations, including a shock–bubble interaction, the Richtmyer–Meshkov instability, the Rayleigh–Taylor instability, the collapse of an air bubble in water and the breakup of a water drop in air, using the Euler or the Navier–Stokes equations, are performed in order to demonstrate the accuracy and capability of the new method. The computational results are compared with experiments and earlier computational studies. The results show that the new method can simulate interface dynamics accurately, including the effect of surface tension. Results for compressible gas–water systems show that the new method can be used for simulations of fluid interface with large density differences.     

Liquid-vapor interface in liquid binary alloys: an ab initio molecular dynamics study

We report the results of an ab initio molecular dynamics simulation of the liquid-vapor interface of two binary liquid alloys, Na(0.3)K(0.7) and Li(0.4)Na(0.6), whose bulk behavior exhibits rather differing ordering tendencies. The study has been performed using samples of 2000 and 3000 particles, respectively, in a slab geometry with periodic boundary conditions. In both cases, the total ionic density distributions along the normal to the interface display some layering with a virtually pure monolayer of the lower surface tension component located outermost at the interface. However, the two systems behave very differently below the interface which can be accounted for by their different ordering tendencies in the bulk.     

Gas—liquid interfaces of room temperature ionic liquids

The structure of the gas-liquid surface of dimethylimidazolium chloride has been studied using atomistic simulation. We find that there is a region of enhanced density immediately below the interface in which the cations are oriented with their planes perpendicular to the surface and their dipoles in the surface plane. There is negligible segregation of cations and anions. The temperature dependence of the surface tension is predicted to be anomalously low or be reversed in sign. The vapour-liquid interfaces between mixtures of water and dimethylimidazolium chloride show similar regions of enhanced density and preferential orientation of the cations. Water molecules also show preferential orientation in the interface region and are preferentially adsorbed on the vapour side of the interface. The surface tension decreases with increase in the mole fraction of water.     

Characterization of water–ethanol electrosprays

The electrospray (ES) ionization method is widely used in the analysis of biological molecules. The stable cone regime spray is usually obtained by adding methanol or ethanol to the electrosprayed dipolar fluid, with the role of reducing fluid surface tension. We investigate the spray onset voltages of water–ethanol mixtures starting from distilled water to pure ethanol. Positive or negative DC is applied separately in our measurements. The experimental setup allows for the study of nozzle-counter electrode gaps of up to 50 cm. Significant variations in the spray onset are observed and analyzed. The onset voltage and the surface tension of water–ethanol binary mixtures appear to be in line with Smith’s formula. The largest variations of onset values are observed for surface tensions of 20 mN/m–40 mN/m in the mixtures. The results of the study may be instrumental to electrospray electrode design and spray control.     

On field effect and diffuse surface scattering

An electric field effect mechanism by diffuse surface scattering is proposed. This new field effect mechanism is dominant at metal oxide interfaces and due to interfacial exchange between conduction electrons and trap states in the oxide.     

Nanopore wall effect on surface tension of methane

Local pressure is known to be anisotropic across the interfaces separating fluids in equilibrium. Tangential pressure profiles show characteristic negative peaks as a result of surface tension forces parallel to the interface. Nearby attractive forces parallel to the interface are larger than the repulsive forces and, hence, constitute the surface tension. In this work, using molecular dynamics simulations of methane inside nano-scale pores, we show this surface tension behaviour could be significantly influenced by confinement effects. The layering structure, characterised by damped oscillations in local liquid density and tangential pressures, extends deep into the pore and can be a few nanometers thick. The surface tension is measured numerically using local pressures across the interface. Results show that the tension is smaller under confinement and becomes a variable in small pores, mainly controlled by the thickness of the liquid density layering (or liquid saturation) and the pore width. If the liquid saturation inside the pore is high enough, the vapour–liquid interface is not interfered by the pore wall and the surface tension remains the same as the bulk values. The results are important for understanding phase change and multi-phase transport phenomena in nanoporous materials.     

Hydrodynamics of binary fluid phase segregation

Starting with the Vlasov-Boltzmann equation for a binary fluid mixture, we derive an equation for the velocity field u when the system is segregated into two phases (at low temperatures) with a sharp interface between them. u satisfies the incompressible Navier-Stokes equations together with a jump boundary condition for the pressure across the interface which, in turn, moves with a velocity given by the normal component of u. Numerical simulations of the Vlasov-Boltzmann equations for shear flows parallel and perpendicular to the interface in a phase segregated mixture support this analysis. We expect similar behavior in real fluid mixtures.     

A diffuse interface Lox/hydrogen transcritical flame model

We present a diffuse-interface all-pressure flame model that transitions smoothly between subcritical and supercritical conditions. The model involves a non-equilibrium liquid/gas diffuse interface of van der Waals/Korteweg type embedded into a non-ideal multicomponent reactive fluid. The multicomponent transport fluxes are evaluated in their thermodynamic form in order to avoid singularities at thermodynamic mechanical stability limits. The model also takes into account condensing liquid water in order to avoid thermodynamic chemical instabilities. The resulting equations are used to investigate the interface between cold dense and hot light oxygen as well as the structure of diffusion flames between cold dense oxygen and gaseous-like hydrogen at all pressures, either subcritical or supercritical.     

模拟盒参数对汽液体系模拟过程的影响

为探讨模拟盒参数对分子动力学模拟影响这个长期被忽略的问题,采用Verlet-List搜寻法和Leapfrog差分算法,对分子数目为1000的汽液共存体系进行分子动力学模拟,得到了系统温度为100K时,汽液两相密度、表面张力随系统切片数、汽相空间尺寸、液膜厚度以及步长等模拟盒参数的变化情况.研究结果表明,系统切片数不会影响表面张力及两相密度;体系z轴无量纲长度为60时,模拟值与实验值吻合较好;液膜无量纲厚度达到13以后,液相密度将趋于稳定,两侧界面分子不会相互影响;分子动力学步长不影响平衡后的系统密度,但对表面张力的计算影响较明显.     

The Compressible Viscous Surface-Internal Wave Problem: Stability and Vanishing Surface Tension Limit

This paper concerns the dynamics of two layers of compressible, barotropic, viscous fluid lying atop one another. The lower fluid is bounded below by a rigid bottom, and the upper fluid is bounded above by a trivial fluid of constant pressure. This is a free boundary problem: the interfaces between the fluids and above the upper fluid are free to move. The fluids are acted on by gravity in the bulk, and at the free interfaces we consider both the case of surface tension and the case of no surface forces. We establish a sharp nonlinear global-in-time stability criterion and give the explicit decay rates to the equilibrium. When the upper fluid is heavier than the lower fluid along the equilibrium interface, we characterize the set of surface tension values in which the equilibrium is nonlinearly stable. Remarkably, this set is non-empty, i.e., sufficiently large surface tension can prevent the onset of the Rayleigh-Taylor instability. When the lower fluid is heavier than the upper fluid, we show that the equilibrium is stable for all non-negative surface tensions and we establish the zero surface tension limit.