Quasi-2D and prewetting transitions of square-well fluids on a square-well substrate

Molecular simulation methodologies are employed to study the first-order transition of variable square-well (SW) fluids on a wide range of weak attractive surfaces. Surface phase diagram of SW fluids of attractive well diameter λ _ff = 1.5, 1.75, 2.0 on a smooth, structureless surface modelled by a SW potential is reported via grand-canonical transition-matrix Monte Carlo (GC-TMMC) and histogram reweighting techniques. Fluids with λ _ff = 1.5 and 1.75 show quasi-2D vapour–liquid phase transition; on the other hand, prewetting transition is visible for a SW fluid with larger well-extent λ _ff = 2.0. The prewetting line, its length, and closeness to the bulk saturation curve are found to depend strongly on the nature of the fluid–fluid and fluid–wall interaction potentials. Boundary tension of surface coexistence films is calculated by two methods. First, the finite size scaling approach of Binder is used to evaluate the boundary tension via GC-TMMC. Second, the results of the boundary tension are verified by virtue of its relation to the pressure tensor components, which are calculated using a NVT-Monte Carlo approach. The results from the two methods are in good agreement. Boundary tension is found to increase with the increase in the wall–fluid interaction range for the quasi-2D system; conversely, boundary tension for thin–thick film, at prewetting transition, decreases with the increase in the wall–fluid interaction range.     

Surface phase transitions of multiple-site associating fluids

Surface phase transitions of Lennard–Jones (LJ) based two- and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor–liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor–liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.     

Wetting transitions in polydisperse fluids

The properties of the coexisting bulk gas and liquid phases of a polydisperse fluid depend not only on the prevailing temperature but also on the overall parent density. As a result, a polydisperse fluid near a wall will exhibit density-driven wetting transitions inside the coexistence region. We propose a likely topology for the wetting phase diagram, which we test using Monte Carlo simulations of a model polydisperse fluid at an attractive wall, tracing the wetting line inside the cloud curve and identifying the relationship to prewetting.     

Quantum mechanical state preparation II-The statistical ensemble as a thermostat

In this sequel to an earlier paper on quantal state preparation (Park & Band, 1972), the simple model used before is adapted and somewhat generalised to enable an exact treatment of the causal evolution of a system immersed in a statistical ensemble constituted of replicas of the system itself. The ensemble is initially in a state of statistical equilibrium, but the initial state of the system is arbitrary. It is established as a purely dynamical theorem that the system is eventually coerced into the same equilibrium state as that of the replicas in the ensemble. From this general result we obtain as a special case mechanical justification for the common assumption in statistical thermodynamics that an ensemble can function as a thermostat.     

The liquid-vapour phase transition in fluid mercury

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.     

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.     

Toward a mean field theory of spin glasses: The TAP route revisited

After a sketchy derivation of TAP equations, we show first how one can recover two known results (Sherrington, Kirkpatrick and Sommers) by direct perturbation resommation à la Martin-Siggia-Rose. Going beyond perturbation, it is recalled that TAP equations can be reduced to a one site self consistent problem involving several order parameters. The existence of a large number of solutions to TAP equations at low T forces us to choose a weight to be attributed to each solution. Also we are forced to reintroduce replicas to compute averages over the ensemble of solutions. In contradistinction to what usually occurs it is argued that here replicas are rather innocent. The uniform (white) average and the microcanonical average are briefly discussed.     

Prewetting and layering in athermal polymer solutions

Coexistence conditions for prewetting and layering at a hard surface in additive hard sphere polymer solutions, where the solvent particles are smaller than the monomers, have been calculated by density functional methods. Various chain lengths and pressures have been investigated. An unexpected finding is that prewetting in these systems may proceed below the bulk critical pressure. We rationalize this behavior in terms of local properties of the pressure tensor. For longer chains, a different behavior is observed where the systems display a lower wetting pressure, i.e., a low pressure bound for surface wetting.     

Prewetting boundary tensions from Monte Carlo simulation

We examine the boundary tension of a model system along the prewetting saturation line. Boundary tensions are evaluated through a combination of finite-size scaling and grand canonical Monte Carlo simulation. The model system consists of Lennard-Jones particles interacting with a single structureless surface. After scaling our dimensionless results with a characteristic force, we obtain a value of 2 x 10(-11) N for the boundary tension at the system's wetting temperature. This estimate is consistent with theoretical and recent experimental values.     

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.     

Are bosonic replicas faulty?

Motivated by the ongoing discussion about a seeming asymmetry in the performance of fermionic and bosonic replicas, we present an exact, nonperturbative approach to both fermionic and bosonic zero-dimensional replica field theories belonging to the broadly interpreted beta=2 Dyson symmetry class. We then utilize the formalism developed to demonstrate that the bosonic replicas do correctly reproduce the microscopic spectral density in the QCD-inspired chiral Gaussian unitary ensemble. This disproves the myth that the bosonic replica field theories are intrinsically faulty.     

Liquid surface and liquid/liquid interface energies of binary subregular alloys and wetting transitions

Energetics and chemistry of liquid surfaces and liquid/liquid interfaces of binary A-B alloys are calculated using a subregular solution model. In this model, two macroscopic energetic parameters are used to produce an asymmetric miscibility gap. They are related to two microscopic parameters which describe the interaction energy between two atoms as a function of the composition of the first coordination shell of each atom. The impact of the asymmetry of the A-B interactions on the surface and interfacial energies and adsorption are analyzed by comparing the results obtained with this subregular model to those calculated for a regular solution. The role of the asymmetry on the prewetting and wetting transitions are also discussed. Calculations performed in the Co-Cu system are in good agreement with experimental data of surface energy.     

Structure of a liquid-vapor interface

The structure of the interface of an argonlike fluid in equilibrium with its own vapor at low temperature is studied using molecular dynamics. The longitudinal pair correlations in the interface are found to be consistent with a simply defined ensemble of local thermodynamic states. However, the transverse correlations exhibit very long-range behavior not predicted by straightforward local thermodynamics. These results strongly suggest that the interface is made up of an ensemble of configurations in each of which the transition from liquid to vapor is locally sharp, but that the transition surface fluctuates strongly in space and time.Supported in part by ERDA, Contract No. EY-76-C-02-3077, and by NSF, Grant DMR-72-03029, through Materials Science Center, Cornell University.The classical theoretical density profile dates back to van der Waals. (1)     

Thermodynamic properties for the triangular-well fluid

The thermodynamic properties of the triangular-well fluid with a well range of up to twice the hard sphere diameter were studied by means of a new developed equation of state and molecular simulation. This EoS is based on the perturbation theory of Barker and Henderson with the first and second-order perturbation terms evaluated by molecular simulation and then a fit with a simple function based on the radial distribution function of the reference fluid. The thermodynamic properties for the triangular-well fluid were also obtained directly by Gibbs ensemble and NPT Monte Carlo simulations. Good agreement is observed between the proposed EoS and the molecular simulation results. A model for the triangular-well solid is also presented; this has been used to calculate the solid–liquid transition line. Very good agreement is obtained with previously report values for this line and for the triple point temperature and pressure.     

Hydrogen and helium films as model systems of wetting

Optical experiments on the wetting properties of liquid ⁴He and molecular hydrogen are reviewed. Hydrogen films on noble metal surfaces serve as model systems for studying triple point wetting, a continuous transition between wetting and non-wetting. By means of optically excited surface plasmons, the adsorbed film thickness for temperatures around, and far below, the bulk melting temperature is measured, and the physical mechanisms responsible for the transition are elucidated. Possible applications for other experiments in pure and applied research are discussed. Thin films and droplets of liquid helium are studied on cesium surfaces, on which there is a first order wetting transition. Our studies concentrate on dynamical observations via surface plasmon microscopy, which provide insight into the morphology of liquid helium droplets spreading at different temperatures. Features corresponding to pinning forces, the prewetting line, and the Kosterlitz-Thouless transition are clearly observed.     

Replica field theories,painlevé transcendents,and exact correlation functions

Exact solvability is claimed for nonlinear replica sigma models derived in the context of random matrix theories. Contrary to other approaches reported in the literature, the framework outlined does not rely on traditional "replica symmetry breaking" but rests on a previously unnoticed exact relation between replica partition functions and Painlevé transcendents. While expected to be applicable to matrix models of arbitrary symmetries, the method is used to treat fermionic replicas for the Gaussian unitary ensemble (GUE), chiral GUE (symmetry classes A and AIII in Cartan classification) and Ginibre's ensemble of complex non-Hermitian random matrices. Further applications are briefly discussed.     

The influence of random changes in the adsorbing potential on phase transitions in a Lennard-Jones fluid confined to energetically heterogeneous slit-like pores

A density functional approach is used to study the adsorption and phase behaviour of a Lennard-Jones (LJ) fluid in slit-like pores with energetically heterogeneous walls, investigating how the randomly varying part of the fluid-solid potential imposed on a periodic ‘back-ground’ potential modifies the phase behaviour of the confined fluid. Non-local density functional theory is employed to describe the system. To study the system with a random external field, the method used is based on investigations of several replicas of the system and on averaging the final thermodynamic results over the replicas.     

Spectroscopic evidence of a wetting and prewetting transition in liquid K-KCl mixtures

Spectroscopic ellipsometry and simultaneous reflectivity measurements of liquid KxKCl1-x solutions clearly exhibit a first order wetting transition in metal-rich melts. At the sample/substrate interface, salt-rich wetting films of mesoscopic thickness ( approximately 100 nm) are observed at and off of coexistence. They are uniquely characterized by the liquid F-center absorption band. However, crossing the prewetting line towards metal-rich concentrations, the F bands disappear. From the observation of the liquid F-center band, it is concluded that a strong undercooling of the wetting films of about 200 K may occur in binary metallic fluids, which is demonstrated here for the first time.     

Heat flux density in the region of droplet contact line on a horizontal surface of a thin heated foil

The evaporating water droplets on a horizontal heated substrate were experimentally studied. The constantan foil 25 μm thick with a size of 42×35 mm² was used as a substrate. The experiments were carried out with a single droplet or with an ensemble of two or three droplets on the foil. The temperature of the lower surface of foil was measured by an IR scanner. To determine the heat flux density at evaporation of liquid near the contact line, the Cauchy problem for the heat conduction equation was solved using the thermographic data. The results of calculations showed that the maximal heat flux density takes place in the region of the contact line and exceeds the average heat flux density from the entire surface of foil. This is explained by the heat inflow from the foil periphery to the droplet due to relatively high value of the coefficient of heat conductivity of the foil material and high evaporation intensity in the contact line region.     

Numerical and experimental validation of variance prediction in the statistical energy analysis of built-up systems

In the statistical energy analysis (SEA) approach to vibration modeling, a complex system is represented as an assembly of coupled subsystems, and the method leads to the prediction of the vibrational energy level of each subsystem. The averaging procedures implicit in the technique imply that the predicted energy is the mean value taken over an ensemble of random structures, such as a set of vehicles leaving a production line. Recently, a new method has been developed to allow the ensemble variance, in addition to the mean, to be predicted within the context of SEA, and the present paper concerns further extension and validation of this work. The theoretical extension concerns the variance of the energy density at a single point in any of the subsystems, and the validation includes both simulation and experimental studies. The simulation results concern plate assemblies, while experimental results are presented both for a single-plate and for a cylinder-plate structure. In each case an ensemble of random structures has been generated by adding small point masses at random locations on the structure. In general, good agreement between the predictions and the validation results is observed.