arXiv subjects

A. Maciolek

Publications and source records attributed to A. Maciolek.

22 records · Page 2Linked to original sources

arXiv2003

Effects of weak surface fields on the density profiles and adsorption of a confined fluid near bulk criticality

The density profile and Gibbs adsorption of a near-critical fluid confined between two identical planar walls is studied by means of Monte Carlo simulation and by density functional theory for a Lennard-Jones fluid. By reducing the strength of wall-fluid interactions relative to fluid-fluid interactions we observe a crossover from behaviour characteristic of the normal surface universality class, strong critical adsorption, to behaviour characteristic of a 'neutral' wall. The crossover is reminiscent of that which occurs near the ordinary surface transition in Ising films subject to vanishing surface fields. For the 'neutral' wall the density profile, away from the walls, is almost constant throughout the slit capillary and gives rise to an adsorption that is constant along the critical isochore. The same 'neutral' wall yields a line of capillary coexistence that is almost identical to the bulk coexistence line. In the crossover regime we observe features in the density profile similar to those found in the magnetisation profile of the critical Ising film subject to weak surface fields, namely two smooth maxima, located away from the walls, which merge into a single maximum at midpoint as the strength of the wall-fluid interaction is reduced or as the distance between walls is decreased. We discuss similarities and differences between the surface critical behaviour of fluids and of Ising magnets.

cond-mat.stat-mech↗

arXiv2003

Phase diagram of a model for 3He-4He mixtures in three dimensions

A lattice model of 3He - 4He mixtures which takes into account the continuous rotational symmetry O(2) of the superfluid degrees of freedom of 4He is studied in the molecular-field approximation and by Monte Carlo simulations in three dimensions. In contrast to its two-dimensional version, for reasonable values of the interaction parameters the resulting phase diagram resembles that observed experimentally for 3He - 4He mixtures, for which phase separation occurs as a consequence of the superfluid transition. The corresponding continuum Ginzburg-Landau model with two order parameters describing 3He- 4He mixtures near tricriticality is derived from the considered lattice model. All coupling constants appearing in the continuum model are explicitly expressed in terms of the mean concentration of 4He, the temperature, and the microscopic interaction parameters characterizing the lattice system.

cond-mat.stat-mech↗

arXiv2000

Influence of Capillary Condensation on the Near-Critical Solvation Force

We argue that in a fluid, or magnet, confined by adsorbing walls which favour liquid, or (+) phase, the solvation (Casimir) force in the vicinity of the critical point is strongly influenced by capillary condensation which occurs below the bulk critical temperature T_c. At T slightly below and above T_c, a small bulk field h<0, which favours gas, or (-) phase, leads to residual condensation and a solvation force which is much more attractive (at the same large wall separation) than that found exactly at the critical point. Our predictions are supported by results obtained from density-matrix renormalization-group calculations in a two-dimensional Ising strip subject to identical surface fields.

cond-mat.stat-mech↗

arXiv1999

Effects of Confinement on Critical Adsorption: Absence of Critical Depletion for Fluids in Slit Pores

The adsorption of a near-critical fluid confined in a slit pore is investigated by means of density functional theory and by Monte Carlo simulation for a Lennard-Jones fluid. Our work was stimulated by recent experiments for SF_6 adsorbed in a mesoporous glass which showed the striking phenomenon of critical depletion, i.e. the adsorption excess "Gamma" first increases but then decreases very rapidly to negative values as the bulk critical temperature T_c is approached from above along near-critical isochores. By contrast, our density functional and simulation results, for a range of strongly attractive wall-fluid potentials, show Gamma monotonically increasing and eventually saturating as the temperature is lowered towards T_c along both the critical (rho=rho_c) and sub-critical isochores (rho<ρ_c). Such behaviour results from the increasingly slow decay of the density profile away from the walls, into the middle of the slit, as T->T_c. For rho < rho_c we find that in the fluid the effective bulk field, which is negative and which favours desorption, is insufficient to dominate the effects of the surface fields which favour adsorption. We compare this situation with earlier results for the lattice gas model with a constant (negative) bulk field where critical depletion was found. Qualitatively different behaviour of the density profiles and adsorption is found in simulations for intermediate and weakly attractive wall-fluid potentials but in no case do we observe the critical depletion found in experiments. We conclude that the latter cannot be accounted for by a single pore model.

cond-mat.stat-mech↗