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

TL;DR

This study investigates how weakening surface interactions affects the density profiles and adsorption behavior of a confined near-critical fluid, revealing a crossover from strong to neutral wall effects using simulations and density functional theory.

Contribution

It demonstrates the crossover in surface critical behavior of confined fluids from strong adsorption to neutral wall behavior, linking it to phenomena in Ising films and critical surface transitions.

Findings

Weakening wall-fluid interactions leads to a transition from strong adsorption to neutral wall behavior.

Density profiles show two maxima that merge into one at the midpoint as interactions weaken.

Capillary coexistence line closely matches the bulk coexistence line in the neutral regime.

Abstract

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.