Casimir amplitudes and capillary condensation of near-critical fluids between parallel plates: Renormalized local functional theory

TL;DR

This paper develops a renormalized local functional theory to analyze the critical behavior and Casimir effects of near-critical fluids confined between parallel plates, revealing phase transitions and enhanced Casimir amplitudes.

Contribution

It introduces a universal renormalized local functional framework for near-critical fluids, accounting for fluctuation effects and capillary condensation phenomena.

Findings

Capillary condensation line appears outside bulk coexistence curve.

Casimir amplitudes are significantly larger than at the critical point.

Transition line ends at a capillary critical point slightly below T_c.

Abstract

We investigate the critical behavior of a near-critical fluid confined between two parallel plates in contact with a reservoir by calculating the order parameter profile and the Casimir amplitudes (for the force density and for the grand potential). Our results are applicable to one-component fluids and binary mixtures. We assume that the walls absorb one of the fluid components selectively for binary mixtures. We propose a renormalized local functional theory accounting for the fluctuation effects. Analysis is performed in the plane of the temperature T and the order parameter in the reservoir \psi_{\infty} . Our theory is universal if the physical quantities are scaled appropriately. If the component favored by the walls is slightly poor in the reservoir, there appears a line of first-order phase transition of capillary condensation outside the bulk coexistence curve. The excess adsorp- tion changes discontinuously between condensed and noncondensed states at the transition. With increasing T, the transition line ends at a capillary critical point T = T_c^{ca} slightly lower than the bulk critical temperature T_c. The Casimir amplitudes are larger than their critical-point values by 10-100 times between the transition line and the bulk coexistence curve and slightly above the capillary critical point.