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

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.