Near-critical confined fluids and Ising films: Density-matrix renormalization-group study

Two-dimensional Ising strips subject to identical surface fields h(1) = h(2)greater than or equal to0 are studied for temperatures above and below the bulk critical temperature T-c and a range of bulk fields h by means of the density-matrix renormalization-group method. In the case of nonvanishing surface fields, the near-critical behavior of the solvation force f(solv), total adsorption Gamma, inverse longitudinal correlation length xi (-1)(parallel to) and specific heat C-H is strongly influenced by the (pseudo) capillary condensation that occurs below T-c. We obtain scaling functions of f(solv), Gamma, and xi (-1)(parallel to). C-H exhibits a weakly rounded singularity on crossing the pseudocoexistence line. We contrast these results with those for the case of free boundaries where. for temperatures slightly below T-c, f(solv) and CH exhibit a sharp extremum away from h = 0. Our results have direct repercussions for the properties of near-critical Ising films in three dimensions and we argue that the long-ranged solvation (Casimir) force in confined fluids should be more attractive in the neighborhood of the capillary critical point than exactly at the bulk critical point.