Phase separation in a binary mixture confined between symmetric parallel plates: Capillary condensation transition near the bulk critical point

Abstract

We investigate phase separation of near-critical binary mixtures between parallel symmetric walls in the strong adsorption regime. We take into account the renormalization effect due to the critical fluctuations using the recent local functional theory [J. Chem. Phys. 136, 114704 (2012)]. In statics, a van der Waals loop is obtained in the relation between the average order parameter $<psi>$ in the film and the chemical potential when the temperature $T$ is lower than the film critical temperature $T_c^{ca}$ (in the case of an upper critical solution temperature). In dynamics, we lower $T$ below the capillary condensation line from above $T_c^{ca}$. We calculate the subsequent time-development assuming no mass exchange between the film and the reservoir. In the early stage, the order parameter $psi$ changes only in the direction perpendicular to the walls. For sufficiently deep quenching, such one-dimensional profiles become unstable with respect to the fluctuations varying in the lateral directions. The late-stage coarsening is then accelerated by the hydrodynamic interaction. A pancake domain of the phase disfavored by the walls finally appears in the middle of the film.