Metastability of freely suspended liquid-crystal films

We investigate the conditions required for the existence of smectic-A liquid-crystal films freely suspended in vapor. This work is based on a molecular density-functional theory developed in earlier studies of wetting and layering transitions of smectic-A films at Liquid-solid and liquid-vapor interfaces. It is emphasized that all freely-suspended films are metastable with respect to either the absence of the film or the formation of the bulk smectic phase, depending on the relative stability of the bulk vapor and smectic phases. Films containing different numbers of layers correspond to local minima of the grand canonical potential, and are ''stabilized'' relative to each other by the presence of sufficiently large metastability barriers. The disappearance of these barriers corresponds physically to the ''rupturing'' of the films. It is demonstrated that surface enhancement of smectic ordering does not play an essential role in the existence of freely suspended films, but may influence the order of disappearance of the layer-layer metastability barriers on changing temperature. We relate these findings to the results of recent experimental studies of ''layer thinning transitions'' in freely suspended films.