First-order Landau-de Gennes model for layer thinning in presmectic free-standing films.

A Landau theory for surface enhanced ordering in free-standing smectic-A films is described, based on a generalization of de Gennes's "presmectic" model to systems which undergo a first-order smectic-isotropic transition in bulk. As found in related work on phase transitions in thin films, the system exhibits three phases, an isotropic liquid, a bulk-like ordered (smectic-A) phase, and a surface-ordered ("quasismectic") phase. Over much of its range, the temperature-thickness boundary between the bulk-ordered and surface-ordered phases is effectively characterized by a power-law relation similar to those observed for layer-thinning transitions in overheated free-standing smectic-A films.