SMECTIC-A ORDERING IN LIQUID-CRYSTAL FILMS

We modify an earlier density-functional theory of inhomogeneous liquid crystals so that it is now able to account for enhanced smectic-A ord ering at a free surface. The main modification is the use of different spatial ranges for different angular components of the intermolecular pair potential. We show that the modified theory is able to generate both partial and complete wetting by the smectic-A phase, as well as d iscrete layering transitions at an interface between isotropic liquid and vapour phases. The major application of the theory is to investiga te the properties and conditions required for the existence of 'layer- thinning' transitions in freely suspended smectic-A films. Despite the existence of surface-enhanced smectic ordering, we find that layer-th inning transitions do not occur in most parameter ranges of the theory , and instead that freely suspended films will generally tend to ruptu re on heating. Thinning transitions analogous to those observed experi mentally are found to occur within very limited parameter ranges, and are associated with melting of the him interior to a nematic rather th an isotropic liquid phase.