Smectic-A ordering in liquid crystal films

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Abstract
We modify an earlier density-functional theory of inhomogeneous liquid crystals so that it is now able to account for enhanced smectic-A ordering 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 discrete layering transitions at an interface between isotropic liquid and vapour phases. The major application of the theory is to investigate 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-thinning transitions do not occur in most parameter ranges of the theory, and instead that freely suspended films will generally tend to rupture on heating. Thinning transitions analogous to those observed experimentally are found to occur within very limited parameter ranges, and are associated with melting of the film interior to a nematic rather than isotropic liquid phase.