Surface-induced alignment at model nematic interfaces

Abstract

We have applied the generalized van der Waals theory to a model liquid crystal that includes, explicitly, all of the second-order terms in the spherical harmonic expansion of the anisotropic intermolecular potential. We have investigated the orientational order induced by each one of these terms, as well as the order resulting from the competition of various terms included in the potential. It was shown that, for appropriate choices of the relative strengths of the spherical harmonic coeKcients, the theory is capable of accounting qualitatively for all the orientational e8'ects observed at nematic interfaces, including tilted orientations. In particular, different molecular alignments at the nematic-vapor and nematic-isotropic interfaces of a given nematogen were described as the result of competing terms in the anisotropic interactions. Additionally, we have shown that temperaturedriven orientational transitions may occur in systems characterized by this type of interaction. For a given choice of parameters, we have also found an orientational transition, which is related to a wetting transition at the nematic-vapor interface. Finally, it was shown that complete wetting of the isotropic liquid-vapor interface by the nematic phase may be destroyed as a result of the competition between di8'erent terms in the potential. Similarly, a reentrant wetting transition at the nematic-vapor interface by the isotropic phase was found, as a result of this competition, at the nematic-vapor interface.