SURFACE-INDUCED ALIGNMENT AT MODEL NEMATIC INTERFACES

We have applied the generalized van der Waals theory to a model liquid crystal that includes, explicitly, all of the second-order terms in t he spherical harmonic expansion of the anisotropic intermolecular pote ntial. We have investigated the orientational order induced by each on e of these terms, as well as the order resulting from the competition of various terms included in the potential. It was shown that, for app ropriate choices of the relative strengths of the spherical harmonic c oefficients, the theory is capable of accounting qualitatively for all the orientational effects observed at nematic interfaces, including t ilted orientations. In particular, different molecular alignments at t he nematic-vapor and nematic-isotropic interfaces of a given nematogen were described as the result of competing terms in the anisotropic in teractions. Additionally, we have shown that temperature driven orient ational 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 wetti ng of the isotropic liquid-vapor interface by the nematic phase may be destroyed as a result of the competition between different terms in t he 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.