The interfacial properties of a model of a nematic liquid crystal

Abstract

We have used the theory developed in the previous paper to study the induced orientational order at simple models of a solid-liquid crystal (LC) interface. The LC is modelled by a Maier-Saupe type of anisotropic pairwise potential while the solid is modelled by an impenetrable wall which exerts an anisotropic external potential on the fluid molecules. Our theory treats both the solid-fluid and fluid-fluid potentials on a microscopic level which allows us to study the role played by these two types of interactions in the alignment of the fluid molecules at the solid surface. In particular, we study in detail the nature of the wetting of the solid-isotropic liquid interface by the nematic phase. We illustrate this by presenting the results of numerical calculations for the density and orientational order parameter profiles at the wetting transition for two different models of the solid substrate. We find that due to the weakness of the first order N/I bulk transition, the surface phase transition for z -3 substrate potentials is also weakly first order, in contrast with the results for isotropic systems. For exponential solid substrates the transition is found to depend on the range of the solid-fluid interaction; for short-ranged potentials the transition is continuous whereas for longer-ranged potentials the transition becomes more strongly first-order as the range of the interaction increases. This type of behaviour is in agreement with the results for isotropic solid-fluid interfaces. We calculate the pre-wetting lines for the two solid-fluid models. We find that the structure of the co-existing films as well as the surface critical point depend rather strongly on the model for the solid-fluid interaction. Finally, we compare the results of our theory with the results obtained from a phenomenological one-order parameter Landau-de Gennes theory and with some recent experimental results. We assess their validity and possible range of application.