Why do most nematic liquid crystals align parallel to inorganic surfaces

Abstract

To a first approximation one may predict the orientation of the director of a thin film of nematic liquid crystal (L.C.) adjacent to a solid surface through the relative magnitude of the L.C. surface tension and solid surface energy. A deeper insight, taking into account the L.C. surface tension anisotropy leads to the conclusion that homeotropic orientation should be observed on high energy surfaces. Most experimental evidence indicates that L.C. orientation over inorganic surfaces is homogeneous. We will present the results of our measurements of inorganic solids surface tension which, in accordance with previously published results, show that inorganic surfaces, in contact with the atmosphere, have an average surface energy of about 40 mJm-2. This result has, in most cases, been explained through the absorption of a water layer. In this note we retain this explanation, as a first approximation. From the criterion for displacement of a liquid 1 by a liquid 2 and known values of the surface energy of inorganic solids, water and liquid crystals, it is shown that water will not be displaced by L.C. molecules. Thin L.C. films lie over a water layer of surface energy 40 ± 5mJm-2 and remain parallel to the solid surface.