NEMATIC SURFACE TRANSITIONS INDUCED BY ANCHORING COMPETITION

The interface properties of a nematic Liquid crystal in contact with a solid substrate are investigated by considering the effect of a posit ion dependent external field on a short range nematic-substrate intera ction. Such a field could be due, for instance, to the van der Waals i nteraction between the substrate and the nematic, or to the electrosta tic interaction due to selective ion adsorption. In all cases a transi tion from homeotropic to planar orientation is expected. This phenomen on is interpreted in terms of different aligning effects, by invoking the competition between a stabilizing short range term and a destabili zing long range term. We show that, according to the value of the shor t range anchoring energy strength, the stable nematic state in the pre sence of the position dependent external field can be homeotropic, dis torted, or planar. The thresholds and the order of the corresponding t ransitions are obtained. Temperature induced surface transitions can b e also interpreted in this framework.