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 position dependent external field on a short range nematic-substrate interaction. Such a field could be due, for instance, to the van der Waals interaction between the substrate and the nematic, or to the electrostatic interaction due to selective ion adsorption. In all cases a transition from homeotropic to planar orientation is expected. This phenomenon is interpreted in terms of different aligning effects, by invoking the competition between a stabilizing short range term and a destabilizing long range term. We show that, according to the value of the short range anchoring energy strength, the stable nematic state in the presence of the position dependent external field can be homeotropic, distorted, or planar. The thresholds and the order of the corresponding transitions are obtained. Temperature induced surface transitions can be also interpreted in this framework.