A LANDAU-DE GENNES MODEL FOR THE TEMPERATURE SURFACE TRANSITION IN NEMATICS

Recently a surface transition driven by the temperature in nematics ha s been observed and discussed [K. Flatischler et al., Mel. Cryst. Liq. Cryst. 198, 119 (1991)]. Such a transition usually appears some degre es below the clearing point: it consists of a change of alignment in t he whole nematic layer, from homeotropic to planar, on heating. The tr ansition could be second or first order, depending on surface treatmen t and bulk properties and is reversible: in the latter case hysteresis takes place. The importance of such a phenomenon is evident, since th e temperature surface transition affects the electro-optical propertie s of a nematic cell. Here we present a new Landau-de Gennes model desc ribing the surface transition which when the cell walls, covered by a thin layer or silicon monoxide, are dipped or in a very dilute solutio n of egg-lecithin in chloroform.