Nematic anchoring transitions on bistable SiO films driven by temperature and impurities

The influence of temperature and impurities on the surface orientation of nematic 5CB on obliquely SiO evaporated glass plates has been studied. We observe on cooling a continuous anchoring transition from the planar alignment to the tilted one. The experimental data have been interpreted by using elastic models developed by Durand and coworkers in which the surface order parameter can vary in space, close to the surface.