Order parameter of a nematic liquid crystal on a rough surface

We measure the surface order parameter Sₛ at the interface between the nematic liquid crystal p-methoxy-benzylidine-p-n-butyl-aniline and a rough glass plate. Roughness is induced by oblique evaporation of SiO. We observe a critical decrease of Sₛ when the surface roughness increases, just above the transition from planar to oblique orientation. This experiment supports the ordoelectric model to explain the oblique orientation of the nematic liquid crystal.