Rubbing strength dependence of surface interaction potential and surface-induced order above the nematic–isotropic transition

Abstract

Optical retardation measurements as a function of rubbing strength are reported for both rubbed polyimide-coated substrates and cells containing the liquid crystal hexylcyanobiphenyl. Our analysis assumes a Sheng surface interaction potential of the form −GSsδ(z), where Ss is the order parameter of the liquid crystal at the interface and G is related to the rubbing strength. Moreover, we assume that there can be a distribution ΔG in the parameter G. Several features are noted: There appears to be a minimum rubbing strength before the onset of substantial surface-induced order in the liquid crystal; the parameter that characterizes the interaction potential between the polyimide and liquid crystal is proportional to the optical retardation of the polyimide-coated substrate; and, in light of our analysis, there are substantial inhomogeneities in the liquid crystal interaction potential and surface order parameter, perhaps even on length scales comparable to the nematic correlation length.