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

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 -GS(s) delta(z), where S-s 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 Delta G 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. (C) 1998 American Institute of Physics. [S0021-8979(98)00323-5].