Microscopic and macroscopic description of anchoring at the nematic-solid substrate interface.
A simple microscopic mean-field model for a flat nematic liquid crystal (NLC) sample in a contact to the solid substrate surface is offered. An interaction between NLC molecules is simulated by the well known McMillan model potential, and an orienting action of the solid substrate surface on NLC molecules is modeled by a short-range external field which acts directly only on molecules within the first molecular layer of the nematic sample adjacent to the substrate surface. For an undistorted NLC sample, the model allows the calculation of local order parameter profiles for different values of strength of this external orienting field and temperature of the sample. These profiles are used in a description of a director field distortion caused by a certain external action and in the calculation of the anchoring energy coefficient used in a macroscopic description of the anchoring at the nematic-solid substrate interface. Dependence of this coefficient on the strength of the short-range orienting field is obtained, and an unequivocal relation between the magnitude of and the orientational order parameter profile near the substrate surface is established. The temperature dependence of the coefficient W calculated from the offered microscopic model is in good agreement with the experimental data on NLC MBBA.