The effect of surfaces on the structural properties of liquid crystals

Abstract

The effect of the surface microrelief on the structural properties of both the nematic and smectic A (SmA) phases, and the effect of the emerging smectic structure induced by the surface at temperatures close to the second-order nematic-SmA (NA) phase transition temperature TNA, on the Freedericksz threshold voltage for homogeneously aligned polar liquid crystals, is theoretically analyzed.It is shown that the orientation distortion of the director field caused by the wave-like microrelief on solid surfaces exponentially decays in the bulk of both phases. Since in the case of the SmA phase, the characteristic depth of deformation is comparable to the length of the molecule, then when the period of undulating relief passes several micrometers, the depth of penetration of the deformation is several tens of micrometers. This is two orders of magnitude greater than the penetration length of the distortion in the case of the nematic phase.The effect of the surface-induced SmA phase on the threshold voltage, for homogeneously aligned of polar liquid crystals is discussed from the energy point of view. It is shown that the Freedericksz critical voltage, for 4-n-octyl-4′-cyanobiphenyl, in a very thin cell near the second-order nematic-SmA (NA) phase transition temperature, exhibits an anomalous increase with decreasing of temperature T towards TNA.