NEMATIC LIQUID-CRYSTAL ANCHORING ON LANGMUIR-BLODGETT-FILMS - STERIC, BIPHILIC, DIELECTRIC AND FLEXOELECTRIC ASPECTS AND INSTABILITIES

A survey of the published data on the type of anchoring (homeotropic, planar or tilted) of nematic liquid crystals on Langmuir-Blodgett (LB) films of charged and dipolar amphiphiles, as a function of surface density of molecules and number of molecular layers in the film, reveals a complex and seemingly controversial picture. A theoretical model of the anchoring, taking into account the non-homogeneous electric field originating from surface charges and dipoles, which dictates the liquid crystal orientation via a competition of dielectric, flexoelectric and ordoelectric torques, predicts a rich variety of anchoring states, depending on the sign of surface charges, the orientation of surface dipoles and the signs of dielectric anisotropy and total flexocoefficient. In specific situations, critical molecular densities and thicknesses for anchoring transitions (either first or second order) from homeotropic to tilted to planar or vice versa are predicted. 'Reentrance' sequences, homeotropic-planar-homeotropic and planar-homeotropic-planar, are also possible, while surface density increases monotonically. Thus, the model resolves the apparent contradictions between various experiments and puts them in a general framework for the first time.