Role of interfacial entropy in the command-surface effect

The paper presents a study of the evolution of the tilt angle with time in ''command-surface'' liquid crystal cells under irradiation with UV light. In these cells, the liquid crystal orientation can be switched via a photoinduced isomerization of azo moieties attached to the surface. The trans and the cia state induce homeotropic and homogeneous alignment, respectively. Polymeric systems assembled with the Langmuir-Blodgett (LB) technique were employed. Not only homeotropic and homogeneous alignment, but also tilted alignment is found. The azimuthal direction of tilt is given by the dipping direction during LB transfer. In order to study the underlying microscopic mechanisms, we investigated the switching kinetics, performed in situ evanescent UV-visible spectroscopy, and measured the anchoring energy. The material parameters of both the command layer and the liquid crystal were systematically varied. The well defined azimuthal direction of tilt is ascribed to packing anisotropies in the command layer induced during LB transfer. The polar tilt angle is determined by the competition between interfacial entropy, on the one hand, which favors mixing and thereby homeotropic alignment, and a polar interaction, on the other hand, favoring homogeneous alignment.