Sliding planar anchoring and viscous surface torque in a cholesteric liquid crystal

Abstract : We propose a new surface treatment allowing to obtain a sliding planar anchoring of nematic (or cholesteric) liquid crystals. It consists of depositing a thin layer of the polymercaptan hardener of an epoxy resin on an isotropic substrate (bare or ITO-coated glass plates). Microscopic observations of defect annihilations and capacitance measurements show that the molecules align parallel to the surface and slide viscously on it when they change orientation, which implies a zero (or extremely small) azimuthal anchoring energy. By contrast, the zenithal anchoring energy W_\theta is found to be larger than 3\times10^{-5} J/m{^2}. We also measured the liquid crystal rotational surface viscosity \gamma_S by a new thermo-optical method using the large temperature variation of the pitch of a compensated cholesteric mixture. We found that the sliding length \gamma_S/\gamma_1 (where \gamma_1 is the bulk rotational viscosity) is very large in comparison with the length of a liquid crystal molecule. This result is explained by a simple model which takes into account the diffusion of the liquid crystal within the polymer layer.