Surface induced alignment transition in a nematic layer with symmetrical boundary conditions

The inner surfaces of a cell of conventional type were covered with an SiO, aligning layer evaporated at an angle α = 60° and subsequently treated with lecithin in order to achieve symmetrical boundary conditions. The alignment of a nematic liquid crystal layer with negative dielectric anisotropy (Δε < 0) was found to be homeotropic below a critical temperature Tc, (low temperature range). Above that particular temperature the homeotropic alignment abruptly transforms into a planar one, which remains with increasing temperature up to the clearing point TNI (high temperature range). The alignment transition was found to be reversible, and is attributed to a packing change of the lecithin layer with the temperature. A simple model based on the different temperature dependence of the anchoring strengths W H, (T) and W P, (T), characterizing the homeotropic and planar alignment respectively, is proposed to explain the surface induced alignment transition.