Optical Studies of Pretransitional Surface Ordering and Disordering in Liquid Crystals

The Landau-deGennes theory predicts interesting pretransitional surface behavior at the interface between a liquid crystal and a smooth or rough substrate. In the former case, we propose a simple form for the liquid crystal-substrate interaction which yields enhanced surface ordering in the nematic and a uniaxial-biaxial transition in the isotropic phase. For the experimentally accessible range of liquid crystal-substrate interaction strengths, the experimentally observed pretransitional behavior in the isotropic phase is in excellent agreement with the theory. For the case of a rough substrate, pretransitional surface disordering in the nematic is predicted and measured directly by evanescent-wave ellipsometry, while pretransitional random-planar surface ordering occurs in the isotropic phase.