Wetting Phenomena at the Free Surface of the Isotropic Phase of a Smectic Liquid Crystal

We report an ellipsometric study of the free surface of a smectic liquid crystal possessing a smectic- A--to--isotropic phase transition. Approaching the transition from above, an ordered surface layer appears at a discontinuous transition one degree above the bulk transition temperature. With decreasing temperature the thickness of the surface layer increases continuously, indicating a possible logarithmic divergence at the bulk transition. The behavior may be described as complete wetting accompanied by a prewetting transition.