Variety of wetting behaviors at the free surface of isotropic liquid crystals

We present an experimental study of the wetting behavior at the free surface of the isotropic phase of various liquid-crystal compounds. For the first time we observe a prewetting transition above a bulk nematic—isotropic transition. Above smectic-A–isotropic transitions of nonpolar compounds, we observe complete wetting, partial wetting with pretransitional increase of surface order, and partial or nonwetting without any pretransitional increase. Above the smectic-A–isotropic transition of a polar (cyano-substituted) compound, a series of single layering transitions occurs at the surface which has already been observed by x-ray reflectivity studies. These layering transitions are not observed in the nonpolar smectic compounds.