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

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Abstract
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