Nematic Wetting at the Free-Surface of 4-Cyano-4'-N-Alkyl-Biphenyls

Abstract

The free surface of a homologous series of low molecular weight liquid crystals 3-cyano-4'-n-alkyl-biphenyls (nCB, n=5,6.7,8) is studied by reflection ellipsometry. On approaching the bulk isotropic-nematic transition temperature T-NI from the isotropic phase, the growth of a nematic layer at the surface is observed. The director orientation in the layer is identified as homeotropic. The ellipsometric data are analyzed in terms of a perturbation calculation. It is shown that for 7CB and 8CB very close to T-NI, where the layer has a thickness d greater than or equal to 50 Angstrom, both d and the order parameter S-0 at the surface can be determined. In all other cases only one parameter, which corresponds to the nematic coverage, can be extracted from the data. Results are discussed in terms of a wetting process, Partial wetting by the nematic phase is observed for n =6,7.8. On approaching T-NI from lower temperatures where the bulk is in the nematic phase a small increase of excess surface order is measured which is interpreted as a homeotropic nematic wetting layer possessing higher order than the nematic bulk.