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 4-cyano-4′-n-alkyl-biphenyls (nCB, n = 5,6,7,8) is studied by reflection ellipsometry. On approaching the bulk isotropic-nematic transition temperature TNI 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≥50 Å, both d and the order parameter S0 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 TNI 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. © 1995 American Institute of Physics.