Wetting behaviour of 5CB and 8CB and their binary mixtures above the isotropic transition

Abstract

High resolution optical microscopy techniques have been employed to study the wetting properties of (5CB) _x /(8CB)_100-x (x=0, 10, 30, 50, 70, 90, and 100 wt %) binary mixture liquid crystal thin films above the isotropic transitions. Dewetting is found to occurr at T _w=65 and 42.5°C for the 5CB and 8CB samples, respectively, and to depend strongly on the 5CB content in the mixtures. First-order wetting transitions were seen for pure 5CB and 8CB samples; a higher order wetting transitions were obtained for the mixtures. For thin film 5CB, a large hysteresis (ΔT=30°C) between wetting and dewetting during the heating and cooling is obtained. This hysteresis is compared with the corresponding values obtained for pure 8CB and mixture samples. Evidence of the formation of nematic layers on the surface of isotropic droplets was found. Attempts to extract values of the contact line tension for these materials are made. In this respect the applicability of the modified Young's equation is questionable.