Computer simulation study of the free surfaces of a liquid crystal model

We present a computer simulation study of the liquid-vapor interface of the Gay-Berne fluid model for a range of temperatures above and below the vapor-isotropic liquid-nematic triple point. The molecular elongation is kappa=3, and two values of the energy anisotropy parameter (kappa'=1 and 1.25) are considered. Our results show that an excess of orientational order is developed at the isotropic liquid-vapor interface. The degree of orientational order grows as the temperature is lowered to its value at the triple point. Molecules in the nematic phase orient themselves parallel to the interface. Combining our results with those obtained previously for kappa=3 and kappa'=5, we conclude that a change in the preferred orientation at the isotropic free surface takes place when kappa' is decreased (for a fixed value of kappa), as the alignment changes from perpendicular to parallel. The behavior of the surface tension and the adsorption of the order parameter at the isotropic free surface close to the triple point are compatible with the existence of wetting at the isotropic liquid-vapor interface by the nematic phase.