A molecular theory of surface tension in nematic liquid crystals

The surface tension at the free surface of a nematic liquid crystal is computed using the Fowler-Kirkwood-Buff approximation to the interface, and a mean field approximation to the molecular pair distribution function. It is found for a Van der Waals interaction between molecules that : a) the molecules always prefer an orientation in the plane of the surface; b) there is a gap at the nematic-isotropic phase transition, but the surface tension is less in the nematic phase; c) there may be observable excess surface order which persists into the bulk isotropic phase. General expressions are given for the surface order parameter using free energy arguments. The effect of a permanent dipole interaction on the surface tension is considered, and it is found for uniaxial molecules, that this interaction also leads to the condition that the molecules be parallel to the surface. LE JOURNAL DE PHYSIQUE TOME 37, OCTOBRE 1976,