ELASTIC ANOMALIES AT THE INTERFACE BETWEEN A NEMATIC LIQUID-CRYSTAL AND ITS VAPOR - A MICROSCOPIC APPROACH

Close to the interface between a nematic liquid crystal (NLC) and anot her medium the elastic constants are expected to become functions of d istance z from the interface and of angle theta between the director n and the unit vector k orthogonal to the interface. Furthermore, due t o symmetry breaking, new elastic contributions that are absent in the bulk may become important close to the interface. In this paper we con sider a simple microscopic model based on van der Waals induced dipole -induced dipole interactions under the special assumption of perfect o rientational order (S = 1). By using this model, we calculate the free energy density and the excess of surface free energy. Some elastic co nstants are obtained by using a numerical procedure. In particular, a new elastic contribution which is linear in the director gradients is obtained for the first time. Close to the interface, these elastic con stants greatly depend on distance z and are simple functions of the sc alar product n . k.