J. Phys. II France
Volume 4, Numéro 9, September 1994
Page(s) 1617 - 1630
DOI: 10.1051/jp2:1994221
J. Phys. II France 4 (1994) 1617-1630

Elastic anomalies at the interface between a nematic liquid crystal and its vapour : a microscopic approach

Sandro Faetti and aurizio Nobili

Dipartimento di Fisica dell'Universita' di Pisa and Consorzio Interuniversitario di Fisica della Materia, Piazza Torricelli 2, 56100 Pisa, Italy

(Received 19 November 1993, revised 25 March 1994, accepted 18 May 1994)

Close to the interface between a nematic liquid crystal (NLC) and another medium, the elastic constants are expected to become functions of distance z from the interface and of angle $\theta$ between the director n and the unit vector k orthogonal to the interface. Furthermore, due to symmetry breaking, new elastic contributions that are absent in the bulk may become important close to the interface. In this paper we consider a simple microscopic model based on van der Waals induced dipole-induced dipole interactions under the special assumption of perfect orientational order ( S = 1). By using this model, we calculate the free energy density and the excess of surface free energy. Some elastic constants 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 constants greatly depend on distance z and are simple functions of the scalar product n $\cdot$ k.

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