EXPERIMENTAL INVESTIGATION OF SURFACE DEFORMATIONS AT THE NEMATIC-ISOTROPIC INTERFACE: A NEW METHOD TO MEASURE THE NEHRING-SAUPE ELASTIC CONSTANT K//1//3 (** plus )

Structural deformations of a nematic-isotropic interface induced by magnetic field have been predicted some years ago by de Gennes. In this paper we extend the de Gennes' theory in order to account for the finite anchoring energy of the director at the interface, for the anisotropy of the elastic constants K//1//1 and K//3//3 and for the effect of the elastic constant K//1//3 introduced by Nehring and Saupe. We obtain a simple analytical expression which relates the maximum tilt angle of the interface to the surface director polar angle, to the elastic constants, to the surface tension and to the magnetic field. Therefore a measurement of this tilt furnishes a direct method to measure the K//1//3 constant which cannot be measured by standard techniques. Theoretical predictions are verified experimentally.