Experimental investigation of surface deformations at the nematic-isotropic interface : a new method to measure the Nehring-Saupe elastic constant K13

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Abstract
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 K11 and K33 and for the effect of the elastic constant K13 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 K13 constant which cannot be measured by standard techniques. The predictions of the theory have been verified experimentally at the nematic-isotropic interface of the liquid crystal 4-cyano-4'-n-heptylbiphenyl (7CB). Both the geometry and the magnetic field-dependence of the surface deformations are found to agree satisfactorily with the theoretical predictions. An upper limit to the elastic constant K13 is obtained from the experiment