The surface tension of nematic liquid crystals

Abstract

The surface tension γ has been measured for two cyanobiphenyls (5CB and 8CB) at temperatures near the nematic-isotropic transition temperature T _c, using the Wilhelmy plate method. Reproducible results were obtained, but only by the exercise of considerable patience: for reasons that are discussed at the end of the paper, it took up to four days for equilibrium to be re-established whenever the temperature was changed. The results suggest that in both materials the equilibrium value of γ increases discontinuously (by approximately 0·3 dyne cm⁻¹) on heating through T _c, and that (dγ/dT) is positive both for the isotropic phase just above T _c and for the nematic phase just below it. A positive value of (dγ/dT) implies excess order in the surface layers. A simple phenomenological theory is outlined, based on the hypothesis that this excess order resides solely in the degree of alignment of the molecules, the conventional nematic order parameter being larger near the surface than in the bulk. The predictions of the theory (which involves only one adjustable parameter) are in partial agreement with the observations, but there are some discrepancies, which suggest that there may also be smectic-like order near the surface. The evidence for smectic-like surface order is more marked for 8CB than for 5CB. The theory of the Wilhelmy plate method, as applied to a nematic liquid, is discussed in an Appendix. A second Appendix contains a novel criticism of one aspect of the molecular model used to describe nematics by Maier and Saupe (1959): it is not legitimate to use this model in the way that Parsons (1976) has done in his attempt to provide a microscopic theory for γ.