Recent Experimental Investigations in Nematic and Cholesteric Mesophases

Abstract

The development of the Frank thermodynamical description of liquid crystals has recently allowed quantitative predictions on the static and dynamical behavior of the nematic and cholesteric mesophases. We report here the results of our experimental investigations to measure the parameters introduced by the theory, in two main directions: (a) the magnetically induced cholesteric to nematic phase transition, in low concentration mixtures of cholesteryl esters in nematic materials, appears as a general method for measuring the “twist” elastic constant of the nematic solvent; (b) the study of the thermal fluctuations of anisotropy in nematic materials, observed by high resolution spectral analysis of the Rayleigh light scattered by the liquid crystal, allows the determination of the Leslie viscosity coefficients. Finally, we report on the observation of a new type of disclination line in cholesteric mesophases (double disclination line).

This paper is a short review of the experimental work performed during the current year by the Orsay Liquid Crystal Group on the physical properties of liquid crystal mesophases. The aim of this effort has been to test recent theoretical predictions concerning some elastic, magnetic and optical properties of liquid crystals, in the frame work of the continuum thermodynamical model. Our results have been partially published in form of letters, and experimental details can be found therin. We would like to emphasize here that the continuum model of liquid crystals is not a formal speculation; it can indeed lead to an accurate characterization of the physical behavior of the mesophases, and it gives powerful methods for the measurement of the anisotropic physical constants involved in elastic and hydrodynamical properties of liquid crystals.

We first describe an experiment of magneto-elasticity, which allows the determination of the “twist” elastic constant⁽¹⁾ of a nematic material. We then report on results concerning the damping of thermal fluctuations of anisotropy in a nematic mesophase, which give at the present time 4 out of the 5 Leslie viscosity coefficients⁽²⁾. Finally, we describe a new type of defect (“disclination”) line recently observed in cholesteric mesophases.

Work partially supported by D.G.R.S.T.(under contract N° 6801 194).