Thermal Diffusivity Measurements in Nematic and Smectic Phases by Forced Rayleigh Light Scattering

Abstract

The forced Rayleigh light scattering technique has been applied to measurements of thermal diffusivity in liquid crystalline phases. In the nematic phase of p-methoxy benzylidene p-n-butyl aniline (MBBA), our results are in good agreement with the data obtained by classical methods. The thermal diffusivity anisotropy in smectic A and B phases has been measured here for the first time in p-butoxy benzylidene p-n-butyl aniline (BBOA). As in the nematic phase, the thermal diffusivity parallel to the long molecular axis is greater than perpendicular to it, i.e. D _| > D _⊥. It is particularly striking that the thermal properties do not change significantly at the nematic to smectic phase transition. This suggests that the thermal diffusivity does not depend on the long range order properties of the various mesophases in a crucial manner, contrary to other transport properties such as the electrical conductivity. A tentative description of the thermal transport in terms of high frequency phonons seems adequate to explain our observations.