Anisotropic Ultrasound Propagation in a Cholesteric Liquid Crystal

We report here a detailed study of the propagation of longitudinal ultrasound through a magnetically aligned cholesteric liquid crystal (a mixture of cholesteryl chloride and cholesteryl myristate, 1.75: 1 by weight). The observed velocity anisotropy is tentatively attributed to dispersion and an attenuation anisotropy was observed for the first time. The temperature dependence of several elastic constants and combinations of viscosity coefficients was determined. A new expression for the attenuation was derived from the hydrodynamic equations, and from a twisted nematic model. A more detailed report of the acoustic Brillouin zone effect is also presented.