A dynamic light scattering study of the viscoelastic twist mode in cholesteric liquid crystals

Although viscoelastic properties of liquid crystals are very important for applications like display technology, there are not many direct techniques to study them. We have used dynamic light scattering to obtain dispersion curves for the twist mode of director fluctuations in a thermotropic cholesteric liquid crystal (cholestrol) mixture, in a wave vector range comparable to the inverse pitch of the cholesteric. In the experiment, light scattered in the vicinity of Bragg reflection is investigated using a hitherto unused scattering geometry where the scattering wave vector q is parallel to the cholesteric helix axis. By fitting the experimental data to a parabolic function of the scattering wave vector, we have determined the twist viscoelastic coefficient k(22)/gamma(1). We have studied the temperature dependence Of k(22)/gamma(1) and found that the viscoelastic coefficient increases rapidly as the cholesteric-to-isotropic transition temperature is approached.