Thermal fluctuations in cholesteric liquid crystals

The thermal fluctuations from equilibrium are more important in liquid crystals than in ordinary condensed matter, as they are a lot stronger. The cholesteric liquid crystals are materials made, usually, of rod-like organic molecules that are chiral (with no mirror symmetry). They arrange themselves in a helical structure with the direction of the rods rotating around a helical axis. The thermal fluctuations in these materials may be the driving force behind the transition, when the temperature is increased, into a “blue phase”, a phase with double twisted structure. We did theoretical calculations of the fluctuations in a cholesteric liquid crystal using a tensor order parameter. The calculations were limited to the case of a wave vector along the helical axis. We found 5 fluctuation modes, corresponding to the 5 components of the tensor order parameter. We studied the temperature dependence of these modes as well as the effect of an electric field (for the case of an electric negative anisotropy).; The experimental work was focused on the investigation of the strongest fluctuation mode by using dynamic light scattering. The relaxation time of the thermal fluctuations was measured at various scattering vectors and a peak was found, as predicted by the theory. The temperature dependence of the relaxation time was investigated. Several materials, with various values of the pitch (periodicity of the helical structure) were investigated. The effect of an electric field on the relaxation time in a material with electric negative anisotropy was studied. We found some unexpected effects that may be the object of future investigations.