Light Scattering Properties of Cholesteric Liquid Crystal Films

It has been observed that cholesteric liquid crystal films exist in two states both of which exhibit characteristic brilliant colors. A transition between the two states can be caused by a variety of disturbances including mechanical perturbation and electrophoretic action.1 Optical scattering data indicate that the fundamental dispersive cause in both the undisturbed state and the disturbed state is the existence of regions of local order characterized by optically anisotropic layers stacked in a helical fashion. Intensity studies indicate that the angular distribution of helical axes is not isotropic in either state. In the undisturbed state the helical axes lie predominantly in the plane of the surface whereas in the disturbed state the helical axes are predominantly normal to the surface. This is confirmed by microscopic observations which show in the undisturbed state regions (with the dimensions of microns) of large birefringence and no optical activity. The disturbed state exhibits one uniform color between crossed nicols along with dispersive optical activity. Typical materials used were mixtures of cholesteryl nonanoate and cholesteryl chloride. The significance of a reflecting substrate is also treated.