Fluctuations and Light Scattering in Cholesteric Liquid Crystals

The normal modes of an incompressible cholesteric liquid crystal are determined from the hydrodynamical equations. There are two important modes which should be readily observable by light scattering. The first mode is a twisting and untwisting of the helical structure. The second mode is a combination viscous-splay mode. Both modes are overdamped. These modes scatter light most strongly for momentum transfers exactly equal to and exactly one-half of those leading to Bragg scattering, respectively. Thus the modes should be easily separated experimentally. The normal modes are also evaluated in the presence of a transverse magnetic field, and the damping constants turn out to be quite sensitive functions of the magnetic field.