Identification criterion for the mechanisms of orientation instability of cholesteric liquid crystals in ultrasonic fields

An identification criterion is proposed and experimentally substantiated for the two independent acoustic mechanisms (vortex and relaxation ones) governing the formation of a system of two-dimensional domains in a planar layer of a cholesteric liquid crystal in the frequency range where the sound wavelength is greater than the step of the cholesteric circuit. The results of measuring the critical compression amplitude in mesophase layers with thicknesses from 40 to 240 μm are presented for frequencies lower and higher than the frequency equal to the inverse relaxation time of the orientation order parameter in a cholesteric liquid crystal. The data are obtained for diluted solutions of cholesterilchloride in a nematic liquid crystal with a circuit step varied from 2 to 30 μm. The concept of the binary nature of the acoustic mechanism that causes destabilization of the planar texture of a cholesteric liquid crystal in the frequency range under consideration is experimentally verified.