On the binary nature of the mechanism of orientational instability in cholesteric liquid crystals in wave fields

The prediction of a theory developed with allowance for the processes of orientational order parameter relaxation in liquid crystals about two independent acoustic mechanisms of orientational instability in the planar layer of a cholesteric liquid crystal (CLC) in a wave field, manifesting itself in the occurrence of a system of 2D domains, is experimentally substantiated for the first time. The distortions of the layer macrostructure for the mesophase of this type was observed in the field of longitudinal waves in a wide frequency range, including frequencies above the relaxation frequency of the orientational order parameter in CLC. The values of the spatial domain period at the effect threshold and the threshold compression amplitudes are determined for CLC layers 10–100 μm thick with a helix pitch of 2–30 μm in the frequency range of 0.3–45 MHz. It is shown that a complete theoretical description of the phenomenon, consistent with the experimental data, can be obtained only with allowance for the action of the convection and the nonlinear relaxation mechanisms of the mesophase layer structure destabilization, which have different natures.