Pattern Dynamics in the Electrohydrodynamics of Nematic Liquid Crystals

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Abstract
Various patterns in the electrohydrodynamic convection of planarly aligned nematic liquid crystals are investigated. We give experimental and theoretical results on the onset of convection in the conduction regime and the dielectric regime as well. The transition to the fluctuating Williams domain (FWD) immediately above the onset of convection in the conduction regime is characterized in detail. At this secondary threshold the straight rolls become unstable and defects appear. During the temporal development of the FWD, defects are continuously created and annihilated, and the defect density behaves rather stochastical in time. At even higher values of the applied voltage we investigate the transition between the two turbulent states DSM1 and DSM2 which has some analogy with TI-TII transition in superfluid HeII. DSM 2 turbulence can be characterized by disclination and therefore called disclination turbulence. We show that this transition is local via nucleation and that the main difference between both states is the vanishing disclination density in the DSM1 state and its finite value in the DSM2. In the high frequency regime we analyse the secondary transition to chevrons and the defect dynamics in this pattern as a periodic defect structure. Furthermore, the influence of a superimposingly applied magnetic field on these patterns is considered.

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