Pattern Dynamics in the Electrohydrodynamics of Nematic Liquid Crystals

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 DSMl and DSM2 which has some analogy with TI-TII transition in superfiuid Hell. DSM2 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 DSMl 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.at Oakland University on June 2, 2015 http://ptps.oxfordjournals.org/ Downloaded from

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“…Whereas disclinations are not usually found in the DSM1 state, numerous disclinations exist in the DSM2 state, which reduces the sample's light transmittance. Without a shear field, the bifurcation voltage between the two states for the present rheometer-confined sample was about 29.7 V, similar to the bifurcation voltage of 31 V for a typical glass cell as reported by Kai et al [20,25]. In contrast, detecting the bifurcation voltage quantitatively under a shear flow was difficult because disclinations are generated even in the DSM1 state.…”

“…For a standard sandwich-type cell under homogenous anchoring conditions, the DSM1 director is influenced by the anchoring force from the glass plates, whereas the DSM2 director remains uninfluenced. As a result, images of transmitted light in the DSM1 and DSM2 states are anisotropic and isotropic, respectively [20,25]. On the other hand, the images of the transmitted light in both states are anisotropic in the current experiments due to the shear flow.…”

“…For a standard sandwich-type cell under homogenous anchoring conditions, there are two types of states, DSM1 and DSM2, distinguished by the existence of disclinations [20,25]. Whereas disclinations are not usually found in the DSM1 state, numerous disclinations exist in the DSM2 state, which reduces the sample's light transmittance.…”

“…In the dielectric regime, a fine oscillatory striation pattern ("chevron") appears at a certain threshold of the electric field that is order of 25 kV/cm for f = 100 Hz [18]. Experimentally, it is known that the chevron is not convection over the whole volume of the sample but a skin convection just close to the electrodes [19,20]. Hence, we contend that f c for Negita's sample was less than 100 Hz such that no convection over the whole volume of the sample was induced.…”

Persistent global power fluctuations near a dynamic transition in electroconvection

“…The set-ups permit the recording of both P (t) and the optical patterns, while at the same time monitoring the transmitted light intensity I(t) integrated over the entire area of the sample. up into segments that move, seemingly at random with speed proportional to ε [12]; this state has been called defect turbulence [3]. Measurements in MBBA, not presented here,…”