Colored noise-induced threshold shifts and phase diagrams in electroconvections

We report noise-induced threshold shifts and phase diagrams in electroconvections (ECs) in a nematic liquid crystal by controlling the frequency band (i.e., cutoff frequency fc) of noise. A crucial relationship between the internal characteristic time (i.e., charge relaxation time ¸·) of the EC system and the correlation time [¸N = 1/(2³fc)] of the external colored noise is found, which determines the role of noise in the nonequilibrium EC system. Modified numerical results using the relationship are quantitatively compared with the present experimental results. From the relationship, two types of phase diagrams can be classified in white-like and colored noises

Electroconvection in nematic liquid crystals in Hele-Shaw cells

We report electrohydrodynamic instability in nematic liquid crystals found in Hele-Shaw cells. Due to the present cell geometry, the convective structures could be directly visualized as surface or bulk flows. An unexpected structure is observed, which is completely different from the well-known patterns in the standard cells. By using the voltage-frequency jump method, the stability of a convective structure in Hele-Shaw cells is discussed in terms of the Busse diagram

ELECTROHYDRODYNAMIC CONVECTION IN PLANAR AND HOMEOTROPIC SYSTEMS OF NEMATIC LIQUID CRYSTALS(Session III : Complex Fluids, The 1st Tohwa University International Meeting on Statistical Physics Theories, Experiments and Computer Simulations)

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Compressed Exponential Relaxation as Superposition of Dual Structure in Pattern Dynamics of Nematic Liquid Crystals

Soft-mode turbulence (SMT) is the spatiotemporal chaos observed in homeotropically aligned nematic liquid crystals, where non-thermal fluctuations are induced by nonlinear coupling between the Nambu-Goldstone and convective modes. The net and modal relaxations of the disorder pattern dynamics in SMT have been studied to construct the statistical physics of nonlinear nonequilibrium systems. The net relaxation dynamics is well-described by a compressed exponential function and the modal one satisfies a dual structure, dynamic crossover accompanied by a breaking of time-reversal invariance. Because the net relaxation is described by a weighted mean of the modal ones with respect to the wave number, the compressed-exponential behavior emerges as a superposition of the dual structure. Here, we present experimental results of the power spectra to discuss the compressed-exponential behavior and the dual structure from a viewpoint of the harmonic analysis. We also derive a relationship of the power spectra from the evolution equation of the modal autocorrelation function. The formula will be helpful to study non-thermal fluctuations in experiments such as the scattering methods.Comment: 17pages, 3 figures, to be published on AIP conference proceedings for "The 4th International Symposium on Slow Dynamics in Complex Systems

Observation and determination of abnormal rolls and abnormal zigzag rolls in electroconvection in homeotropic liquid crystals

Direct evidence for two different types of normal rolls and of zigzag rolls in homeotropically aligned nematic liquid crystals in a magnetic field is reported. The conventional normal rolls have the reflection symmetry in the xy plane. The instability, however, breaks the reflection symmetry y→-y on the director and then the abnormal rolls are expected to be observed. We have investigated the instability experimentally and discussed it in terms of the recent numerical results by Plaut et al. [Phys. Rev. Lett. 79, 2367 (1997)]. Due to the new instability, the abnormal zigzag rolls are also found below the Lifshitz frequency

Prewavy instability of nematic liquid crystals in a high-frequency electric field

A kind of electrohydrodynamic instability, the prewavy instability, in nematic liquid crystals is reported. The characteristic of the instability was optically investigated and discussed in comparison with some similar instabilities. Obviously the instability partially shows an isotropic feature around the nematic-isotropic transition temperature. Owing to the characteristic properties of the flow and the spatial periodicity, it should be distinguished from a previously proposed isotropic instability