Multiplicative Noise-Induced Electrohydrodynamic Pattern Formations by Controlling Electric Conductivity

We report multiplicative noise impacts on electroconvections (ECs) in a nematic liquid crystal. By controlling the intensity and cutoff frequency of a superposed electric noise (on a sinusoidal field for ECs), we investigate the variation in the characteristics of ECs such as thresholds and pattern diagrams in high-conductivity cells (σ ∼ 10−6 Ω−1 m−1), in comparison with that in usual conventional cells (σ ∼ 10−8 − 10−7 Ω−1 m−1). Unpredictable threshold behaviors, unknown pattern formations such as isotropic liquid bubbles and EC-sustained phases, and undesirable dielectric breakdown induced by thermal focusing are found in high-conductivity cells

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

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

Electrohydrodynamic Pattern Formation in Nematic Liquid Crystals by External Pure Noise

Pure noise-induced electrohydrodynamic convections (EHCs) in nematic liquid crystals are presented in comparison with ac field-induced ones. There exists a characteristic cutoff frequency fc*of noise dividing EHC patterns qualitatively. Sufficiently colored noise with fc < fc* can induce a variety of well-ordered patterns such as Williams domains, fluctuating Williams domains, grid patterns, and dynamic scattering modes (highly developed turbulence). The amplitude of the primary pattern (Williams domains) and its wavelength are investigated with varying intensity VN and/or cutoff frequency fc of noise. The present noise-induced EHCs are discussed on the basis of the conventional (ac-based) Carr-Helfrich mechanism

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

Pattern formation of chevrons in the conduction regime in homeotropically aligned liquid crystals

We report on chevrons (herringbonelike patterns) observed in homeotropically aligned liquid crystals with high electric conductivity. We focus our attention on two types of chevrons observed in the conduction regime. The threshold voltage and the characteristic double periodicity of chevrons (i.e., the short wavelength λ1 of the striated rolls and the long wavelength λ2 of the chevron bands) have been measured as functions of the applied electric frequency f. With the aid of a crossed polarizer set, we have, in addition, determined the director field which shows a periodic in-plane rotation for our chevrons (with a wavelength λ2). We arrived at the types of chevrons after qualitatively different bifurcation sequences with increasing voltage. The frequency dependence of λ2 also shows a qualitatively different behavior with respect to the two types of chevrons. The experimental results are discussed in terms of recent theoretical investigations

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

Phase Noise Can Induce Stochastic Resonance?

We investigated stochastic resonance (SR) of ac-driven electroconvection (EC) in nematic liquid crystals by employing the phase noise of the ac field. The phase noise smoothly varies the threshold of EC, as the amplitude noise does it. Surprisingly, a kind of phase stochastic resonance (PSR) is discovered in appropriately colored phase noises; thus, it provides nonmonotonic behavior of the threshold in which a peak of the pattern amplitude of EC is found as a function of the phase noise intensity. The colored and white phase noises are discussed for understanding the PSR