Jong-Hoon Huh

Prewavy Pattern: A Director-Modulation Structure in Nematic Liquid Crystals

Abstract A periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported. It is characterized by a periodic modulation of the director in the xy-plane, which can be distinguished from electroconvection patterns with modulation in the xz-plane. The prewavy instability is investigated in detail; a phase diagram in the frequency-voltage plane, the voltage dependence of the azimuthal rotation angle, the voltage and frequency dependence of the wavelength of the prewavy pattern, and the director field in the prewavy pattern are provided. This prewavy pattern always evolves into the so-called chevrons above onset of convection. The wavelength of the chevrons and the orientation of their alternating zig and zag rolls depend on the director structure of the prewavy pattern.

Formation Scenarios for Nonlinear Patterns in Electroconvection under Controlling Goldstone Modes in Magnetic Field

We report novel scenarios for the transitions from a stationary periodic state to spatio-temporal chaos (STC) for electroconvection in homeotropically aligned nematics, in which a constant magnetic...

Transition Properties of the Soft-Mode Turbulence in the Homeotropic Electroconvection Superimposing Magnetic Fields

The experimental study on transition processes of the spatio-temporal chaos (STC) as well as periodic patterns in homeotropically aligned nematic liquid crystals is reported under superimposing magnetic fields with electric fields. The strong dependence of the threshold for STC on the strength of magnetic fields is observed above certain normalized voltage e th , where the two types of STC are found out below and beyond the characteristic value of the magnetic field H * = 620 G. The periodic patterns were observed for the normalized voltage e below e th . Below H * the soft-mode turbulence appears due to the Goldstone mode related to a continuous rotational-symmetry which can be controlled by the application of H . Beyond H * , the conventional STC called defect turbulence, which is similar to STC in the planar system, is observed.

Dynamical aspects of spatiotemporal chaos at the onset of electroconvection in homeotropic nematics

The systematic experimental study of direct transition “nonconvective state – spatiotemporal chaos” in electrohydrodynamic convection of a homeotropically aligned nematic is carried out. The transient patterns, both below and beyond the Lifshitz point, are studied and classified. The patterns initially correspond to oblique and normal rolls, respectively. In all the cases the transition to chaos undergoes slow spatiotemporal modulations of the local roll orientation. This results in the final dynamical patterns isotropic on large spatial scales. The dynamics of the final patterns are also studied with a quantitative analysis of the power spectra of turbulent modes and their autocorrelation functions. The exponential decay of the correlations in time is proved.

Noise-induced electrohydrodynamic patterns in nematic liquid crystals

We report on noise-induced patterns and threshold characteristics for the ac-driven electrohydrodynamic system in nematic liquid crystals. The wave number-dependence of the threshold for the stochastic system was experimentally investigated with controlling intensity rate of noise. The results are discussed in comparison with the previous experimental and theoretical ones. Moreover, unexpected patterns as well as pre-transitional ones are presented.

Pure Noise-Induced Pattern Formations in a Nematic Liquid Crystal

We report pure noise-induced pattern formations in a homeotropically aligned nematic liquid crystal. Williams domains and stripe patterns are induced by colored and white noises, respectively, and the Fredericks transition is also found, which has no dependence on correlation time of the external noise. Furthermore, threshold behavior in the presence of a regular (sinusoidal) field superposed with the noise was investigated to understand the intrinsic role of the noise on the mechanisms driving noise-induced patterns and transitions.

Noise Effects on Threshold of Electrohydrodynamic Convection in Nematic Liquid Crystals

New experimental aspects of multiplicative noise effects on the onset of ac-driven electroconvection in nematic liquid crystals are presented. In the superposition of a stochastic field with a deterministic one, the variation in the threshold voltage V c (of the deterministic one) was investigated as a function of the intensity ( V N ), cutoff frequency ( f c ), and correlation time (τ N ) of the stochastic noise. In particular, it is observed that the noise effects vanish for certain stochastic and deterministic conditions.

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 τσ) of the EC system and the correlation time [τN = 1/(2πfc)] 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.

Difference in Noise-Induced Threshold Shift between Planar and Homeotropic Electroconvections in Nematic Liquid Crystals

We report the threshold shift induced by externally applied noises for electrohydrodynamic convections (EHCs) in both planarly and homeotropically aligned nematic liquid crystals as well as in both the conduction and dielectric regimes. Owing to the difference in timescales among the intrinsic properties of EHCs, externally applied noises, and deterministic fields, the stabilization or destabilization effects induced by noises are observed. In particular, the difference in the threshold shift between both alignments is found, and discussed in terms of the EHC mechanisms for both alignments. Moreover, a noticeable noise-induced threshold shift is observed in the dielectric regime, which is markedly different from that in the conventional conduction regime.

Temperature Dependence of Electromechanical Effects in a Swollen Polydomain Liquid Crystalline Elastomer

We report electromechanical effects in a swollen polydomain liquid crystalline elastomer (POLY-LCE). Owing to the director reorientation, we could directly observe its deformation in the unconstrained elastomer gel state. Using optical image analysis, the deformation amplitude, threshold voltage, and temporal characteristics of the POLY-LCE were quantitatively measured with varying voltage and temperature. From the experimental results we discuss the deformation mechanism and property of the POLY-LCE.