Dynamics of defects in electroconvection patterns

Abstract

PACS. 61.30.Gd -Orientational order of liquid crystals; electric and magnetic field effects on order. PACS. 61.72.Cc -Kinetics of defect formation and annealing. PACS. 61.72.Ff -Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, X-ray topography, etc.).Abstract. -In homeotropically aligned nematics with negative dielectric anisotropy the electrohydrodynamic instability occurs above a bend Fréedericksz transition. In the presence of a magnetic field H parallel to the liquid crystal slab, ordered roll patterns with a welldefined uniform wave vector k id appear above the onset of convection. By rotating the cell around an axis perpendicular to the slab by a small angle α, one can manipulate the system into a state with wave vector k = k id + ∆ k, where ∆ k is roughly perpendicular to k id . We have studied experimentally the motion of defects, which then move essentially perpendicular to the rolls. The direction as well as the magnitude of the velocity as a function of ∆ k agrees with predictions of the weakly nonlinear theory. In particular, we obtain evidence for the nonanalyticity for ∆ k → 0.

Characterization of the vortex-pair interaction law and nonlinear mobility effects

“…Precisely, the total (integrated) error power is 4.18 times greater for the constant mobility fit. Note that, while to zero order a constant mobility is a reasonable approximation, consistent with previous observations [3,19,20], the role of a logarithmic mobility is fundamental from a theoretical point of view, as it renormalizes the effects of an infinite mobility. Finally, we checked how the interaction is modified by a third vortex.…”

Convective Patterns in Liquid Crystals Driven by Electric Field

“…In the normal-roll regime, the stationary structure is characterized by the condition q H. However, when changing the field direction one can easily induce a temporary wavevector mismatch ∆q = q new − q old which relaxes via a glide (v q) motion of defects. Experiments have confirmed the validity of detailed theoretical predictions, both with respect to the direction (v ⊥ ∆q) and the magnitude (consistent with logarithmic divergence at |∆q| → 0) of the defect velocity v [42].…”

Dynamics of laser-induced electroconvection pulses

“…The EC process is in some respects the anisotropic counterpart of Rayleigh-Bénard convection (RBC), with the EC electric field replacing the vertical thermal gradient in RBC. 1 However, the large aspect ratio in EC, along with the small dissipative transport coefficients and the small sample thickness (usually tens of microns), are important experimental advantages for EC that have allowed, for example, recent precision measurements of fluctuations below the onset of a steady-state periodic pattern, 4,15,16 along with detailed characterizations of above-onset defect dynamics, [17][18][19] fluctuations, 7 brokensymmetry modes, 20,21 and spatiotemporal chaos. 22 From experimental, numerical, and theoretical perspectives, pattern-forming systems (including EC) have usually been studied in the limit in which the overall size of the complete pattern is much larger than the pattern wavelength.…”