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Colloidal systems comprising solid or fluid particles dispersed in nematic monodomains are known to be a convenient means to study topological defects. Recent experiments have shown that twist-bend nematic $({N}_{\mathrm{TB}})$ droplets in a nematic matrix act as colloidal particles that lead to the formation of elastic dipoles, quadrupoles, and their ordered clusters. In this study, we examine the effect of low-frequency $(f\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}\mathrm{mHz})$ electric fields on such defect configurations. We find that (i) the hyperbolic hedgehogs of elastic dipoles shift toward the negative electrode in static fields and perform oscillatory motion in AC fields, indicating the presence of nonvanishing flexoelectric polarization in the field-free state; (ii) the elastic dipoles, propelled by forces of backflow due to coupled flexoelectric and dielectric distortions, drift uniformly along their axes with the ${N}_{\mathrm{TB}}$ drops in lead; (iii) the translational velocity ${v}_{d}$ increases linearly with both $f$ and the diameter of ${N}_{\mathrm{TB}}$ drops; and (iv) with increasing applied voltage U, ${v}_{d}(U)$ exhibits a monotonic, slightly nonlinear variation at $f\ensuremath{\le}200\phantom{\rule{0.16em}{0ex}}\mathrm{mHz}$, tending toward linearity at higher frequencies.

We report, for a rodlike nematic liquid crystal with small positive dielectric and conductivity anisotropies, and in the 90°-twisted configuration, low frequency (

We report on the anisotropic electrohydrodynamic states formed over a wide temperature range (∼45 °C) in a planarly aligned bent-core nematic liquid crystal driven by fields of frequency in the range 0.1 Hz-1 MHz. Three different primary bifurcation scenarios are generated in the voltage-frequency (V-f) plane, depending on the temperature T. These, under increasing T, are characterized by the pattern sequences (i) in-plane longitudinal rolls (ILR)→in-plane normal rolls 1 (INR1), (ii) Williams rolls (WR)→ILR→INR1, and (iii) WR→INR2→INR1. Temperature-induced ILR→INR2 transition, the first example of its kind, points to elastic anisotropy as possibly the determining factor in wave vector selection. In the ILR and INR states, at threshold, the director modulations are predominantly azimuthal, and the streamlines, mainly normal to the wave vector, lie in the sample plane. Well above threshold, growing director deviations lead to narrow disclination loops that evolve in regular arrays, with their area density being exponential in voltage. The defects drift in a coordinated manner along the flow lines with a speed that scales nonlinearly with voltage; they mediate in the eventual occurrence of turbulence. The current theories of anisotropic convection based on static electrical parameters fail to account for the observed high-frequency instabilities. The study includes (i) a quantitative characterization of the critical parameter functions V(c)(f), V(c)(T), q(c)(f), and q(c)(T), with q(c) denoting the critical pattern wave number, and (ii) measurement of electrical and elastic parameters of relevance to electroconvection; the latter show anomalous features supporting the cluster hypothesis.

We report, for a nematic liquid crystal with a low conductivity anisotropy, an ac field generated transition from a uniformly planar to a periodically modulated director configuration with the wave vector parallel to the initial director. Significantly, with unblocked electrodes, this instability is not excited by dc fields. Additionally, in very low frequency square wave fields, it occurs transiently after each polarity reversal, vanishing completely during field constancy. The time of occurrence of maximum distortion after polarity reversal decreases exponentially with voltage. The time dependence of optical phase change during transient distortion is nearly Gaussian. The pattern threshold Vc is linear in sqrt[f], f denoting the frequency; the critical wave number qc of the modulation scales nearly linearly as sqrt[f] to a peak at approximately 50 Hz before falling slightly thereafter. The observed Vc(f) and qc(f) characteristics differ from the predictions of the standard model (SM). The instability may be interpreted as a special case of the Carr-Helfrich distortion suppressed in static fields due to weak charge focusing and strong charge injection. Its transient nature in the low frequency regime is suggestive of the possible role of gradient flexoelectric effect in its occurrence. The study includes measurement of certain elastic and viscosity parameters relevant to the application of the SM.

Competing modes of electric-field-driven bifurcation have been examined in a planarly aligned nematic liquid crystal with a small positive dielectric anisotropy and large electrical conductivity. The line of bifurcation into the electroconvective state is linear in frequency not only up to the codimension-2 (C2) point but well beyond. This contrasts with the recent theoretical prediction of the absence of further bifurcations within the Freedericksz state, above the C2 frequency. Further, in the frequency regime beyond C2, within the space-filling electroconvective state, a strongly localized dynamical state (SLDS) appears at a voltage threshold which again is linear in frequency. Occurrence of the SLDS through a secondary forward bifurcation is significant in view of the reported observations on the so-called worms, an instability belonging to the SLDS, as the first form of convection seen at the primary subcritical bifurcation.