We report on measurements of dielectric permittivity epsilon, electrical conductivity sigma, elastic moduli k(ii), and rotational viscosity gamma for a bent-core nematic liquid crystal. The static permittivity anisotropy epsilon(a) = epsilon(parallel)-epsilon(perpendicular) is negative; at a given temperature in the interval 107-123 degrees C, epsilon(parallel) shows two relaxations falling in the frequency bands 20-200 kHz and 0.9-2 MHz; epsilon(perpendicular) also shows a relaxation between 0.9 and 5 MHz. The conductivity anisotropy sigma(a) = sigma(parallel)-sigma(perpendicular) is negative at low frequencies; it changes sign twice at frequencies f(1) and f(2) that increase with temperature, in the ranges 6.5-10 and 95-600 kHz, respectively. Surprisingly, the splay modulus k(11) is considerably greater than the bend modulus k(33) in the entire nematic range. Viscous relaxation is more complex than in calamitic systems involving at least a two-step process. The gamma values are an order of magnitude greater compared to calamitics.

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 the results of investigations on the anisotropic electrohydrodynamic states arising in a highly conducting, planarly aligned, bent-core nematic liquid crystal driven by ac fields of frequency f in the range from 10 Hz to 1 MHz. Pattern morphologywise, two f regimes are distinguished. The low-f regime, wherein the primary bifurcation is to a state of periodic longitudinal stripes (LS), extends to an unprecedentedly large f, in the range 150-550 kHz, depending on the temperature T. This is followed by the high-f regime wherein periodic normal stripes (NS) constitute the primary instability. Both instabilities involve predominant director modulations and streamlines in the layer plane. The transitional frequency between the two regimes is linear in temperature. The curve V(c)(f) shows a nonlinear increase for the LS state and decrease for the NS state. V(c)(T) is an ever increasing curve close to the nematic-isotropic point for both states. The wavenumber of LS varies directly as V, and that of NS shows nearly the same behavior. The pattern period versus f is increasing for LS but decreasing for NS. Both instability states exhibit complex, light-polarization-dependent lens action. Well above the threshold, disclination loops of regular geometry appear along the stripes. They drift in a coordinated manner along the flow lines. At very high voltages, the instability turns strongly time dependent. The current models of anisotropic convection based on static electrical parameters fail to account for the observed instabilities.

Bent-core liquid crystals have set the first example of forming polar superstructures from achiral molecules. Polar switching studies in smectic phases have revealed several exciting sub-phases which have never been observed in rod-like liquid crystals. In this study, mesomorphic and polar switching properties of three bent-core compounds belonging to a homologous series have been investigated using polarizing optical microscopy (POM), differential scanning calorimetry, XRD studies, electro-optics, and dielectric spectroscopy. These achiral, unsymmetrical four-ring bent-core liquid crystals with a polar fluoro substituent at one end and n-alkoxy chain at the other terminal end possess azo, ester, and imine linkages between the four phenyl rings and different lateral substituents. The compounds 16-F and 18-F exhibit orthogonal smectic phase with antiferroelectric polar order, and additionally, the compound 16-F exhibits a short range nematic phase with a polar order. The compound 7-F exhibits broad enantiotropic nematic mesomorphism without appearance of any smectic ordering at low temperature. The smectic and nematic phases were identified by their optical textures observed by POM. Distinct polarization current peaks under triangular wave voltage are observed for all the compounds in the entire mesophase range. Relaxation phenomena corresponding to polar associations of the molecules are observed in dielectric spectroscopy. The cybotactic nature of the nematic phase is established by the XRD and electro-optic studies of 16-F. Although it is rather difficult to form mesophase when the number of aromatic rings is reduced from five, we have successfully demonstrated the bent-core compounds with four-ring which exhibit orthogonal smectic phases as well as nematic mesomorphism with unusual cybotactic signature.

We examine here the Bobylev-Pikin flexoelectric instability in an initially planar monodomain of a bent-core nematic liquid crystal, which is negative in conductivity and dielectric anisotropies. Experiments employing dc excitation reveal the domain density to be linear in field, as predicted; however, the instability threshold has a negative temperature coefficient indicating the effective flexomodulus as nonquadratic in order parameter. The dc threshold is also determined as a function of simultaneously acting ac voltage; a theoretical fit is found for the data taking into account elastic anisotropy, whereby the relevant flexoelectric and elastic parameters are estimated. Remarkable morphological changes occur under an increasing field. Half-strength disclinations of opposite topological charge evolve within the flexostructure rendering the wavevector orientation degenerate in the layer plane. Dipolar and quadrupolar topological defect patterns, akin to the singularities in cholesteric fingerprint texture, lead finally to fan like objects. The morphological equivalence between the periodic flexoelectric state and a layered lattice as realized here is attributable to a much lower energy of bend type distortion compared to splay, unlike in a calamitic.