Drifting Periodic Structures in a Degenerate-Planar Bent-Rod Nematic Liquid Crystal Beyond the Dielectric Inversion Frequency

Dislocations and metastable chevrons in the electroconvective inplane normal roll state of a bent core nematic liquid crystal

We report experimental results on the formation, dynamics, and annihilation of edge dislocations of opposite topological charge in the electroconvective inplane vortex state of a bent core nematic liquid crystal. The approach of paired, oppositely charged defects toward each other is a two-step process. Near constant velocity at large separation and accelerated motion close to annihilation are found, as in the case of nematic rolls belonging to standard electroconvection. Periodic arrays of dislocations of alternating polarity form upon a sudden, strong elevation of the control parameter. Chevron structures that appear between undulatory defect chains are metastable, and their decay with time is accompanied by an exponential reduction in the dislocation density. The initial periodicity of defect chains also drops exponentially with increasing field.

Nematic biaxiality in a bent-core material

The results of a recent investigation of the nematic biaxiality in a bent-core mesogen (A131) are in apparent disagreement with earlier claims. Samples of mesogen A131 used in the two studies were investigated with polarized optical microscopy, conoscopy, carbon-13 NMR, and crossover frequency measurements. The results demonstrate that textural changes associated with the growth of biaxial nematic order appear at ∼149  °C. The Maltese cross observed in the conoscopic figure gradually splits into two isogyres at lower temperatures indicating phase biaxiality. Presence of the uniaxial to biaxial nematic phase transition is further confirmed by temperature trends of local order parameters based on 13C chemical shifts in NMR experiments. Frequency switching measurements also clearly reveal a transition at 149  °C. Differences between the two reports appear to be related to the presence of solvent, impurities, and/or adsorbed gases in samples of A131 used in the study of Van Le [Phys. Rev. E 79, 030701 (2009)].

Electroconvection in nematic mixtures of bent-core and calamitic molecules

The onset of electroconvection in binary mixtures of a bent-core and a rodlike nematic has been characterized by measuring the threshold voltage U(c) and the critical wave number of the pattern in a wide range of frequencies f. In the mixtures rich in bent-core molecules, a "conductive-prewavy2-patternless-prewavy1" morphological sequence has been detected with an unusual negative slope of U(c)(f) at high frequencies. This latter scenario seems to be related to the bent-core component, as it disappears with increasing the concentration of rodlike molecules. In addition, one of the parameters most relevant for electroconvection, the electrical conductivity, has also been varied by ionic salt doping. It has been found that the above effect of the banana-shaped molecules on the electroconvection scenarios can be suppressed by the conductivity.