Patterned flexoelectric instability in a bent-core nematic liquid crystal

Abstract

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.

Suppression of spatially periodic patterns by dc voltage

“…This behaviour is in accordance with previous findings on FD in other nematics [16,17]. For low frequencies of the ac voltage component (f = 2 Hz, Fig.…”

“…Experimental studies on electro-optics in nematics at combined ac+dc driving are so far very scarce. E.g., the dc threshold of FD was found to increase upon superposing ac voltage in a calamitic [16] as well as in a bent-core nematic [17]; however, the instabilities at pure ac driving or at a small dc bias voltage have not been studied. Combined driving was also applied for the nematic 5CB (4-pentyl-4'-cyanobiphenyl) having ε a > 0 and σ a > 0 and thus exhibiting nonstandard EC [18].…”

Electric-field-induced patterns and their temperature dependence in a bent-core liquid crystal

“…[44] Bent-core nematics, due to the combination of their molecular shape with a transverse component of their dipole moment, are ideal candidates for compounds with a large flexoelectric response. Indeed, determination of the flexoelectric coefficient of some BCN materials from the observed flexoelectric domains [45] and flexoelectro-optic effect [46] confirmed this theoretical anticipation. Moreover, it was reported that under certain conditions (e.g.…”

“…[33] Bent-core nematic (BCN) systems (including mixtures of bent-core and rod-like molecules) may also exhibit EC patterns. [34][35][36][37][38][39][40][41][42][43][44][45] The patterns reported so far can mostly be classified as non-standard EC, however, with some characteristics substantially different from those in calamitic LCs.…”

Unusual polarity-dependent patterns in a bent-core nematic liquid crystal under low-frequency ac field

“…The bentcore nematics (BCNs) studied so far mostly exhibit various nonstandard EC patterns [7][8][9][10][11][12][13][14][15][16][17][18][19][20], whose characteristics may substantially differ from those in calamitic LCs. Moreover, there are some unprecedented scenarios, like prewavy patterns at ultrahigh ( / > 50 kHz) frequencies with negative slope of Uc(f), which have been found so far only in BCN compounds [7,[11][12][13], These banana-specific features still wait for explanation.…”