Patterned Electroconvective States in a Bent-Core Nematic Liquid Crystal

Abstract

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.

Guidelines for the analysis of free energy calculations

“…These discrepancies originate from undersampling in the tail regions of the ΔU ij distributions [32], which yields biased free energy estimates. BAR [1] eliminates the bias in ΔG estimation by including both forward, ΔU ij , and reverse, ΔU ji, potential energy differences in the analysis.…”

“…In addition to looking for consistency between free energy estimates from several methods discussed in Section 2.3, the Overlapping Distribution Method (ODM) [11,32]—introduced by Bennett [1] under the name of the Curve-Fitting Method— provides another useful technique for spotting trouble. It is a helpful tool for assessing consistency, when combined with another free energy estimator.…”

Physics of liquid crystals of bent-shaped molecules

“…Subsequent EHC studies on other BCNs were carried out by a number of research groups(Y. Xiang et al 2009;Stannarius et al 2007;Tadapatri et al 2010a;Tadapatri, Krishnamurthy, and Weissflog 2010;Tanaka et al 2009). Studies on a (+ −) material (Heuer et al 2008) revealed transition between longitudinal and normal rolls.…”

“…Srigengan et al 2018)(P. Sathyanarayana et al 2010;Majumdar et al 2011;Tadapatri et al 2010b;Salter et al 2011;Görtz et al 2009)(Kaur et al 2012) or in mixtures of bent-core and rod-shaped molecules (Brindaban Kundu, Pratibha, and Madhusudana 2007) (P Sathyanarayana and Dhara 2013), often by a factor of 3-5. Such a dramatic reduction of K 33 as compared to rod-like mesogens cannot be easily reconciled with Eq.…”

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.…”