Converse flexoelectric effect in a bent-core nematic liquid crystal

Abstract

Flexoelectricity is a unique property of liquid crystals; it is a linear coupling between electric polarizations and bend and/or splay distortions of the direction of average molecular orientation. Recently it was shown [J. Harden, Phys. Rev. Lett. 97, 157802 (2006)] that the bend flexoelectric coefficient in bent-core nematic liquid crystals can be three orders of magnitude higher than the effect with calamitic (rod-shaped) molecular shape. Here we report the converse of the flexoelectric effect: An electric field applied across a bent-core liquid crystal sandwiched between thin flexible substrates produces a director distortion which is manifested as a polarity-dependent flexing of the substrates. The flex magnitude is shown to be consistent with predictions based upon both the measured value of the bend flexoelectric constant and the elastic properties of the substrates. Converse flexoelectricity makes possible a new class of microactuators with no internal moving parts, which offers applications as diverse as optical beam steering to artificial muscles.

Statistical mechanics of bend flexoelectricity and the twist-bend phase in bent-core liquid crystals

“…Our theoretical results can be compared with recent experiments. For the flexoelectric effect, the most relevant comparison is with the experiments of Harden et al [3,4], which found a surprisingly large bend flexoelectric coefficient in bent-core liquid crystals, about three orders of magnitude larger than the typical value in rodlike liquid crystals. This observation is at least qualitatively consistent with our concept that bent-core liquid crystals are near an incipient polar phase, and hence are very sensitive to any slight polar perturbations.…”

“…In general, the director deformation might be either splay or bend, but one would expect the bend flexoelectric effect to be dominant in bent-core liquid crystals. Indeed, recent experiments by Harden et al [3,4] have found that bent-core liquid crystals have a surprisingly large bend flexoelec-tric coefficient, up to 35 nC/m, roughly three orders of magnitude larger than the typical value of 3-20 pC/m in rod-like liquid crystals.…”

Extraordinary Magnetic Field Effect in Bent-Core Liquid Crystals

“…Thermotropic BCMs represent a new class of LCs exhibiting substantially different physical properties than traditional linear (calamitic) materials [20,21]. These include chiral mesophases composed of achiral BCMs [22], giant flexoelectricity [23,24], biaxial N order [25,26], a ferroelectric response in the N phase [27], and a large flow birefringence [28]. The consensus emerging is that the N phase of BCMs constitutes a new type of mesophase, namely, a cybotactic nematic (N cyb ) phase unrelated to pretransition cybotaxis [16,29], in agreement with experimental [16,27,30,31] and theoretical findings [26,32].…”

Determination of the flexoelectric coefficient (e1–e3) in nematic liquid crystal by using fully leaky optical-guided mode

“…5 However, Harden et al 6 proposed that the flexoelectric coefficient in bent-core nematics is 10 3 times higher than that in rod-like systems; the discrepancy in the results provides another reason for examining the flexoelectric coefficient. [7][8][9][10][11][12][13] Various experimental techniques have been developed to measure flexoelectric coefficients; [14][15][16][17][18][19][20][21][22] most of these techniques use the hybrid-aligned nematic (HAN) cell because the LC material filled in HAN cell contains splay and bend distortions in the initial state and thus present evident flexoelectric properties.…”