Strong flexoelectric behavior in bimesogenic liquid crystals

Abstract

In this paper, we demonstrate strong flexoelectric coupling in bimesogenic liquid crystals. This strong coupling is determined via the flexoelectro-optic effect in chiral nematic liquid crystals based on bimesogenic mixtures that are doped with low concentrations of high twisting power chiral additive. Two mixtures were examined: one had a pitch length of p∼300nm, the other had a pitch length of p∼600nm. These mixtures exhibit enantiotropic chiral nematic phases close to room temperature. We found that full-intensity modulation, that is, a rotation of the optic axis of 45° between crossed polarizers, could be achieved at significantly lower applied electric fields (E<5Vμm−1) than previously reported. In fact, for the condition of full-intensity modulation, the lowest electric-field strength recorded was E=2Vμm−1. As a result of a combination of the strong flexoelectric coupling and a divergence in the pitch, tilt angles of the optic axis up to 87°, i.e., a rotation of the optic axis through 174°, were observed. Furthermore, the flexoelastic ratios, which may be considered as a figure-of-merit parameter, were calculated from the results and found to be large, ranging from 1.3to2C∕Nm for a temperature range of up to 40°C.

The limits of flexoelectricity in liquid crystals

“…The implications of approaching this limit for distorted equilibrium structures 15,16 suggest an interesting avenue of research. Further, in some device configurations-such as the flexoelectric effect in chiral nematics 3,[5][6][7][8][9][10][11][12][13] -the magnitude of the observed flexoelectric response is governed by a ratio of the form |e/K |. Generally, the higher the ratio |e/K |, the lower the electric field needed to drive the device.…”

“…7,17 These reduce the driving voltage for certain electro-optic devices, 7 and produce unusually stable blue phases. 15,18 It is important to ask how far the development can continue; what are the maximum flexoelectric coefficients practically achievable?…”

“…1 In the ground state, n is uniform, and an elastic energy cost is associated with distortions of n. Flexoelectricity is a coupling between certain such distortions and electric polarization; an applied electric field may induce distortion, and distortion may induce polarization. 2 The effect is of interest in devices such as display panels, [3][4][5][6][7][8][9][10][11][12][13] and electro-mechanical transducers for sensing or energy-harvesting applications. 14 More fundamentally, flexoelectricity has recently been studied as a factor influencing the energetics of distorted equilibrium structures.…”

Uniform Lying Helix Alignment on Periodic Surface Relief Structure Generated via Laser Scanning Lithography

“…molecules [6][7][8]. These mixtures exhibit a large flexoelectric ratio e=k, and a relatively small dielectric constant De extending the linear range of the flexoelectric effect to angles as large as 30 [9].…”

Flexoelectric coefficient measurements in the nematic liquid crystal phase of 5CB

“…3,4 The converse effect-polarization-induced distortion-may be exploited in electro-optic devices such as display panels, providing a mechanism whereby the optical properties can be manipulated by an applied voltage. [5][6][7][8][9][10][11][12][13][14][15] Values of the flexoelectric coefficients are required for inclusion in theoretical models, to test simulations, [16][17][18][19] and to inform the development of devices. Thus, for fundamental and applied reasons, it is important to be able to measure the flexoelectric coefficients of a given material.…”