Abstract
ABSTRACTThe ‘flexoelastic ratio’ is commonly used to characterise the electro-optic behaviour of chiral nematic liquid crystal (LC) devices that exhibit flexoelectro-optic switching. There has been renewed interest in this electro-optic effect of late as new LC materials and mixtures have been developed that exhibit large tilt angles, Ø, of the optic axis (Ø ≥ 45°) whilst maintaining a fast response time (< 1 ms). In this study, we compare the different flexoelastic ratios that are obtained for fixed and variable-pitch chiral nematic LCs for materials with a tilt of the optic axis as large as Ø = ± 45°. We show that for large tilt angles of the optic axis the values for the two different flexoelastic ratios measurably diverge. Of the two ratios, we propose that for large tilt-angle mixtures it is more appropriate to use the flexoelastic ratio that is derived for the case when the pitch of the helix is constrained. In addition, a simple way of determining the ‘pitch-constrained’ flexoelastic ratio is presented that involves identifying the electric field amplitude at the point for which the transmission levels are the same for both positive and negative electric field polarities.
Highlights
Flexoelectricity in nematic liquid crystals (LCs) was first considered by Meyer in 1969 and is a phenomenon that involves the direct coupling between the electric polarisation and director curvature distortions [1,2]
In chiral nematic LCs, the coupling between an electric field and flexoelectricity leads to a macroscopic rotation of the optic axis, which may be as fast as 100 μs when the pitch of the helix is small (< 300 nm) [3,4,5]
The most commonly studied configuration involves a uniform lying helix (ULH) alignment of the chiral nematic LC whereby the helical axis is aligned parallel to the device substrates [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
Summary
Flexoelectricity in nematic liquid crystals (LCs) was first considered by Meyer in 1969 and is a phenomenon that involves the direct coupling between the electric polarisation and director curvature distortions [1,2]. By virtue of this linear dependence, and provided that the pitch of the helix is known, the flexoelastic ratio can be determined by measuring the electro-optic tilt angle, Ø, over a range of applied electric field amplitudes and extracting the gradient [in accordance with Equation (2)].
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