Supramolecular and macromolecular functional helical superstructures are ubiquitous in nature and display an impressive catalog of intriguing and elegant properties and performances. In materials science, self-organized soft helical superstructures, i.e., cholesteric liquid crystals (CLCs), serve as model systems toward the understanding of morphology- and orientation-dependent properties of supramolecular dynamic helical architectures and their potential for technological applications. Moreover, most of the fascinating device applications of CLCs are primarily determined by different orientations of the helical axis. Here, the control of the helical axis orientation of CLCs and its dynamic switching in two and three dimensions using different external stimuli are summarized. Electric-field-, magnetic-field-, and light-irradiation-driven orientation control and reorientation of the helical axis of CLCs are described and highlighted. Different techniques and strategies developed to achieve a uniform lying helix structure are explored. Helical axis control in recently developed heliconical cholesteric systems is examined. The control of the helical axis orientation in spherical geometries such as microdroplets and microshells fabricated from these enticing photonic fluids is also explored. Future challenges and opportunities in this exciting area involving anisotropic chiral liquids are then discussed.

Stimuli-Driven Control of the Helical Axis of Self-Organized Soft Helical Superstructures.

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 obs...

Strong flexoelectric behavior in bimesogenic liquid crystals

In this paper, we report on the flexoelastic and viscoelastic ratios for a number of bimesogens compounds with the same generic structure. Values are obtained indirectly by measuring the flexoelectro-optic response in the chiral nematic phase. By varying the molecular structure we alter the bend angle, transverse dipole moment, and length of the molecule. First, to examine the influence of the bend angle we use a homologous series whereby the only alteration in the molecular structure is the number of methylene units in the aliphatic spacer, n . Results show that the flexoelastic ratio, e/K , and the effective flexoelectric coefficient, e , both exhibit an odd-even effect with values for n=odd being greater than that for n=even . This is understood in terms of an increase in the bend angle of the molecule and an increase in the transverse dipole moment. Second, in order to investigate the impact of the dipole moment, we have altered the mesogenic units so as to vary the longitudinal dipole moment and used different linkages in the aliphatic spacer in an attempt to alter the transverse dipole moment. Qualitatively, the results demonstrate that the odd-spaced bimesogen with larger transverse dipole moments exhibit larger flexoelastic ratios.

Structure-flexoelastic properties of bimesogenic liquid crystals

Previous work has shown that when short-pitch cholesteric liquid crystals are aligned in cells with the helical axis in the plane of the substrate in a well-defined direction, then application of an electric field across the cells may result in microsecond in-plane switching due to the flexoelectric effect. Moreover, the response to the electric field is linear and temperature independent. Hence this device geometry has great potential since fast, temperature-independent switching with analogue gray scale may be realized. Up to now, however, the alignment of the helix in the plane of the substrate achieved, for example, by rubbed polyimide layers has been rather unstable, since it relaxes to a Grandjean texture in a time scale determined by the cell thickness. This time is generally of the order of minutes. Here we report a method for the attainment of stable anchoring of the unidirectional lying helix texture using periodic anchoring conditions.

Alignment of cholesteric liquid crystals using periodic anchoring

Flexoelectric coupling gives rise to a linear electro-optic response in cholesterics (flexoelectrooptic effect) with a uniformly lying helix (ULH) structure and this electro-optic effect is strongly reliant on the homogeneity and quality of the texture. The ULH structure, unfortunately, is complicated in itself and may be perturbed by factors such as dielectric coupling, surface/liquid crystal interactions and phase transitions, and often there is a tendency for relaxation into the Grandjean texture (standing helix structure) with time. Hence, in order to exploit the flexoelectro-optic effect in cholesterics any instability of the ULH structure must be ruled out. We have overcome these problems by incorporating a polymer network by means of photopolymerization of a reactive monomer added to the cholesteric. The volume stabilized ULH structure still exhibits the flexoelectro-optic effect, it is stable and it is also retained after heating to the isotropic phase and going back to the cholesteric phase. In a...

Volume-stabilized ULH structure for the flexoelectro-optic effect and the phase-shift effect in cholesterics

The linear electro‐optic effect in short‐pitch cholesterics is based on the linear coupling of the medium with an applied electric field. It has a number of remarkable properties. The electric field causes the optic axis to tilt in a plane parallel to the surfaces of the cell glass plates, giving the same symmetry as the electro‐optic effects in the smectic C* phase (surface stabilized ferroelectric liquid crystals and deformed helix mode) and the smectic A* phase (soft mode/electroclinic effect). For shutters and displays this guarantees a very wide viewing angle. The induced tilt is a linear function of the applied field, at least for small fields, which gives a well‐controlled continuous grey scale. Furthermore, it is practically independent of temperature. Response times of the order of 100 μs are easily achievable. The most interesting development in this effect would be to extend the linear regime to much larger tilt angles, in particular to 22.5°, where light could be modulated from 100% to zero transmission. In order to do this the perturbation from the quadratic dielectric coupling has to be ruled out or minimized, which requires materials with essentially zero dielectric anisotropy. This has been done, and it has been found that the bare flexoelectric‐induced tilt has a surprising range of linearity: The linear response in tilt could be followed up to about 30° after which the high electric field caused breakdown. The response time is typically about 100 μs and below.

M. Buivydas

Papers

Linear electro‐optic effect based on flexoelectricity in a cholesteric with sign change of dielectric anisotropy