Lesley A. Parry-Jones

Alignment of the Uniform Lying Helix Structure in Cholesteric Liquid Crystals

The uniform lying helix (ULH) configuration in cholesteric liquid crystals, where the helical axis of a chiral nematic is aligned uniformly in the plane of two confining substrates is of interest for both electro-optic and photonic applications. However, the formation of the well-aligned ULH structure required is non-trivial. Since the helical axis tends to align at an angle with respect to the surface alignment direction, cells where both substrates have planar alignment layers can cause the formation of a two-domain ULH structure. Here we investigate the orientation and nature of the domains as a function of field application. Further we show that it is possible to obtain mono-domain structures when the alignment directions on each substrate are skewed so as to match the angle that the helix forms with respect to the alignment at the electric field applied. This improves the electro-optic contrast of devices.

Zenithal bistability in a nematic liquid-crystal device with a monostable surface condition

The ground-state director configurations in a grating-aligned, zenithally bistable nematic device are calculated in two dimensions using a Q tensor approach. The director profiles generated are well described by a one-dimensional variation of the director across the width of the device, with the distorted region near the grating replaced by an effective surface anchoring energy. This work shows that device bistability can in fact be achieved by using a monostable surface term in the one-dimensional model. This implies that is should be possible to construct a device showing zenithal bistability without the need for a micropatterned surface.

Measurement of the Flexoelectric Coefficients e 1 and e 3 in Nematic Liquid Crystals

The flexoelectric coefficients e 1 and e 3 for splay and bend are measured using two experiments. The first experiment measures the sum (e 1 + e 3) using a π-cell and applying an ac voltage across the device. The second experiment measures the difference (e 1 − e 3) using a TN-cell and applying an in-plane quasi-dc voltage.

Comparison of Theoretical and Experimental switching curves for a zenithally bistable nematic liquid crystal device

Zenithally bistable nematic liquid crystal devices have been fabricated from a interferometrically defined microgrooved surface and a flat substrate treated with homeotropic surfactant. Two stable states are observed in the devices, one where the liquid crystal is aligned homeotropically (VAN) and the other where the alignment is planar at the grooved surface (HAN). The time-voltage switching curves have been measured using an automated addressing and detection system for switching from HAN to VAN and viceversa. The values of the surface energy and the flexoelectric e11+e33 coefficient have been estimated by comparing these with theory.

Conoscopic Observations of a Homeotropically Aligned Antiferroelectric Liquid Crystal Device: A Comparison of Theory and Experiment

A homeotropically aligned sample of CS4001 is examined conoscopically both in the ground state and under the application of an electric field parallel to the smectic layers. The results compare well with those predicted by a combination of an extended Jones optical method and a theoretical model of the helix distortions. The comparison not only gives support to the underlying theory, but allows the determination of values for the elastic constant associated with distortions to the helical structure, and also a combination of the interlayer coupling coeffients.

Measurement of Twist Elastic Constant in Nematic Liquid Crystals using Conoscopic Illumination

A method of imaging the birefringence and optic axis orientation of a sample as a function of angle of incidence using conoscopic illumination and a rotating input polariser has previously been demonstrated on polymerized liquid crystals. In this work, we apply this technique to a planar nematic device with in-plane electrodes, which cause a twist in the director profile. The conoscopic images are compared with theoretical predictions based on a combination of a 1D nematic and an extended Jones optical method. The comparison allows values for the twist elastic constant K22 and the azimuthal surface anchoring energy to be determined.

Optical Determination of Twist Elastic Constant of the Chiral Smectic Liquid Crystal SCE8

When a “thick” device is filled with a chiral smectic liquid crystal material, a helix is allowed to evolve within the device. In this paper, such a device was fabricated and filled with the ferroelectric liquid crystal material, SCE8. Application of voltage to the device will cause a deformation of the helix, and hence a deviation of the optic axis. The optical response of the device due to the applied voltage, was obtained experimentally. The helical deformation was also theoretically modeled using the continuum approach. In this paper, two different theoretical models will be presented. The first model is a one dimensional model which calculates the director variation along the helical axis. The second model is a two dimensional model which calculates the director variation through the thickness of the device, and along the helical axis. Comparison of the theoretical and experimental results allows us to calculate the twist elastic constant, B 3 . The values of B 3 for SCE8 obtained using the two different models will be discussed.

A Unified Description of Switching in Antiferroelectric Liquid Crystals

We present a unified descrption of switching in antiferroelectric liquid crystals which provides an explanation for a number of observed phenomena. In particular, the model succeeds in explaining the long debated pretransitional effect in AFLCs. In addition, the thresholded transitions between the antiferroelectric and ferroelectric states are explained in terms of a quadrupolar ordering term between adjacent layers. Experimentally, the quadrupolar term is found to be so large in commercial material CS4001 that the ferroelectric state is always an energy minimum. The frequency dependence of the hysteresis loop is predicted by including the influence of domain switching into the theoretical model.

Switching Behaviour of Antiferroelectric Liquid Crystals

We present a model of antiferroelectric liquid crystals that explains a number of phenomena observed in their switching properties. These include the pretransitional effect, the thresholded transitions between the antiferroelectric and ferroelectric states, the frequency dependence of the hysteresis loop and the origin of the v-shaped switching observed in some antiferroelectric materials.

Smectic Layer Structures in Complex Geometries—Modelling Complex Layer Structures in Smectic Liquid Crystals

In order to understand the operation of liquid crystal devices based on tilted smectic materials it is highly important to also understand the formation of the smectic liquid crystal layer structures. However, in many situations this is not easy to do. It is well established that in devices filled with materials that have an upper lying SmA phase a “chevron” structure forms when cooled from the SmA to the tilted smectic phase due to the layer thinning combined with layer packing constraints. But it is less well understood how the structure is influenced by the application of an electric field. Experimental work has been done in both ferroelectric and anti-ferroelectric phases which shows a transition from chevron towards bookshelf structure. In this paper we present some of our recent theoretical work in the area of smectic layer structures where “melting” of the smectic layer ordering is allowed.