E. P. Raynes

Method for the measurement of the K22 nematic elastic constant

A technique has been developed for the measurement of the K22 twist elastic constant in nematic liquid crystal materials. This involves the measurement of the Freedericksz transition voltages in untwisted linear and π-twist regions in a wedge cell geometry. The method avoids the need for the accurate determination of the cell thickness and cholesteric pitch and is far more straightforward to implement than other methods in the literature. The validity of this method is demonstrated for the well-characterized material E7.

Study of elastic constant ratios in nematic liquid crystals

A technique has recently been proposed for determining the elastic constant ratio K22∕K11 in nematic liquid crystals [E. P. Raynes, C. V. Brown, and J. F. Stromer, Appl. Phys. Lett. 82, 13 (2003)]. This technique has been applied to five nematic materials that cover a range of K33∕K11 elastic constant ratio values. An unusually high value of K22∕K11 is found in one material. High values of the ratio K22∕K11 tended to occur in materials that also have high values of the ratio K33∕K11. The results are validated with independent measurements of K22 from cholesteric helix unwinding.

Investigation into chiral active waveplates

An examination into a chiral liquid crystal active waveplate device using a one-dimensional model, giving numerical results, is presented. The model calculates the director and flow configuration by minimizing the free energy of the chiral nematic modeled with fixed boundary conditions. The static case of varying the flexoclectric coefficients, electric field magnitude, and dielectric anisotropy is examined and it is shown that both flexoelectricity and dielectric effects alter the birefringence of the device in the presence of an electric field. The dynamic reaction to a rotating electric field is then examined.

Measurement of the twist elastic constant of nematic liquid crystals using pi-cell devices

We present a fast and accurate method for measurement of the twist elastic constant of nematic liquid crystals using pi-cell devices. We observe that there are two bifurcations during the transition from the bend state to the planar 180° twisted state, each of which leads to a voltage threshold. The Freedericksz threshold voltage due to a tilt bifurcation occurs at a lower voltage and is “softened” by the required surface pretilt in the device. The higher of the two thresholds is due to a bifurcation between left- and right-handed 180° twisted states and we term this the T to V state threshold. This voltage threshold is independent of device thickness, and is not softened by the pretilt. We show that the T to V state threshold is highly sensitive to changes in the twist elastic constant, K22, and use this threshold to evaluate the magnitude of K22 for a variety of materials. These results show very good agreement with those obtained using the standard magnetic Freedericksz twist threshold experiment. This...

Modeling of dual frequency liquid crystal materials and devices

We present a director-based model of the dual frequency nature of liquid crystals based on a Debye-type relaxation of the permittivity in the direction parallel to the director. This relaxation is governed by a first order differential equation in terms of the polarization and electric field along the long axis. We demonstrate that this equation can be used as an extension to the well-known Eriksen-Leslie-Parodi theory. Since solution is in the time domain, the frequency response of the applied waveform need not be calculated. Consequently, the device response to arbitrary applied waveforms can be modeled. As an example, we present the switching response of a dual frequency addressed hybrid-aligned nematic cell, and suggest some optimization of the addressing scheme.

Measurement of the Sum (e 1 + e 3) of the Flexoelectric Coefficients e 1 and e 3 of Nematic Liquid Crystals using a Hybrid Aligned Nematic (HAN) cell

A hybrid aligned nematic cell (HAN) is often used to measure the sum (e 1 + e 3) of the flexoelectric coefficients e 1 and e 3. However, a HAN cell has the difficulty of an internal offset voltage (internal bias) caused by the homeotropic alignment. We present a method to determine the internal bias and then take it into account in the flexoelectric measurement.

The influence of chirality on the difference in flexoelectric coefficients investigated in uniform lying helix, Grandjean and twisted nematic structures

Measurements of the difference in flexoelectric coefficients (e 1 – e 3), using the sign convention as originally defined by Meyer, are reported from three experiments employing the flexoelectro-optic effect in different geometries. The uniform lying helix (ULH) structure is used to measure the tilt angle of the liquid crystal director with respect to the helix axis for an applied electric field, in order to infer a value for (e 1 – e 3). Alternatively, measurements of the flexoelectric difference can be made by considering the transmission through a device with an in-plane electric field aligned in either the Grandjean structure for highly chiral materials, or a twisted nematic (TN) structure for largely achiral materials. The results from the Grandjean and ULH structures show the equivalence of the measurement techniques with helix axis either perpendicular or parallel to the substrates. Further comparison of these results with the measurement from the achiral TN device shows that the difference in flexoelectric coefficients displays no dependence on chirality, demonstrating that flexoelectricity is purely associated with splay and bend director deformations, as expected from symmetry considerations.

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.

Electric-field-induced disclination migration in a Grandjean-Cano wedge

The behavior of a disclination line in a Grandjean-Cano wedge is investigated under an electric field applied normal to the cell substrates. The focus of attention is the first disclination, separating the untwisted and the π-twisted area. This disclination line shows a field dependent migration process that was observed in the cholesteric mixture ZLI-1132/S811. The dynamics of this migration process is studied experimentally and is compared with two theoretical models. One model utilizes a two-dimensional lattice Boltzmann approach to predict the initial migration movement, whereas a one-dimensional continuum model allows a full analysis of the experimental results.

Symmetric H state lifetime in splayed nematic liquid crystal devices

The dynamic behavior of splayed nematic devices during sudden voltage application is investigated in detail, both experimentally and through simulation. The symmetric H state forms transiently under such drive conditions, and we show that the lifetime of this state is strongly dependent on the liquid crystal material parameters and the applied voltage. The decay of this state to the asymmetric H states is shown to occur via domain growth: isolated regions of the device form the asymmetric H states, and these domains then grow, taking over the areas containing the symmetric H state.