Field-Induced Transitions in Cholesteric Liquid Crystals: Dynamics and Application to Displays

Cholesteric liquid crystals sandwiched between two parallel substrates have three states: planar, focal conic and homeotropic. The material is transformed from one state to another by application of a suitable electric field. Dynamic equations for the transitions from the field-induced homeotropic to the planar and focal conic states are studied. In the homeotropic-planar (HP) transition, the liquid crystal transforms rapidly to a transient planar state (p = {K_{33}over K_{22}}p_{o}) through a conical relaxation process, then further relaxes to the stable planar state (p = p_{o }). The HP transition can only proceed when the bias voltage is lower than rm V_ {HP}. In the homeotropic-focal conic (HF) transition, the liquid crystal relaxes to the focal conic state at a bias voltage between rm V_ {HF} and rm V_{HP }, where rm V_{HF } is higher than rm V_ {HP}. At a critical bias voltage significantly larger than rm V_{HF}, the liquid crystal transforms from the focal conic to the homeotropic state. These transitions are experimentally studied by dynamic dielectric measurement. The results confirm the existence of the transient planar state in the HP mode and the measured transition time agrees with the theoretical prediction. The chiral concentration dependence of the rotational viscosity is determined and the large hysteresis in the homeotropic-focal conic transition is confirmed. The ratio between the measured bias voltages of the hysteresis loop agrees with the theoretical calculation. A three-stage dynamic drive scheme for a reflective cholesteric display is based on the rapid homeotropic-transient planar transition and the hysteresis in the homeotropic -focal conic transition. By utilizing the pipeline algorithm, this scheme enables commercially viable bistable cholesteric displays for a wide variety of applications. The row and column wave forms are designed to eliminate cross-talk; 320 x 320 pixel displays are successfully driven by a prototype driver with a frame time of 440 ms. The image shows good contrast ratio and reflectance. Schemes are proposed to further reduce the frame time, particularly for full-page size displays of higher pixel density.