The homeotropic to planar transition in cholesteric liquid crystals.

摘要

Liquid crystal devices based on Bragg-reflections from cholesteric helices have recently been the focus of a large amount of research. Such reflections occur when the cholesteric helices are roughly parallel to the incident light direction. If the helical axes are at a large angle to the incident light direction, light is not reflected back to the viewer. Operation of such displays is based on electrical switching between these states.; One transition necessary for switching these systems, the Homeotropic to Planar transition, has a transition time more than an order of magnitude longer than the other transitions involved. This transition is known to proceed through a one-dimensional relaxation to a “Transient Planar” state with less twist than the equilibrium state, followed by a previously unexplained transition to a planar state with an equilibrium pitch. For display applications, it is desired to understand this transition and reduce its switching time. In this work, we have characterized the dynamics of this transition.; We used various experimental techniques to obtain an initial understanding of helix and domain dynamics during the transition, including optical retro-reflection studies, characterization of domain dynamics, and investigation of system capacitance. When a sufficient knowledge had been gained from such studies, we investigated a simple cholesteric system with three twists in the equilibrium state (compared to 14 for the reflective display system we analyzed) through time-resolved microphotography. Observations suggested that Transient Planar transformed to Planar through a locally two-dimensional process initiated by a twist-energy driven Helfrich-like modulation.; Based on these observations, a full model for the transition was proposed. 2D computer simulation was used to model the Homeotropic to Planar transition for actual display devices. Analysis of the simulated liquid crystal director structures reproduced the initial experimental data that had been acquired.; The Homeotropic to Planar transition in cholesteric liquid crystals has now been explained for the first time, and verified through computer simulations based on the acquired data. Due to this work, cholesteric liquid crystal displays may now be optimized for various applications through computer simulation.