Studies of Transitions in Cholesteric Liquid Crystal Displays

P. Watson, J. E. Anderson, V. Sergan, C. M. M Titus, J. Ruth, P. J. Bos

The primary goal of this study has been to develop a thorough understanding of the physics involved in director transitions occurring in cholesteric liquid crystal displays. Some of the these transitions (such as the understanding of a focal conic helix with an applied field) are fairly simple in nature, and have been well understood for some time. Some other transitions, particularly those involving twisting from the homeotropic state via the transient planar state, are very complex, and are just recently being understood.

The focus of our studies has been determining the dynamics of the cholesteric director on relaxing from the homeotropic state. A strong voltage (~40V) is required to keep the liquid crystal in the homeotropic state. When this voltage is removed quickly, the director relaxes to the Transient Planar state, which has significantly less twist than the equilibrium state. While the reflecting planar state of a typical 5 micron cholesteric display may have 14 360° twists, the transient planar state may have only 5 or 6 twists. The transient planar state then relaxes slowly to the equilibrium planar state.

Similarly, if the high voltage is not totally removed, but merely reduced, the liquid crystal may relax to a state that for historical reason is called "Focal Conic". This state has the helical axis in the plane of the cell, unlike the planar and transient planar states, which have their helical axes along the cell-normal direction. The Focal Conic state is approximately transparent.

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