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Transition dynamics of gray-scale bistable cholesteric displays

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  • Liquid Crystal Displays
  • Chemistry
  • Philosophy


In this dissertation, planar cholesterics subjected to an ac electric field are studied in the vicinity of the cholesteric-nematic phase transition. Short-pitch chiral nematic mixtures are used such that d/p >>1 and the dielectric anisotropy is positive. Two anomalies have been recorded for the first time. The first, which occurs at low frequency is a constant birefringence measured above the unwinding threshold. This is contrary to the normal response at high field, since the liquid crystal molecules should align along the field direction. Below the unwinding threshold, the distorted helical structures are observed vibrating energetically and the helical axes lie effectively in-plane. When the threshold is exceeded, helix unwinding begins at the spacers and the nematic phase grows from there. The domain boundary of the nematic phase expands until all the local boundaries meet and degenerate into the nematic phase. Hence the pitch of the distorted helical structures diverges. The second is a kink which accompanies the phase transition at several hundreds of hertzs. In this case, the motion of the domain boundaries becomes non-uniform and the helix unwinding undergoes two stages. The initial stage is similar to that at low frequency but the nematic phase is not homogeneous. Distorted helical structures with resemblance to isolated fingerprint texture coexist and are unwound from finger-like tips. A higher field at final stage is required to unwind the structures completely. At high frequency, this kink disappears and the unwinding threshold saturates in this regime. According to de Gennes and many experts, the physical flow effects and orientational effects are much more complex on a helical structure: in particular the apparent bulk viscosity of a cholesteric sample may often be l05 times larger than the friction coefficients defined in Leslie equations. Therefore, a phenomenological proposition is made to describe these anomalous transition behaviours and frequency dependence. Nevertheless, the findings are crucial for bistable cholesteric displays since the unwinding field determines the maximum operation voltage and the domain size for gray-scale control. In recognition of the importance, an addressing sequence of high and low bipolar pulses is used to control the domain size and hence the reflectivity This addressing method is different from the current developments based on amplitude or pulse width modulation utilizing homeoplanar transition. It can be easily encoded in binary data format and incorporated into our driving scheme developed for low electrical requirements. A sizeable reduction in drive voltage is also possible by doping with chemicals such as Benzophenone. Therefore, we are able to demonstrate the gray-scale operation with an addressing time no longer than 5ms/line and under cost-effective measures. So far only a few groups have made this accomplishment.

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