Enhancing lifetime in a bistable smectic A liquid crystal device

In this paper, we consider the electro-optic properties of a bistable display device based upon the smectic A phase which can function without polarizers. This scheme is attractive for low-power, high-efficiency devices with indefinite storage. The bistable modes are opaque (write), due to electrohydrodynamic instabilities from motion of ionic dopant at dc/low ac frequencies, and clear (erase), due to dielectric reorientation. The two regimes are delimited by a critical frequency at which ionic motion is damped out. From consideration of current flow through the cell and contrast ratio, it is shown that the display lifetime is significantly dependent on the write mode drive frequency e.g. operation at 100 Hz extends lifetime by a factor greater than 10 2 compared with 1 Hz. The origin of the limited lifetime at dc/low ac applied fields is explained via an electrolytic purification mechanism; operation at frequencies above a threshold (e.g. 50 Hz) significantly reduces this process. It is also proposed that, although this phenomenon must be minimized for dynamic scattering, the technique should be of benefit generally for in situ purification of liquid crystalline samples in other electro-optic effects where ionic conductivity is an undesirable property.