The desired wall orientation was ob

The desired wall orientation was obtained by the wellknown method of rubbing the electrodes with a cotton swab. (The molecules of the liquid crystal align parallel to the direction of rubbing.) A twist of 90° was achieved by melting the material between "crossed" electrodes. The cell was placed between two polarizers which were parallel or perpendicular to the direction of rubbing and illuminated at normal incidence. When no voltage was applied to the electrodes the liquid crystal rotated the light by 90°. Applying dc or ac voltages we found that the rotatory power of the cell can practically be reduced to zero. The effect showed no hysteresis and the cell returned to its initial state when the voltage was turned off.The angle of rotation as a function of voltage for a mixed liquid crystal is shown in Fig. 2(a) for a 10-J..!-thick sample. Also plotted for the same driving frequency of 1 kHz is the transmission with parallel polarizers [Fig. 2(b)]. The brightening effect has a threshold of only 3 V (rms) and is roughly 90% complete at 6 V. The behavior in PEBAB is frequency independent from about 0.1 Hz to 80 kHz. At higher frequencies turbulences begin to show, destroying the optical homogeneity of the cell. The threshold for PEBAB is even lower (1 V at 1 kHz) and the brightening effect is almost complete at 3 V. Under dc operation, higher voltages were required for the brightening effect to occur, the threshold being at 2. 5 V for PEBAB and 6 V for the mixed liquid crystal. Also, the appearance of the cell was not as homogeneous as with ac. This may have been due to electro-hydrodynamic effects. The voltage needed for a given brightness fell slightly with increasing temperature, but the change was less than 10% over the entire nematic range.