Secondary hydrodynamic structure in dynamic scattering

A hydrodynamic regime in dynamic scattering has been observed in which the relaxation time depends on the voltage at turnoff. This regime manifests itself, above a threshold voltage, with microscopic regions of increased on‐axis optical density which grow until the whole cell area is converted. It has been found in all nematic materials which exhibit dynamic scattering. With originally perpendicular homeotropic orientation, the speed of relaxation increases with the square of the voltage at turnoff. With originally parallel homeotropic orientation, the speed of relaxation decreases with increasing voltage at turnoff. The time t to fill a given area depends on the current density and the voltage above a second threshold V_m, according to the empirical relation J(V ‐ V_m)t = const. For MBBA, the second threshold is about 15 V, and the constant 10⁻³ J/cm². The relation is independent of current density, temperature, and thickness (greater than 10⁻⁴ cm). This phenomenon may explain variations in response as seen by different workers. A hydrodynamic model is proposed, with contributions attributable to the dielectric anisotropy.