On the Kinetics of Texture Changes in Electric Field Induced Cholesteric‐to‐Nematic Transitions

Abstract

The kinetics of dc electric field induced texture changes in cholesteric phases were studied. The rapid changes in the transmission of polarized light, in the optical rotation and in the fluorescence intensity of a (dimethylaminonitro‐stilbene) fluorescence probe were measured. In a parallel static experiment the field induced texture changes as observed with a polarizing microscope were related to the above physical parameters. In addition the pitch was measured as a function of the field strength. The structural changes may be divided into three independent processes: grid deformation (at threshold field Fc1), rotation of the helix axis without change in the pitch (between F < F < 0.75 FCN) and unwinding of helical structure (at the critical field FCN). Hysteresis effects were observed for layer thicknesses comparable to the pitch. The relaxation from the field induced nematic to the planar state occurs in two separated steps characterized by different relaxation times: the retwisting of the helical structure and the rotation of the helix axis.The rapid transition to the nematic state occurs also in separated steps characterized by different rise times. Judged from the static experiment the first step involves the grid deformation and the second the rotation of the helix axis by 90°. The unwinding of the helix could not be observed by our means.For optical rotations larger than 90° the transmission transient exhibits oscillatory character. Very rapid rise times of the order od 70 μsec have been found in nematic systems doped with cholesteryl chloride.