TRANSIENT SHEAR-FLOW OF NEMATIC LIQUID-CRYSTALS - MANIFESTATIONS OF DIRECTOR TUMBLING

Calculations of a variety of transient flow phenomena based on the Leslie-Ericksen model for nematic liquid crystals are presented. Emphasis is placed on the behavior of nematics subject to director tumbling. A wide range of complicated transient phenomena such as oscillatory responses are predicted when the Ericksen number is sufficiently high to cause significant rotation of the director at steady state. Particular attention is given to relaxation processes such as constrained elastic recoil and stress relaxation that arise from the interaction of the relaxing director profile with the viscous response of the nematic. These relaxation processes are dramatically different in tumbling and flow aligning systems, due to the saturation of the director profile at high Ericksen number for the flow aligning case. A qualitative physical model of a textured polymer liquid crystal is presented in which the dynamics on a local scale governed by the distance between defects are assumed to be similar to the macroscopic dynamics presented in these calculations. By assuming (i) director tumbling and (ii) the existence of a limiting Ericksen number in polymer systems, a wide range of published observations in lyotropic liquid crystalline polymer solutions may be qualitatively reproduced. © 1990, The Society of Rheology. All rights reserved.