Structural response of nematic liquid crystals to weak transient shear flows

Using conoscopy, we have investigated the dynamic director response of nematics to oscillatory shear and step shear strain. In oscillation, the ratio of the amplitude of the director rotation to the strain amplitude and the phase difference between the director rotation and the applied strain are measured as functions of frequency for both 5CB and 8CB. Comparison with the Leslie—Eric ksen theory allows extraction of several important material parameter ratios. In particular, data in the high frequency limit yield X, the material parameter which indicates flow-aligning (X > l) or tumbling (X < 1) behavior. For 5CB at 32.5 °C, X-1.03, while 8CB at 34 °C gives X 0.42. An analysis is presented for director field relaxation following distortion induced by a step strain. Step strain experiments on 5CB and 8CB provide an independent measurement of viscoelastic properties. Aside from confirming the flow classification of these model nematics, these new experimental methods establish a framework for the study of director dynamics in polymeric nematic liquid crystals.