Tumbling dynamics in a nematic surfactant solution in transient shear flows

We report rheological and structural studies of a nematic surfactant soluti on (CPCl/hex) in transient shear flows. Upon step changes in steady shear f low conditions, the shear stress exhibits damped oscillations that scale wi th shear strain, attributed to director tumbling under shear. In situ small -angle x-ray scattering spectroscopy is used to measure changes in average micellar orientation during similar transient shear protocols; the measured orientation parameter also exhibits damped oscillations reflecting the und erlying tumbling dynamics. Stress and structure data are compared with pred ictions of the Larson-Doi tumbling polydomain model. The model predictions are qualitatively similar to the experimental observations. In particular, orientation is observed to initially decrease upon shear flow reversal in C PCl/hex, in agreement with the Larson-Doi predictions but in contrast to pr evious results for model lyotropic solutions of poly(benzyl glutamate). She ar stress and average orientation are closely related during the oscillator y response to transient flows. The data suggest that higher orientation in the flow direction leads to low instantaneous stresses, and vice versa. The Larson-Doi model does not predict this relationship. (C) 1999 The Society of Rheology. [S0148-6055(99)01503-5].