A new x-ray shear cell capable of probing complex fluid structure in the flow-gradient (“1–2”) plane is used to study orientational dynamics of nematic surfactants in transient shear flows. Two surfactant systems are studied, one of which (SDS/decanol) apparently exhibits shear aligning dynamics and the other of which (CPyCl/hexanol) exhibits director tumbling. Data on the SDS/decanol system exhibit what appear to be common characteristics of aligning nematics: constant orientation state in steady shear; a single undershoot of long duration in average orientation upon flow reversal; and no significant orientation change upon step increase or decrease in shear rate or upon flow cessation. These data are discussed in light of similar measurements on shear aligning thermotropic liquid-crystalline polymers, and polydomain model predictions of transient orientation for aligning nematics. Shear reversal data on CPyCl/hexanol show an oscillatory response typical of tumbling materials, but in better qualitative agreement with the Larson–Doi polydomain model than similar recent measurements on lyotropic polymer solutions.
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