Effects of Shear-Induced Density Flucuations on the Microstructure and Rheology of Complex Fluids

It is well established that shear flow can enhance naturally occuring density flucuations in flowing fluids and positively reinforce such fluctuations with sometimes dramatic consequences.  This often arises when the shear-induced density fluctuations can couple to the underlying thermodynamic phase behavior of a complex fluid. Macroscopic consequenses include phenomena such as shear-induced demixing, shear-induced phase separation, shear banding, shear-thickening, and flow-instabilities.   In this work, we investigate the behavior of the microstructure during such transitions for a variety of self-assembled surfactant solutions and colloidal gels using developments in experimental techniques in rheo and flow-small angle neutron scattering and rheo-light scattering.  Of particular interest are new insights afforded by spatiotemporially resolved microstructure measurements in the plane of shear flow.  We demonstrate:  how a shear-induced buckling instability can lead to the formation of multilamellar vesicles from a lamellar phase; how a shear-induced disentanglement-entanglement transition can lead to shear-banding in polymerlike micelles; how shear-induced concentration gradients result in shear-induced nematic formation; how shear-induced branching leads to phase separation under shear flow for branched wormlike micelles; and how colloidal gels shear by the generation of density fluctuations.   Comparisions between systems and with theoretical predictions provide insights into the coupling between flow, microstructure and thermodynamics in complex fluids.