Direct observation of flow-concentration coupling in a shear-banding fluid.

We present the first direct evidence of flow-concentration coupling in a shear-banding fluid using the first measurements of scanning narrow-aperture flow ultrasmall-angle neutron scattering. Transmission measurements in the flow-gradient shear plane determine the spatial concentration profile of the material. Shear banding in a model fluid composed of wormlike micelles near an equilibrium isotropic-nematic transition is shown to be concomitant with shear-induced demixing, providing new insight into the nonequilibrium phase behavior of the fluid. The new experimental approach and analysis apply generally to any fluid, and will enable more rigorous testing of theories for shear-banding and shear-induced phase separation.