Flow-structure relationship of shear-thickening surfactant solutions

The shear-thickening transition in dilute surfactant solutions is investigated using rheology and small-angle neutron scattering. Steady shear experiments on a new thickening system, the tosylate of cethyltrimethylammonium, revealed a continuous increase of the apparent viscosity above a critical strain fate (gamma)over dot(c). Concentration and temperature variations of (gamma) over dot(c) are derived, and the former is found to be in contradiction with a theoretically predicted gelation. From the scattering under shear, we establish a clear correlation between flow and structure. In the thickening region, the patterns are due to the superposition of two coexisting states, one viscoelastic entangled sheared network and one still purely viscous made of short aggregates. At higher (gamma) over dot(c), the former state dominates and its increasing orientation results in shear-thinning.