We have combined rheology and small-angle neutron scattering experiments to investigate dilute aqueous solution of cetyltrimethylammonium tosylate (CTAT). Rheology experiments have been performed under controlled strain conditions as function of concentration (φ ) 0.05 -1%) and temperature (T ) 23-45 °C). At low surfactant concentration and under steady shear conditions, the CTAT exhibits a continuous increase of the apparent viscosity with increasing shear rates, also termed a shear-thickening transition. The critical shear rate for shear thickening varies strongly with the temperature according to an Arrhenius behavior, but very weakly with concentration. On the same solutions that were studied by rheology, neutron scattering was performed at rest to search for a correlation between the rheological features of the shear-thickening transition and the equilibrium micellar structure. We have found that (i) for all concentrations that display the thickening transition, the local morphology is that of cylindrical micelles, with a radius R ) 21.0 ( 0.5 Å; (ii) all spectra exhibit a scattering correlation peak due to strong electrostatic repulsion between charged micelles; (iii) the shear-thickening transition in CTAT displays the same rheological features for solutions prepared either in the dilute (φ < φ*) or in the semidilute (φ* < φ < 0.8%) regimes.

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 rate γc. Concentration and temperature variations of γc 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 γ, the former state dominates and its increasing orientation results in shear-thinning.

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We report on the shear-thickening transition observed in dilute aqueous solutions of cetyltrimethylammonium tosylate (CTAT) at concentrations φ ∼ 0.2%. We have re-examined the kinetics of the shear-thickening transition using start-up experiments at rates above the critical shear rateγc. Using simple well-defined protocols, we have found that the transient mechanical response depends dramatically on the thermal and on the shear histories. Using the same protocols, flow birefringence experiments were carried out. The gap of a Couette cell containing the sheared solution has been visualized between crossed polarizers in steady shear conditions, as well as in start-up experiments. We show that the birefringent shear-induced phase starts from the inner cylinder and grows along the velocity gradient direction, as in a shear banding situation. However, aroundγ c we have not observed a regime of phase coexistence (isotropic and birefringent).