Shear-Enhanced Orientation and Concentration Fluctuations in Wormlike Micelles: Effect of Salt

Small angle light scattering (SALS) is used to investigate the effect of excess salt on flow-induced orientation and concentration fluctuations in a semidilute aqueous solution of cetyltrimethylammonium bromide and sodium salicylate. In the low shear rate regime, solutions at all salt concentrations studied showed a butterfly scattering pattern, which can qualitatively be explained on the basis of the Milner modification to the Helfand−Fredrickson model. At high shear rates, however, novel scattering patterns are observed, evolving from a flattened butterfly pattern with a broad scattering maximum, at relatively low salt:micelle ratios, Ψ ≤ 4, to a 4-fold symmetric pattern at 5 ≤ Ψ ≤ 6, and finally to a 2-fold symmetric pattern oriented in the vorticity direction at 7 ≤ Ψ < 10. No scattering was obtained at Ψ ≥ 10. These patterns were in qualitative agreement with the Porte model of a structural evolution from an entangled to a multiconnected network as the salt concentration is increased as well as with the flow-induced 1-D gel phase predicted by Turner and Cates. A quantitative estimate of the diffusion coefficient, D, and stress relaxation time, τ s, were obtained by considering the relaxation behavior of specific scattering vectors, q, on flow cessation. The values showed a trend consistent with the structural evolution. Furthermore, τ s, and the characteristic stress relaxation time, τ r, obtained from small-amplitude oscillatory rheological measurements, were in qualitative agreement.