Structural transitions in wormlike micelles

We investigate the effect of excess salt and simple shear on the dynamics and structure of semi-dilute aqueous solutions of cetyltrimethylammonium bromide and sodium salicylate. Small-amplitude oscillatory rheological measurements suggest a structural evolution from an entangled to a multi-connected network as the salt concentration is increased. Steady-shear measurements, however, show a significant departure from the Cox-Merz rule. At low salt concentrations, this departure occurs at high shear rates with ^{* –0.92±0.08} and ^{–0.51±0.06} and is attributed to the formation of large shear-induced structures. The critical shear rate _c at which the Cox-Merz rule fails approximates the inverse of the terminal relaxation time, . At high salt concentrations, however, the departure occurs at both low and high shear rates and is attributed to the formation of a multi-connected network. Small-angle light scattering (SALS) under shear was used to probe the mesoscopic structure and revealed novel scattering patterns exhibiting two-fold symmetry at low salt concentration and four-fold symmetry at high salt concentration. The SALS patterns were in qualitative agreement with the formation of large scale anisotropic structures at high shear rates and a multi-connected network at high salt concentrations.