Effect of silica colloids on the rheology of viscoelastic gels formed by the surfactant cetyl trimethylammonium tosylate

The effects of the addition of submicrometer-sized colloidal silica spheres on the linear and nonlinear rheology of semidilute solutions of a viscoelastic gel are studied. For a 1.4 wt% solution of the surfactant CTAT, a peak in the zero-shear rate viscosity η0 is observed at approximately equal weight percents of silica and CTAT. This peak shifts to lower silica concentrations on increasing either the CTAT concentration or the surface charge on silica and disappears when the CTAT concentration is increased to 2.6 wt%. The increases in η0 and the high frequency plateau modulus G0 on the introduction of SiO2 are explained by considering the increasingly entangled wormlike micelles that are formed due to the enhanced screening of the electrostatic interactions. The observed decrease in the values of G0 and η0 at higher concentrations of silica particles is explained in terms of the formation of surfactant bilayers due to the adsorption of the positively charged cetyl trimethylammonium to the negatively charged silica.