  The effects of the addition of sub-micrometer sized colloidal silica spheres
on the linear and nonlinear rheology of semi-dilute 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 $\eta_{\circ}$ 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
2.6wt%. The increases in $\eta_{\circ}$ and the high frequency plateau modulus
G$_{\circ}$ on the introduction of SiO$_{2}$ 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 G$_{\circ}$ and $\eta_{\circ}$ 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.
