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

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.Comment: 28 pages, includes 8 eps and 2 png figures; accepted for publication in Jl. Colloid Interface Sc

Dynamical Behaviour in the Nonlinear Rheology of Surfactant Solutions

Several surfactant molecules self-assemble in solution to form long, flexible wormlike micelles which get entangled with each other, leading to viscoelastic gel phases. We discuss our recent work on the rheology of such a gel formed in the dilute aqueous solutions of a surfactant CTAT. In the linear rheology regime, the storage modulus $G^{\prime}(\omega)$ and loss modulus $G^{\prime\prime}(\omega)$ have been measured over a wide frequency range. In the nonlinear regime, the shear stress $\sigma$ shows a plateau as a function of the shear rate $\dot\gamma$ above a certain cutoff shear rate $\dot\gamma_c$. Under controlled shear rate conditions in the plateau regime, the shear stress and the first normal stress difference show oscillatory time-dependence. The analysis of the measured time series of shear stress and normal stress has been done using several methods incorporating state space reconstruction by embedding of time delay vectors.The analysis shows the existence of a finite correlation dimension and a positive Lyapunov exponent, unambiguously implying that the dynamics of the observed mechanical instability can be described by that of a dynamical system with a strange attractor of dimension varying from 2.4 to 2.9.Comment: 12 pages, includes 7 eps figure

Experimental study of nonlinear dust acoustic solitary waves in a dusty plasma

The excitation and propagation of finite amplitude low frequency solitary waves are investigated in an Argon plasma impregnated with kaolin dust particles. A nonlinear longitudinal dust acoustic solitary wave is excited by pulse modulating the discharge voltage with a negative potential. It is found that the velocity of the solitary wave increases and the width decreases with the increase of the modulating voltage, but the product of the solitary wave amplitude and the square of the width remains nearly constant. The experimental findings are compared with analytic soliton solutions of a model Kortweg-de Vries equation.Comment: The manuscripts includes six figure