Quantitative description of the non-linear flow properties of viscoelastic surfactant solutions

Abstract

We have systematically studied the non-linear flow properties of certain viscoelastic surfactant solutions. While the dynamic features of these systems can be described by a modified reptation theory, one obtains in the regime of large velocity gradient stress overshoot in start-up flow experiments and shear thinning properties in the steady-state regime. These properties of viscoelastic surfactant solutions can be described with a non-linear Maxwell material, first proposed by Giesekus. From these models it is easy to calculate steady-state values of the shear viscosity η(∞, $\dot \gamma$) and the first normal stress coefficient ω ₁ (∞, $\dot \gamma$). It is often observed that these properties obey viscometric functions such as the Yamamoto relation and the Gließle mirror relationships. It turns out that the experimental results are in fairly good agreement with theoretical predictions of the one-mode Giesekus model, especially by adjusting the only free parameter with α=0.5. For these conditions, the Cox-Merz rule, the Yamamoto relation, and both Gleißle mirror relations are automatically derived from the theoretical model. It is very interesting that the non-linear Maxwell material can be used for the qualitative description of non-linear flow properties of viscoelastic surfactant solutions, such as the shear viscosity of the first normal stress difference.