Myristyl dimethylamine oxide surfactant solutions: model systems for rheological research.

Aqueous surfactant solutions of entangled, rod-shaped micelles are often characterized by monoexponential stress-relaxation processes. This special phenomenon leads to relatively simple theoretical descriptions, and viscoelastic surfactant solutions can, therefore, also be used as simple model systems for studying fundamental principles of flow. Herein, we present a detailed study of the nonlinear rheological properties of aqueous myristyl dimethylamine oxide surfactant solutions. In the regime of small deformations, shear stresses, or shear rates, the dynamic features of the viscoelastic solutions are characterized by the simple equations of a Maxwell material. At elevated values of shear stresses or deformations, however, this simple model fails and nonlinear features, such as normal stresses, stress overshoots, or shear-thinning properties occur. All these phenomena can be described by a Maxwell-type differential constitutive equation, which was first proposed by Giesekus. It turns out that the experimental results are in fairly good agreement with the theoretical predictions, if the anisotropy factor alpha is equal to 0.5. Besides transient data and nonlinear steady-state measurements, many semiempirical laws, such as the Cox-Merz rule, the Yamamoto relation, the Laun equation, and the Gleissble mirror relationships are approximately satisfied. The dynamic properties discussed in this paper are of general importance and they are equally observed in different materials such as polymer, dye, or protein solutions.