NONLINEAR FLOW PROPERTIES OF VISCOELASTIC SURFACTANT SOLUTIONS

This paper gives a quantitative description of the viscoelastic proper ties of aqueous solutions of entangled rod-shaped micelles. The experi mental data are compared with the theoretical predictions of a special constitutive equation which is based on the concept of deformation-de pendent tensorial mobility. In the regime of small deformations, shear stresses or shear rates, the dynamic features of the viscoelastic sol utions are characterized by the equations of a simple Maxwell material . These phenomena are linked to the average lifetime of the micellar a ggregates and the rheological properties are controlled by kinetic pro cesses. At these conditions one observes simple scaling laws and linea r relations between all rheological quantities. At elevated values of shear stresses or deformations, however, this simple model fails and n on-linear properties as normal stresses, stress overshoots or shear-th inning properties occur. All these phenomena can be described by a con stitutive equation which was first proposed by H. Giesekus. The experi mental results are in fairly good agreement with the theoretical predi ctions, and this model holds for a certain, well defined value of the mobility factor alpha. This parameter describes the anisotropic charac ter of the particle motion. In transient and steady-state flow experim ents we always observed alpha = 0.5. Especially at these conditions, t he empirically observed Cox-Merz rule, the Yamamoto relation and both Gleissle mirror relations are automatically derived from the Giesekus model. The phenomena discussed in this paper are of general importance , and can be equally observed in different materials, such as polymers or proteins. The viscoelastic surfactant solutions can, therefore, be used as simple model systems for studies of fundamental principles of flow.