Nonmonotonic Constitutive Laws and the Formation of Shear-Banded Flows

We consider constitutive models of viscoelastic behaviour which predict a shear stress which is a nonmonotonic function of the shear rate. It is known that a homogeneous shear flow is unstable when the shear stress decreases with shear rate. We use a novel simulation technique (the Lagrangian-Eulerian method for the fluid dynamics combined with Öttinger's stochastic method for the constitutive equation) to solve one- and two-dimensional models of plane Couette flow for an integral constitutive equation describing entangled wormlike micelles. The results are compared with those of a `toy' model (with a differential constitutive equation). We show that the steady state actually consists of bands of different shear rate. Such a flow is strongly inhomogeneous, and our preliminary results indicate that the constitutive equation must be modified to allow for spatial variations in the viscoelastic stress.