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We study the early stages of the shear banding instability in semidilutewormlike micelles using the non-local Johnson-Segalman model with a two-fluidcoupling of the concentration (phi) to the shear rate (gamma_dot) and micellarstrain (tensor{W}). We calculate the ``spinodal'' limit of stability for sweepsalong the homogeneous intrinsic flow curve. For startup ``quenches'' into theunstable region, the instability in general occurs before the homogeneousstartup flow can attain the intrinsic flow curve. We predict the selected timeand length scales at which inhomogeneity first emerges. In the ``infinitedrag'' limit, fluctuations in the mechanical variables (gamma_dot and\tensor{W}) are independent of those in phi, and are unstable when the slope ofthe intrinsic flow curve is negative; but no length scale is selected. Forfinite drag, the mechanical instability is enhanced by coupling to phi and alength scale is selected, in qualitative agreement with recent experiments. Forsystems far from an underlying zero-shear demixing instability this enhancementis slight, while close to demixing the instability sets in at low shear ratesand is essentially demixing triggered by flow.

Early Stage Kinetics in a Unified Model of Shear-Induced Demixing and Mechanical Shear Banding Instabilities