  After surveying the experimental evidence for concentration coupling in the
shear banding of wormlike micellar surfactant solutions, we present flow phase
diagrams spanned by shear stress (or strain-rate) and concentration in the
two-fluid, non-local Johnson-Segalman (d-JS-phi) model. We also present
macroscopic flow curves for a range of (average) concentrations. For any
concentration high enough to give shear banding, the flow curve shows the usual
non-analytic kink at the onset of banding, followed by a coexistence
``plateau'' that slopes upwards. As the concentration is reduced, the width of
the coexistence regime diminishes, then terminates at a non-equilibrium
critical point. We outline the way in which the flow phase diagram can be
reconstructed from a family of such flow curves measured for several different
average concentrations. This reconstruction could be used to check new
measurements of concentration differences between the coexisting bands. Our
d-JS-phi model contains two spatial gradient terms describing the interface
between the shear bands. The first is in the viscoelastic constitutive
equation, with a characteristic (mesh) length, l. The second is in the
(generalised) Cahn-Hilliard equation, with the characteristic length, xi, for
equilibrium concentration-fluctuations. We show that the phase diagrams depend
on the ratio r=l/xi, with loss of unique state selection at r=0. We also give
results for the full shear-banded profiles, and study the divergence of the
interfacial width at the critical point.
