Observation of shear banding in worm-like micelles by NMR velocity imaging

We report an observation of shear-banding, under Couette how, of the worm-like surfactant system cetyl pyridinium chloride/sodium salicylate (respective concentrations of 100 mM and 60 mM in pure water). The method, based on NMR velocity imaging, allows the direct measurement of velocity, and consequent calculation of shear rate, at a spatial resolution of around 10 mu m, sufficient to resolve apparent slip at the inner wall from shear banding within the bulk of the fluid. Above a critical shear rate (around 1 s(-1)) a high shear rate band is observed in the annulus between concentric rotating cylinders, the width of the band being of order 30 mu m with a corresponding shear rate of order 500 s(-1). The precise location of the band is sensitive to boundary conditions at the inner wall and is shown to be susceptible to significant broadening.