Transition to shear banding in pipe and Couette flow of wormlike micellar solutions

We used both conventional rheometry and nuclear magnetic resonance (NMR) ve locimetry to study shear banding in a solution of 200 mM cetylpyridinium ch loride and 120 mM sodium salicylate in 0.5 M sodium chloride. The solution behaved as a Maxwell fluid up to frequencies of 10 Hz. Theoretical predicti ons of critical strain rate and shear stress were in good agreement with me asurements obtained using controlled strain rate rheometry. Using NMR veloc imetry, we observed convincing evidence of shear banding in capillary flow with a band of very high, approximately constant, shear rate next to the wa ll that grew in thickness with increasing apparent shear rate. We believe t hat the shear rate in this band ( similar to 600 s(-1)) marks the beginning of the hypothesized high shear rate limb of the flow curve. We also observ ed shear banding in both the cylindrical Couette and cone-and-plate geometr ies. Shear banding started at shear rates that were approximately the same as the critical shear rate measured with the mechanical rheometer. With inc reasing shear rate in the fluid, more than two shear bands were sometimes e vident, although they exhibited dynamical instabilities. That is, the highe st shear rate band was variable in both magnitude and position. (C) 1999 Th e Society of Rheology. [S0148-6055(99)00104-2].