SHEAR-FLOW OF WORMLIKE MICELLES IN PIPE AND CYLINDRICAL COUETTE GEOMETRIES AS STUDIED BY NUCLEAR-MAGNETIC-RESONANCE MICROSCOPY

The nonlinear viscosity of the wormlike surfactant system cetyl pyridi nium chloride/sodium salicylate (60 mM/100 mM in water) has been inves tigated in both pipe and cylindrical Couette geometries, using nuclear magnetic resonance to image both velocity end diffusion. In pipe flow we observe transitions from Newtonian to non-Newtonian viscosity, to spurt, to unstable flow, and then to a regime where fluctuations an ra pid on the timescale of a few milliseconds. In the Couette cell we obs erve apparent slip at the inner wall as well as a high shear rate band located away from the wall in the body of the fluid. The banding phen omenon, which has its counterpart in the pipe flow, is consistent with double valuedness in the stress versus rate of strain relationship fo r this fluid. (C) 1997 The Society of Rheology.