A Study of the ”Spurt Effect” in Wormlike Micelles Using Nuclear Magnetic Resonance Microscopy

The non-Newtonian flow properties of the wormlike surfactant system, cetyl pyridinium chloride/sodium salicylate in water, have been studied using dynamic and steady rheometry and nuclear magnetic resonance velocity imaging. The NMR measurements of velocity profiles across a 5.0 mm diameter glass tube reveal a discontinuity in the flow behaviour, once a critical shear strain rate of around I s~~ is exceeded, a manifestation of the so-called "spurt effect". Rheological measurements show that three characteristic regimes are observed. Below 0.2 s~~ the system is near-Newtonian. Between 0.2 and 0.8 s~~, shear-thinning behaviour is observed. Above this a multi-valued shear-rate regime is found at constant stress. This "spurt" regime exhibits shear rates up to values of around 50 to 100 s~~, at which an upturn in the shear stress is found. The rheological flow curves are characteristic of those predicted by a Wagner model and in turn are found to be broadly consistent with the velocity profiles as measured by