Abstract
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 1 s -1 is exceeded, a manifestation of the so-called spurt effect. Rheological measurements show that three characteristic regimes are observed. Below 0.2 s -1 the system is near-Newtonian. Between 0.2 and 0.8 s -1 , 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 -1 , 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 NMR.
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