Fluctuations in shear-banded flow seen by NMR velocimetry

Magnetic-resonance velocimetry measurements, on the wormlike micelle system 10% w/v cetylpyridinium chloride/sodium salicylate in brine, reveal the existence of widely varying velocities at each point across the gap of a concentric cylinder Couette cell under controlled strain rate conditions. The correlation time for velocity fluctuations is longer than 54 ms, the velocity encoding time in the NMR experiment, and shorter than the 4 h experiment time. Using a constitutive model comprising two high-shear-rate branches, along with stress fluctuations, we are able to accurately represent the observed velocity distributions.