Particle image velocimetry of the unstable capillary flow of a micellar solution

The unstable capillary flow of the micellar system formed by cetylpyridinium chloride 100 mM/sodium salicylate 60 mM (CPyCl/NaSal) in tridistilled water was studied in this work using a combination of particle image velocimetry (PIV) and rheometrical measurements. The experiments were run in a pressure controlled capillary rheometer at a temperature of 26°C and covered all the different flow regimes occurring in the nonmonotonic flow curve characteristic of micellar solutions. First, we show the suitability of the PIV technique to study the unstable capillary flow of this micellar system. Then, and more important, we provide evidence of the development of shear banding and the velocity profiles for the different flow regimes, including the transition to the high shear branch because of spurt. The velocity profiles at low shear rates exhibited a Newtonian behavior, followed by a still Newtonian with apparent slip up to the onset of spurt. There was a jump of one order of magnitude in the shear rate with a transient of a few minutes once spurt was triggered. The velocity profiles obtained during the transition to the high shear rate branch were plug-like, with a large increase in the velocity close to the capillary wall. When the transition was completed, the velocity profiles corresponding to the high shear rate branch showed the development of a shear band close to the capillary wall, whose thickness increased along with the shear rate. Finally, the true flow curve reconstructed from the velocity profiles provided additional evidence of the existence of a shear band that grows from the capillary wall.