Velocity measurements for shear flows of CTAB/NaSal aqueous solutions between parallel plates

摘要

Shear flows of wormlike micellar solutions between parallel plates were experimentally investigated using aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) as test fluids. The solution having a molar concentration ratio of NaSal to CTAB of 2.0 was primarily considered. The velocity between the plates was measured by particle tracking velocimetry. In addition, flow-induced birefringence and shear stress were measured using a parallel-plate rheometer. The measurements were carried out for shear rates at which shear thinning or shear thickening was observed. Banded velocity profiles were found at every shear rate considered in the present experiments, and the profile was stable at relatively low shear rates but fluctuated with time at high shear rates. Furthermore, multi-band velocity profiles were observed in the shear-thinning region. The results of velocity measurements in the shear-thickening region suggest that the velocity profile largely varies with time, accompanying the movement of the position of a band interface. Moreover, both birefringence and shear stress fluctuated with time at high shear rates. These results suggest that the fluctuation of the velocity profile is related to the structural change in the micellar network that leads to the change in the thickness fraction of oriented and isotropic layers or to three-dimensional flows induced by material instabilities.