Visualization of shear banding and entry Poiseuille flow oscillations in a micellar aqueous solution

A study of the plane Poiseuille flow of a micellar aqueous solution of cetylpyridinium chloride 100 mM and sodium salicylate 60 mM was performed in this work. The experiments were run at 27.5 °C under controlled pressure using a transparent flow cell, where simultaneous measurements of transmitted light intensity, pressure drop and flow rate were performed in order to asses the flow stability. Particle image velocimetry (PIV) was also used to analyze the flow kinematics upstream of the contraction. Different regimes were observed in the flow curve, including shear banding and spurt. In the high shear rate branch the flow became unstable and was composed by asymmetric shear bands of structured and isotropic fluid, which oscillated with respect to the zero-shear stress plane. Symmetric lip vortices were observed to grow upstream of the contraction, and then to oscillate and decrease their average length under unstable flow. The shear bands downstream of the contraction oscillated in the same way as upstream vortices, with a frequency that increased along with the flow rate. Finally, the oscillating flow upstream of the contraction produced jets or spurts of highly oriented material followed by recoiling, akin to those reported in the melt fracture regime of polymers.