Visualization of vorticity banding in a shear thickening micellar solution in couette flow

The shear thickening behavior and flow instabilities of an equimolar semidilute aqueous solution of cetylpyridinium chloride and sodium salicylate were studied in this work by using rheometry coupled with flow visualization. The experiments were conducted at 25°C in a stress controlled Couette rheometer with a transparent flow cell. A detailed flow curve was obtained for this solution, which includes five different regimes. At very low shear rates a Newtonian behavior was found, followed by a shear thinning one in the second regime. In the third regime, a strong orientation of the micelles consistent with shear banding served as a precursor of the shear induced structures (SIS) and shear thickening. The fourth and fifth regimes in the flow curve were separated by a spurt-like behavior, and they clearly evidenced the existence of shear thickening accompanied by stick–slip oscillations at the rotating cylinder. The use of the combined method allowed the detection of SIS. The visualization of this solution in the shear thickening regime showed the build-up of SIS (turbid fluid) in the shear banding regime followed by large variations in shear rate and viscosity, corresponding to the oscillation of clear and turbid bands stacked in the vorticity direction. These alternating bands and shear rate variations are in agreement with the presence of the vorticity banding instability as well as with the creation and destruction of SIS.