Shear thickening and shear induced band formations in solutions of wormlike micelles

An equimolar mixture of a cationic surfactant, cetylperidinium chloride (CPyCl) and the salt sodium salicylate (NaSal) forms wormlike micelles in aqueous solutions. Under shear, the solution shows a pronounced shear-thickening behavior, which is coupled with oscillations in shear rate and the apparent viscosity. In this shear-thickening regime the formation of shear-bands is observed, which also oscillate in position and intensity. Fast Fourier Transformations (FFT) of the oscillating shear rate and intensity signals show a single dominating frequency in the power spectrum analysis. This characteristic frequency as well as the amplitude of shear rate oscillation is found to increase with stress. Experiments performed in transparent parallel-plate geometry show dampening of the shear rate oscillations and increase in the characteristic frequency with decrease in the gap. Rheo-small angle light scattering and rheo-optical techniques confirm the formation of different kinds of structures at smaller gaps.