Time-periodic flow induced structures and instabilities in a viscoelastic surfactant solution

Using various optical and mechanical techniques we report on a shear induced structure (SIS) that results in instabilities during flow. The solution in this study is an equimolar solution of cetylpyridinium chloride and sodium salicylate. In this investigation we further probe the hypothesis of Rehage et al. that an SIS occurs [1]. We find that not only does an SIS occur but as the solution is subjected to stronger flows it becomes turbid before forming ring-like structures which alternate in intensity both spatially and temporally. Since the rings alternate periodically it is evident that the flow not only forms these structures but is also capable of destroying them as well. Simultaneously measuring birefringence and mechanical properties we observe the same periodic oscillations in the birefringence. On the other hand these ring-like structures are present in the parallel plate, cone and plate, and Taylor Couette flow cells. These rings are similar in appearance to those seen for elastic instabilities in Boger fluids seeded with mica flakes. In a constant stress rheometer the shear rate exhibits overshoots and fluctuations again resembling the behavior of elastic instabilities in Boger fluids.