TIME-PERIODIC FLOW-INDUCED STRUCTURES AND INSTABILITIES IN A VISCOELASTIC SURFACTANT SOLUTION

Using various optical and mechanical techniques we report on a shear i nduced structure (STS) that results in instabilities during flow. The solution in this study is an equimolar solution of cetylpyridinium chl oride and sodium salicylate. In this investigation we further probe th e hypothesis of Rehage et al. that an SIS occurs [I]. We find that not only does an SIS occur but as the solution is subjected to stronger f lows it becomes turbid before forming ring-like structures which alter nate in intensity both spatially and temporally. Since the rings alter nate periodically it is evident that the flow not only forms these str uctures 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 r ing-like structures are present in the parallel plate, cone and plate, and Taylor Couette how cells. These rings are similar in appearance t o those seen for elastic instabilities in Boger fluids seeded with mic a flakes. In a constant stress rheometer the shear rate exhibits overs hoots and fluctuations again resembling the behavior of elastic instab ilities in Boger fluids. (C) 1998 Elsevier Science B.V. All rights res erved.