Wormlike micellar fluids have received considerable attention in recent years for a number of reasons. They are of industrial interest and find use in oil recovery, waste water treatment, and consumer products. They are also considered as a model for systems such as suspensions, foams, and granular materials that display shear-banding – that is, discontinuities in the velocity gradient in the flowing material. Finally, in small amplitude oscillatory shear flow, wormlike micelle solutions can display remarkably simple rheological behavior, described by a Maxwell model with a single relaxation time and plateau modulus. Thus, they display viscoelastic behavior that has strong analogies with the behavior of polymer solutions and follow similar scaling laws. Here we describe a series of experiments to probe the hydrodynamic stability of wormlike micellar solutions in complex flows ranging from shear-dominated flows between concentric cylinders to the mixed shear and extensional flow due to the settling of a sphere under gravity. While the flow behavior and stability in some cases is consistent with that of polymer solutions, a number of interesting differences emerge. These differences suggest the sensitivity of the results to the precise formulation of the wormlike micellar system, to the presence of branching, and to the importance of extensional flow.
Susan J. Muller is Professor of Chemical & Biomolecular Engineering, at the University of California, Berkeley and Associate Dean of the Graduate Division. She has worked on problems related to polymer fluid mechanics, processing, rheology, and microfluidics for over 25 years at MIT, Schlumberger Cambridge Research, AT&T Bell Laboratories, and the University of California, Berkeley. She has worked extensively in the area of viscoelastic instabilities in Taylor-Couette flows, the role of polymer-solvent interactions in determining macroscopic flow behavior, shear-induced migration of polymers in dilute solutions, and the behavior of DNA and other microscale objects in microfluidic devices. Recent research on large scale flows has focused on drag reducing polymers; recent work on microscale flows has focused on the design of microfluidic devices to trap and manipulate DNA and vesicles, and the use of microfluidics to make and study suspensions of particles. As Associate Dean, she oversees graduate student support issues across the campus (fellowships, awards, grants, and campus employment) and supports of the campus’ diversity, equity, and inclusion initiatives. Muller holds a B.S.E. in chemical engineering from Princeton University, and a Ph.D. in chemical engineering from M.I.T.