CAREER: Kinetics and Molecular Origin of Shear Banding in Wormlike Micellar Fluids

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Micelles are aggregates of surfactants that can form structures of various shapes, including long flexible strands that are called wormlike micelles. Micelles are used in a variety of products to control the physical properties of the material during its processing and use. Under flow, solutions of wormlike micelles sometimes exhibit a curious phenomenon known as shear banding in which the micelles segregate into multiple thin bands. Shear banding is important, because it can strongly influence the properties of the solution during processing and affect the quality of the final product. This CAREER award will support a comprehensive investigation of shear banding of wormlike micellar solutions. Multiple experimental techniques will be used to understand how the concentration of the wormlike micelles, their end-to-end lengths, and the physical properties of the micellar solution influence shear banding. The results will be useful to process engineers and scientists who work with a wide variety of materials, because shear banding is also observed in other industrially important materials such as emulsions, foams, oil-sands and colloidal suspensions. The research team will involve diverse groups of students in the research. To demonstrate the importance of the research to younger students, the team will produce FlowKits for teachers and students to explore the unusual properties the complex fluids in the classroom.Understanding the mechanisms of shear banding in wormlike micellar solutions remains a critical challenge due to limitations of the traditional techniques to probe fluid microstructures. This CAREER award will circumvent these limits and experimentally examine a) flow-induced micellar breakage and b) flow-induced concentration gradients and c) the effects of viscoelastic instabilities on the mechanisms of shear banding via Rheo-DOSY-NMR and Rheo-qNMR techniques. This will be accomplished by measuring spatio-temporal evolution of micellar length and concentrations across the gap of a Taylor-Couette flow, i.e. flow between two concentric cylinders, for a broad range of systems and conditions. The research will establish the connections among fluid properties (elasticity, inertia, micellar entanglement), processing conditions (flow ramp up and geometry curvature) and macroscale kinetics of shear banding flow formation. A combination of experiments and simulations will provide the first systematic comparisons between experiments and various models of shear banding in terms of the local micellar length, concentration gradients, velocity profiles and shear stresses. These experimental results will provide new data which are essential for verifying existing theories and/or developing new predictive models. Results will help engineers to rationally design novel shear banding materials with improved properties for enhanced oil recovery, drag reduction, and many consumer products.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.