POWRE: Rheology and Microstructure of Dilute Surfactant Systems in Complex Flows

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Surfactants, or surface active agents, pervade almost every aspect of our daily lives. Biological functions, healthcare products, cleaning materials, paints and many other products and processes rely on the unique properties of these molecules. Many of these solution properties arise from the fact that surfactants aggregate in solution, forming a myriad of structures with different properties. The ability to control aggregate structure requires a fundamental knowledge of the influence of system parameters. Although this understanding exists for equilibrium situations, the changes in structure caused by external forces, such as flow, are not well studied. Flow is a pertinent variable as most surfactant-based systems are subject to flow in application, transport and processing. Thread-like micelles, or one-dimensional aggregates, form in a number of surfactant solutions. In the case of dilute solutions of rod-like (stiff) micelles with low levels of added salt, a transition to a shear-thickened state has been observed and fairly well characterized. A qualitative understanding of the shear-induced transition exists both at a macroscopic and structural level and this understanding is becoming more quantitative. Most studies have focused on simple shear fields and Couette rheometry. The influence of flow fields other than shear has not been studied, yet information from these types of studies are required for a complete understanding of the phenomenon. In this proposed research, the intent if to simultaneously probe macroscopic flow behavior and microstructural changes in more complex flow fields. Initial thrusts will involve inhomogeneous shear flow. The study will then extend to flow fields with increasing levels of elongation to begin to understand the influence of strong flows. Long term goals include the study of elongational flows, both at the macroscopic and microstructural levels. Results from this research will provide information necessary for the development of struct ure based constitutive equations for these micellar solutions, verify the potential of these fluids n specific engineering applications (such as drag reduction) and answer specific questions about the nature of the shear-induced transition which cannot be elucidated with shear alone.