Rheology and fluid dynamics of surfactant solutions with flow-induced structure

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Surfactants are molecules comprised of a head that is water soluble bonded to an oily tail that repels water. Because of this dual nature, surfactants dissolved in water form assemblages called micelles, in which the oily tails form the center, separated from the surrounding water molecules by the water-preferring head groups. Micellar surfactant solutions are used in a wide range of applications from fracking fluids in the oil and gas industry to heat-transfer fluids in the chemical process industries to various personal care and cleaning products. During flow, surfactant solutions that form long thin "wormlike" micelles undergo a process where individual micelles combine to form much larger assemblages. These are called flow-induced structures and can have dramatic effects on the flow properties of the solution. These structures form in turbulent flow of wormlike micelle solutions leading to substantial reduction in the energy cost for pumping the fluid. Despite the increasingly frequent application of these so-called drag-reducing surfactant systems, this structure-formation phenomenon remains poorly understood. This work will advance the development of mathematical models for predicting the flow behavior of wormlike micelle solutions and use these models to understand why they are so effective in reducing energy consumption in flow processes. The project will also involve a substantial outreach component: in a service-learning course, undergraduate engineering students will develop project-based lessons in fluid mechanics and then go out to the Boys and Girls Clubs to share these with kids from underserved minority populations.The long-term aim underlying this project is (1) development of models for self-assembling complex fluids like micellar solutions that areas computationally tractable for fluid dynamics analyses, and (2) use of these models to generate insights and predictions into the behavior of this class of fluids in flows characteristic of engineering applications. The work will further develop a recently-proposed micromechanical constitutive model called the reactive rod model, putting it firmer physical ground and making extensive comparisons to experimental data. It will also analyze the flow of an structure-forming fluid in a circular Couette device, where many of the interesting features of flow-induced structure in surfactant solutions have been studied experimentally. Finally, it will elucidate the interaction between turbulence, flow-induced structure and drag reduction using computations with the model. Comparisons with results for polymer solutions will be made, to uncover differences between turbulent drag reduction in polymer and surfactant solutions. Furthermore, comparisons of simulation results with experimentally observed features of turbulence in surfactant solutions will be made. Flow-induced structure formation is widespread in complex fluids and this work takes first steps into analysis of the fluid dynamics of these materials and thus toward new insights into the origins and consequences of their fascinating behavior.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.