CAREER: Surfactant impact on laminar drag reduction of realistically-textured superhydrophobic surfaces

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Drag-reducing coatings have been investigated to reduce the large pressure differences that can arise in microfluidic devices and in micro-cooling applications, as well as to enhance control in chemical or biological analysis. Superhydrophobic surfaces is a promising type of drag-reducing coating, which uses a combination of microscopic texture and hydrophobicity to retain a thin coating of air bubbles when submerged in a liquid, like water. However, surfactants, which are naturally present in the environment or in many working liquids, can drastically curtail the achievable drag reduction through a mechanism that is highly sensitive to the details of the coating texture. This project will use simulations, theory, and experiments to investigate drag reduction phenomenon in laminar flow microchannels with superhydrophobic surfaces in the presence of surfactants. The project will encompass several educational activities, including university course developments, an undergraduate research program, a hands-on activity for local junior high school outreach events, and a K-12 learning activity targeting the Next Generation Science Standards.This project aims to predict superhydrophobic drag reduction in laminar flow with surfactants, as a function of texture geometry, bulk flow properties, surfactant type and concentration. This will be achieved by examining superhydrophobic surfaces of progressively increasing complexity, using a combination of numerical simulations, theory, and experiments. The numerical simulations will enable precise control of surfactant properties and concentration and solve the equations for the transport of mass, momentum, and surfactant in the fluid interior and along the air–water interface. The results will be used to develop tractable theoretical models, which will yield insight about how flow properties and texture geometry affect the resulting drag. The theoretical models will also be used for rapid drag reduction predictions. In the experiments, superhydrophobic surfaces will be placed in a microfluidic system, and confocal microscopy will be used to measure velocity fields near the air-water interface. The experiments will validate the simulations and theory. This three-pronged computational, theoretical, and experimental investigation will advance our understanding of surfactant effects on drag using superhydrophobic surfaces in realistic laminar flow conditions and lead to reliable prediction and mitigate strategies.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.