Superhydrophobic surfaces (SHS) combine hydrophobicity and microscopic surface patterning, enabling them to retain a layer of air when submerged in water. SHS have the potential to vastly reduce fluid drag, yielding benefits in applications ranging from microfluidics to maritime transportation. However, experiments have provided inconsistent results, with many SHS yielding little or no drag reduction. It has been hypothesized that naturally-occurring surfactants could be responsible, by creating adverse Marangoni stresses. However, testing this hypothesis has proven challenging. Experiments with purified water already show large interfacial stresses and, paradoxically, adding surfactants yields barely measurable drag increases, thus casting doubt on this hypothesis. Mathematical modeling is also arduous, as the flow/surfactant problem comprises six nonlinearly-coupled partial differential equations, for which exact solutions are unlikely to be found. To test the surfactant hypothesis, we introduce simulations inclusive of surfactant transport, which reveal that Marangoni stresses can immobilize the air–water interface even for concentrations below typical environmental values. We confirm these findings through new time-dependent experiments. Although the general problem comprises ten dimensionless groups, a scaling analysis reveals that surfactant effects are predicted by a single parameter, expressing whether the interface is longer than a “mobilization length” associated with the surfactant properties. These findings provide strategies to design SHS capable of drag reduction in realistic conditions.
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About the Speaker
Paolo Luzzatto-Fegiz received an undergraduate degree in Aeronautics from the University of Southampton, an MSc in Applied Mathematics from Imperial College, and a PhD in Aerospace Engineering from Cornell University. He was a postdoctoral fellow at the Woods Hole Oceanographic Institution and at Churchill College, Cambridge, before joining UCSB. His work received the Acrivos Award and the Gallery of Fluid Motion Award from APS-DFD, a Northrop Grumman Teaching Award, and an NSF CAREER award.
Host: Professor Kyle Smith