Title: Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces,
Abstract: Superhydrophobic surfaces (SHSs) have the potential to achieve large drag
reduction for internal and external flow applications. However, experiments
have shown inconsistent results, with many studies reporting significantly
reduced performance. Recently, it has been proposed that surfactants,
ubiquitous in flow applications, could be responsible, by creating adverse
Marangoni stresses. Yet, testing this hypothesis is challenging. Careful
experiments with purified water show large interfacial stresses and,
paradoxically, adding surfactants yields barely measurable drag increases. This
suggests that other physical processes, such as thermal Marangoni stresses or
interface deflection, could explain the lower performance. To test the
surfactant hypothesis, we perform the first numerical simulations of flows over
a SHS inclusive of surfactant kinetics. These simulations reveal that
surfactant-induced stresses are significant at extremely low concentrations,
potentially yielding a no-slip boundary condition on the air--water interface
(the "plastron") for surfactant amounts below typical environmental values.
These stresses decrease as the streamwise distance between plastron stagnation
points increases. We perform microchannel experiments with thermally-controlled
SHSs consisting of streamwise parallel gratings, which confirm this numerical
prediction. We introduce a new, unsteady test of surfactant effects. When we
rapidly remove the driving pressure following a loading phase, a backflow
develops at the plastron, which can only be explained by surfactant gradients
formed in the loading phase. This demonstrates the significance of surfactants
in deteriorating drag reduction, and thus the importance of including
surfactant stresses in SHS models. Our time-dependent protocol can assess the
impact of surfactants in SHS testing and guide future mitigating designs.