Effects of trace amounts of surfactants on drag reduction for superhydrophobic surfaces
Joint work with Francois Peadecerf, Julien Landel, and Ray Goldstein (University of Cambridge)
Large drag reductions (of the order of 50% or more) have been measured for laminar flows over superhydrophobic surfaces (SHS), making them attractive for applications in pipelines, ships and submarines. However, experiments involving turbulent flows (typical of these applications) have often yielded limited and erratic drag reductions. A complete explanation for this issue has so far proved elusive.
We propose that trace amounts of surfactants (unavoidable in the environment and in large-scale experiments) might yield poor performances of SHS, by producing Marangoni stresses when the edges of the SHS pattern are not aligned with the local flow velocity.
To explore our hypothesis, we develop simulations (inclusive of surfactants) for flow over a textured SHS in a micro-channel (which may exemplify a viscous sublayer). The texture consists of gaps and ridges perpendicular to the flow. Even small amounts of surfactant seem to prevent any drag reduction.
To test our hypothesis, we flow de-ionised water with known surfactant concentrations through custom-built SHS micro-channels, while performing micro-PIV. At negligible concentrations, we find higher velocities above the gaps, as expected by classical models. However, as the concentration increases, we discover that the slip velocity drops to very small values even in the presence of a plastron.
Our results show that the drag-reducing potential of superhydrophobic surfaces can be strongly limited in realistic flow conditions (see full paper in Proceedings of the National Academy of Sciences).