DRAG ON SUPERHYDROPHOBIC SHARKSKIN INSPIRED SURFACE IN A CLOSED CHANNEL TURBULENT FLOW

Salvinia leaf and sharkskin are prime examples of nature's marvel. Salvinia leaf-inspired superhydrophobic surfaces keep themselves clean and reduce drag in fluid flow. Sharkskin also reduces drag in turbulent flow and inhibits biofouling. Therefore, the prospect of having a drag-reducing surface with both salvinia leaf and sharkskin properties is attractive. However, fabricating such a surface is difficult, and the current fabrication methods require at least two separate steps. In addition, the mechanisms of drag reduction of salvinia leaf and sharkskin are different, and their combined effect on the flow field is not well understood. In this study, we produced a PTFE surface that mimics sharkskin in its surface pattern and copies the superhydrophobic nature of the salvinia leaf in its microstructure. This surface was fabricated by laser machining and tested in a closed channel under turbulent flow conditions. We measured the pressure drop at different Reynolds numbers on this surface both in pre-wet and non-pre-wet conditions and compared the result with pressure drop data on four other PTFE samples: two types of non-superhydrophobic sharkskin inspired surface (riblets), a superhydrophobic surface, and a non-machined surface. Both the non-superhydrophobic riblets and the superhydrophobic sample reduced drag compared to the non-machined surface. However, we observed a lack of drag reduction by the superhydrophobic riblets sample. We presented a qualitative explanation for the lack of drag reduction and concluded that the modifications of the flow field by the two drag reduction mechanisms are not beneficial for overall drag reduction in our experiment.