Superhydrophobic Drag Reduction for Turbulent Flows in Open Water

Despite two decades of research, drag reduction with superhydrophobic (SHPo) surfaces has never been experimentally confirmed under the most coveted condition, i.e., high Reynolds number flows in open water, thus casting doubt on its ultimate impact. Here, we present large drag reductions (approximately 30%) by SHPo surfaces tested on the sea at Rex estimated to be as high as 6.5 x 10(6). To achieve success, we prepare 4 x 7 cm(2) silicon samples of re-entrant microtrenches for maximum plastron retention and test them as a portion of hull surface underneath a motorboat using a custom-developed comparative shear sensor. The state of plastron on the sample surfaces are observed throughout the tests via a custommade miniature underwater camera. The successful results attest the importance of microscopic nuances of SHPo surfaces for plastron retention and drag reduction, guiding directions for future applications.