C.-J. Kim
UCLA,
United States
Keywords: superhydrophobic surface, drag reduction, hydrodynamic drag reduction, plastron
Summary:
The potential friction-reducing ability of superhydrophobic (SHPo) surfaces, which may capture a thin air layer (called plastron) under water, have been studied by many over the last two decades. However, despite many reports of successful SHPo drag reduction in laboratory settings, a success in highly turbulent flows on the open water in natural environment was reported only recently with a passenger motorboat (Xu et al., Phys. Rev. Appl., vol. 13, 2020, 034056) and in a towing tank (Xu et al., J. Fluid. Mech, vol. 908, 2021, A6). The success strongly suggested that most of the puzzling episodes of "lab success and field failure" in the past were caused simply by the inability to maintain the plastron in field conditions as well as the difficulty to accurately monitor the plastron during flow tests. To maintain a full plastron at typical boat speeds (tested up to ~14 knots; shear rate ~ 83000 s-1; friction Reynolds number ~5500), we have developed high-performance SHPo surfaces by advancing the micro electro mechanical systems (MEMS) technologies. To replace a part of hull surface with two 4 cm x 7 cm surface samples – one smooth and one SHPo, we have developed a unique low-profile shear comparator and installed it flush underneath a 4 m long motorboat. To avoid the widely-popular but often-misleading practice of confirming the existence of plastron with the silvery sheen appearance, we adopted a new observation technique developed for field tests (Yu et al., Langmuir, vol. 37, 2021, pp. 1206-1214). The resulting drag-reduction data are found to collapse to one curve when plotted against the slip length in wall unit. In addition to reporting ~30% of drag reduction with longitudinal micro-trench SHPo surfaces, the results attest the importance of microscopic nuances of SHPo surfaces even for the macroscale flows of water vessels.