Turbulent drag reduction measured on a flat plate covered with silicon tiles of longitudinal superhydrophobic microtrenches in a high-speed water tunnel

Reduction of frictional drag is examined on a 31 cm × 15 cm flat plate covered with superhydrophobic (SHPo) longitudinal microtrenches in a high-speed water tunnel capable of imposing wall shear levels representative of operating water vessels. Experimentation capable of testing substantially larger (~ 16× larger in area than before) microtrench-array surfaces in a large, high-speed water tunnel using a standard measurement method is developed, thereby providing high-quality, rigorously controlled experimental data to support and validate turbulent SHPo drag reduction. High-performance microtrench surfaces featuring hierarchical roughness and re-entrant trench tops are microfabricated on silicon wafers, diced into 7.6 cm × 4.7 cm tiles, and assembled as a 4 × 3 array on the drag plate interfaced with a drag balance for shear-drag measurement. The water tunnel enables control of water pressure and air saturation and allows a low-grazing-angle view for detecting plastron depinning. Two microtrench geometries are tested: (i) 50 µm pitch, 90% gas fraction, 100 µm depth, and 10 mm length, designed primarily for plastron stability and (ii) 100 µm pitch, 92% gas fraction, 200 µm depth, and 20 mm length for larger slip. The former geometry achieves around 30% drag reductions at flow speeds corresponding to a wall shear stress of about 100 Pa, surpassing the previous record of 70 Pa and reaching conditions relevant to practical vessel operation.