Robust air cavity generation on sacrificial microstructures for sustainable underwater drag reduction

Reducing fluid frictional drag at the solid–liquid interface is a promising strategy for improving the hydrodynamic properties of the structure in water, though so far it has remained unattainable without robust air cavities. Herein, we report a durable generation strategy of robust air cavity on the rough microstructured surface, which could achieve stable drag reduction even after 2000th water entry test. It is worth noting that the generation strategy is almost independent of the wear of surface microstructure, as the worn microstructures still keep a rough morphology and would alter the capillary driving force and prevent the spreading of the liquid film along the structure body. Therefore, the triple contact line is pinned at the solid–liquid interface and induces the generation of a complete air cavity. Comprehensive evaluation, including the mechanical and chemical stability tests, confirm that the microstructured spheres could produce robust cavities even after harsh destruction, and they also reduce the hydrodynamic drag by more than 70.8% at a higher Reynolds number of ∼4.9 × 104. Finally, the boundary slip at the solid–liquid interface of the microstructured surface is simulated, which concludes that the decrease in the contact angle at air–liquid interface and fraction of solid–liquid contact area on the wall would enhance the slip length of fluid, thus resulting in an obvious decreasing of frictional resistance at the solid–liquid interface. We believe that the present work provides a perspective on the sustainable construction of the robust cavity which may have important potential application value in the field of drag reduction.