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

Abstract

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.

Armored Superhydrophobic Surfaces with Excellent Drag Reduction in Complex Environmental Conditions

“…Extensive research work has focused on drag reduction and various strategies have been developed to construct an air layer on a structural body’s surface so as to produce significant slip in contact with fluid flow, reducing the shear forces at the boundary and producing drag reduction properties. − Vakarelski et al used the Leidenfrost effect to completely encase a metal sphere falling into a liquid in a stable streamlined air cavity, which eliminates contact between the surface of the sphere and liquid, and the drag coefficient of the streamlined sphere-in-cavity structure is approximately 0.02–0.03. − Liu et al conducted an experimental study on the drag reduction performance of mixed solutions of polymers and surfactants, and the experimental results showed that the mixed solutions could inhibit the generation of vortices more effectively and improve the drag reduction efficiency . Wang et al investigated the mechanism of microstructured spheres to reduce the critical velocity of air cavity entrainment, analyzed the effect of surface morphology effect on hydrodynamic properties, and further revealed the mechanism of surface morphology effect-induced drag reduction in underwater air cavities by numerical simulation. − Liao et al used femtosecond lasers to prepare superhydrophobic glass microchannels with drag-reducing properties, which were able to significantly reduce the flow drag of microfluidics . Zhang et al were inspired by the fine hairs and cilia of living organisms to prepare a hair-like superhydrophobic surface by electrostatic value linting and subsequent surface modification, which has excellent mechanical stability and enables an effective drag reduction effect .…”