Bioinspired Cavity Regulation on Superhydrophobic Spheres for Drag Reduction in an Aqueous Medium

Hydrodynamic drag not only results in high-energy consumption for water vehicles but also impedes the increase of vehicle speed. The introduction of a low-viscosity gas lubricating film is assumed to be an effective and promising method to reduce hydrodynamic drag. However, the poor stability of the gas film and massive extra energy consumption restricts the practical application of the gas lubricating method. Herein, inspired by the microhairs with low surface energy wax covering the abdomen of water spiders, superhydrophobic sphere surfaces were designed. Attributed to numerous neighboring nanoneedle branches with low surface energy chemicals, an air-entrained cavity with a large surface area was captured and stabilized by the superhydrophobic sphere, changing its shape from a sphere to a streamlined body. The cavity continued attaching to the superhydrophobic sphere without bursting at a depth of 70.0-90.0 cm underwater and reduced the hydrodynamic drag by more than 90%. This work provides a simple, cost-effective, and energy-efficient way to stabilize the underwater gas-liquid interface to achieve a reduction in the hydrodynamic drag.