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.

Stabilizing lubricating gas films at the solid–liquid interface is a promising strategy for underwater drag reduction. It has been restricted by the enormous extra energy input and the poor stability of superhydrophobic coatings. Cavity encapsulation is a valid method to improve and maintain the formation of the air layer on the solid surface, which is created by the rapidly impacting process on a water surface. The wettability of solid objects (the combination of the surface roughness and chemical component) and liquid properties played a key factor in determining the water impact process for cavity entrainment. However, inspired by the striking behavior of basilisk lizards and their toe’s shape, we found that the geometric shape of solid objects plays an equally important role in cavity entrainment and stabilization, which is often ignored. Herein, we present a universal strategy to retain the air cavity on the cylinder surfaces. The cavity can be retained not only on the surface of superhydrophobic cylinders but also on the surface of hydrophobic, hydrophilic, and even superhydrophilic cylinders, without bursting at a depth of 70.0–90.0 cm underwater. The retaining cavity enfolds the profile and upper sides of the cylinder and changes its shape to a streamlined body to achieve underwater drag reduction. In addition, optimizing the cylinders’ shapes by increasing the fillet radii significantly improved the drag reduction efficiency from 64.2 to 70.5%.

forces are always experienced. Thus, hydrodynamic drag reduction is crucial for avoiding speed loss and improving energy efficiency. [1] In general, there are two main forms of hydrodynamic drag resistance, i.e., skin friction (friction drag force) and the form drag force. Skin friction is the friction between a fluid and the surface of a solid moving through it or between a moving fluid and its enclosing surface as a result of fluid viscosity. Skin friction is the key factor for a streamlined body, such as a submarine or torpedo. [2] Form drag occurs from the pressure difference due to the physical dimensions of the object obstructing and altering the flow of the fluid (i.e., flowseparation phenomena). On a blunt body, the form of drag underwater is much greater than the skin friction. [3,4] Consequently, for hydrodynamic drag reduction, it is essential to reduce the form of drag on a blunt body.The innovative approach of form drag reduction is realized by changing the shape of the blunt body to a streamlined body through introducing a cavity. [3][4][5][6][7] The cavity can be generated by the principle of natural cavitation on underwater objects because the water near the head of the object is vaporized to generate cavitation when the speed of the object is high enough. [8,9] However, at low speeds, the cavity greatly depends on the air-capture process when the object impacts the water surface. [10,11] If the water disturbance and water splashing outward along the object create an open splash crown for air to enter during the impacting process, more air is drawn into the cavity with the object continuously falling. Then, due to the hydrostatic pressure of the surrounding water and the capillary force, the middle of the cavity gradually shrinks until it is clipped, and then a streamlined cavity form. [12] The behavior of water regulation on the object interfaces (i.e., water disturbance and water splashing outward) is significantly relayed on the characteristics of the object interface, including wettability, [13][14][15] microstructure, [16,17] and surface energy distribution. The static unwetted interfaces, such as the superhydrophobic [18][19][20][21][22][23][24][25] and Leidonfrost [26][27][28][29] interfaces, prevent water from penetrating the space between the microscale and/or nanoscale structures, leading to a Cassie-Baxter wetting regime and then water splashing outward. A superhydrophobic interface is generally characterized by low surface free energy Form drag of the blunt object occurs from the pressure difference due to the physical dimensions of the object obstructing and altering the flow of the fluid. The innovative approach of form drag reduction is real ized by changing the shape of the blunt object to a streamlined body by introducing a cavity. The construction of superhydrophobic interfaces is widely considered as the best solution for introducing a cavity, because water is prevented from penetrating the space between the nanoscale structures, leading to a Cassie-Baxter wetting regime and the...