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

Abstract

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.

Bioinspired Universal Approaches for Cavity Regulation during Cylinder Impact Processes for Drag Reduction in Aqueous Media: Macrogeometry Vanquishing Wettability

“…With the attached cavity, the hydrodynamic drag force can be successfully reduced by 80−90% at a releasing height of 40.0 cm, which was close to the value of the superhydrophobic sphere (Supporting Information S15), 40 indicating the universal ability of the brilliant drag reduction effect of the underwater cavity. Moreover, the effective density of the cavity on cylinders was a bit larger than that of water, indicating that the main contribution of the upward drag force was by the buoyancy force F B (Figure S16).…”

“…52 The retaining cavity changed the outline shape to a streamlined shape. The streamlined outline shape suppressed the flow separation to reduce pressure-induced drag 40 (more analysis can be found in Supporting Information S10). Thus, the cavity-retaining cylinders significantly reduced the hydrodynamic drag, and the cylinders with larger releasing height showed a better drag reduction effect.…”

“…Cavity entrainment regulates the solid body shape to a streamlined shape, causing the pressure-induced drag resistance force to effectively reduce. The wettability of solid objects (the combination of the surface roughness and chemical component), the dynamic wetting process, and liquid properties play a key factor in determining the water impact process for cavity entrainment. − …”

Focus on Bioinspired Textured Surfaces toward Fluid Drag Reduction: Recent Progresses and Challenges

“…The stability and lifetime of the air layer are improved by optimizing the surface microstructures. [106][107][108] Du et al proposed to restore the Cassie state by injecting air continuously so that the slip can re-form on the solid surface. [109] The method is expected to preserve a stable air-water interface on an SHS to maintain the drag reduction effect in engineering applications.…”

Underwater Drag Reduction and Buoyancy Enhancement on Biomimetic Antiabrasive Superhydrophobic Coatings

“…The noble quality of the lotus leaf is highlighted by the phrase “keeping clean in the dirty soil”, which reflects the self-cleaning property of the lotus leaf. Inspired by this unique property, SHB functional surfaces have been intensively explored in various fields, such as self-cleaning, , oil–water separation, water droplet energy harvesting, drag reduction, , and so on. − However, the biggest drawback of SHB surfaces is its poor mechanical properties and resistance to wear . Thus, researchers have used different strategies, such as passive resistance strategies and active regeneration strategies, to construct robust SHB surfaces. , For example, Verho et al used hierarchical roughness to improve the wear resistance of the surface, where the microscale structure protected the fragile nanostructure from wear .…”