Bioinspired hydrophilic strip patterning on superhydrophobic spheres for stabile underwater cavities and enhanced drag reduction

Efficient drag reduction is critical for improving the energy efficiency of underwater vehicles, where a significant portion of propulsion energy is consumed in overcoming hydrodynamic resistance. When a superhydrophobic surface impacts water at a sufficient velocity, a streamlined cavity forms, effectively minimizing drag. However, at lower initial impact velocities, the cavity dissipates rapidly due to static water pressure and flow separation, leading to increased drag and reduced velocity. Numerical simulations using STAR-CCM+ reveal that cavity dissipation enlarges the flow separation zone at the sphere’s tail, significantly increasing pressure-induced drag. To enhance cavity stability and sustain drag reduction, this study proposes a bioinspired strategy that integrates hydrophilic strips onto superhydrophobic steel spheres via spraying and laser processing. The results demonstrate that the superhydrophobic/hydrophilic heterogeneous structure effectively anchors the gas-liquid-solid three-phase contact line, traps air bubbles, and stabilizes the cavity. Compared to fully coated spheres, localized hydrophilic strips (applied only at or above the equator) exhibit superior cavity stabilization, leading to prolonged drag reduction. Further optimization reveals that when the width and spacing of the hydrophilic strips are both 0.25 mm, the cavity remains most stable, achieving a maximum drag reduction efficiency of 63.3%. This study presents a scalable, cost-effective surface engineering approach for stabilizing underwater cavities, providing valuable insights into next-generation drag reduction technologies for underwater vehicles and robotic systems.