Bioinspired Drag Reduction of Cavities Induced by Janus Nanostructure Interfaces on Hydrophobic Spheres Plunging into Water with a Low Speed

Abstract 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 realized 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 then water splashing outward. Here, by introducing Janus nanostructure coating on hydrophobic spheres, it is found that the position (impacting direction) and coverage ratio (r) of the nanostructures, accurately manipulate the performance of water film on the sphere surface. When the nanostructure faces upward, the detached thin film develops to a splash crown only if r increases to 1/3 or even above. Correspondingly, the remaining open aperture aids in increasing the subsequent air entrainment, and an air‐entrainment cavity is trapped on the sphere surface. However, if the nanostructure faces downward, only a tiny region of the nanostructures (r ≤ 1/8) can help a thin liquid film directly develop into a splash crown and cavity.