A Durable Hierarchical ZnO/Epoxy Superhydrophobic Coating for Dual-Mode Drag Reduction of Surface and Underwater Vehicles

Superhydrophobic coatings can reduce hydrodynamic drag without external energy input, but their application in marine vehicles is hindered by limited durability and by studies addressing only a single navigation mode. Here, a durable ZnO/epoxy superhydrophobic coating with a hierarchical flower-like micro/nanostructure was fabricated by spray coating followed by chemical immersion, and its wettability, durability, and drag-reduction performance were systematically evaluated by model ship towing tests, steel-ball free-fall tests, interfacial friction measurements, and numerical simulations. The coating showed a water contact angle of 161.8 ± 3.1° and a roll-off angle below 1°, and remained superhydrophobic after 200 cm of sandpaper abrasion, 50 h of underwater immersion, 100 repeated water-entry cycles, and immersion in pH 1–11 solutions. The superhydrophobic model ship achieved drag-reduction efficiencies of up to 31%, while the 20 and 30 mm superhydrophobic steel balls achieved 19% and 28%, respectively. Interfacial friction measurements and numerical simulations revealed that the retained air layer induces wall slip on the water surface, whereas the entrained spindle-shaped cavity delays flow separation underwater, reducing both frictional and pressure drag. These results demonstrate a feasible coating strategy for energy-saving marine vehicles operating at or below the water surface.