Study of the drag reduction performance on steel spheres with superhydrophobic ER/ZnO coating

Superhydrophobic surface can effectively reduce the pressure drag of the falling sphere, which grants it potential application prospects in drag reduction of underwater vehicle. Herein, a superhydrophobic steel sphere was fabricated by coating a ZnO/epoxy resin solution onto the steel sphere and subsequently reacted with a mixed acetic acid/stearic acid/ethanol solution. The prepared superhydrophobic coating had a contact angle of 159.3 +/- 1.7 degrees and a sliding angle below 1 degrees. By comparing the terminal velocity observed in the fall of smooth and superhydrophobic steel spheres, the drag reduction efficiency for superhydrophobic steel spheres can reach up to 19 %. The drag reduction effect of superhydrophobic coating depends strongly on the Reynolds number and the plastron. The plastron directly influences the fluid flow around the surface, thereby reducing the drag. Thus, this superhydrophobic coating may offer a new strategy for drag reduction of underwater vehicles.