Thin Vapor layers can reduce drag owing to early drag crisis

The drag of a solid sphere moving in a fluid is known to be only a function of the Reynolds number, and diminishes rapidly at the drag crisis around Re ∼ 3 X 10⁵. Metallic spheres coated with commercially available superhydrophobic agents help to reduce the drag experienced by the spheres falling in water, as compared to unmodified and uncoated spheres. Freshly dipped spheres have a thin air-plastron on their surface, which modifies the boundary condition. These thin air layers (∼1–2 μm) can reduce the drag force by around 80% within the Reynolds numbers from 10⁵ to 3 × 10⁵, owing to an early drag crisis transition*. These effects can have significant implications for the future of sustainable air-layer-based energy saving technologies.

*Jetly, Vakarelski and Thoroddsen, Soft Matter, 14, 1608-1613 (2018).