Skipping under water: Buoyant sphere hydrodynamics at the air-water interface

We investigate the hydrodynamic behavior of a buoyant sphere pulled horizontally along the water interface. At low velocities, the sphere remains buoyant and stays on the water's surface. However, at higher velocities, the sphere exhibits oscillatory motion, diving below and toward the free surface akin to underwater skipping. The sphere frequently breaks the water surface, forming air cavities. These underwater air cavities become horizontal and are attached to the sphere's surface near the flow separation point (~π/2). We employed high-speed imaging techniques to examine the hydrodynamic effects by varying the pulling angles and counterweight-induced velocity. We particularly examine the transition from oscillatory underwater skipping to the sphere's water exit, particularly above the critical Froude of 1.2. In this regime, the interaction between the buoyant sphere and the surrounding fluid becomes complex, revealing the influence of the air cavity on drag and lift coefficients and the overall hydrodynamics of the sphere. Finally, we investigate a novel phenomenon where a horizontally pulled sphere exhibits steady motion with an attached air cavity shaped like an inverted wing.