Hydrodynamics of accelerated spheres pulled along the air-water interface

We report the experimental study of floating spheres accelerated horizontally along an air-water interface. At low speeds, the spheres float on the surface, however, as speed is increased, the behavior of the spheres becomes more erratic as the spheres oscillate above and below the surface, forming horizontal cavities underwater and large skipping above. To a certain extent, the underwater horizontal cavities are attached to the sphere surface just above the equator instead of entirely wrapped around it and exhibit narrower shapes and lengths. High-speed imaging is used to capture the generation of the horizontal air cavities and the effect of the pulling angle on the transition from the floating to the skipping mode. The hollow spheres are in the subcritical Reynolds number range of Re ≈ 2 × 10⁴ to 2 × 10⁵ with density ratios between 0.5 g/cm³ and 0.65 g/cm³. We show when stable skipping modes occur and unravel the dynamics of the erratic hydrodynamic behavior.