A combination of speed and near-horizontal orientation is essential for getting a stone to skip across the surface of a body of water. As the stone reaches the surface, it gets thrust back skyward by a cushion of compressed air (see Physics Today, December 2014, page 70). Now Tadd Truscott of King Abdullah University of Science and Technology in Saudi Arabia and his colleagues have demonstrated that some objects can skip underwater. When propelled horizontally at the surface with sufficient speed, buoyant spheres can descend and traverse the water with a wavelike motion, as if they are skipping.
Truscott and his team performed their experiments inside a laboratory water channel monitored by high-speed cameras. They attached fishing line to a hollow aluminum ball, whose density was 75% that of water, and threaded it through a pulley system. The researchers could adjust both the speed of the ball, by varying the mass of a counterweight on the pulley, and the angle at which the ball was pulled.
At low speeds, the partially submerged ball would simply get dragged along the surface. Things got more interesting as the counterweight mass increased. Truscott and colleagues observed as the speedier ball descended, turned upward, and then dipped again just before reaching the surface (see panel I of the figure). The researchers attribute the “underwater skipping” to the forces generated by vortices in the ball’s wake.
The motion of the ball changed when it was pulled at a slight angle. After an initial dive, the ball would ascend and briefly surface before diving back down (see panel II and the video). Because it pierced the surface, the ball generated an air cavity, which the ball then pulled underwater and dragged along throughout its undulating journey. Ultimately, the ball rose to the surface and, buoyed by the trailing air cushion, launched out of the water (see panel III), much like a perfectly thrown skipping stone. (F. Kamoliddinov et al., Phys. Fluids 35, 072106, 2023.)
