February 5, 2016

Walking on Water: USU Researchers Unravel Science of Skipping Spheres

A high-speed camera captured this image of an elastic sphere bouncing off the water surface in a tank.
(photo: Chris Mabey)

(February 5, 2016)  It takes a perfect flick of the wrist and just the right angle to get a disk-shaped stone to skip across the surface of the water multiple times. So why is it so easy to get such impressive water-skipping performance from an elastic ball with only a mediocre launch?

Researchers at Utah State University’s College of Engineering say they have some answers that may offer new insight into water impact physics — an important area of study in naval applications and maritime and ocean engineering.

In collaboration with scientists at the Naval Undersea Warfare Center in Newport, R.I., and Brown University, assistant professor of mechanical engineering Tadd Truscott and his associates at USU’s Splash Lab have unraveled the physics of how elastic spheres bounce on water more easily than rigid ones. Truscott and his collaborators published their findings in the latest edition of Nature Communications — an online open access interdisciplinary journal.

Assistant Professor Tadd Truscott, right, works with graduate student Nathan Speirs at the Splash Lab. Truscott is studying
material compliance as it relates to water impact physics. 

The team uses high-speed cameras to capture images of elastic spheres bouncing across tanks of water in a laboratory. They found that elastic spheres skip along the water surface by deforming into an ideal disk-like geometry that resembles a stone one might find near the shore. Due to the sphere’s deformed shape, the water exerts a larger lifting force on elastic spheres than stones.

Truscott’s study not only reveals the physics of how elastic spheres interact with water, but also predicts how many skips will occur. In addition, the team found that elastic spheres can bounce off the water surface from much higher impact angles compared to rigid spheres — a big clue into why these elastic objects are much easier to skip.

Skipping objects along the water surface has a wide range of applications from simple aquatic toys, to naval operations like the WWII-era Wallis Bomb, or the water-walking locomotion of the Basilisk lizard.

Truscott’s setup may look like fun and games, but behind the scenes he and his team are conducting highly technical research with funding from the U.S. Navy. His work could help make inflatable boats and other soft-hull vessels safer for passengers and, on a more playful note, improve the design of water toys.

One such toy, the Water Bouncing Ball, or Waboba for short, was the inspiration for this study.

journal reference (Open Access) >>