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.