New high-speed videos show that a hot sphere falls through
liquid more than twice as fast as a cold sphere, a finding that may
someday help ease high-speed submarines
through the sea.
The spheres can move faster thanks to the same phenomenon that
makes water droplets skate across a frying pan, known as the
Leidenfrost effect. A thin layer of vapour forms beneath the water
droplet and protects it from evaporating quickly, allowing it to
levitate and skitter about freely.
It turns out the effect works just as well in reverse. An
international team of physicists showed
that heating a sphere to the point where the Leidenfrost effect
takes over cuts the drag the sphere experiences in free fall by 85
The researchers, led by Ivan Vakarelski of the King Abdullah
University of Science and Technology in Saudi Arabia, heated a
steel sphere to 482 degrees Fahrenheit and magnetically suspended
it in a special liquid with a low boiling point.
The sphere immediately formed a layer of vapor about 200
micrometers thick, which rippled and bubbled around it. But when
the sphere got too cool, around 266 degrees Fahrenheit, the
protective layer dissolved in a dramatic burst of bubbles (around
35 seconds in the video on Wired.com).
To test whether that layer actually lubricates the sphere's
movement, the researchers sank balls heated to between 77 and 536
degrees Fahrenheit. Balls that were hot enough for Leidenfrost fell
at a rate of about 11.5 feet per second, more than two and a half
times as fast as the coolest spheres (video above).
The results could help develop new energy-saving technologies for high-speed underwater vehicles,
the researchers write.