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 vapor 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 percent.

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 above).

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 below).

The results could help develop new energy-saving technologies for high-speed underwater vehicles, the researchers write.

Videos: Ivan Vakarelski/Physical Review Letters

Citation: “Drag Reduction by Leidenfrost Vapor Layers.” Ivan U. Vakarelski, Jeremy O. Marston, Derek Y. C. Chan, and Sigurdur T. Thoroddsen. Physical Review Letters, Vol 106, Issue 21, May 23, 2011. DOI: 10.1103/PhysRevLett.106.214501.

See Also: