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Dancing droplets reveal physics at work

Magnetic fields can deflect liquid oxygen

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12:27pm, May 17, 2012
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Throw water into a hot pan, and it will sizzle so fast that the drops actually levitate across the surface. Physicists have now taken this phenomenon, called the Leidenfrost effect, a step further: Using magnets, the scientists directed droplets of liquid oxygen to speed up, slow down and change course as they scoot across a sheet of glass.

Magnetic fields force the tiny blobs to travel in a mesmerizing dance, says David Quéré, a physicist at ESPCI Paris Institute of Technology in France. He and his colleagues describe the work in an upcoming Physical Review E.

Leidenfrost drops form when a drop hits a surface much hotter than the liquid’s boiling temperature. The liquid evaporates so quickly that the droplet starts to float on its own vapor, cushioned from below. This insulating layer also reduces friction between the droplet and the surface. Given a push, a droplet 1 millimeter across can slide for several meters before finally slowing down and stopping.

“There are some interesting questions about these frictionless systems,” Quéré says. So he decided to look at how magnetic fields might influence this bizarre motion. He chose to study oxygen because it boils at such a low temperature that it naturally forms Leidenfrost drops at room temperature. In its liquid form, oxygen is also paramagnetic, meaning that it is attracted to the poles of a magnet.

The scientists threw oxygen droplets onto a sheet of glass, beneath which lay a small circular magnet. Depending on the angle at which the drop approached the magnet, the drop either got deflected away or swooped partway around the magnet like a comet swinging past Jupiter. Occasionally, the magnet’s field captured a droplet into a permanent orbit. “You can hang these drops on the ceiling if the magnet is strong enough,” Quéré says.

Friction between the drops and the surface changes as the droplets change shape, he adds — allowing scientists to better explore the forces that control friction.

The Leidenfrost effect occurs in many industrial processes that involve liquid droplets and hot surfaces, and so physicists have long wanted to better understand and control it, says Tuan Tran, a physicist at the University of Twente in the Netherlands. In 2006, scientists reported using a textured surface to dictate the way drops moved. The new work using magnets, Tran says, “could be seen as a second cornerstone in controlling Leidenfrost droplets.”

The team is now investigating how water drops containing magnetic-sensitive particles behave in the same situation, which may open the door to more practical applications.

An oxygen droplet floating on its vapor (approaching from left at 12 centimeters per second) can be deflected by a small magnet (moving from bottom center at 7 centimeters per second) held beneath a sheet of glass. Such movies reveal the elusive behavior of droplets subject to what’s known as the Leidenfrost effect.
Credit: K. Piroird et al/Physical Review E 2012

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