Dancing Balls Lead to a Physics Discovery

Posted on Categories Discover Magazine

Toss a few droplets of water in a hot pan and they seem to come alive, skittering to and fro as if trying to escape. Try the same thing with balls of hydrogel, and they actually could break free. The spheres bounce animatedly about a hot pan, emitting a piercing, shrieking noise as they do so.

Both tricks are due to something called the Leidenfrost effect, which describes the instantaneous vaporization that occurs when water touches a hot surface. If enough steam is produced, it can be enough to levitate water droplets above a pan, allowing them to race about without evaporating.

The same effect occurs in hydrogels, substances that are nearly entirely water, but with just a bit of a polymer to hold them together. When they touch a frying pan, however, they don’t just levitate, they full-on bounce, reaching heights up to several times their diameter. This can go on for more than a thousand bounces, and usually only ends when the hydrogel itself ruptures. Even when dropped from a very low height, the balls soon bounce higher, gaining energy from the pan instead of losing it as would be expected.

The phenomenon first came to attention thanks to a Russian YouTube user who tossed a few in his griddle after frying pancakes. This intrigued a group of researchers at Leiden University in the Netherlands, who say the behavior had never been studied before. In a paper published Monday in Nature Physics they do just that, using high-speed cameras to watch the hydrogel balls jump and scatter. It’s an entirely new form of the Leidenfrost effect, they say.

The process behaves like a tiny steam engine, they say. The burst of vapor created when the hydrogel first hits the hot pan deforms the ball, which, because of its elastic nature, is able to store the energy. The ball bends first inwards and then springs back, providing the kick necessary to propel the ball back into the air. This happens more than once; over the course of a single bounce, the gel actually contacts the surface many times, creating numerous “kicks” and forming waves that propagate around the sphere. Vapor trapped briefly under the ball being released is also to blame for the screaming noise.

The researchers say that their findings could be of use in powering soft robots of the future. Turning heat into motion via heating coils and hydrogels could provide a source of locomotion without a bulky, inflexible motor.

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