Physicists create tractor beam made of water

A water-based tractor beam that could contain oil spills and manipulate floating objects cannot yet be explained by mathematical theory.

Dr Punzmann and Professor Shats testing the tractor beam. ANU/Stuart Hay

A surprising new technique created by a team of physicists led by Professor Michael Shats at the Australian National University reveals that simple wave generators can be used to move floating objects in a number of directions -- even against the waves.

"We have figured out a way of creating waves that can force a floating object to move against the direction of the wave," said Dr Horst Punzmann from the Research School of Physics and Engineering. "No one could have guessed this result."

The technique allows the researchers to finely control objects floating in the water. Using ping pong balls, the team figured out the size and frequency of the waves required to move the object in the desired direction.

It is not, though, the waves themselves that are responsible for the movement. Rather, according to advanced particle tracking tools developed by team members Dr Nicolas Francois and Dr Hua Xia, the waves generate currents on the water's surface, which in turn move the balls.

"We found that above a certain height, these complex three-dimensional waves generate flow patterns on the surface of the water," said Professor Shats. "The tractor beam is just one of the patterns, they can be inward flows, outward flows, or vortices."

Anyone can replicate these experiments in a bathtub, but no mathematical theory yet exists that can explain them, Dr Punzmann said.

The full research can be found online in the journal Nature Physics.

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