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An unusual kind of water wave can repeatedly toss a water droplet into the air thousands of times without breaking it.

For the past decade, at the University of Chile and his collaborators have been studying how wave patterns emerge on the surface of water when it is shaken or otherwise disturbed. Recently, they made the accidental discovery that, under the right conditions, a wave can bounce a droplet of water up and down thousands of times in a row.

The finding came while Mujica and colleagues were making a type of wave called a soliton 鈥� instead of having ripples at the surface, these are formed of a single undulating bump. The team made solitons using a shaking tank of water and a ruler that drags along the surface. Sometimes, a water droplet would fall from the ruler back into the water, and occasionally one would bounce.

To better understand what was going on, the team then started purposefully pipetting single drops of water onto solitons and recording the subsequent motion with a high-speed camera. Out of thousands of experiments, the droplet bounced in 459 of them. Sometimes, this would happen thousands of times for up to 90 minutes.

For it to work, the droplet had to be added at the right time in the soliton鈥檚 undulation, says Mujica. This is because the drop deformed whenever it hit the wave, and for a long and stable juggle, the changes in the drop鈥檚 shape had to synchronise with the soliton鈥檚 motion. It didn鈥檛 merge with the soliton because a thin layer of air formed between the two when they collided.

鈥淭he conditions for the droplet bouncing are really delicate,鈥� says at the University of Li猫ge in Belgium. He says that if the droplet doesn鈥檛 bounce high enough the air layer can dissipate and the droplet and wave would merge, while if the droplet bounces too high it can fall too forcefully and break the air layer, which would also cause merging.

at the Massachusetts Institute of Technology says that the way the soliton and the droplet must play off each other to maintain stable motion is reminiscent of experiments with quantum systems. Quantum particles are oddly wave-like and must synchronise their waving with motions of objects they interact with.

One parallel is in light waves that interact with quantum particles in so-called 鈥�optical tweezers鈥�. Optical tweezers are typically used to push particles around, which inspires one practical use of the 鈥渏uggling soliton鈥�, says Mujica. 鈥淓ventually we may be able to grab drops and move them in an intelligent way,鈥� he says.