Swiss researchers managed to make a toothpick float in the air using acoustic waves

Sound-wave-induced levitation of a toothpick

Researchers in Switzerland have managed to make particles, liquid droplets and even a toothpick levitate on acoustic waves in controlled conditions.

The team from the Swiss Federal Institute of Technology (ETH) managed to hold the objects in mid-air and manipulate them, making them rotate or react chemically.

The technique presented in the journal Proceedings of the National Academy of Sciences is completely innovative as, so far, it has only been possible to levitate objects using magnetic or electric fields, or employing the neutral buoyancy in liquids.

"Typical state-of-the-art methods of contactless handling of matter are based on electromagnetic principles and have interesting capabilities but also clear limitations in terms of particle size and/or inherent requirements of special material properties,” said Dimos Poulikakos, professor of thermodynamics at the ETH, and the project supervisor.

"Acoustic levitation is both contact-free and material-independent, also without requiring laborious sample preparation."

Contactless moving of objects can have significant implications in, for example, chemical engineering or biotechnology where contact with surfaces can spoil the chemical substances and interfere with the reaction processes.

“The process of controlled movement can run in parallel with several objects, making it interesting for industrial applications,” Poulikakos said.

Poilukakos worked on the project together with a post-doctoral student Daniele Foresti. They discovered that the basic limitation for the method to function is the object’s size, which has to correspond to half the wavelength of the acoustic wave used.

The objects are held in the air by waves between an emitting surface and a reflector. While gravity pulls the object downward, the wave pulls it upwards, resulting in the forces cancelling each other out. When the objects are trapped in stable regions of the energy field called nodes, they can be lifted and moved around with precision. The concept is similar to the air jet from a fan keeping a ping-pong ball in the air.

In theory, the concept has been known since the 1900s. Until now, however, no one has managed to control the motion of objects riding on acoustic waves in mid-air. Foresti achieved this goal by switching on multiple emitter-reflector modules in parallel next to each other. He varied the acoustic waves from module to module in order to transfer particles or droplets of liquid from one module to the next.

In a test run, Foresti used this method to move a granule of instant coffee on to a droplet of water and merge the two. In a further experiment, he mixed two droplets of liquid with different pH values, one alkaline and the other acidic; the resulting droplet contained a fluorescent pigment that glows only at a neutral pH value.

The researchers already tested the method with droplets and particles of several millimetres in diameter. The excitation of the acoustic waves has to be chosen after careful theoretical analysis - if the acoustic force exceeds the surface force of a certain liquid, the droplet is atomised explosively.

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