Sound waves levitate and move objects : Nature News & Comment


Contact-free manipulation could help to protect samples from contamination.

Sodium and water are pushed together by sound waves.


Water droplets, coffee granules, fragments of polystyrene and even a toothpick are among the items that have been flying around in a Swiss laboratory lately — all of them kept in the air by sound waves. The device that achieves this acoustic levitation is the first to be capable of handling several objects simultaneously. It is described today in the Proceedings of the National Academy of Sciences1.

Typically, levitation techniques make use of electromagnetism; magnetic forces have even been used to levitate frogs2. It has long been known that sound waves could counter gravity, too, but so far the method has lacked practical application because it could do little more than keep an object in place.

To also move and manipulate levitating objects, Dimos Poulikakos, a mechanical engineer at the Swiss Federal Institute of Technology (ETH) in Zurich, and his colleagues built sound-making platforms using piezoelectric crystals, which shrink or stretch depending on the voltage applied to them. Each platform is the size of a pinky nail.

The platforms emit sound waves which move upward until they reach surface lying above, where they bounce back. When the downward-moving reflected waves overlap with the upward-moving source waves, the two ‘cancel out’ in the middle, at so-called node points. Objects placed there remain stuck in place because of the pressure of sound waves coming from both directions (see picture below).

A liquid droplet is levitated in the space between sound-emitting platforms (bottom) and a sound-reflecting surface (top).

Written By: Josh Howgego
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  1. I think I am seeing that exothermal ( exothermic? ) reactions in those flashes when the sodium metal ( highly reactive ) come in contact with water.

    Now, how cool is this? I can see biological applications here. Instead of the contamination that can result in plating bacteria you just suspend the bacteria and it’s food in an aseptic center space.

  2. I’m curious what the restrictions are for mass. Just how big can an object be? If you could produce enough magnitude to levitate a person, how much damage would that sound cause to the person or surrounding area?

  3. When one hears “levitation,” it’s easy for the mind to run wild with possibilities. While the described application for this technology seems limited, I think the real innovation is the manipulation of the complex waves necessary to accomplish this level of control.

    Yes, the size of the safe nodal area would be somewhat relative to the fundamental frequency of the standing wave. So you’d need a lot of power to generate a low enough frequency to safely lift something large. Also, without going into the math, it takes a lot of dBs before you get PSIs. More mass would require more power. At some point, air pressure is more efficiently created by turbines.

    While I concede that this innovation would not work in a vacuum, I must point out that the technology doesn’t require that air be the carrier of the sound. Since “sound” is really just a description of patterns of movement within a system, the same harmonic manipulation could be used in any media, provided that one has an efficient transducer to excite the media. The same levitation isolation could be used under water, in tanks of inert gas, or possibly even in piles of aggregate or corn.

    Also, since all waves behave similarly, even when they are non-mechanical, what application does this technology have in the electromagnetic manipulation of waves? I dare not speculate across such a spectrum.

    I wonder if the people at Planter’s peanuts could use this to negate the granular convection in their mixed nuts? I just don’t like Brazil nuts.

    Whatever comes of this new tech, bravo to its innovators.

  4. Before I give my opinion, just a quick note: I lived and worked for a few short months near Zurich.

    I am not in the least surprised that, in Switzerland, adult scientists are required to make up for the straight-laced upbringing to which most Swiss children are subjected.

    Those of us who live in the rest of the Free World did these experiments, without adult supervision, before we entered High School, no?


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