Levitating objects .... with sound!

It has been known for 100 years that sound waves can exert a force (the acoustic radiation pressure effect) but not until now has this principle been used to manipulate physical objects.

I'll keep this post pretty short because the maths involved is pretty obscene but for those who are slightly masochistic and would like to take a look, the paper by Foresti et al. was published yesterday in PNAS and can be viewed here.

The basic principle used here is there is an emitting platform (that as the name suggests emits the sound wave) and then placed opposite that, there is a reflecting platform and these platforms are placed at particular places based on some complex math. The figure below is a diagram of the apparatus taken directly from the paper.

Setup of droplet manipulator (Foresti et al. (2013)

I won't get into it but take my word that the design of this system and the calculations involved is an example of some beautifully thought out elegant science and props to those involved in its development.

Now, it's all very well to make an object levitate using a sound wave, but they go one step further and actually move the object with these sound waves which again has some pretty complex equations behind it. The sound emitting mechanism is made with piezoelectric crystals, which shrink or stretch depending on the voltage applied to them and hence can alter the sound wave produced quite finely and accurately.

As a proof of principle they did several awesome things! First, I will show you the video of them moving and mixing droplets of water seen in the video below:

Then as true scientists, they wanted to make this useful for something very important to most scientists ... Making coffee ... Check it out:

Yep, they are mixing a granule of instant coffee and water in thin-air to make a drop of coffee! That video is slowed down by 100-times so you can see the fine details.

Next they took a step larger:

I don't know about you, but I find that pretty incredible!

The applications for this are not as wide-ranging as you might think unfortunately. The object being levitated and the sound waves have a very intimate relationship, the diameter of the object needs to half of the wavelength of the soundwave, so big objects are out of the question at this stage. However, it does have some exciting applications for chemistry as many chemical reactions and science experiments are influenced by the chemistry of being on the surface of something so we can now investigate questions free from surface interference.

So aside from making coffee in mid-air, the benefits of this amazing technology remain to be reaped by scientists ... For now.


Foresti D., Nabavi M., Klingauf M., Ferrari A. & Poulikakos D. (2013). Acoustophoretic contactless transport and handling of matter in air, Proceedings of the National Academy of Sciences, 110 (31) 12549-12554. DOI: 10.1073/pnas.1301860110