Acoustic waves have allowed for many more objects and liquids to be levitated in mid-air than ever before. Such feats are usually performed with magnets, but the objects floating on the magnets needed to have magnetic properties. ETH, a group of researchers in Zurich, successfully levitated a toothpick in mid-air and controlled its movements using acoustic waves. With the same technique they also merged droplets of liquid.
The postdoctoral researcher involved in this study at the Swiss Federal Institute of Technology is Daniele Foresti. Foresti made it possible for any object, regardless of its properties, (except maybe weight which is still at the experimental stage) to levitate in mid-air through the use of acoustic waves. This allows for the study of some chemical reactions involving processes that would otherwise be compromised with the contact of a surface.
Levitation via acoustic waves allows for a vast number of objects and liquids to be experimented on. The current methods we have that allow objects to reach a levitated state are magnetic levitation, electrical fields, and suspension in liquids with the help of buoyancy. The problem with magnets is the difficulty of controlling the movement of liquids. The liquids are required to have some sort of magnetic properties. Floating liquids in another liquid requires the use of immiscible liquids (not forming a homogeneous mixture when added together) such as oil and water. Acoustic waves take care of these limitations. The only limitation is the maximum diameter of the object being levitated. It has to correspond with half the acoustic wavelength being used. Levitation is achieved when all of the acting forces (gravity and acoustic waves) reach equilibrium. The force of the acoustic waves counteracts the force of gravity, which allows this state to occur.
Making Coffee on Acoustic Waves
The scientific term used to describe this phenomenon is the acoustic radiation pressure effect. This is not a new discovery, rather an alteration to a previous one that goes back to the findings of Rayleigh. However, unlike in previous studies, we can now control movement with acoustic waves. This was done by putting together multiple modules that create the acoustic waves and by switching them on to vary their wave action. This varying process is how liquid is able to move from one module to the next. Foresti used this method to merge a water droplet and a bit of instant coffee together to form coffee!