Acoustic Levitation: Suspending Matter with Sound

Acoustic levitation is a technique through which small objects can be levitated in the air by sound waves. While it may seem to oppose gravity, it is in fact an application of wave phenomena such as standing waves and pressure variation. The mechanism relies upon highly controlled acoustic interference patterns that create fixed points on which particles can be levitated without touching them.

Principles of Operation
The most significant acoustic levitation concept is creating a standing wave. When a sound wave reflects off itself—between an ultrasonic transducer and a reflector most commonly—it can produce points of constructive and destructive interference. The points, or nodes (minimum pressure fluctuation) and antinodes (maximum pressure fluctuation), occur alternately at equal spacing along the wave.

If a small body, say a droplet or a polystyrene bead, is placed at or close to a pressure node, then the acoustic radiation force of the high-frequency sound waves will oppose the gravitational force on the body. The object remains suspended at a stable position in the acoustic field.

Experimental Setup
An ultrasonic levitation system can employ a single transducer or multiple high-frequency ultrasonic transducers outside the human range of hearing (usually about 40 kHz). These can be directed against a reflector in order to create a standing wave. More complex setups will use sets of transducers to move objects in two or three dimensions, which allow the items to travel or rotate under direction.

The transducers make use of the piezoelectric effect, where certain materials create vibrations when subjected to an electric voltage. The vibrations generate the high-frequency sound waves utilized in levitation.

Applications and Relevance
The advantage of acoustic levitation is that it allows contactless manipulation of materials. This is especially useful in fields where contamination must be avoided, such as the manufacture of medicines, or where fragile samples must not be handled, such as in biological or aerospace research. In microgravity environments, levitation methods such as this become even more relevant.

Broader Context
Although the forces are tiny, acoustic levitation shows how sound, which is usually thought of as a sensory or fleeting phenomenon, can be employed to create a measurable and useful force on matter. It integrates a few concepts from undergraduate physics, including wave mechanics, pressure gradients, and resonance.

In presenting this topic earlier in the semester, I was impressed by how intuitive but underappreciated these effects are. While based on well-known physical principles, the visual effect of levitation is still somewhat counterintuitive, and it is thus a powerful educational and outreach tool.