Acoustic levitation is a method that uses a resonant ultrasonic standing wave to suspend small objects. When placed in an acoustic field, these small objects experience a net force that can be used to levitate the objects in air.
To understand how acoustic levitation works, you first need to know a little about:
🔹Gravity: gravity is a force that causes objects to attract one another. This can be simply put through Isaac Newton's law of universal gravitation.
🔹Air: air is a fluid that behaves essentially the same way liquids do. Like liquids, air is made of microscopic particles that move in relation to one another. The particles in gasses, like the ones that make up air, are simply farther apart and move faster than the particles in liquids.
🔹Sound: sound is a vibration that travels through a medium, like a gas, a liquid or a solid object. A sound's source is an object that moves or changes shape very rapidly. The sound wave travels as the moving molecules push and pull the molecules around them. Each molecule moves the one next to it in turn. Without this movement of molecules, the sound could not travel, which is why there is no sound in a vacuum.
In summary: Acoustic levitation uses sound traveling through a fluid -- usually a gas -- to balance the force of gravity.
A basic acoustic levitator has two main parts -- a transducer, which is a vibrating surface that makes sound, and a reflector. Often, the transducer and reflector have concave surfaces to help focus the sound. A sound wave travels away from the transducer and bounces off the reflector.
Acoustic levitation is used in a variety of research disciplines, particularly in the study of phase transitions and materials susceptible to contamination, or as a stabilisation mechanism in microgravity environments.
The photo above demonstrates the acoustic levitation of a 50-mm polystyrene sphere, the first spherical object to be acoustically levitated that is larger than the acoustic wavelength.