Acoustic Levitation
My final year project for my Physics MSci was to design and construct a non-resonant acoustic levitator - a device which uses sound to levitate objects.
Sound exerts radiation pressure which can be used to support objects against gravity and cause them to levitate. A tiny polystyrene ball can be seen floating in the centre of these three transducers which are emitting high intensity ultrasonic frequencies.
Acoustic levitators are most powerful when set up in a resonant system like the one shown here. The sound is reflected back and forth between the transducer and the reflector and sets up a standing wave which is holding lots of polystyrene balls in the nodes of this field.
Our challenge was to create a non-resonant system which has to be much more powerful to hold anything up as there is no standing wave, just the interference between the three sets of waves as they pass each other once. The advantage of such a system is that it doesn't rely on reflecting waves, so the object being levitated can be manipulated by changing the phase of the transducers.
It also means the sample can be heated and cooled. Heating a sample in a resonant system causes the air around it to heat up too, which changes the speed of sound in air and affects how far apart the reflecting surfaces have to be to still have an integer number of wavelengths between them to set up the standing wave.
Acoustic levitation is useful for all sorts of things in science:
- Containerless mixing - for super precise amounts of liquid to make sure nothing is left on the walls of the beaker
- Contactless containment - no contamination of samples from a beaker
- No nucleation - no points for things like ice crystals to start growing from while levitated
- A 3D environment - samples being free-floating instead of flat on a surface allows for things like realistic cell growth
We made the transducers out of aluminium and brass which sandwich together some piezoelectric rings. These rings have a voltage applied across them which causes them to vibrate at a frequency we choose, producing the sound waves which are then amplified by the metal body of the transducer.
Such a hands-on project was exactly what I wanted from my degree and the first time the little polystyrene ball jumped into the air before our eyes it was properly jaw-dropping. This device feels like something out of a sci-fi movie and was totally worth all the tiny electric shocks from the piezos.
10/10, would build again.
