A tiny bead hangs in the gap between two facing arrays of ultrasonic transducers. There is no thread to follow and no platform underneath it. The apparatus is supplying a sound field whose average force can support the bead against gravity.
The striking part is not merely an upward push. A useful levitator must also keep its object from drifting away. The field has to provide an equilibrium and restoring forces around it. Holding a bead in a small region of space is a more demanding task than briefly launching it into the air.
Make a pattern that stays in place
Sound in air consists of changes in pressure and particle motion. When appropriately matched waves travel in opposite directions, their interference can produce a standing-wave pattern. Some positions have small pressure oscillations; others have large ones.
A pressure node is a location of minimal pressure variation in that pattern. It is not a hole containing no air, and it is not necessarily a place where air particles never move. Pressure and particle velocity have different spatial patterns in a standing sound wave.
The object interacts with that field through scattering and the way it responds to pressure and motion. Averaged over many rapid oscillations, the interaction can produce an acoustic radiation force. Calling it an average force helps resolve the apparent paradox of an object staying near one position while the sound continually oscillates.
Suitable small particles in air can be trapped near pressure nodes. The exact equilibrium depends on the object and field, with gravity shifting where support balances weight. Describing every levitated object as sitting precisely at an ideal node would conceal that balance.
A smaller distance than the sound's wavelength
One widely used research arrangement, TinyLev, has two opposing curved arrays. A 2022 research paper using that apparatus describes 72 transducers operating at 40 kilohertz. Those numbers describe that particular device, rather than every acoustic levitator.
Using an illustrative room-temperature sound speed of 343 metres per second, divide by 40,000 cycles per second. The wavelength is about 8.6 millimetres. In an ideal standing wave, successive pressure nodes are separated by half a wavelength, roughly 4.3 millimetres in this example.
This provides a useful sense of scale when looking at a photograph. A tiny bead and a closely spaced row of suspended particles belong to a sound pattern with millimetre-scale structure. The visible gap is not evidence of a broad, uniform cushion capable of holding anything placed inside it.
Changing the frequency changes the wavelength, while changing the geometry and phase relationships changes the field. A practical trap also has to provide sideways stability. A one-dimensional drawing of high and low pressure is a starting point, not a complete description of the apparatus.
Watch for support, motion and processing
The University of Bristol's account of TinyLev links a demonstration video featuring foam and droplets. Watch it as an observation exercise: find one moment when an object remains supported, one when a small disturbance changes its position, and one where its material changes what you can see.
For a more direct research example, Foresti and colleagues' 2013 paper includes supplementary movies of contactless droplet handling. Their apparatus manipulated its acoustic field to move droplets, bring them together and merge them.
- Support: what visible evidence shows that an object stays above a surface?
- Transport: does the object move between locations while remaining suspended?
- Processing: do two droplets merely approach, or do they actually coalesce?
A single still image can establish much less than the sequence. It may show a gap without showing how long the object remained there. Distinguishing these three events turns an impressive clip into evidence about what the system can do.
The holder changes the experiment
Keeping a droplet away from a solid container can remove some interactions with a wall. That is useful when studying materials or handling tiny samples. It does not make the droplet isolated from its environment.
The sample still exchanges heat and matter with the surrounding air. It may evaporate, circulate internally or change shape. The sound field itself can influence the sample, so the method of holding it becomes part of the experimental conditions.
In the 2013 work, the researchers controlled humidity for biological droplet experiments to slow evaporation. They also measured and modelled droplet movement rather than assuming the field's intended shape guaranteed an exact trajectory.
This is an important distinction between a demonstration and a measurement tool. The demonstration establishes that suspension is possible. A quantitative experiment must establish what the suspension does to the quantity being studied.
Ask what happens after a nudge
Imagine three situations. In the first, an upward force exactly equals an object's weight at one position, but becomes smaller if the object falls slightly. In the second, the upward force grows after that small fall. In the third, there is no upward force.
Which could provide a restoring response?
The second arrangement can oppose the downward displacement. The first can become unstable: falling reduces support and encourages further falling. The third provides no levitation. A complete trap must also be checked for other directions and for the actual object's response.
No ultrasonic assembly is needed for this exercise; the research media supplies the observation. The key question is how the force changes when position changes. Acoustic levitation becomes more interesting once the floating bead is understood as a controlled relationship between wave, object and surroundings.
Sources and further reading
- University of Bristol: When Physics Girl came to Bristol
- Li et al. (2022): Building programmable multicompartment artificial cells incorporating remotely activated protein channels
- Foresti et al. (2013): Acoustophoretic contactless transport and handling of matter in air
- Simon Fraser University Sonic Studio: Standing Wave
- OpenStax University Physics: Sound Waves
