Acoustic levitators are already pretty intriguing devices, in that they use opposing sound waves to suspend small objects in mid-air. Now, however, scientists from Brazil's University of São Paulo have created what they claim is a better acoustic levitator. It's less fussy about the exact orientation of its components, making it more feasible for use in practical applications.

In a typical acoustic levitator, ultrasound waves generated by a transducer travel downwards to a concave reflector located a short distance away. The reflected waves subsequently travel back up towards the transducer. Midway between the transducer and reflector, however, the upward-traveling reflected waves meet up with downward-traveling newly-emitted waves (in some cases, two transducers are used instead of a transducer and a reflector).

The point where the waves interfere with one another is known as a standing wave, and the pressure created at that point is sufficient to hold small, light objects in place, such as drops of liquid or even tiny pieces of growing cartilage.

Usually, the transducer and reflector must be perfectly aligned and kept a precise distance apart, otherwise the sound waves won't resonate properly and the standing wave will collapse. The unique geometry of the São Paulo device, however, allows it to operate as a non-resonant acoustic levitator. This means that both the alignment and the distance between its two main components can be changed – even while it's suspending an object – without creating any problems. Additionally, by changing the orientation of the reflector, the hovering object can be moved around in mid-air.

The device in action (left) and a simulated standing wave pattern (right)

So far, the device has only been used to suspend 3-mm polystyrene spheres. It is hoped that once further developed, however, it could be used to levitate larger items, such as hazardous materials, pharmaceuticals, or other things that would be best left untouched.

It could also possibly find use in the field of manufacturing. "Modern factories have hundreds of robots to move parts from one place to another," said project leader Marco Aurélio Brizzotti Andrade. "Why not try to do the same without touching the parts to be transported?"

A paper on the research was recently published in the journal Applied Physics Letters.

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