Brit boffins build 'tractor beam' out of sound

Researchers from Spain and the British city of Bristol have found a way to move objects using sound.

In their Nature Communications paper, the six researchers explain that “Sound can levitate objects of different sizes and materials through air, water and tissue ... This allows us to manipulate cells, liquids, compounds or living things without touching or contaminating them. However, acoustic levitation has required the targets to be enclosed with acoustic elements or had limited manoeuvrability.”

The paper describes a new “single-sided emitter” that can “translate, rotate and manipulate particles.”

The research also introduces an “holographic acoustic elements framework that permits the rapid generation of traps and provides a bridge between optical and acoustical trapping. Acoustic structures shaped as tweezers, twisters or bottles emerge as the optimum mechanisms for tractor beams or containerless transportation.”

Here's how Bristol University explains one goes about making acoustic pincers:

The researchers used an array of 64 miniature loudspeakers to create high-pitch and high-intensity sound waves. The tractor beam works by surrounding the object with high-intensity sound and this creates a force field that keeps the objects in place. By carefully controlling the output of the loudspeakers the object can be either held in place, moved or rotated.

That array of loudspeakers is depicted in the video below and the remarkable thing about it is that they're all in the one unit: previous attempts at this kind of thing have required more than one speaker array.

Youtube Video

The paper and video both describe small plastic beads being controlled, so this is obviously not something that is going to be used to draw the Millennium Falcon into the Death Star any time soon. Not that the authors care: their current goal is “in vivo manipulation since the device could be applied directly onto the skin with the manipulation taking place inside the body; similar to an ultrasound scanner but for manipulating particles”. Targeted drug delivery and moving your kidney stones around are among the applications the researchers think could emerge from their work. ®

^* Expanded polystyrene particles ranging from 0.6 to 3.1 mm diameter are levitated above single-sided arrays. The acoustic transducers (10 mm diameter) are driven at 16 Vpp and 40 kHz. (a–c) The particles can be translated along 3D paths at up to 25 cm s−1 using different arrangements and without moving the array. (c–e) The traps are strong enough to hold the spheres and counteract gravity from any direction. (f) Asymmetric objects, such as ellipsoidal particles, can be controllably rotated at up to 128 r.p.m. Scale bars represent 2 mm for the particle in a and 20 mm for the rest. ®