Acoustic Hologram Levitates Without Physical Contact

Holograms are tridimensional light-fields that can be projected from a two-dimensional surface. (Image courtesy of Asier Marzo/Bristol University.)

A team of engineers has built a sonic “tractor beam,” coming one step closer to fulfilling a dream of science fiction fans everywhere.

According to the paper by engineers from Bristol University, “acoustic waves can exert radiation forces and form acoustic traps at points where these forces converge, permitting the levitation of particles of a wide range of materials and sizes.”

Their technique for the tractor beam uses high-amplitude sound waves to generate an acoustic hologram that is able to lift, hold and manipulate small objects.

The device uses an array of 64 miniature loudspeakers on a single-axis arrangement to create high-pitch, high-intensity sound waves. By directing this high-intensity sound, it creates a force field able to levitate small polystyrene objects.

By carefully controlling the output of the loudspeakers, the object can be held in place, moved or rotated.

As illustrated in the image above, expanded polystyrene particles ranging from 0.6 to 3.1 mm in diameter are levitated above single-sided arrays. The acoustic transducers (10 mm in 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. (Image courtesy of Asier Marzo/Bristol University.)

The team’s research shows three different shapes of acoustic force fields that work as tractor beams. The first is an acoustic force field that resembles a pair of fingers or tweezers. The second is an acoustic vortex in which the objects become stuck and then trapped at the core. The third is best described as a high-intensity cage that surrounds the objects and holds them in place from all directions.

Acoustic trap configurations. L to R: twin trap, vortex trap and bottle trap generated with a flat 20 x 20 array 12 cm above the center. (Image courtesy of Asier Marzo/Bristol University.)

Applications envisioned by the team include a sonic production line that could transport or assemble delicate objects, or a miniature version able to grip and transport drug capsules or microsurgical instruments through living tissue.

At this point, the technique can only manipulate small objects made of lightweight polystyrene, so the spaceship-towing tractor beams of Star Trek fame won’t be available any time soon.

But for the earth-based applications alone, this project is worth keeping an eye on.

The full paper is available in the journal Nature Communications and can be read here.