These sound waves can levitate and move particles in new ways

A new device captures particles within a sound field that can be shifted three dimensionally

ultrasonic speaker

UP IN THE AIR  A new levitation device uses ultrasonic speakers to create “sound fields” that contain regions of high-intensity sound (white) and low-intensity sound (black) that can trap particles (green) in midair.  

A. Marzo and B.W. Drinkwater/PNAS 2018

A new machine uses ultrasonic waves to make particles dance in midair like marionettes on (invisible) strings.

Unlike other devices that also use sound radiation to manipulate matter, the new system can move several objects in different directions at once (SN: 4/19/14, p. 8). This kind of levitation technology, described online the week of December 17 in Proceedings of the National Academy of Sciences, could assemble microelectronics or maneuver small objects inside the body for medical treatment.

In the new device, an array of 256 speakers, each about 1 centimeter wide, faces another, identical speaker array across a distance of 23 centimeters. The speakers emit sound waves at frequencies too high to hear, creating an intricate sound field between the two arrays. This sound field has high intensity sound regions that repel particles, and pockets of relative quiet that trap particles. By controlling the timing of the ultrasonic waves released by each speaker, researchers can shift these quiet, particle-toting regions around in three dimensions.

ULTRASONIC CHOREOGRAPHY Unlike other devices that could levitate only one object or move around particles as a group, a new device (left) uses ultrasonic waves to independently steer up to 25 millimeter-sized particles in different directions simultaneously (shown here manipulating six foam balls). The device (shown in a computer simulation on the right) uses two grids of ultrasonic speakers (rainbow-colored dots at top and bottom) to create sound fields between the speaker arrays. In these sound fields, particles get trapped in relatively quiet regions (black) between regions of high intensity sound (white).

While laser tweezers can steer much smaller microscopic objects, sound waves can hoist micrometer- to centimeter-sized cargo (SN: 10/27/18, p. 16). The new device’s 40-kilohertz sound waves can juggle up to 25 foam beads at a time, each 1 to 3 millimeters across. A future version that emits higher frequencies could nab smaller objects, like cells that are mere micrometers across.

The device’s ability to move many particles in all three dimensions represents “an excellent progression” in acoustic levitation technology, says Christine Démoré, an ultrasound scientist at Sunnybrook Research Institute in Toronto not involved in the work.

Since sound waves travel through tissue, acoustic tweezers may someday deliver drugs to specific organs, clear out kidney stone debris or steer implanted medical devices to new locations inside the body, says coauthor Bruce Drinkwater, an ultrasonic engineer at University of Bristol in England.

The technology could also facilitate hands-off construction of delicate microelectronic components, says coauthor Asier Marzo, a computer scientist at the Public University of Navarre in Pamplona, Spain. Or it could be used in manipulating levitated particles into free-floating, 3-D images to create futuristic displays à la Princess Leia in Star Wars (SN: 2/17/18, p. 16).

SURFING SOUND WAVES Inside a new levitation device, ultrasonic sound waves can push and pull several particles in different directions simultaneously. This kind of technology could someday allow completely hands-off construction of small electronics or manipulation of medical devices inside the body.

Previously the staff writer for physical sciences at Science News, Maria Temming is the assistant managing editor at Science News Explores. She has bachelor's degrees in physics and English, and a master's in science writing.

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