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Watch Sound Waves Levitate Small Objects In Different Directions

A new type of acoustic levitation called "holographic acoustic tweezers" is mesmerizing to watch.

by Daniel Oberhaus
Dec 19 2018, 5:46pm

Image: YouTube

Researchers from the UK and Spain have developed a device that can simultaneously levitate individual objects ranging from micrometers to centimeters in size in different directions using only sound waves.

“Acoustic levitation,” the ability to move particles using only sound waves, has been explored by researchers for years, but until now it has only been able to move small objects along one axis at a time.

In a study published this week in Proceedings of the National Academy of Sciences, the researchers dubbed their technique “holographic acoustic tweezers” in homage to its likely use cases. They say their device could be used to manipulate tiny biomedical devices inside the human body without invasive surgery or to do advanced manufacturing at incredibly small scales.

When a normal sound wave is produced, it has peaks and valleys. If you imagine a particle riding along that wave, it would ride along the peaks and valleys from point A to point B. A standing wave, by contrast, is produced when a wave is reflected off of something back toward itself, or is offset by a second wave.

This interference results in the wave having a number of fixed points. Acoustic levitation works when an acoustic system produces a standing wave and objects are trapped in these fixed points, or nodes. Here’s what a standing wave looks like, with the nodes marked in red below:

File:Standing wave.gif

A similar principle is at work in the new research, which uses arrays of ultrasonic (beyond the range of human hearing) emitters to create nodes where particles can be suspended in three-dimensional space.

These nodes can then be manipulated to form a desired arrangement of particles, or to perform complex maneuvers using algorithms which subtly change the way the ultrasonic waves are produced by the array.

According to the study, the researchers were able to use their device to manipulate up to 25 millimeter-scale objects in three dimensions.

This may become a common tool for doctors in the future, but for now the research is confined to the laboratory, where it looks more like an art project than the future of medicine.