These Acoustic Tweezers Can Levitate Bits of Matter

Acoustic trapping is the phenomenon in which sound waves are made capable of moving matter thanks to the acoustic radiation force that they produce. A trained opera singer shattering a wine glass is an example of this phenomenon. Scientists have been studying this phenomenon since the 1980s, and they have used devices called “acoustic tweezers.”

"When an acoustic wave interacts with a particle, it exerts both an oscillatory force and a much smaller steady-state 'radiation' force," wrote the American Physical Society. "This latter force is the one used for trapping and manipulation. Radiation forces are generated by the scattering of a traveling sound wave, or by energy gradients within the sound field."

When tiny particles encounter this radiation, they tend to be drawn toward regions of certain pressure and velocity within the sound field. Researchers can exploit this tendency by engineering sound waves that "trap" — or suspend — tiny particles in the air. Devices that do this are often called "acoustic tweezers."

One of the common challenges of acoustic trapping is that sound waves bounce off reflective surfaces, which disrupts the device. This new acoustic tweezer has a solution to this problem: by adjusting ultrasonic transducers in order to create overlapping sound waves.

By changing how the transducers emit sound waves, the team can move the acoustic trap through space, which moves the bit of matter.

So far, the device is only able to move millimeter-sized pieces of matter with varying degrees of success. "When we move a particle, it sometimes scatters away," the team noted. Still, improved acoustic trapping and other no-contact lifting technologies — like optical tweezers, commonly used in medicine — could prove useful in many future applications, including cell separation, nanotechnologies, and biological research.