A blue close-up image of particles moving through a bright channel

Acoustic tweezers put the pinch on particles

There are no physical structures in this video. The “walls” guiding the particles through the liquid are actually a complex combination of sound waves created by a new technique called a shadow waveguide. Credit: Junfei Li, Duke University

A ‘shadow waveguide’ sounds like something out of a sci-fi movie, but it’s actually a technique developed by engineers to manipulate matter with sound.

Researchers from Duke University in the US have just come up with a new type of acoustic tweezers, which use sound waves to move small objects without ever touching them. They have applications spanning from micro-robotics to drug delivery to biomedical science (such as targeting tumours or performing surgery).

This new method employs complex acoustic patterns to control tiny particles suspended in liquid. As described in a paper in Science Advances, the team used two sound sources to create a tightly confined acoustic field within a chamber and move the particles through it.

Previous techniques have demonstrated that acoustic tweezers can trap, rotate and move a variety of particles. But there have been limitations – current set-ups often use multiple sound sources that influence particles in lockstep with each other. Manipulating particles independently has been achieved by building solid channel structures within the chamber, but this can damage particles and slow their movement.

This new shadow waveguide technique can influence individual particles without the need for any internal structures.

“We wanted to inject acoustic wave energy into the chamber and use a structure just outside of the chamber to control the shape of the sound waves inside,” explains co-author Steve Cummer, from Duke University.

“The result is sort of like an optical fibre for sound that shapes the sound propagation and intentionally leaks some of its energy into the chamber – a sort of sound shadow – to control the particles inside with virtual channels.”

The shadow waveguides are created by 3D-printed moulds, filled with polydimethylsiloxane (PDMS), a type of silicone, which has air channels built into it. These channels dictate where and how sound waves enter the chamber, and thus how particles should be controlled. The guiding mould remains outside the chamber, but PDMS has very similar properties to water, allowing sound waves to easily travel from the guide into the chamber.

Using this set-up, the team placed two sound sources at either end of the chamber and used them to move particles across it, demonstrating precisely controlled speed along complex paths.

The next step: flexibility.

“Acoustic devices are very difficult to make reconfigurable, but we would love to figure out a way to make that possible because it would be a dramatic improvement in this technique’s usability,” says co-author Junfei Li, also from Duke.

“For now, we’re looking for specific challenges that we could adapt these shadow waveguides to address, to move it from a proof-of-concept demonstration to a more sophisticated application.”