Transforming acoustic waves with a chip

Acoustic waves are best known as the invisible delivery agents bringing voices, car horns or our favourite song to our ears. But the waves can also move physical objects, like an item vibrating atop a concert speaker — offering the power to turn sound into a tool.

Assistant Professor of mechanical engineering Zhenhua Tian and his team at the Virginia Tech College of Engineering have explored how to use acoustic waves as invisible grabbers to manipulate fluid flows and tiny particles on electronic chips. The work has significant potential in the medical field, where acoustic wave chips could play a role in non-invasive surgery or do the work of a centrifuge, pulling particles from blood.

A central challenge, however, has been that the standard technology that produces acoustic waves on electronic chips — a device called an interdigital transducer (IDT) — doesn’t make the kind of highly customisable curved and overlapping waves that Tian’s team needs to trap objects, route wave information, or transport fluids. The solution? Make a new wave-producing technology themselves, all contained on a chip. The research behind it has been published in Nature Communications.

Making the chip

Tian’s team uses acoustic waves to grab small objects like blood clots in the body and tiny cells in a petri dish, but the plane acoustic waves produced by an IDT didn’t make that possible. Think of it like trying to move a ping pong ball with the flat of your hand — you can roll it along a surface, but you can’t pick it up and freely move it. Tian’s team needed acoustic wave fingers for complex movement and manipulation at the microscale.

Image caption: Zhenhua Tia’s team created a number of versions of this device, capable of directing acoustic energy to move microscopic objects. Image credit: Alex Parrish, Virginia Tech.

To create crisscrossing acoustic waves tuned to work together required reimagining not only the shape of the acoustic transmitter, but also the electrodes that create the energy patterns coming out of it.

The team developed several versions of the new tool that could operate at different scales, carry different degrees of power, and generate on-chip waves with different energy profiles. Team members encoded it with a highly customisable phase distribution, enabling new ways to tilt, curve and harmonise acoustic waves. This new collection of mechanisms came together on an electronic chip, an all-in-one instrument that, with a few adjustments, could make long jets of acoustic energy with more range and power than a traditional IDT could.

The metamaterial difference

Team members didn’t just create a new tool; they created a new metamaterial for the job. Their chip is more than just a new kind of fabric or a new flavour of ice cream. It is engineered with materials and acoustics that can reshape acoustic energy to change its function.

The reason? Adaptability. Tian’s team engineered the chips to precisely control the energy flow of acoustic waves for different purposes, such as wave routing or the manipulation of fluids and particles. That offers potential applications for non-invasive surgery, biosensors, microfabrication and semiconductor cooling.

Tian’s team will continue to explore these tools’ use in new applications. There have been promising results when the PIM was deployed for controlling acoustic waves in both liquid and solids, making a wide horizon for the future of the technology.

Top image caption: (From left) Postdoctoral researcher Jiali Li, Assistant Professor Zhenhua Tian, and postdoctoral researcher Luyu Bo analyse acoustic waveforms in Tian’s lab. Image credit: Alex Parrish, Virginia Tech.