Acoustic levitation – which is exactly what it sounds like – has seen some incredible feats in the last several years, becoming sophisticated enough to hold and keep a small object in mid-air as well as manipulating it using sound waves. Now, Japanese researchers report a new approach of achieving non-contact pickup that they believe is a world-first. They were able to pick up an object using "acoustic tweezers" (something akin to a tractor beam made of sound) from a flat reflective surface.
The team from Tokyo Metropolitan University employed a hemispherical ultrasonic array of ultrasound transducers to generate a 3D acoustic field that could trap and lift a small polystyrene ball. These arrays are commonly used in creating acoustic tweezers, but if the particle you want to pick up is sitting on something that reflects sound waves, the device fails. To circumvent this obstacle, the scientists approached the problem in a new way, detailing their breakthrough in the Japanese Journal of Applied Physics.
To overcome the limitation of technology, each channel of the array had the phase and amplitude of its soundwaves optimized. This allowed the creation of an acoustic trap right at the precise position to grab the particle without getting into the mess of dealing with the reflected soundwaves.This meant the device could pick up a little polystyrene ball from a reflective surface and move it up and down like a tractor beam. This hugely expands upon what is already possible to achieve with soundwaves.
The science behind this is fascinating. Sound waves emitted by the array overlap in a small region where their pressure is enough to keep a small object suspended in mid-air. It's also possible to move the place where they overlap and how they overlap so that the little floating object can be moved about and manipulated.
A hemispherical array of ultrasound transducers with phase and amplitude control is driven to create an acoustic field that can trap and lift a polystyrene ball off a reflective surface. Tokyo Metropolitan University
Acoustic levitation has been demonstrated in multiple ways. From a levitation gauntlet that allows you to manipulate things mid-air to robotic arms equipped with hemispherical ultrasonic arrays, to approaches that could one day potentially levitate a human.
Similar approaches to moving and suspending matter have been achieved with light for a very long time. The late Professor Arthur Ashkin won the Nobel prize in 2018 for his invention of optical tweezers, which allow the manipulation of microscopic objects. This technique is often used in chemistry and biology.
Acoustic tweezers have applications in similar fields where hands-on manipulation might damage or decrease the product one is making. It might also be useful in medicine for both non-invasive therapies and drug delivery. If there’s an approach that can benefit from not being contaminated, acoustic tweezers has the potential to be a truly revolutionary technology.