Acoustic fields used to levitate polystyrene ball

Optical trapping is a technology which has allowed scientists to move tiny objects around for many years, enabling manipulation with no contact. In fact, one of the recipients of the 2018 Nobel Prize for Physics, Arthur Ashkin, was recognised for his work on optical trapping. Despite the many possibilities opened up by this technology, it has some limitations. Chiefly, it places limits on the properties of objects which can be manipulated.

Acoustic trapping is an alternative technology which uses sound waves instead of light waves; the technology is sometimes referred to as 'acoustic tweezers'. Scientists hope that they may be able to manipulate individual cells and other tiny objects using the technology. Sound waves, unlike light waves, can be applied to a wider range of object sizes and materials, even for particles of millimetre size.

Ultrasound frequencies tend to be used for manipulating particles, as the very high amplitude of sound required to produce the necessary forces to counteract gravity would otherwise produce noise at unbearable volumes.

While acoustic tweezers have not been around for as long as their optical counterparts, the technology has great promise for lab settings and beyond. However, some technical challenges remain. In particular, it is difficult to individually and accurately control huge arrays of ultrasound transducers in real time and produce the correct fields to lift objects far from the sources themselves (particularly near reflective surfaces).

Researchers from Tokyo Metropolitan University developed a new approach for lifting millimetre-scale objects from a reflective surface, using a hemispherical array of transducers. Rather than addressing individual elements, their approach splits the array into more manageable blocks and uses an inverse filter to identify the best phases and amplitudes to produce a 3D acoustic field to trap objects some distance from the transducers themselves.

By adjusting how they drive the blocks over time, they can change the position of their target field and move the particle they have trapped. Their findings are supported by simulations of the 3D acoustic fields that are created by the arrays and through a real-world demonstration using a polystyrene ball.

Although challenges remain in keeping particles trapped and stable, this exciting new technology promises big advances towards transforming acoustic trapping from a scientific curiosity to a more practical tool.