Acoustic tweezers don't need to touch an object to pick it up

Researchers from Tokyo Metropolitan University have devised a new technology that allows the manipulation of small objects without having to touch them. The small objects are manipulated using sound waves. Researchers used a hemispherical array of ultrasound transducers to generate a 3D acoustic field that stably trapped and lifted a small polystyrene ball off a reflective surface.The team says their technique employs a method similar to laser trapping in biology but is adaptable to a wider range of particle sizes and materials. Moving objects without touching them is not uncommon in biology and chemistry, where technology known as optical trapping has been in use for years. However, the use of laser light has drawbacks, particularly in the limits the technique places on the property of objects that can be moved. Acoustic trapping is seen as an appealing alternative because it uses sound instead of optical waves.

Sound waves can be applied to a wider range of objects and materials, allowing for successful manipulation of millimeter-sized particles. Acoustic levitation and manipulation shows promise in both the laboratory setting and beyond despite being relatively new compared to its optical counterpart. However, some significant technical challenges have to be overcome.

One of those challenges is that it's not easy to individually and accurately control large arrays of ultrasound transducers in real-time. It's difficult to get the right sound fields to lift objects far from the transducers, particularly near surfaces that reflect sound. Researchers came up with a new technique to lift millimeter-sized objects off a reflective surface using a hemispherical array of transducers. The method used to drive the array doesn't involve complex addressing of individual elements.

Rather they split the array into manageable blocks and use an inverse filter to find the best phase and amplitude to drive them to make a single trap at a distance from the transducer. The team can adjust how they drive the blocks over time, allowing them to change the position of the target field and move the particle that is trapped. The findings are supported by simulations of 3D acoustic fields created by the arrays and by experiments using a polystyrene ball.