Levitating a sphere similar in size to the acoustic wavelength

Acoustic levitation, the use of sound waves to suspend objects in mid-air, is gaining momentum as a contactless tool, enabling researchers to avoid contamination in applications like pharmaceutical analysis and electronic microassembly.

However, most techniques are limited to objects smaller than half the sound wavelength. It is difficult to precisely distribute the acoustic pressure across the surface of the object to create enough suspending force, as well as restoring forces, in all directions.

To tackle this issue, researchers used a single-beam levitation technique to lift a sphere that is similar in size to the acoustic wavelength. They achieved this by employing a computationally fast numerical sound field model based on spherical harmonics expansion to calculate the acoustic radiation force on the sphere.

They formulated an optimization problem that maximizes sphere stability while keeping the net force balanced. In experiments, they used a 16-by-16 array of 40 kilohertz transducers to levitate a polystyrene sphere with a diameter of 8.9 millimeters. They discovered the sphere is stably levitated in a twin tuning forks trap that results from a superposition of two twin trap signatures and a bottle trap signature.

“Our findings could possibly facilitate the dynamic manipulation of larger objects in mid-air by superimposing multiple signatures to form an acoustic trap at the desired location, rather than iteratively solving a computationally expensive optimization problem,” author Sebastian Zehnter said. “To move the particle around, one only needs to translate the trap signature toward a new location.”

Furthermore, it could also be possible to rapidly translate and rotate macroscopic objects by gradually shifting and rotating the holographic signature of the acoustic trap towards the next desired position or orientation.

Source: “Acoustic levitation of a Mie sphere using a 2D transducer array,” by Sebastian Zehnter, Marco A. B. Andrade, and Christoph Ament, Journal of Applied Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0037344 .