Acoustic arrays levitate more sizeable spheres

Acoustic levitation uses sound arrays to make small objects hover. Combining several floating components together could lead to new 3D visualizations with each object as a sort of pixel. Acoustic levitation could also be useful in experiments as a trap for chemicals that require clear surroundings and limited contact with other materials.

However, the larger an object, the more difficult it is to levitate. The acoustic array must accordingly increase in size, and the computation time for controlling its driving signals becomes costly. Reducing that cost is important for controlling the levitating objects in real time.

Andersson developed a mathematical method to stabilize the levitation of a 20mm sphere using less computation time than previous experiments. By optimizing a physics-based cost function, Andersson determined the best driving signals for the acoustic array.

“Essentially, we target the force field in a meaningful way using the cost function,” said author Carl Andersson. “We say the net force on the object has to be zero to counteract gravity. Then we want the force to converge towards where we want the object to be, so that if it moves away, then it’s actually pushed back.”

They explicitly make the force field conservative to prevent a buildup of inertia or momentum. A softplus transform was used to polish the results, making the optimization more robust by considering each criterion less important as it became close to fulfilled.

“If you scale up the array and scale the levitating distance so that you preserve the geometry, you should be able to go bigger, in terms of levitating object size,” said Andersson.

Source: “Acoustic levitation of multi-wavelength spherical bodies using transducer arrays of non-specialized geometries,” by Carl Andersson, The Journal of the Acoustical Society of America (2022). The article can be accessed at https://doi.org/10.1121/10.0010358 .