Direct measurement of forces in air-based acoustic levitation systems

TL;DR

This paper introduces a novel in situ measurement method for acoustic forces in air-based levitation systems, capable of high-resolution mapping of primary and secondary forces between objects.

Contribution

The authors develop a technique similar to an atomic force microscope that measures acoustic forces directly with high precision, including complex secondary scattering forces.

Findings

Measured acoustic potential in levitation cavity

Directly quantified scattering force between objects

Demonstrated force-displacement curve for granular raft deformation

Abstract

Acoustic levitation is frequently used for non-contact manipulation of objects and to study the impact of microgravity on physical and biological processes. While the force field produced by sound pressure lifts particles against gravity (primary acoustic force), multiple levitating objects in the same acoustic cavity interact via forces that arise from scattered sound (secondary acoustic forces). Current experimental techniques for obtaining these force fields are not well-suited for mapping the primary force field at high spatial resolution and cannot directly measure the secondary scattering force. Here we introduce a method that can measure both acoustic forces in situ, including secondary forces in the near-field limit between arbitrarily shaped, closely spaced objects. Operating similarly to an atomic force microscope, the method inserts into the acoustic cavity a suitably shaped probe tip at the end of a long, flexible cantilever and optically detects its deflection. This makes it possible to measure forces with a resolution better than 50 nN, and also to apply stress or strain in a controlled manner to manipulate levitated objects. We demonstrate this by extracting the acoustic potential present in a levitation cavity, directly measuring the acoustic scattering force between two objects, and applying tension to a levitated granular raft of acoustically-bound particles in order to obtain the force-displacement curve for its deformation.