Determine the true close-range acoustic interaction between levitated spheres

Determine the physical origin and quantitative magnitude of the enhancement of acoustic interparticle forces at close range, including the role of multiple rescattering beyond the first Born approximation, in order to establish the correct nonreciprocal contact force for acoustically trapped spheres.

Background

The model computes the König interaction using a first-Born, single-scattering approximation, even though the two spheres are in contact and therefore may undergo significant multiple rescattering. The paper notes that measurements and numerical calculations suggest that the true close-range interaction may be enhanced by roughly a factor of two, but the mechanism responsible for this enhancement is unresolved.

Resolving this issue would improve the quantitative predictions for the nonreciprocal force, the instability threshold, and the resulting dynamical phase diagram, especially for the strongly asymmetric bead pairs used in the simulations. The problem is included because the paper explicitly states that the underlying physics of the observed enhancement has not been settled.