Acoustodynamic mass determination: accounting for inertial effects in acoustic levitation of granular materials

Acoustic traps use forces exerted by sound waves to confine and transport small objects. The dynamics of an object moving in the force landscape of an acoustic trap can be significantly influenced by the inertia of the surrounding fluid medium. These inertial effects can be observed by setting a trapped object in oscillation and tracking it as it relaxes back to mechanical equilibrium in its trap. Large deviations from Stokesian dynamics during this process can be explained quantitatively by accounting for boundary-layer effects in the fluid. The measured oscillations of a perturbed particle then can be used not only to calibrate the trap but also to characterize the particle. By fitting particle trajectories to a harmonic oscillator model that accurately accounts for boundary-layer effects, we can measure the density of a particle and the acoustic force on it.