Dipolar and quadrupolar mode dissipation of spherical probes spinning in vortex beam acoustical tweezers

Abstract

In this work, the possibility of simultaneously trapping and rotating single polystyrene beads, or clusters, against gravity with an ultrasonic vortex beam is demonstrated. The induced rotation of a single particle is compared to a torque balance model accounting for the acoustic response of the particle. Two dominating dissipation mechanisms of the acoustic orbital angular momentum responsible for the observed rotation are examined. The first takes place in the bulk of the absorbing particle, while the second arises as the angular momentum flux is dissipated in the viscous boundary layer surrounding the particle. Importantly, even in the long-wavelength (Rayleigh) regime, it is crucial to fully model the dissipation of the dipolar and quadrupolar vibrational modes of the particle in a slightly viscous fluid such as water. Further results suggest that, while the induced outer (Eckart) rotational flow can be neglected in water, it can play an important role in other viscous and complex fluids.