Orbital motion of a particle levitated in a standing-vortex acoustical trap

Abstract

Stable acoustical tweezers and particle manipulation using vortex beams have recently been accomplished in air [Nature Commun. 6: 8661 (2015)] and in water [Phys. Rev. Lett. 116: 024301 (2016)]. In both configurations, a particle is trapped along the propagation axis of a focused vortex, where the acoustic field exhibits a pressure node. More recently, a stable orbital motion of a particle trapped off-axis was achieved by alternating between two counter-rotating vortices [Phys. Rev. Lett. 120: 044301 (2018)]. However, this kind of orbital motion had not been possible so far with a steady helical field having a single angular momentum state. In this work, we present the first experimental evidence of the simultaneous 3D trapping and stable orbiting of millimeter-sized particles in air due to angular momentum transfer in a standing wave trap created by the interference of two vortices. In contrast with previous work, the two vortices have the same helicity with respect to the laboratory reference frame, and thus the angular momentum of this field is well-defined. A description of the particle kinematics as a function of different control parameters is presented, along with a comparison between experimental measurements of the field and theoretical simulations.