Manipulation and levitation of particles with acoustic vortices

Abstract

Acoustic waves with screw dislocations at their wavefronts, or acoustic vortices, are characterized by an azimuthal phase dependence. The time average of an acoustic vortex forms a circular region of high pressure which can be used to trap particles, and by manipulating the vortex axis, it can controllably translate them. In addition, acoustic vortices carry orbital angular momentum which can be transferred to absorbing particles causing them to rotate. This paper describes various new observations of these phenomena. In the first example, microparticles are manipulated in a host liquid at high ultrasonic frequencies (2 MHz), and a controlled translation and rotation is demonstrated. In this viscously dominated system, small particles rotate slowly with the liquid and larger particles are drawn into the centre of the vortex. The particle dynamics are explained as a fine balance between the transfer of angular momentum and the action of acoustic radiation forces. In the second example, much larger particles are levitated in air in the low ultrasonic range (40 kHz) and controlled translation and rotation is again observed. Here, viscosity is shown to play a lesser role in the particle dynamics, and conditions are observed under which the particles are rapidly ejected from the vortex core.