Dynamics of levitated objects in acoustic vortex fields

Abstract

Acoustic levitation in gaseous media provides a tool to process solid and liquid materials without the presence of surfaces such as container walls and hence has been used widely in chemical analysis, high-temperature processing, drop dynamics and bioreactors. To date high-density objects can only be acoustically levitated in simple standing-wave fields. Here we demonstrate the ability of a small number of peripherally placed sources to generate acoustic vortex fields and stably levitate a wide range of liquid and solid objects. The forces exerted by these acoustic vortex fields on a levitated water droplet are observed to cause a controllable deformation of the droplet and/or oscillation along the vortex axis. Orbital angular momentum transfer is also shown to rotate a levitated object rapidly and the rate of rotation can be controlled by the source amplitude. We expect this research can increase the diversity of acoustic levitation and expand the application of acoustic vortices.

Figure 1

Potential-well structures of first-order acoustic vortex fields generated by three to six peripherally placed acoustic sources. The symbol “S” denotes an acoustic source. (a–d) The potential-well structures are displayed as three-dimensional isosurfaces of the normalized Gor’kov potential, (e–h) normalized Gor’kov potential in x-z plane and (i–l) x-y plane.

Figure 2

Photographs (top view) of levitated expanded polystyrene objects used to visualize the experimentally achieved potential-well structures.

Figure 3

Simulation of the acoustic radiation force on a rigid sphere in acoustic vortex field. Results are for the 4-transducer arrangement and a rigid sphere of radius 1.5 mm. (a) Effect of device dimension, R, on the vertical component of acoustic radiation force obtained by respectively integrating acoustic radiation pressure over the top (z > 0) hemisphere (F _t) and bottom (z < 0) hemisphere (F _b) of a rigid sphere located at (0,0,0). Using R = 0.92λ, the insert shows the distribution of the normalized Gor’kov potential in the x-z plane. (b) z-component of the total force on the sphere as a function of location on the z-axis. (c) The x-component of force on the sphere as a function of location on the x-direction. (d) The x-component force on the sphere as a function of location on the line (0.1λ,0,z).

Figure 4

Levitating dense objects in acoustic vortex fields (top view). (a–d) The levitated objects are silicone oil, silicone bubble, water droplet and Chimonanthus (Wintersweet) flower, respectively. Note that the array with three transducers is used for (a and b) and the array with four transducers for (c and d).

Figure 5

Drop oscillation in acoustic vortex field in air. (a) The drop formation process and the vertical drop oscillation in the central trap. The dashed line across the images gives the stable height of the final drop. (b) Drop shape oscillation in the central trap. The vortex field is generated by four transducers.

Figure 6

High-speed spinning of an expanded polystyrene particle in acoustic vortex field. The vortex field is generated by four transducers. The particle of radius 1.5 mm is levitated in the central trap of the acoustic vortex field. (a) The experimental rotation rate of the particle as a function of the voltage applied to drive the transducers. Rotation measured using a high speed camera (see Methods). (b) The simulated phase distribution of the acoustic field in the x-y plane. (c) The simulated normalized density distribution of the orbital angular momentum in the x-y plane.