Optimal acoustic trapping of Mie particles

Abstract

Acoustical tweezers are now widely used to manipulate micro-particles. Typically, such devices use lenses or a phased array to create a pressure field in the vicinity of the particle that acts to confine the particle. This paper uses an efficient finite element scattering model in conjunction with a global optimizer to find the phases of an ensemble of planar incident waves that maximize the minimum trapping stiffness. It is shown that for small spherical particles, a vortex of unity topological charge is optimal and for larger, Mie-sized spheres, higher topological charges are required. A symmetric trap that carries no net angular momentum also emerges from the optimization. The use of a numerical scattering model enables the optimal trap for non-spherical objects to be explored. For cylindrical objects, it is shown that vortex traps perform poorly, and in some cases fails to trap altogether. Conversely, optimal traps with uniform stiffnesses exist for all the cylinders considered. In a similar scenario, a uniform-stiffness trap is found for an object shaped as a frog and imported from a three-dimensional-printing file. Hence, this paper provides a route to improved trapping and the trapping of a wider range of objects than was previously possible.