Single beam acoustical tweezers based on focused beams: A numerical analysis of 2D and 3D trapping capabilities

TL;DR

This paper numerically investigates the trapping capabilities of focused acoustic beams, demonstrating 3D and 2D trapping of various particles, droplets, and microorganisms based on their properties and resonances.

Contribution

It provides a comprehensive numerical analysis of focused acoustic beam trapping, expanding understanding beyond previously limited experimental reports of 3D traps.

Findings

3D trapping of more compressible particles is possible beyond Rayleigh regime

2D trapping of particles with positive contrast factor can be achieved via resonances

Focused beams can trap a wide range of particles, droplets, and microorganisms

Abstract

Selective single beam tweezers open tremendous perspectives in microfluidics and microbiology for the micromanipulation, assembly and mechanical properties testing of microparticles, cells and microorganisms. In optics, single beam optical tweezers rely on tightly focused laser beams, generating a three-dimensional (3D) trap at the focal point. In acoustics, 3D traps have so-far only been reported experimentally with specific wavefields called acoustical vortices. Indeed, many types of particles are expelled (not attracted to) the center of a focused beam. Yet the trapping capabilities of focused beams have so-far only been partially explored. In this paper, we explore numerically with an angular spectrum code the trapping capabilities of focused beams on a wide range of parameters (size over wavelength ratio and type of particles). We demonstrate (i) that 3D trapping of particles, droplets and microorganisms more compressible than the surrounding fluid is possible in and beyond Rayleigh regime (e.g. polydimethylsiloxane, olive oil, benzene, and lipid sphere) and (ii) that 2D trapping (without axial trap) of particles with positive contrast factor can be achieved by using the particles resonances.