Acoustic Bessel beam with combined optical trapping

Whilst the main strength of optical trapping techniques is arguably its precision and dexterity, the complimentary technique of acoustic trapping offers massive scalability and potentially larger forces. Acoustic traps commonly use ultrasonic standing waves to trap particles within the nodes of a pressure field, often over distances upwards of a few cm. Here, an acoustic Bessel beam has been created using a piezoelectric cylinder whereby particles are trapped within the entire 14 mm-diameter of the transducer (1.5 cm2 trapping area). In optics, Bessel beams have the ability to trap particles over axial distances of several hundred microns. In this acoustic case, the Bessel function shape of the field is formed within the entire length of the cylinder (10 mm). Polymer spheres ranging from 1 μm to 100 μm in diameter are trapped simultaneously within the nodes of the standing wave field, in this case the concentric rings of a Bessel beam. The smaller particles within this field (< 5m) have also been trapped optically using a single beam optical tweezer, as the acoustic force scales such that it becomes comparable to that of the optical trap. This allows for a large range of particle sizes to be simultaneously trapped in a single device, and for large arrays (hundreds of mm2) to be formed acoustically within which particles can be individually optically trapped. This result demonstrates the complementarity of optical and acoustic trapping which makes it possible to trap large area arrays of particles whilst retaining the dexterity to manipulate individual particles.