Acoustic Tweezers with Electrical Controllability on Rotation of Trapped Particle

This paper describes an electrically controllable acoustic tweezers capable of holding and rotating mm-sized particles through varying the frequency of the electrical voltage applied to the tweezers. The tweezers is composed of a pair of vertically placed acoustic tweezers, in parallel and center aligned, so that the trapping zone of each of the tweezers set overlaps with each other's. Each single transducer is built on a 2.03 mm thick lead zirconate titanate (PZT) substrate with 18 symmetric beamforming sectors (pie shaped when viewed from top) arranged for 3 focal lengths (17.0, 18.5 and 20.0 mm) defined by air-cavity acoustic lens. Acoustic waves generated from the pair of the two tweezers produce Bessel beam zone with acoustic energy well, where a particle gets trapped and held. Once a particle is captured, rotational manipulation is achieved by fine tuning the tweezers' driving frequency, which impacts the trapping zone quite slightly, enabling gravity to provide an asymmetric force that rotates the trapped particle. Our experiments show that a trapping of mm-sized particle is achieved at 1.17 MHz driving frequency for both transducers, and tuning of the frequency by about 100 Hz generates rotation of the trapped particle. The on-demand rotational manipulation is shown to be effective in rotating mm-size polyethylene particles and 24 - 36 hours-post-fertilization zebrafish embryos that are 1.3 - 1.5 mg in weight.