Selective assembly of particles using synchronized acoustic tweezers with oblique incidence

The precise assembly of microparticles and cells shows considerable promise in areas such as bubble-bubble dynamics, reproductive cell selection and fertilization, microchemical reaction measurement, and beyond. Recently, a synchronized acoustic tweezer configuration was theoretically proposed to selectively trap individual particles and subsequently assemble them at a single location [Gong and Baudoin (2019); Gong and Baudoin (2020)]. However, experimental implementation of this method has been limited because the physical overlap of the two acoustic transducers disrupts the effective superposition of the two vortex beams. To address this challenge, we propose an improved design in which the axes of the vortex beams are rotated using oblique incidence. This arrangement ensures (i) effective superposition of the two vortex beams for particle trapping and assembly, and (ii) the feasibility of bringing the two transducers into close proximity without interference. The three-dimensional acoustic radiation forces acting on Rayleigh particles are calculated via Gor'kov potential theory. Numerical simulations performed with different tilt angles of the vortex beams confirm the feasibility of assembling two selectively trapped particles into one spot. This work provides guidance for future experimental designs and demonstrations of particle assembly using acoustic tweezers.