Mie particle assembly by a converging ultrasound field and acoustic interaction forces

Abstract

There has been growing interest in exerting radiation forces to trap and cluster randomly distributed cells in body fluid, micro-particles in water, or microorganisms in fluid-like culture media. Acoustic standing waves are extensively utilized as a patterning tool to assemble particles at nodes or anti-nodes but, for frequencies above megahertz, the nodal distances become too small for particle separation. We study a mechanism to propel particles towards one central space by creating a converging wave field without significant reflections. This is achieved by strong decay of the main ultrasound beam by multiple scattering. When two opposing traveling waves are suitably decayed towards their meeting point, a converging wave field is created for particle assembly via radiation forces. This paper describes the theoretical prediction model based on the translation addition theorem and the partial-wave expansion method. The predicted acoustic pressure fields are compared with full numerical simulations for a limited number of particles, and the attenuation coefficient is validated by the existing experimental data. The results demonstrate that the converging wave field is formed, and the acoustic radiation force vectors in a bulk acoustic wave (BAW) device are directed towards the central space for meaningful clustering of micro-particles from their host fluid.