Engineering the acoustic field with a Mie scatterer for microparticle patterning

The utilization of acoustic fields offers a contactless approach for microparticle manipulation in a miniaturized system, and plays a significant role in medicine, biology, chemistry, and engineering. Due to the acoustic radiation force arising from the scattering of the acoustic waves, small particles in the Rayleigh scattering range can be trapped, whilst their impact on the acoustic field is negligible. Manipulating larger particles in the Mie scattering regime is challenging due to the diverse scattering modes, which impacts the local acoustic field. The rapid movement of free-moving Mie scatterers in an acoustic standing wave field makes it difficult to study the interaction between a sound field and a Mie scatterer in an engineering context. Here, a combined approach that integrates theoretical analysis and experimental investigation was developed to explore the influence of a Mie scatterer on the acoustic field by fabricating an acoustic trapping device featuring a fixed Mie scatterer at its center. We demonstrate that an insonified Mie scatterer can operate as an acoustic emitter in water, enabling dynamic and versatile modulation of the total acoustic field. Such a scatterer can interact with one or multiple incident propagating acoustic waves, leading to the generation of a localized standing wave field in the vicinity of the scatterer. This local field can be controlled by the relative location of the scatterer with respect to the incident field leading to control over the transformation from an incident 1D acoustic field into a 2D acoustic field. This control paves the way for localized and multi-scale micro-object manipulation.