Agglomeration of particles by a converging ultrasound field and their quantitative assessments

Abstract

The acoustic radiation force resulting from acoustic waves has been extensively studied for the contact-free generation of organized patterns. Microscopic objects normally cluster at pressure nodes, but the size of the clusters is restricted by the saturation limit. Here, we present a two-dimensional bulk acoustic wave (BAW) device to propel particles of various sizes. It is shown experimentally that, when particles are large, significant acoustic energy is scattered and is partly absorbed by the matched layers in front of the sensors. The acoustic radiation force then forms a convergent acoustic pressure field which agglomerates the large polystyrene (PS) particles towards the central region instead of the pressure nodes. The transition from the nodal-array patterns to the central agglomeration depends on particle size, particle concentration, and load voltage. A parametric study reveals that the particles can agglomerate with a cluster ratio greater than 70%, and this ratio can be improved by increasing the load power/voltage supplied to the transducers. With its ability to perform biocompatible, label-free, and contact-free self-assembly, the device introduced offers a new possibility in the fabrication of colloidal layers, the recreation of tissue microstructure, the development of organoid spheroid cultures, the migration of microorganisms, and the assembly of bioprinting materials.