The Effect of Microchannel Cavity on the Bulk Acoustic Wave-Induced Acoustofluidics: Numerical Investigation

Abstract

Acoustofluidics is emerging as an effective approach to manipulating microparticles and cells no matter their optical, electrical, and magnetic properties and no requirement of pre-processing. Standing field in a microfluidic channel produced by a bulk acoustic wave (BAW) could accumulate the microparticles at the plane of the pressure node. In order to further accumulate them from a plane (2D) to a line (1D), a new strategy without significant change of the systematic setup (i.e., adding another orthogonal standing field) was proposed and evaluated numerically in a full-sized model. Concave cavity on the conventional rectangular microchannel leads to a slight increase of the maximum acoustic pressure and distortion of the wavefront, but two more vortexes close to the edge of the bottom cavity and directional acoustic radiation forces in the middle line of the microchannel (the upper part pointing downwards while the lower part upwards). Subsequently, most of the microparticles are accumulated in a very small region in the middle line of the microchannel. The effect of the cavity geometry on such a novel phenomenon was investigated. With the increase of the diameter of the cavity from 170 μm to 260 μm, the resonant frequency of the microchannel, the maximum acoustic pressure, and the maximum acoustic streaming velocity increased by 13%, 78%, and 7.1 fold, respectively. When shifting the center of the cavity, the position of 1D accumulated microparticles could be changed correspondingly. In summary, the characteristics of acoustofluidics are highly dependent on the microchannel geometry. Microparticle accumulation with a significant reduction to one dimension using only one acoustic standing field is theoretically possible by introducing an appropriate concave cavity in the conventional rectangular microchannel.