Numerical simulation of continuous separation of microparticles in two-stage acousto-microfluidic systems

Abstract

Numerical modelling of acousto-microfluidic particle manipulation systems cannot only be used to explain the complex phenomena observed in experiments, but can also be applied to optimise their performances. In this work, we present numerical simulations of continuous-flow-based two-stage acoustic microparticle separations with a reduced-fluid model, which is consisted of three main parts: (1) an acoustic focusing zone; (2) a transition zone; and (3) an acoustic separation zone. The acoustophoresis of microparticles of various sizes in the fluid channel was modelled based on Newton's second law, where the acoustic radiation forces and the flow-induced drag forces, the main driving terms for particle motion, were solved from the Gorkov equation and the Navier-Stokes equations, respectively. It was found that an acoustic focusing process configured with appropriate force amplitudes can focus all particles to the same flow vector before entering the separation zone and thus can improve the separation efficiency, and that a sheath flow injected from the transition zone can push the sample flow onto the side boundaries, which can broaden the effective separation range for more robust separations. Based on the mechanism analyses, we here numerically demonstrated acoustofluidic separation of 5 different particle fractions simultaneously in a continuous microfluidic channel ending with 9 equally spaced outlets. We also predicted here that, with carefully designed acoustic and flow fields, it is capable to acoustically separate two different particle fractions with a diameter difference of 4% (difference in acoustic mobility of only ~1.08).