Acoustic particle migration and focusing in a tilted acoustic field

Surface acoustic wave-based particle/bioparticle manipulation has emerged as a promising tool for disease diagnosis. The effects of the titled angle of the acoustic field θ and the microchannel aspect ratios β on the particle migration mode, the force of particle, and the three-dimensional focusing behavior are studied by using simulation and high-speed microscopic visualizations experiments. The acoustic field tilt range is from 0° to 15°, and the wavelength is 160 μm. Particle migration trajectory is observed from high-speed photographic images. Compared with most parallel acoustic fields, the particle migration efficiency of the tilted acoustic field is higher because the acoustic radiation force (Fr) continues to act on the particles in the lateral direction. The tilted angle of the acoustic field is not a fixed value (usually 15°), and there is an optimal angle to match the maximum lateral migration of the target particles. A model is put forward to predict the optimal acoustic field tilt-angle for acoustofluidic devices, which can achieve 96% separation of 15 μm target particles. The change in the direction of the Fr drives the particles to create two typical migration states during the lateral migration process, named continuous migration and intermittent migration. The phenomenon of multi-layer particle focus in the vertical Z-direction of the microchannel is experimentally observed for the first time, which mainly depends on whether the microchannel has enough height to make multiple acoustic pressure nodes in the vertical direction. Two or even three layers of particle focus lines can be observed in the vertical direction at the microchannel aspect ratios β > 0.5. The research results provide new insight into the high-throughput development of microfluidic devices.