Particle separation in surface acoustic wave microfluidic devices using reprogrammable, pseudo-standing waves

We report size and density/compressibility-based particle sorting using on-off quasi-standing waves based on the frequency difference between two ultrasonic transducers. The 13.3 MHz fundamental operating frequency of the surface acoustic wave microfluidic device allows the manipulation of particles on the micrometer scale. Experiments, validated by computational fluid dynamics, were carried out to demonstrate size-based sorting of 5-14.5 μm diameter polystyrene (PS) particles and density/compressibility-based sorting of 10 μm PS, iron-oxide, and poly(methyl methacrylate) particles, with densities ranging from 1.05 to 1.5 g/cm³. The method shows a sorting efficiency of >90% and a purity of >80% for particle separation of 10 μm and 14.5 μm, demonstrating better performance than similar sorting methods recently published (72%-83% efficiency). The sorting technique demonstrates high selectivity separation of particles, with the smallest particle ratio being 1.33, compared to 2.5 in previous work. Density/compressibility-based sorting of polystyrene and iron-oxide particles showed an efficiency of 97 ± 4% and a purity of 91 ± 5%. By varying the sign of the acoustic excitation signal, continuous batch acoustic sorting of target particles to a desired outlet was demonstrated with good sorting stability against variations of the inflow rates.