Acoustophoretic particle manipulation in droplet microfluidics at higher resonance modes

In this paper we investigate focusing of microparticles in the presence of multiple pressure nodes inside aqueous droplets by using bulk acoustic waves. The microfluidic chip s for droplet generation and particle encapsulation (within the droplets) were fabricated using anisotropic wet - etching of a silicon wafer. Subsequently, piezoelectric transducers featuring different thicknesses were glued on the chips to build the final devices. The transducer thicknesses were chosen as to match the acoustic resonances of the embedded micro channel at the fundamental frequency, the first and the second harmonics. The actuation of the devices at the first three resonance modes enabled the positioning of the microparticles in one, two or three bands, in accordance with the presence of pressure nodes within the droplet contained in the microchannel. This acoustic particle manipulation technique opens up for new possibilities to perform biological assays using droplet microfluidic platforms