Residue-free acoustofluidic manipulation of microparticles via removal of microchannel anechoic corner

Surface acoustic wave (SAW)-based acoustofluidics has shown significant promise to manipulate micro/nano-scale objects for biomedical applications, e.g. cell separation, enrichment, and sorting. A majority of the acoustofluidic devices utilize microchannels with rectangular cross-section where the acoustic waves propagate in the direction perpendicular to the sample flow. A region with weak acoustic wave intensity, termed micro -channel anechoic corner (MAC), is formed inside a rectangular microchannel of the acoustofluidic devices where the ultrasonic waves refract into the fluid at the Rayleigh angle with respect to the normal to the substrate. Due to the absence of a strong acoustic field within the MAC, the microparticles flowing adjacent to the microchannel wall remain unaffected by a direct SAW-induced acoustic radiation force (ARF). Moreover, an acoustic streaming flow (ASF) vortex produced within the MAC pulls the particles further into the corner and away from the direct ARF influence. Therefore, a residue of particles continues to flow past the SAWs without intended deflection, causing a decrease in microparticle manipulation efficiency. In this work, we introduce a cross-type acousto-fluidic device composed of a half-circular microchannel, fabricated through a thermal reflow of a positive photoresist mold, to overcome the limitations associated with rectangular microchannels, prone to the MAC formation. We investigated the effects of different microchannel cross-sectional shapes with varying contact angles on the microparticle deflection in a continuous flow and found three distinct regimes of particle deflec-tion. By systematically removing the MAC out of the microchannel cross-section, we achieved residue-free acoustofluidic microparticle manipulation via SAW-induced ARF inside a half-circular microchannel. The pro-posed method was applied to efficient fluorescent coating of the microparticles in a size-selective manner without any residue particles left undeflected in the MAC.

Automated summary

Surface acoustic wave (SAW)-based acoustofluidics has significant potential for biomedical applications. This study introduces a cross-type acousto-fluidic device with a half-circular microchannel to overcome the limitations associated with rectangular microchannels and achieve residue-free microparticle manipulation.