Microchannel anechoic corner for size-selective separation and medium exchange via traveling surface acoustic waves

We demonstrate a miniaturized acoustofluidic device composed of a pair of slanted interdigitated transducers (SIDTs) and a polydimethylsiloxane microchannel for achieving size-selective separation and exchange of medium around polystyrene particles in a continuous, label-free, and contactless fashion. The SIDTs, deposited parallel to each other, produce tunable traveling surface acoustic waves (TSAWs) at desired locations, which, in turn, yield an anechoic corner inside the microchannel that is used to selectively deflect particles of choice from their streamlines. The TSAWs with frequency fR originating from the right SIDT and propagating left toward the microchannel normal to the fluid flow direction, laterally deflect larger particles with diameter d1 from the hydrodynamically focused sample fluid that carries other particles as well with diameters d2 and d3, such that d1 > d2 > d3. The deflected particles (d1) are pushed into the top-left corner of the microchannel. Downstream, the TSAWs with frequency fL, such that fL > fR, disseminating from the left SIDT, deflect the medium-sized particles (d2) rightward, leaving behind the larger particles (d1) unaffected in the top-left anechoic corner and the smaller particles (d3) in the middle of the microchannel, thereby achieving particle separation. A particle not present in the anechoic corner could be deflected rightward to realize twice the medium exchange. In this work, the three-way separation of polystyrene particles with diameters of 3, 4.2, and 5 μm and 3, 5, and 7 μm is achieved using two separate devices. Moreover, these devices are used to demonstrate multimedium exchange around polystyrene particles ∼5 μm and 7 μm in diameter.