Concentration and mixing of particles in microdrops driven by focused surface acoustic waves

The concentration and indeed mixing of particle suspensions are important procedures in microfluidic processes. Relying on diffusion for mixing leads to unreasonably long mixing times, and enhancing or avoiding it by promoting chaotic mixing typically relies on complex geometries. Concentric circular and elliptical single-phase unidirectional transducers (SPUDTs) were used to focus the SAW, with a straight SPUDT as a benchmark for comparison. Due to the increased wave intensity and asymmetry of the wave, we found both circular and elliptical SPUDTs concentrate particles in under one second, one order of magnitude faster than the straight SPUDT and several orders of magnitude faster than conventional microscale devices. The concentric circular SPUDT was found to be most effective at a given input power since it generated the largest azimuthal velocity gradient within the fluid to drive particle shear migration. On the other hand, the concentric elliptical SPUDT generated the highest micromixing intensity due to the more narrowly focused SAW radiation that substantially enhances acoustic streaming in the fluid.