Particle concentration via acoustically driven microcentrifugation: microPIV flow visualization and numerical modelling studies

Through confocal-like microparticle image velocimetry experiments, we reconstruct, for the first time, the three-dimensional flow field structure of the azimuthal fluid recirculation in a sessile drop induced by asymmetric surface acoustic wave radiation, which, in previous two-dimensional planar studies, has been shown to be a powerful mechanism for driving inertial microcentrifugation for micromixing and particle concentration. Supported through finite element simulations, these insights into the three-dimensional flow field provide valuable information on the mechanisms by which particles suspended in the flow collect in a stack at a central position on the substrate at the bottom of the drop once they are convected by the fluid to the bottom region via a helical spiral-like trajectory around the drop periphery. Once close to the substrate, the inward radial velocity then forces the particles into this central stagnation point where they are trapped by sedimentary forces, provided the convective force is insufficient to redisperse them along with the fluid up a central column and into the bulk of the drop.