Particle concentration and mixing in microdrops driven by focused surface acoustic waves

We report the use of focused surface acoustic waves (SAWs) generated on $128°$ rotated $Y$-cut $X$-propagating lithium niobate $(LiNbO3)$ for enhancing the actuation of fluids and the manipulation of particle suspensions at microscale dimensions. In particular, we demonstrate increased efficiency and speed in carrying out particle concentration/separation and in generating intense micromixing in microliter drops within which acoustic streaming is induced due to the focused SAW beneath the drop. Concentric circular and elliptical single-phase unidirectional transducers (SPUDTs) were used to focus the SAW. We benchmark our results against a straight SPUDT which does not cause focusing of the SAW. Due to the increased wave intensity and asymmetry of the wave, we found both circular and elliptical SPUDTs concentrate particles in under 1 s, which is 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.