Fig. 4. Concentrating micro-objects in a Petri dish via acoustic streaming vortices.

(A) Schematics of concentrating micro-objects in a Petri dish using an acoustic streaming vortex generated by a tilted piezoelectric transducer. Traveling acoustic waves generated by the piezoelectric transducer first transmit to the bottom of the Petri dish through a couplant layer and then leak into the fluid layer in the Petri dish. Traveling waves in the fluid layer further induce an acoustic streaming vortex that drives micro-objects to the center of the Petri dish for enrichment. (B to D) Photos taken at 0, 5, and 10 s after the piezoelectric transducer is switched on at the transducer’s resonance frequency of 3.0 MHz. The time-sequential images show that 5-μm polystyrene particles are gradually concentrated at the center of the Petri dish. (E) Schematics of concentrating micro-objects in a sessile droplet at the center of a Petri dish using an acoustic streaming vortex generated by an IDT. High-frequency narrow-beam traveling acoustic waves generated by the IDT can transmit to the droplet and further induce an acoustic streaming vortex that drives micro-objects in the droplet to the vortex center. The oblique incident angle is induced by the difference between the wavelengths of SAWs and bulk acoustic waves. (F to H) Fluorescence microscopy images taken at 0, 6, and 12 s after the IDT is switched on at the IDT’s resonance frequency of 10.3 MHz. The time-sequential images show that 5-μm polystyrene particles (green) are gradually concentrated at the center of the acoustic streaming vortex. Meanwhile, the fluorescence intensity gradually increases and becomes much higher at 12 s than the intensity at the beginning. Photo credit: Zhenhua Tian, Duke University.