Piezo-driven acoustic streaming in an electrowetting-on-dielectric digital microfluidics device

We report the integration of a lead zirconate titanate, Pb[Zr_xTi_1-xO_3 ] (PZT), piezoelectric transducer disk into the top plate of an otherwise conventional electrowetting-on-dielectric (EWD) digital microfluidics device to demonstrate on-demand induction of circulating fluid flow within single 200 nL droplets. Microparticle image velocimetry was used to measure in-plane velocity distributions for PZT excitation voltages that ranged from 0 to 50 V_RMS . Intra-droplet streaming velocities in excess of 2.0 mm·s^-1 were observed without droplet breakup or damage to the EWD device layer. Additionally, we found median intra-droplet streaming velocity to depend quadratically on PZT excitation voltage up to the stress limit of the interfacial boundary. Our approach offers an alternative device architecture for active micromixing strategies in EWD digital microfluidics laboratory-on-chip systems.