Acoustic Bubble-Induced Microstreaming for Biochemical Droplet Mixing Enhancement in Electrowetting (EW) Microfluidic Platforms.

This paper describes a new type of electrowetting (EW)-driven digital microfluidic (DMF) platform incorporated with an acoustic bubble for improving the viscous droplet mixing performance on an open surface. First, it is experimentally verified that the working principle of the two different actuation methods: EW-driven droplet actuation on an open surface and acoustically driven bubble excitation in an aqueous medium, respectively. Then, it is investigated that the acoustic excitation of the bubble injected into the droplet. When an acoustically excited bubble inside the droplet generates microstreaming at the natural frequency of the bubble, the microstreaming inside the droplet is visualized using fluorescent particles and a high-speed camera. To examine the effect of the microstreaming inside the droplet on mixing performance, the mixture droplet consisting of safranin and glycerol (60 wt%) is used in this experiment. The mixing efficiency is measured through the mixing index calculated by image analysis. The mixture droplet with an acoustically excited bubble is completely mixed within 40 seconds owing to the microstreaming generated by the acoustically oscillating bubble at its natural frequency. On the other hand, the same mixture droplet with only a diffusion process remains unmixed for more than 5 mins. The transportation of a droplet ($10~\mu \text{l}$) containing a bubble ($800~\mu \text{m}$ in diameter) and the removal of the bubble from the droplet are demonstrated using an EW chip. Finally, as a proof of concept, the manipulation of droplets with the enhanced mixing performance on the proposed digital microfluidic platform is demonstrated by the combination of EW droplet actuation and acoustic bubble excitation. [2021-0057] [ABSTRACT FROM AUTHOR]