Mixing enhancement of viscous droplet using an oscillating bubble on a digital microfluidic device

In this paper, we present a novel digital microfluidic (DMF) device that utilizes an acoustically excited bubble for droplet mixing. The proposed DMF device not only manipulate droplets but also generate microstreaming inside the droplet, by using electrowetting (EW) and bubble oscillation, respectively. We use the microstreaming which is generated by a bubble oscillating by a piezoactuator. The design of the proposed DMF device with holes for bubble generation on each EW electrode were devised. When a droplet is initially placed on an EW electrode with a hole, the bubble is trapped inside the hole. Then, when the piezoactuator is activated, the microstreaming is induced by the acoustically oscillating bubble. To analyze this microstreaming, visualization experiment and velocity measurement were conducted. Furthermore, we investigated the performance of droplet mixing by quantifying mixing index of liquid droplets through image processing. The proposed device would be useful to handle micro-droplets with various properties in medical and biological fields.