Dynamic capacitive sensing of droplet parameters in a low-cost open EWOD system

Electrowetting on dielectric (EWOD) based digital microfluidics (DMF) emerges as a modern paradigm for lab on a chip (LOC) application, as it offers advantages of portability, automation, higher sensitivity and high throughput in diagnosis applications. One of the challenges in designing of EWOD devices for such applications is managing many droplets on the single substrate. The controlling of the droplet traffic requires a feedback control system whose sensor can continuously monitor the droplet parameters like contact angle, position, size, and velocity. In this work, we have demonstrated a low-cost open EWOD based dynamic sensing system, which is capable of tracking multiple droplets on the single substrate in real time. Droplet parameters are measured by sensing the capacitance value between electrode pad and ground wire. This makes the systems scalable, portable, robust, highly sensitive and inexpensive. We have used printed circuit board (PCB) based open EWOD device, where polydimethylsiloxane (PDMS) is used as a dielectric as well as a hydrophobic layer. The analytical model for droplet velocity has been developed, which matches quite well with experimental results. The droplet sensing approach presented in this work, adds to the functionality of DMF by allowing measurement with top ground configuration.