Abstract
Picoliter droplets actuated on an electrowetting-on-dielectric (EWD) actuator are demonstrated. In this study, the physical scaling of electrodes for 33 μm and 21 μm EWD devices resulted in droplets of 12 pl and 5 pl being dispensed respectively in conjunction with 3 μm SU8 gaskets. The stacked multi-layer insulators in the actuators consisted of 200 nm tantalum pentoxide (Ta2O5) and 200 nm parylene C films deposited and coated with 70 nm of CYTOP. The voltages for dispensing droplets on chips without any external pressure sources are 17.1 Vrms and 22 Vrms for these two sets of devices. A 12 pl droplet can be split into two 6 pl daughter droplets at 18.7 Vrms with 33 μm electrode devices. Droplet manipulation is also demonstrated with paramagnetic beads and buffer solutions with proteins. In addition, electrodes with interlocking protrusions and special featured reservoir gasket are designed to facilitate droplet dispensing on these scaled EWD devices. In order to improve sealing of the two-piece sandwich EWD structure, a soft material, Norland Optical Adhesive (NOA), was coated on the top plate along with pressure on top. We demonstrate that based on fundamental theories and experiments, the dimensional scaling of EWD devices has not yet met a limitation as long as the EWD device can be sealed well.