Simplified ground-type single-plate electrowetting device for droplet transport

The current paper describes a simpler ground-type, single-plate electrowetting configuration for droplet transport in digital microfluidics without performance degradation. The simplified fabrication process is achieved with two photolithography steps. The first step simultaneously patterns both a control electrode array and a reference electrode on a substrate. The second step patterns a dielectric layer at the top to expose the reference electrode for grounding the liquid droplet. In the experiment, a 5 μm thick photo-imageable polyimide, with a 3.3 dielectric constant, is used as the dielectric layer. A 10 nm Teflon-AF is coated to obtain a hydrophobic surface with a high water advancing angle of 116° and a small contact angle hysteresis of 5°. The droplet movement of 1 mM methylene blue on this simplified device is successfully demonstrated at control voltages above the required 45 V to overcome the contact angle hysteresis.