Asymmetric electrodes for droplet directional actuation by a square wave on an open surface

The large-scale electrowetting-on-dielectric (EWOD) digital microfluidic platforms always require complex electrode-addressing schemes and high-cost fabrication methods of dielectric and hydrophobic layers. Hence, the voltage polarity-dependent electrowetting was used to actuate droplets with asymmetric printed circuit board (PCB) electrodes on an open slippery liquid-infused porous surface (SLIPS) chip. The V-shaped, convex-shaped, and curve-shaped coplanar asymmetric electrode arrays were designed to achieve the directional motion of a droplet by a square wave voltage signal. The operation ranges of voltage amplitude and frequency for continuously actuating a 10 µl droplet were obtained on the three asymmetric electrode arrays. Our results found that the curve-shaped electrode array has a broader voltage operation range than the other two for continuous pumping of a 10 µl droplet. It is because the droplet has a long effective width of contact line with the front curve-shaped electrode and does not overlap with the rear electrode during the movement. Meanwhile, a reconfigurable convex-shaped electrode structure was also proposed to achieve bidirectional actuation of a droplet by controlling the square wave voltage. This work provides a low-cost, straightforward electrode-addressing scheme for designing digital microfluidic devices.