Droplet Actuation on Gradient Electrowetting Surface

Droplet manipulation plays a crucial role in various applications, for example, microfluidics and water harvesting. Among various driving mechanisms, electrowetting-on-dielectrics (EWOD) is one of the most promising technologies due to its high flexibility and programmability. Until now, discrete electrodes on the substrate have been used to generate an asymmetric electrowetting force by selectively activating electrodes and then activating droplets individually. However, EWOD with fixed electrode geometry by definition limits the reconfigurability and thereby the flexibility of actuating drops along arbitrary paths. Here, we report a continuous droplet-driving method based on gradient electrowetting (GEW) on a photoactive semiconductor surface. The asymmetric electrowetting force drives droplets due to the continuous electric potential gradient in an amorphous silicon (α-Si) layer, which is established when a current is imposed through the chip. The distribution of the electrostatic potential within the photoconductive α-Si film can be manipulated by illuminating the sample with suitable optical patterns using a commercial projector. This leads to a freely programmable driving force without the need for pre-defined electrode patterns. GEW enables continuous droplet driving and merging, demonstrating a simplified approach to droplet manipulation that avoids complex control circuitry. It provides a complementary method to simplify the EWOD system architecture.