A droplet energy harvesting and actuation system for self-powered digital microfluidics†
When a water droplet slides down a hydrophobic surface, a major energy it possesses is kinetic energy. However, people may ignore another important energy source: triboelectrification. To quantify and utilize triboelectrification energy, a phenomenon is presented in this study: one droplet slides down a tilted chip with a hydrophobic coating and patterned electrodes, triboelectrification happens and the induced charges are transferred to another horizontally placed chip with copper wires, on which another droplet is actuated by the transferred charges. The mechanism of this phenomenon is triboelectrification, electrostatic induction and EWOD (electrowetting on dielectrics). When an 80 μL droplet slides down the chip, the induced charges build up a potential difference between the electrodes of 46 V. With this potential difference, the droplet actuation is achieved not only on the horizontal chip, but also on the vertical chip. By patterning a comb-shaped electrode, functions for droplet manipulations are achieved. Theoretical analysis is conducted to quantify the frictional force, gravitational force and driving force (EWOD force). The presented concept and device could be employed for a self-powered digital microfluidics (DMF) system, replacing the bulky and energy consuming voltage sources which are commonly used in DMF devices.