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Technology
A new Approach to High-Power Energy Harvesting is proposed to convert mechanical to eleclterical energy based on reverse electrowetting [1] . This energy generates through a novel microfluidic phenomenon. Reverse electrowetting is a process by which liquid motion is converted into electrical energy. It can generate a power up to 103Wm−2, by this technology.
Reverse electrowetting (REWOD) concept
At first, The principle of electrowetting is defined in here. The shape of a liquid droplet on a solid layer is characterized by the contact angle θ between the liquid and the solid layer (Figure 1). Electrowetting is the phenomenon whereby an electric field can modify the wetting behavior of a droplet in contact with an insulated electrode. The shape of a droplet on an insulated layer can be altered by applying a voltage difference U between substrate and liquid [2].

Figure 1. Principle of electrowetting [2]
The REWOD process is conceptually straight forward. The droplet and the electrode are connected to the external electrical circuit that provides a constant bias voltage between the droplet and the electrode. External mechanical actuation is used to move the droplet in such a way as to force a decrease of its overlap with the dielectric-film-coated electrode. This results in the decrease of the total charge that can be maintained at the droplet liquid – solid interface. The excessive electrical charge then flows back through the electrical circuit that connects the droplet and the electrode, generating electrical current that can be used to power the external load.

Figure 2. Schematics of the electric circuit. variable capacitor C is the REWOD unit, which represents a harvester set-up, that is, a set of droplets in contact with the electrode grid [1].
The droplet thus acts like a variable capacitor. As the areal overlap at the liquid–solid interface decreases, so does its capacitance, and excess charge flows back through the electrical circuit. In the REWOD process, fluidic actuation can be accomplished in a number of different geometries, as shown in Figure 1, such as out-of-plane vibration[1].

Figure 3. Schematics of droplet actuation mechanisms [1]
Ref.
[1]. Tom Krupenkin, and J. Ashley Taylor, Reverse electrowetting as a new approach to high-power energy harvesting, February 2012, Nature Communications.
[2]. Herman Oprins, and Martine Baelmans, On-Chip Electrowetting Cooling,2006, http://www.electronics-cooling.com.