Micro-energy harvesting from conductive droplets
Relying on conductive droplets sliding along the surface of a micro-fabricated electret film, this non-resonant approach could generate electrical power from low frequency vibrations, explains the paper.
When a charged droplet slides across the interdigital electrodes situated below the charged electret film, it causes a capacitance variation that is used to generate electric power. A prototype of the fluidic energy harvester demonstrated a peak output power at 0.18uW with a single droplet having a diameter of 1.2mm and sliding on a 2-um thick electret film. This electrostatic transduction which is motion-induced is possible thanks to the bias provided by the internal fixed charge polarization of the electret film.
While traditional electrostatic energy harvesters often rely on spring-mass configurations optimised to operate in resonance at specific frequencies, this new type of energy harvesters broadens the frequency range in which energy can be harvested, including the low frequency sources such as human body motion.
Using a fluid droplet as a proof mass is also interesting because it avoids wear and can potentially display very low friction if a sufficient contact angle is achieved, explains the paper. The team used mercury droplets sliding on a thin charged polytetrafluoroethylene (PTFE) electret with interdigital electrodes (IDEs) patterned beneath, but it is also investigating the use of ionic liquid marbles with hydrophobic properties. When the droplet is located at the center of the two adjacent electrodes, the capacitance reaches its peak value as the droplet overlaps the two electrode faces, then it falls to one tenth of this value when the droplet is centered on one of the electrode fingers – see figure 1.
Fig. 1: The variable capacitance induced by a conductive droplet sliding on a charged film (the electret) beneath which are patterned thin electrodes. (a) and (b) indicates different droplet position versus the interdigital electrodes; (c) is the equivalent circuit.
There are embedded electric charges in the dielectric film, so the device is electrically polarized but the droplet as a whole is still charge neutral. Due to fringing fields, the motion will not only cause a capacitance variation, but also generate a position dependent open circuit voltage.
The 100nm thick interdigital electrode was patterned on a 4-inch Pyrex 7740 glass wafer using gold sputtering and wet etching. The fingers were 500um wide and had 500um gaps between adjacent fingers. The IDE was then coated by a 2um thick PTFE film by RF magnetron sputtering, accumulating electrons in the thin film during the process, to yield an average surface potential of -16V. The energy harvester prototype was tested at various tilted angles, with the droplet sliding downward with a constant acceleration. When the droplet starts to slide, the output voltage begins to fluctuate dramatically with positive and negative peaks alternating as the droplet travels across each pair of fingers of the IDEs.
As the droplet keeps accelerating, the time difference between two sequential peaks decreases with time and the maximum output voltage occurs when the droplet reaches its maximum velocity at the other end of the interdigital electrode structure. The maximum output powers for single droplets with diameters at 1.0mm and 1.2mm were around 0.13uW and 0.18uW while the mean output powers for one traversal of the IDE were 5.12nW and 7.78nW, respectively. These results could be scaled up by designing arrays of such devices, with multiple droplets or ionic marbles, each in a channel of its own.
Fig. 2: Typical output voltage and power versus time in the prototype finger with a spacing of 500μm: and a droplet diameter of 1.2mm.
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