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Spring 2015

The Jaw-Dropping Jumping Droplet Generator

A condensation powered and pollution-free energy harvesting device

Nicholas Quackenbush

A high speed time-lapse over 21 milliseconds depicting a water droplet (highlighted in green) spontaneously jumping from the superhydrophobic copper oxide surface to the copper surface. Adapted with permission from Appl. Phys. Lett. 105, 013111 (2014). Copyright 2015, AIP Publishing LLC.

A team of researchers at the Solid-State Solar-Thermal Energy Conversion Center (S3TEC) has discovered an unexpected way to convert thermal energy, or heat, into electricity. Their prototypical device is built from re-purposed and nano-engineered copper heat sinks, a standard part of any computer. This pollution-free generator harvests thermal energy from naturally forming condensation. Unlike a similar device made in the 1800s, their new approach is solely powered by moisture in the atmosphere, requiring no pumps or other inputs. In a paper in Applied Physics Letters, the authors explain that condensed water droplets spontaneously jump from one surface to another, carrying electric charges and in turn generating electricity.

The device is based on the idea that some surfaces like water and some do not; the latter group are called hydrophobic surfaces. For example, a bare wood surface will get wet but a varnished surface is hydrophobic and will protect the wood below by repelling the water. The result is water beading or droplet formation, which makes it easy for the water to be wiped off. The extreme case of this phenomenon, referred to as a superhydrophobic surface, is what allows the droplets to jump. After coating the surface with a nano-engineered "varnish," the surface becomes so hydrophobic that when two droplets coalesce into a larger one, there is enough energy unleashed for the larger droplet to jump off the surface entirely on its own. The superhydrophobic surfaces can be extremely useful as self-cleaning or anti-icing surfaces because they will naturally get rid of any water droplets. However, the S3TEC team looked at this a different way—is there a practical use for the jumping droplets, themselves?

As interesting as these surfaces are, the droplets themselves indeed have some surprising properties. Following the spontaneous jump from the surface, each droplet actually possesses a small amount of positive charge and leaves behind a negative charge. This is what drives this generator. The device is built so that the water droplets jump from the superhydrophobic copper oxide surface across a gap of about 5 millimeters and hit the regular copper surface. As the copper surface gleans the positive charge, a potential difference, or voltage, is created between the two surfaces. The two surfaces are connected only through an external circuit where the voltage can drive an electrical current and do useful work.

This technology is expected to be reasonably cost effective because the surface nano-engineering is a relatively simple process that should be readily adaptable for large-scale manufacturing. "Of course, we have just shown the proof-of-concept," explains lead researcher Evelyn Wang of the Massachusetts Institute of Technology. "The important metric is not just getting the cost down, but getting the power output-to-cost ratio high."

Through their experiments, combined with modeling, the team reports that their design in the near future should be able to produce about 1 μW/cm2 of power, or a millionth of a Watt for every square centimeter of wet superhydrophobic surface. This doesn't sound like a lot of power—at that rate you would need more than the area of a football field to power a 60-watt light bulb! However, they suggest that future models with clever high-surface-area designs should be able to achieve much more power for their size, making this a viable technology to power small electronics such as charging the battery in a cell phone. This would be possible whenever the environment is moist and sufficiently warmer than the device. "In particular, this approach could be potentially used in remote areas near lakes or streams where power is limited," says Wang.

The idea of using water droplets to generate electricity is not a new one. Sir William Thomson, otherwise known as Lord Kelvin, created a similar apparatus in 1867. However, because his design relied on gravity as the driving force, it required pumps to lift the water droplets. The S3TEC design harvests atmospheric thermal energy, which would otherwise be completely unused. With this prototype, putting a nano-engineered modern twist on an old idea, these researchers have demonstrated a promising new strategy for energy harvesting.

More Information

Miljkovic N, DJ Preston, R Enright, and EN Wang. 2014. "Jumping-Droplet Electrostatic Energy Harvesting." Applied Physics Letters 105:013111. DOI: 10.1063/1.4886798


The majority of the work was supported by the Solid-State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Additional funding was provided by the Office of Naval Research. D.J.P. acknowledges funding received by the National Science Foundation Graduate Research Fellowship. R.E. acknowledges funding received from the Irish Research Council for Science, Engineering, and Technology, co-funded by Marie Curie Actions under FP7. Bell Labs Ireland thanks the Industrial Development Agency (IDA) Ireland for their financial support.

About the author(s):

  • Nicholas Quackenbush is a Ph.D. candidate studying physics under the advisement of Louis Piper at Binghamton University. Nicholas is a member of the NorthEast Center for Chemical Energy Storage (NECCES). His research focuses on using synchrotron X-ray spectroscopic techniques to investigate materials for lithium-ion battery cathodes and other smart energy devices.

Pulling Power from a Drop of Water

Devices built from repurposed computer parts utilizing water in the air could power small, remote devices

Condensed water droplets spontaneously jump from a superhydrophobic surface to another surface, carrying electric charges and in turn generating electricity.

Hundreds of miles from the nearest transmission line, small mobile electronics still need power. Water droplets could be the answer. Scientists at Solid-State Solar-Thermal Energy Conversion Center (S3TEC) built a pollution-free, pump-free device that creates power from the moisture in the air. By altering copper heat sinks standard in any computer, the researchers created a designer surface that repels water in the extreme. Water droplets jump off the designer surface and land on a plain surface nearby. Each drop carries a positive charge with it when it jumps and leaves a negative one behind. The result? The surfaces have enough charge to drive an electric current. Future models could create enough energy to power sensors and other small electronics. The center is led by the Massachusetts Institute of Technology.

More Information

Miljkovic N, DJ Preston, R Enright, and EN Wang. 2014. "Jumping-Droplet Electrostatic Energy Harvesting." Applied Physics Letters 105:013111. DOI: 10.1063/1.4886798

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.