Charging Your Phone
with Jumping Droplets


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The experimental chamber setup, with a high-speed camera looking into the chamber. Image: Nenad Miljkovic and Daniel J. Preston / MIT

Researchers at Massachusetts Institute of Technology looking for an improved heat-transfer surface to be used as a condenser in a power plant, made a chance discovery that water droplets jumping away from certain water-repellent surfaces during condensation can gain an electric charge in the process.

Surprised about the charge and excited about the potential, the team followed up by demonstrating that this process can generate small amounts of electricity. This means that one day this phenomenon of the jumping droplets may be the basis for a passive charging device that can power small electronic devices using just humidity in the air, useful where other sources of power are inaccessible.

What happens, according to the team, is that droplets on a superhydrophobic surface convert surface energy to kinetic energy as they merge to form larger droplets. This sometimes causes the droplets to spontaneously jump away, which enhances heat transfer by 30% compared to current state-of-the-art dropwise condensation techniques. The team also found that the jumping and accompanying transfer of heat could be further enhanced by a nearby metal plate with an opposite charge attracting the droplets.

Droplets jump from a superhydrophobic copper-oxide fin to a hydrophilic (water-attracting) copper fin. Image: Nenad Miljkovic and Daniel J. Preston / MIT

 

 

The discovery came as the team was working on designing structured surfaces and engineering different types of surface designs to enhance condensation heat transfer, says Dr. Evelyn Wang, associate professor of mechanical engineering at MIT. Condensation is a very important process in, for example, heat exchanger design, she explains. “While focusing on copper-oxide structured surfaces, we noticed that there seemed to be some positive charge on these droplets as they were interacting with the surface and departing from the surface,” she says.

The team created a comb-like structure using an off-the-shelf heat exchanger with one side being superhydrophobic and the plate on the other side hydrophilic. As the droplets jump, they carry charge and if the two plates are connected through an external circuit, the charge difference can be harnessed to provide power. A charging device could be envisioned as two arrays of metal plates, like fins on a radiator, are constructed so that the plates alternate and are very close but not touching.

“Based on our basic modeling we know that important parameters are [maximizing] the jumping velocity or maximizing the flux,” Wang says. “If we can enhance cooling, we will get much higher potential of the power density. It sounds relatively small but if you think of small-scale electronics and compare this to other systems that can provide that level of power, say thermoelectrics or chemical batteries, you can think about something like powering your cell phone eventually.”

The charging device could be simple, consisting of interwoven flat metal plates, according to Dr. Nenad Miljkovic, postdoctoral associate on the team who recently joined the University of Illinois at Urbana-Champaign as an assistant professor.

Although the amount of power generated so far is miniscule, Miljkovic believes it can be increased to at least 1 microwatt (one millionth of a watt) per square centimeter, enough to power some electronic devices.

The system, although passive, does have one key requirement, which is that it must be colder than the ambient temperature to be able to condense. However, for powering  automated environmental sensors, for example, only a small amount of energy may be needed, according to Miljkovic, who notes that any location where dew forms could produce enough power for a few hours.

Wang adds that being close to a cave or river and having access to ice to use as a cooling source are also possibilities. “That may limit some of the potential,” she says, but regardless, being the first to report this mechanism is very exciting.

“This is a first step,” she says. “The next is demonstrating viable power density. There is a still a lot to be understood fundamentally. Within the next year, we should get to more realistic power densities that can be applicable.”

Even so, she says it may take five years or so before a commercial product is available. “We’ve demonstrated the physics,” she points out. “We also have to think about packaging and robustness over time. These take a lot more time than a demonstration.”

Nancy S. Giges is an independent writer.

 

One day jumping droplets may be the basis for a passive charging device that can power small electronic devices using just humidity in the air.

Dr. Evelyn Wang, associate professor of mechanical engineering, MIT

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November 2014

by Nancy S. Giges, ASME.org