Nenad Miljkovic and Daniel J Preston

Imagine a situation in which a small device could charge smartphones or other electronic items using only the humidity in the air, while also producing clean water. A team from MIT believes it has made a discovery that could allow such a device to be constructed, cheaply and simply.

A study conducted by the research team last year resulted in the discovery that water droplets gain an electrical charge when they spontaneously jump away from superhydrophobic surfaces during condensation. The same researchers have now found out that the small amount of electricity generated during this process can be used to power electronic devices.

Mechanical engineers Nenad Miljkovic and Evelyn Wang have published their new findings in the journal Applied Physics Letters.

A device that utilised this discovery to charge smartphones could be created from a series of interleaved flat metal plates. Miljkovic used copper in his tests, but any metal that conducts a current could be used, including cheaper materials like aluminium.

Nenad Miljkovic and Daniel J Preston

Miljkovic says that if he was using a cube measuring 50cm on each side made of conductive metal, it would take around 12 hours to fully charge a smartphone. The charging process might well be slow using such a device, but it may be a case of someone needing it when in a remote area with no electricity, and no other choice. The device could operate passively if the metal plates were interleaved but not actually touching.

The original findings that suggested the bouncing droplets gained a charge was where an accidental discovery was made, as part of a heat transfer experiment. By placing a metal plate close to the droplets jumping off the superhydrophobic surface, they were attracted towards it thanks to its opposite charge. This enhanced both their ability to bounce and transfer heat.

Now they have discovered that all that is needed is to cover that metal plate in a hydrophilic surface, and the droplet will boing from one to the other carrying the charge as it goes.

Obviously the whole system would rely on the air around the device being humid and remaining humid for the duration of the charging process. It might not be the case that the system is actually used for charging something as demanding as a smartphone though -- it could potentially be used to power remote, automated environmental sensors that would only require a very tiny charge. This means as long as there was a chance of dew or condensation for even a few hours a day, the system could prove useful.

"Getting power from a condensation process is definitely a novel idea, as condensation is mainly used for thermal management," said Boston University mechanical engineer Chuanhua Duan -- who was not involved in the study -- in a statement. "Recent studies of condensation on superhydrophobic surfaces [have] extended its applications in self-cleaning and anti-icing, but no one has correlated condensation with energy-harvesting before."


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