Bouncing water droplets could be used to charge phones (Wired UK)

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

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

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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