LATEST MECHANICAL ENGINEERING RESEARCH UPDATE (Oct. 2, 2013)

Discovery of Charged Droplets Could Lead to More Efficient Power Plants:

— In a completely unexpected
finding, MIT researchers have discovered that
tiny water droplets that form on a
superhydrophobic surface, and then "jump" away
from that surface, carry an electric charge. The
finding could lead to more efficient power plants
and a new way of drawing power from the
atmosphere, they say.

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The finding is reported in a paper in the journal
Nature Communications written by MIT postdoc
Nenad Miljkovic, mechanical engineering
professor Evelyn Wang, and two others.
Miljkovic says this was an extension of previous
work by the MIT team. That work showed that
under certain conditions, rather than simply
sliding down and separating from a surface due
to gravity, droplets can actually leap away from
it. This occurs when droplets of water condense
onto a metal surface with a specific kind of
superhydrophobic coating and at least two of the
droplets coalesce: They can then spontaneously
jump from the surface, as a result of a release of
excess surface energy.
In the new work, "We found that when these
droplets jump, through analysis of high-speed
video, we saw that they repel one another
midflight," Miljkovic says. "Previous studies have
shown no such effect. When we first saw that, we
were intrigued."
In order to understand the reason for the
repulsion between jumping droplets after they
leave the surface, the researchers performed a
series of experiments using a charged electrode.
Sure enough, when the electrode had a positive
charge, droplets were repelled by it as well as by
each other; when it had a negative charge, the
droplets were drawn toward it. This established
that the effect was caused by a net positive
electrical charge forming on the droplets as they
jumped away from the surface.
The charging process takes place because as
droplets form on a surface, Miljkovic says, they
naturally form an electric double layer -- a layer
of paired positive and negative charges -- on
their surfaces. When neighboring drops coalesce,
which leads to their jumping from the surface,
that process happens "so fast that the charge
separates," he says. "It leaves a bit of charge on
the droplet, and the rest on the surface."
The initial finding that droplets could jump from
a condenser surface -- a component at the heart
of most of the world's electricity-generating
power plants -- provided a mechanism for
enhancing the efficiency of heat transfer on
those condensers, and thus improving power
plants' overall efficiency. The new finding now
provides a way of enhancing that efficiency even
more: By applying the appropriate charge to a
nearby metal plate, jumping droplets can be
pulled away from the surface, reducing the
likelihood of their being pushed back onto the
condenser either by gravity or by the drag
created by the flow of the surrounding vapor
toward the surface, Miljkovic says.
"Now we can use an external electric field to
mitigate" any tendency of the droplets to return
to the condenser, "and enhance the heat
transfer," he says.
But the finding also suggests another possible
new application, Miljkovic says: By placing two
parallel metal plates out in the open, with "one
surface that has droplets jumping, and another
that collects them … you could generate some
power" just from condensation from the ambient
air. All that would be needed is a way of keeping
the condenser surface cool, such as water from a
nearby lake or river. "You just need a cold
surface in a moist environment," he says. "We're
working on demonstrating this concept."
The research team also included graduate
student Daniel Preston and Ryan Enright, who
was a postdoc at MIT and the University of
Limerick and is now at Bell Labs Ireland, part of
Alcatel-Lucent. The work received funding from
the U.S. Department of Energy through the MIT
Solid-State Solar-Thermal Energy Conversion
Center, the Office of Naval Research and the
National Science Foundation.