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Scientists had known that cicada wings are super-water-repellent,
or super-hydrophobic. This is different from a great many substances
that are simply water-repellent, or hydrophobic — for instance, oil and
water famously do not mix. But a number of surfaces such as lotus leaves
can make themselves even more water-repellent by covering themselves
with microscopic bumps, so water drops can float on top much as mystics
can lie on beds of nails. For example, cicada wings are covered in rows
of waxy cones about 200 nanometers or billionths of a meter high. In
comparison, the average human hair is roughly 100 microns or millionths
of a meter wide.
When it rains the super-hydrophobic nature of cicada wings can help
them get clean — droplets rolling or splashing off them can remove
soil, dust, pollen and microbes. But what if there is no rain,
especially in the four to six weeks adult cicadas have to live above
ground before dying?
Now scientists find rain is not necessary to keep cicada wings
clean. Apparently, grime can simply leap right off them, given dew.
Mechanical engineer Chuan-Hua Chen at Duke University in Durham,
N.C., and his colleagues were investigating a number of natural and
artificial super-hydrophobic surfaces when they noticed drops of water
at times rapidly disappeared. They were mystified by this behavior for
years until they made observations from a different angle — they used a
high-speed video camera to watch the droplets from the side of these
materials instead of from above.
"That's when we saw them jumping upward," Chen recalled.
The scientists found that when these surfaces are exposed to water
vapor, dew can condense on them. When growing droplets fused together,
the merged drop then leapt off the super-water-repellant surfaces. These
drops, each up to a few microns to a few hundred microns wide, can jump
up to a few millimeters in the air.
"We've since found this happens on almost all normal
super-hydrophobic surfaces," Chen said. "If you take a lotus leaf or any
of the many other super-water-repellant surfaces out there and you let
it cool in your freezer and then take it out, as humidity in the air
condenses on it, you can see with your bare eyes that water drops will
jump in the air."
When small water droplets combine on super-water-repellent
surfaces, a single bigger drop results that has less surface area than
its original parts. As such, energy that is no longer needed to flatten
that water across the surface the smaller droplets once occupied gets
released, popping the drop upward, Chen explained.
"These findings show that super-hydrophobic surfaces don't need
water driven by gravity to take contaminants away — jumping droplets can
do so," Chen said.
"This is a great piece of work that highlights a mechanism that has
not been conventionally considered for self-cleaning," said mechanical
engineer Evelyn Wang at the Massachusetts Institute of Technology, who
did not take part in this research.
Chen and his colleagues found jumping droplets could remove glass,
plastic or pollen particles up to 100 microns wide from cicada wings,
including contaminants that could not be removed by wing vibration or
wind flow.
"Mostly cicadas hang vertically on trees, which means once
condensates jump, either gravity or the air will take them away," Chen
said. "In the worst case scenario, the wing is held parallel to the
ground, but even then, the droplets don't jump straight up, but always
have some horizontal momentum. After they fall down, they leap back up
again, and after a few jumps they leap away from the edge of the wing."
These findings not only can help explain the mystery of how cicada
wings keep clean, but could also lead to improved artificial
self-cleaning materials. Jumping droplets could also help remove heat
from power plants, Chen said.
"I think this work is very exciting and shows the diverse
possibilities and applications with the jumping droplet mechanism," Wang
said.
Chen and his colleagues detailed their findings online April 29 in the journal Proceedings of the National Academy of Sciences.
-Charles Q. Choi, Inside Science News Service
-Charles Q. Choi, Inside Science News Service
Charles Q. Choi is a freelance science writer based in New York City who has written for The New York Times, Scientific American, Wired, Science, Nature, and many other news outlets.
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