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How lotus keeps itself clean and dry while growing from mud |
Duke University engineers have unlocked a secret that has puzzled scientists for
years - how lotus keeps itself clean and dry while growing from mud. Using ultra
high-speed camera, a powerful microscope, scientists, for the first time, observed
water as it condensed on the leaf's surface, and more importantly, how the water
condensate left the leaf. The trick lies in the surface of the plant's large leaves,
and the subtle vibrations of nature. The leaves are covered with tiny irregular
bumps spiked with even tinier hairs projecting upward. When a water droplet lands
on this type of surface, it only touches the ends of the tiny hairs. The droplet
is buoyed by air pockets below and ultimately is repelled off the leaf. "We faced
a tricky problem - water droplets that fall on the leaf easily roll off, while
condensate that grows from within the leaf's nooks and crannies is sticky and
remains trapped," said Jonathan Boreyko, a third-year graduate student at Duke's
Pratt School of Engineering, who works in the laboratory of assistant professor
Chuan-Hua Chen. "Scientists and engineers have long wondered how these sticky
drops are eventually repelled from the leaf after their impalement into the tiny
projections. After bringing lotus leaves into the lab and watching the condensation
as it formed, we were able to see how the sticky drops became unsticky," Boreyko
said. The key was videotaping the process while the lotus leaf rested on top of
the woofer portion of a stereo speaker at low frequency. Condensation was created
by cooling the leaf. It turned out that after being gently vibrated for a fraction
of a second, the sticky droplets gradually unstuck themselves and jumped off the
leaf. Chen said: "This solves a long-standing puzzle in the field. People have
observed that condensation forms every night on the lotus leaf. When they come
back in the morning the water is gone and the leaf is dry. The speaker reproduced
in the lab what happens every day in nature, which is full of subtle vibrations,
especially for the lotus, which has large leaves atop long and slender stems."
The results of these experiments, showing for the first time that water droplets
spontaneously "jump" off a highly water-repellent, or superhydrophobic, surface,
will allow engineers to make use of man-made surfaces resembling the lotus to
improve the efficiency of modern engineering systems, such as power plants or
electronic equipment, which must be cooled by removing heat through water evaporation
and condensation. The results were published early on-line in the journal Physics
Review Letters. |
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