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The lotus leaf's amazing beading abilities is something materials scientists envy. (Ralf Pfeifer.)

The lotus leaf's amazing beading abilities is something materials scientists envy. (Ralf Pfeifer.)

11/06/2006

Can't Touch This

By Patrick Barry

Rising from muddy waterways to bloom in the sunshine, untouched and
unsullied, the lotus flower has inspired religious feelings in Hindus
and Buddhists for millennia. Now it's inspiring materials scientists
as well.

For years, scientists have tried to understand the remarkable water-
repellant properties of the lotus plant, and to mimic them in manmade
materials. Several such materials have been created, but most suffer
from poor durability — a showstopper for practical applications.

A group of researchers at the Georgia Institute of Technology may
have found a solution. They have devised a material with both lotus-
like properties and, in their own lab tests, good long-term
stability. Such surfaces could eventually help keep solar panels
free of obscuring dust or prevent a person's cells from sticking
to implanted medical devices.

Pour water on a lotus leaf and the droplets bead up so strongly that
they almost form little spheres. Tilt the leaf ever so slightly, and
the droplets race off, carrying dirt and dust with them. The leaves
and flower petals are left completely clean and dry, as if they had
never been touched.

The key to this remarkable property lies in the microscopic contours
of the leaf's surface. Zoom in on a lotus leaf and you'll see an
undulating landscape of peaks and valleys, each roughly a thousandth
of a millimeter across. Looking even closer reveals smaller
protrusions, like fields of boulders covering the hills. These
"boulders" are only about a millionth of a millimeter across — the
width of about 10 atoms.

This rough surface means that a water droplet only touches about
three percent of the leaf surface beneath it. "There's an airpocket
in the valley, so the water droplet doesn't sink into the valley,"
said C.P. Wong, Regents Professor of materials science at Georgia
Tech and lead scientist for the group. Give that last three percent
a waxy coating, and the water droplet is left with nowhere to grab hold.

Wong's group created a similar surface by spraying layers of
silicone, fluorocarbons, titanium dioxide, and silicon dioxide onto a
substrate. The resulting material beaded water like a lotus leaf and
remained intact after durability testing.

The first application of this new material could help keep cities'
power grids up and running. The ceramic insulators that hold high-
voltage power lines tend to accumulate dirt and dust. Eventually,
this buildup can cause a short circuit that damages the grid.
Cleaning these insulators by hand is both dangerous and expensive, so
the National Electric Energy Testing Research and Applications
Center, which funded Wong's research, is interested in self-cleaning
insulators.

Coating those insulators with this new material could allow the
occasional rain to rinse away dust and dirt, Wong says.

However, Kenneth Lau, an assistant professor of chemical engineering
at Drexel University, says that more tests are needed to demonstrate
that this new material will be durable enough. "It remains to be
seen if Dr. Wong's research will yield any improvements in
[durability]," Lau said via email.

If those tests go well, lessons learned from the lotus plant may soon
help keep entire cities of people illuminated.

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