| tallex 2006-10-18, 9:25 pm |
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Researchers Mimic Lotus Leaves For Self-cleaning PV Arrays, Non-stick
MEMS
Interesting research. There are many potential applications for this,
regards
tallex
http://www.medicalnewstoday.com/med...hp?newsid=54133
Researchers at the Georgia Institute of Technology
are mimicking one of Nature's best non-stick surfaces
to help create more reliable electric transmission
systems, photovoltaic arrays that retain their
efficiency, MEMS structures unaffected by water
and improved biocompatible surfaces able to prevent
cells from adhering to implanted medical devices.
Based on a collaboration of materials scientists and
chemical engineers, the research aims to duplicate
the self-cleaning surfaces of the lotus plant, which
grows in waterways of Asia. Despite growing in muddy
conditions, the leaves and flowers remain clean because
their surfaces are composed of micron- and nano-scale
structures that - along with a waxy coating - prevent
dirt and water from adhering. Despite their unusual
surface properties, the rough surfaces allow photosynthesis
to continue in the leaves.
"When rain hits the leaves of the lotus plant,
it simply beads up," noted C.P. Wong, a Regents
Professor in Georgia Tech's School of Materials
Science and Engineering. "When the leaves are also
tilted at a small angle, the beads of water run off
instantaneously. While the water is rolling off, it
carries away any dirt on the surface."
The self-cleaning action of the lotus plant has
intrigued researchers for decades, and recent studies
done by researchers in several different groups have
demonstrated the reasons behind the plant's unique
abilities.
The plant's ability to repel water and dirt results from an
unusual combination of a superhydrophobic (water-repelling)
surface and a combination of micron-scale hills and valleys
and nanometer-scale waxy bumps that create rough surfaces
that don't give water or dirt a chance to adhere.
"Because of the combination of nano-scale and micron-scale
structures, water droplets can only contact about three
percent of the surface," Wong said. "They're just not
touching very much of the lotus surface as compared to
a smooth surface."
To address several unique applications, Georgia Tech
researchers have attempted to duplicate the two-tier
lotus surface using a variety of materials, including
polybutadiene. But that organic compound isn't suitable
for coatings that are exposed to sunlight because
ultraviolet radiation breaks down its carbon bonds.
So to address their first lotus application - self-cleaning
insulators used on high-voltage power lines - the researchers
had to develop another material.
Supported by the National Electric Energy Testing Research
and Applications Center (NEETRAC), that project would solve
a problem that plagues electric utilities. The build-up of
dirt and dust on ceramic or silicone insulators used by
high-voltage power lines can eventually create a short
circuit that can damage the electric distribution network.
It's impractical to manually clean the insulators.
Wong and collaborators Yonghao Xiu, Lingbo Zhu and Dennis
Hess have developed a lotus surface able to withstand
ultraviolet radiation using a combination of silicone,
fluorocarbons, and inorganics such as titanium dioxide
and silicon dioxide. Their prototype coating has shown
excellent durability in long-term testing.
Supported by the National Science Foundation, NASA and
other agencies, Georgia Tech is also pursuing other work
based on lotus applications:
* Use of carbon nanotube bundles to create the surface
bumps needed to prevent dust from accumulating on the
surfaces of photovoltaic (PV) cells, space suits and
other equipment intended for use on the moon or Mars
- where there's no rain. Arranging patterns of nanotube
bundles a few microns apart and applying a weak electrical
charge should help keep dust away and maintain maximum
efficiency in the PV cells that power space missions.
* Application of lotus coatings to prevent "stiction,"
which is the strong adhesive force that can form between
the structures of micro-electromechanical systems (MEMS)
and substrates. The magnitude of these forces can be
enough to deform the structures, resulting in device
failure. With its superhydrophobicity and surface
roughness, a lotus surface coating can prevent stiction,
Wong said.
* A two-tier surface system composed of hexagonally-packed
silica spheres on which gold nanoparticles were deposited.
The resulting chemical and physical structures were studied
to establish the impact of surface hydrophobicity and roughness
on the measured contact angles on the rough surfaces.
* Lotus surfaces for use in implantable medical devices to
prevent cells from attaching to form blood clots. If successful,
this application could replace anti-clotting materials
that are coated onto implantable devices such as stents
used to hold blood vessels open.
The lotus plant is yet another example of how researchers
can learn surprising lessons from what Nature has provided,
Wong noted.
"It's not easy to get dust and dirt off a smooth surface,"
he said. "Though it seems counterintuitive, the roughness
actually helps the cleaning process. We believe this lotus
surface will have many potential applications."
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