Salvinia molesta infesting a body of water. Photo source: Wikipedia. |
A
floating weed that clogs waterways around the world has at least one
redeeming feature: It’s inspired a high-tech waterproof coating intended
for boats and submarines.
The Brazilian fern Salvinia molesta
has proliferated around the Americas and Australia in part because its
surface is dotted with oddly shaped hairs that trap air, reduce
friction, and help the plant stay afloat.
In the November 1 issue of the Journal of Colloid and Interface Science,
Ohio State University engineers describe how they recreated the
texture, which resembles a carpet of tiny eggbeater-shaped fibers. The
plastic coating they created in the laboratory is soft and plush, like a
microscopic shag carpet.
In
nature, air pockets trapped at the base of Salvinia’s hairs reduce
friction in the water and help the plant float, while a sticky region at
the tips of the eggbeaters clings lightly to the water, providing
stability.
It’s
the combination of slippery and sticky surfaces that makes the texture
so special, said Bharat Bhushan, Ohio Eminent Scholar and the Howard D.
Winbigler Professor of mechanical engineering at Ohio State.
“The
Salvinia leaf is an amazing hybrid structure. The sides of the hairs
are hydrophobic—in nature, they’re covered with wax—which prevents water
from touching the leaves and traps air beneath the eggbeater shape at
the top. The trapped air gives the plant buoyancy,” he said.
A drop of water sits atop eggbeater-shaped hairs on a Salvinia molesta leaf. Image courtesy of Ohio State University. |
“But
the tops of the hairs are hydrophilic. They stick to the water just a
tiny bit, which keeps the plant stable on the water surface.”
In
tests, the coating performed just as the Salvinia hairs do in nature.
The bases of the hairs were slippery, while the tips of the hairs were
sticky. Water droplets did not penetrate between the hairs, but instead
clung to the tops of the eggbeater structures—even when the coating
sample was turned on its side to a 90-degree vertical.
With
commercial development, the coating could reduce drag and boost
buoyancy and stability on boats and submarines, Bhushan said.
optical micrograph of eggbeater-shaped hairs on the leaf surface of the aquatic plant Salvinia molesta. The hairs reduce friction and help the plant stay afloat. Ohio State University engineers have replicated the surface in a plastic coating that could reduce drag and boost buoyancy on boats and submarines. Images courtesy of Ohio State University. |
Bhushan
and master’s student Jams Hunt compared the stickiness of their plastic
coating to the stickiness of the natural Salvinia leaf using an atomic
force microscope. The two surfaces performed nearly identically, with
the plastic coating generating an adhesive force of 201 nanoNewtons
(billionths of a Newton) and the leaf generating 207 nanoNewtons.
That’s
a very tiny force compared to familiar adhesives such as transparent
tape or even masking tape. But the adhesion is similar to that of
another natural surface studied by Bhushan and other researchers: gecko
feet.
“I’ve
studied the gecko feet, which are sticky, and the lotus leaf, which is
slippery,” Bhushan said. “Salvinia combines aspects of both.”
Bhushan
develops biomimetic structures—artificial structures created in the lab
to mimic structures found in nature. The gecko feet inspired him to
investigate a repositionable, “smart” adhesive, and the lotus leaf
inspired the notion of glass that repels water and dirt.
He
came to study Salvinia through a colleague in the university’s
Biological Sciences Greenhouse, who provided samples of the plant for
the study.
Salvinia
molesta, also known as giant salvinia, is native to Brazil, and is a
popular plant for home aquariums and decorative ponds around the world.
It needs no dirt, but lives solely in the water—even moving water such
as rivers and lakes.
At
some point, the hearty plant escaped from people’s homes into the wild.
Now it has proliferated into commercial waterways in North America,
South America, and Australia, where it has become an invasive species.
While
the plant is a nuisance to ships today, it could ultimately provide a
benefit if a commercial coating based on its texture became available.
Bhushan has no plans to commercialize it himself, though.
“With
this study, we’ve gotten deep insight into a very simple concept [how
the Salvinia leaf works]. That’s where the fun is,” he said. “Besides,
I’ve already moved on to studying shark skin.”
Nanoscale biomimetics studies of Salvinia molesta for micropattern fabrication
