Inspired by the extraordinary characteristics of polar bear fur,
lotus leaves and gecko feet, engineering researchers have developed a
new way to make arrays of nanofibers that can form coatings that are
sticky, repellent, insulating or light emitting, among other
possibilities.
"This is so removed from anything I've ever seen that I would have
thought it was impossible," said Joerg Lahann, a professor of chemical
engineering at the University of Michigan (U-M) and senior author of a
paper on this work in Science.
The somewhat serendipitous discovery was made by researchers at U-M
and the University of Wisconsin (U-W), who revealed a new and powerful
method for making arrays of fibers that are hundreds of times thinner
than a human hair.
Polar bear hairs are structured to let light in while keeping heat
from escaping. Water-repelling lotus leaves are coated with arrays of
microscopic waxy tubules. And the nanoscale hairs on the bottom of the
feet of gravity-defying geckos get so close to other surfaces that
atomic forces of attraction come into play. Researchers looking to mimic
these superpowers and more have needed a way to create the minuscule
arrays that do the work.
"Fundamentally, this is a completely different way of making nanofiber arrays," Lahann said.
The researchers have shown that their nanofibers can repel water like
lotus leaves. They also grew straight and curved fibers and tested how
they stuck together – finding that clockwise and counterclockwise
twisted fibers knitted together more tightly than two arrays of straight
fibers.
They also experimented with the optical properties of the fibers, by
making a material that glowed. They believe it will be possible to make a
structure that works like polar bear fur, with individual fibers
structured to channel light.
But molecular carpets weren't the original plan. Lahann's group was
working with the group of Nicholas Abbott, at the time a professor of
chemical engineering at UW-Madison, to put thin films of polymers on top
of liquid crystals. Liquid crystals are best known for their use in
displays such as televisions and computer screens, but the researchers
wanted to employ them to make sensors that could detect single
molecules.
Lahann brought expertise in producing thin films, while Abbott led
the design and production of the liquid crystals. In typical
experiments, Lahann's group evaporated single links in the polymers and
then coaxed them to condense onto the surface of the liquid crystals.
But the thin polymer films sometimes didn't materialize as expected.
"The discovery reinforces my view that the best advances in science
and engineering occur when things don't go as planned," Abbott said.
"You just have to be alert and view failed experiments as
opportunities."
Instead of coating the top of the liquid crystal, the links slipped
into the fluid and connected with each other on the glass slide. The
liquid crystal then guided the shapes of the nanofibers as they grew up
from the bottom, creating nanoscale carpets.
"A liquid crystal is a relatively disordered fluid, yet it can
template the formation of nanofibers with remarkably well-defined
lengths and diameters," Abbott said.
And the liquid crystals didn't just make straight strands. Depending
on the liquid crystal, they could generate curved fibers, like
microscopic bananas or staircases.
"We have a lot of control over the chemistry, the type of fibers, the
architecture of the fibers and how we deposit them," Lahann said. "This
really adds a lot of complexity to the way we can engineer surfaces
now; not just with thin two-dimensional films but in three dimensions."