Michigan University. US: A new process that can sprout microscopic spikes on nearly any type
of particle may lead to more environmentally friendly paints and a
variety of other innovations.
Made by a team of University of Michigan engineers, the "hedgehog
particles" are named for their bushy appearance under the microscope.
Their development is detailed in a study published in the Jan. 29 issue
of Nature.
The new process modifies oily, or hydrophobic, particles, enabling
them to disperse easily in water. It can also modify water-soluble, or
hydrophilic, particles, enabling them to dissolve in oil or other oily
chemicals.
The unusual behavior of the hedgehog particles came as something of a
surprise to the research team, says Nicholas Kotov, the Joseph B. and
Florence V. Cejka Professor of Engineering.
"We thought we'd made a mistake," Kotov said. "We saw these particles
that are supposed to hate water dispersing in it and we thought maybe
the particles weren't hydrophobic, or maybe there was a chemical layer
that was enabling them to disperse. But we double-checked everything and
found that, in fact, these particles defy the conventional chemical
wisdom that we all learned in high school."
The team found that the tiny spikes made the particles repel each
other more and attract each other less. The spikes also dramatically
reduce the particles' surface area, helping them to diffuse more easily.
One of the first applications for the particles is likely to be in
paints and coatings, where toxic volatile organic compounds (VOCs) like
toluene are now used to dissolve pigment. Pigments made from hedgehog
particles could potentially be dissolved in nontoxic carriers like
water, the researchers say.
This would result in fewer VOC emissions from paints and coatings,
which the EPA estimates at over eight million tons per year in the
United States alone. VOCs can cause a variety of respiratory and other
ailments and also contribute to smog and climate change. Reducing their
use has become a priority for the Environmental Protection Agency and
other regulatory bodies worldwide.
"VOC solvents are toxic, they're flammable, they're expensive to
handle and dispose of safely," Kotov said. "So if you can avoid using
them, there's a significant cost savings in addition to environmental
benefits."
While some low- and no-VOC coatings are already available, Kotov says
hedgehog particles could provide a simpler, more versatile and less
expensive way to manufacture them.
For the study, the team created hedgehog particles by growing zinc
oxide spikes on polystyrene microbeads. The researchers say that a key
advantage of the process is its flexibility; it can be performed on
virtually any type of particle, and makers can vary the number and size
of the spikes by adjusting the amount of time the particles sit in
various solutions while the protrusions are growing. They can also make
the spikes out of materials other than zinc oxide.
"I think one thing that's really exciting about this is that we're
able to make such a wide variety of hedgehog particles," said Joong Hwan
Bahng, a chemical engineering doctoral student. "It's very controllable
and very versatile."
The researchers say the process is also easily scalable, enabling
hedgehog particles to be created "by the bucketful," according to Kotov.
Further down the road, Kotov envisions a variety of other applications,
including better oil dispersants that could aid in the cleanup of oil
spills and better ways to deliver non-water-soluble prescription
medications.
"Anytime you need to dissolve an oily particle in water, there's a
potential application for hedgehog particles," he said. "It's really
just a matter of finding the right commercial partners. We're only just
beginning to explore the uses for these particles, and I think we're
going to see a lot of applications in the future."
The study is titled "Anomalous Dispersions of Hedgehog Particles" and
based upon work partially supported by the Center for Solar and Thermal
Energy Conversion, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science, Office of Basic Energy
Sciences. Support has also been provided by the NSF and the U.S.
Department of Defense.
Kotov is also a professor of chemical engineering, biomedical
engineering, materials science and engineering and macromolecular
science and engineering.