Oil-repelling Material Shows Promise for Hazardous Waste Cleanup
Institute of Technology (MIT) engineers have designed the first simple
process for manufacturing materials that strongly repel oils. The
material, which can be applied as a flexible surface coating, could
have applications in aviation, space travel and hazardous waste
cleanup, university officials said on Dec. 6.
For example, the material could be used to help protect parts of
airplanes or rockets that are vulnerable to damage from being soaked in
fuel, such as rubber gaskets and o-rings.
"These are vulnerable points in many aerospace applications," said
Robert Cohen, the St. Laurent Professor of Chemical Engineering and an
author of a paper on the work that appeared in the Dec. 7 issue of Science.
"It would be nice if you could spill gasoline on a fabric or a
gasket or other surface and find that instead of spreading, it just
rolled off," Cohen said.
Creating a strongly oil-repelling, or "oleophobic" material, has
been challenging for scientists, and there are no natural examples of
such a material.
"Nature has developed a lot of methods for waterproofing, but not so
much oil-proofing," said Gareth McKinley, MIT School of Engineering
Professor of Teaching Innovation in the Department of Mechanical
Engineering and a member of the research team. "The conventional wisdom
was that it couldn't be done on a large scale without very special
The tendency of oils and other hydrocarbons to spread out over
surfaces is due to their very low surface tension (a measure of the
attraction between molecules of the same substance).
Water, on the other hand, has a very high surface tension and tends
to form droplets. For example, beads of water appear on a freshly waxed
car (however, over a period of time, oil and grease contaminate the
surface and the repellency fades). That difference in surface tension
also explains why water will roll off the feathers of a duck, but a
duck coated in oil must be washed with soap to remove it.
The MIT team overcame the surface-tension problem by designing a
material composed of specially prepared microfibers that essentially
cushion droplets of liquid, allowing them to sit, intact, just above
the material's surface.
When oil droplets land on the material, which resembles a thin
fabric or tissue paper, they rest atop the fibers and pockets of air
trapped between the fibers. The large contact angle between the droplet
and the fibers prevents the liquid from touching the bottom of the
surface and wetting it.
The microfibers are a blend of a specially synthesized molecule
called fluoroPOSS, which has an extremely low surface energy, and a
common polymer. They can be readily deposited onto many types of
surfaces, including metal, glass, plastic and even biological surfaces
such as plant leaves, using a process known as electrospinning.
The researchers also have developed some dimensionless design
parameters that can predict how stable the oleophobicity or
oil-resistance between a particular liquid and a surface will be. These
design equations are based on structural considerations, particularly
the re-entrant nature (or concavity) of the surface roughness, and on
three other factors: the liquid's surface tension, the spacing of the
fibers, and the contact angle between the liquid and a flat surface.
Using these relationships, the researchers can design fiber mats
that are optimized to repel different hydrocarbons. They have already
created a non-woven fabric that can separate water and octane (jet
fuel), which they believe could be useful for hazardous waste cleanup.
Robert Cohen: http://web.mit.edu/cheme/people/faculty/cohen.html.