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As engineers, chemists and physicists race to design the nanomachines
predicted to revolutionize everything from manufacturing to health
care, other scientists are worrying about the friction—not to
mention adhesion, indentation, dissipation, corrosion, and even downright
self-destruction.
Dr. Jacqueline Krim, an NC State professor of physics in the College
of Physical and Mathematical Sciences, is one of a new breed of scientists
called nanotribologists who know that friction at the nanoscale is
no small matter. Recent progress in nanotribology has demonstrated
that the laws of macroscale friction simply don’t apply to atomic
scale devices, and that the problems friction can generate are overwhelming
in machine components with such astoundingly small dimensions.
“The technology has crashed head-on with fundamental physics
and
chemistry,” says Krim. “At the atomic scale, friction has
very little to do with surface roughness. Some dry surfaces slide
against each other easier than wet ones. And contrary to what we know
about industrial scale machines, gravity (related to volume or weight)
is a negligible contributor to friction when opposing objects are
only a few atoms or molecules thick.”
Nanotribology [from the Greek tribo, to rub] is such a new science
that Krim herself coined the term in 1986 while working at Northeastern
University. A number of textbooks mention her pioneering work. She
works today in the Nanoscale Tribology Laboratory on NC State’s
Centennial Campus with six graduate research assistants and two
post-doctoral associates. With annual funding from the National Science
Foundation, the Department of Energy, and the U.S. Air Force in the
half-million-dollar range, she runs one of the world’s top research
groups studying the fundamental origins of friction.
Krim’s goal is to develop a lubricant that can be used at extreme
temperatures without vaporizing or freezing, reducing both heat generation
and wear. Either heat or wear could inflict mortal wounds on nanomachines,
where melting or shearing off a surface layer of atoms could render
a device useless.
To meet their research goal, Krim’s group must first understand
why a
lubricant lubricates a particular surface. Their most recent discovery,
now awaiting publication, occurred on a project sponsored by the Air
Force. Researchers at Wright Patterson Air Force Base were searching
for jet engine materials and lubricants that could withstand higher
temperature stress, reducing the need for heavy, energy-guzzling engine
cooling
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In response, Krim set out to compare how fast the lubricant
particles begin to bind or stick to the materials they are supposed
to lubricate. She was astonished by the discovery that a difference
in how the lubricant’s molecules flexed and slipped after they
stuck was what made the difference in effectiveness. “Nothing
is totally flat at the molecular scale, and microscopic irregularities
of surfaces touch and push into one another as materials slide. If
the lubricant begins to stick to one surface, but the molecular bonds
have just the most miniscule amount of flex, sometimes that’s
all that’s needed to slide the contact points of the two surfaces
past each other.” (See illustration below.) Using a quartz microbalance,
she and her students observed that in this case, only a picosecond
of slip—one millionth of a
millionth of a second—was the critical factor.
Armed with this unforeseen result, Krim is advising Air Force researchers
about possible parameters for design of new
lubricants and materials that allow the flexible bonding. “It
can save the Air Force both time and money to have this experiment
done at a university,” she says. “An even greater advantage
to them may be that now any of my graduate
students could step into a job in the Wright Patterson research lab
fully trained to work on this problem.”
Krim says her discovery will be just as important for nanomachines
as for jet engine parts. “Scientists working on nanoscale devices
must understand friction at the atomic scale or their devices won’t
survive the heat they generate.” For now, she’s one of perhaps
a hundred nanotribologists in the world. But she predicts that as
the need to conserve both energy and raw materials becomes more urgent,
nanotechnologists’ rush to understand basic frictional processes
can be expected only to accelerate.
For
more information, please visit
http://www.physics.ncsu.edu:8380/nanotribology/ |
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