Exploiting friction can make MEMS work
nvision the topography of a packed-gravel road. While not “smooth”
like the bottom of a brand-new leather-soled shoe or even an asphalt
street, the road probably appears relatively flat and functional. Yet
the varied shapes of the small stones that comprise it create millions
of tiny points of contact between it and a vehicle’s tires. These
contacts determine the amount of friction between the tire and the road.
The gravel road offers a life-sized picture
of what materials’ surfaces look like under a microscope. “Surfaces
are always rough,” says Assistant Professor Rob
Carpick.
And at the nanoscale, where there is a greater
surface-to-volume ratio, friction, adhesion and bonding can be such
strong factors that micromachines—currently used in applications
ranging from airbag triggers in cars to computer projection systems—won’t
work at all.
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"Topography"
of a silicon MEMS device
(20K
JPG) |
Building on seed projects with both Sandia and
Argonne National Laboratories, Carpick, Professor Mike
Plesha and research associate Anirudha
Sumant have received a three-year, $525,000 grant from the Department
of Energy Nanoscale Science Engineering and Technology program to study
friction at the nanoscale and how it applies to micro-electromechanical
systems (MEMS).
“People have been making all sorts of
elaborate and incredible micromachine devices for many years, but there
are no commercial MEMS devices on the market that involve surfaces that
contact and slide,” says Carpick.
The project’s goal is to enable researchers
to design robust, reliable MEMS devices. And the trio’s results
could open a whole new market for micromachines such as motors, pumps,
actuators, communications devices and miniature mechanical combination
locks with sliding parts—all of which have been nearly impossible.
“There’s really no theory of friction
at the micro- and nanoscale that would be relevant for these MEMS devices,”
says Carpick. “So what we are trying to do is measure friction
at the level of a single point of contact.”
The group will use Carpick’s atomic-force
microscope to characterize friction at that contact point, called an
asperity, based on variables such as pressure, sliding speed, type of
material and environmental conditions. Once the three have established
data for friction at a single point, they will develop models with multiple
contact points, ultimately using a finite-element approach, or approximation
method, to analyze and design MEMS devices.
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The atomic-force microscope
(diagrammed here) can "see" MEMS' tiny surfaces
(12K
JPG) |
Plesha, who is developing the models, brings
a wealth of experience solving engineering contact and friction problems
in geological systems at considerably larger sizes—up to the kilometer
scale. “But we think that, in certain specific ways, a lot of
the ideas are actually applicable,” says Carpick of the larger
scale. “And that’s one of the things I think is very cool,
very fascinating, about this project: that we might make some connections
to friction at the earthquake-geological fault level.”
Wouldn’t it make more sense just to grind
and polish “rough” surfaces smooth? You’d think, says
Carpick, because if they’re rough, you’d imagine gouging
and scratching. “But if they’re too smooth, what happens
is they touch everywhere and then you get a very high friction force
because the area of contact is too large,” he says.
And in fact, scientists at Sandia and elsewhere
can control how rough or smooth they make a surface. Right now, however,
they don’t know which works best. “Ultimately, our goal
is to be able to tell them, ‘If you want a micromachine that’s
going to last, this is how you should design your surface,’”
says Carpick.
