Physicists have a pretty good idea of what to expect
when friction and adhesion occur in the visible
world. You jam on the brakes, for instance, and
your tires and the highway interact to stop your
car. You glue two pieces of wood together, and
they stick.
But how slippery or sticky are things that are too
small to see? When solid surfaces no more than a
thousand atoms across brush past each other, will
they respond like the rubber and the road? Will they
adhere like the wood and the glue?
The answer turns out to be "It depends," according
to Johns Hopkins physicists who used computer modeling
to examine how friction and adhesion operate on
the atomic level.
"Any surface made of individual atoms has 'bumps'
of atomic dimension, and being able to vary the placement
of atoms [in the computer models] allowed us to quantify
the influence of atomic structure," said Mark
O. Robbins, a professor in the Henry A. Rowland Department
of Physics and Astronomy in the university's Krieger
School of Arts and Sciences.
The
modeling showed that surfaces from a few to a thousand
atoms across, with the same shape, but with different
local structures, or "bumps,"
behave quite differently, even if those surfaces
are made of the same material, Robbins said. Local
stresses and adhesion forces can vary by a factor
of two or more, and friction can change tenfold,
he said.
The research is reported in the June 16 issue of
the journal Nature by Robbins and graduate student
Binquan Luan. Their findings could one day help in
the successful design of nanomachines, the name given
to devices built by manipulating materials on an
atomic scale.
"Everyone
knows that matter is made up of discrete atoms,
yet most models of mechanical behavior ignore this
and think of atoms as being 'smeared'
into an artificial continuous medium," Robbins
said. "This approach works well when describing
the behavior of larger machines, but what happens
when the whole machine is only a few to a thousand
atoms across? The answer is crucial to the function
of man-made nanomachines and many biological processes."
Robbins'
and Luan examined contact between solid surfaces
with "bumps"
whose radii varied from about 100 to 1,000 atomic
diameters. Bumps that size might be typical of nanomachine
surfaces or the tips of atomic force microscopes
used to measure mechanical properties at the atomic
scale.
Using
computer simulations, the team followed the displacements
of up to 10 million atoms as the solid surfaces
were pushed together. They then compared these
displacements and the total adhesion and friction
forces to calculations of the same forces using the
standard "continuum theory," the model
that views matter as having smeared rather than discrete
atoms.
"Knowing the exact atomic structure and how
each atom moved allowed us to test the two key assumptions
of continuum theory," Robbins said. "While
it described the internal response of solids down
to nearly atomic scales, its assumption that surfaces
are smooth and featureless failed badly" at
the atomic level.
In
a "News and Views" paper accompanying
the Nature article, Jacob Israelachvili of University
of California, Santa Barbara, noted that these results
have fundamental implications for the limits of theories
that try to "smear out" atomic structure,
as well as indicating "how surfaces might be
tailored in desirable ways ... if atomic-scale details
are taken into consideration."
This work is important because of the growing interest
in nanotechnology, in which unwanted adhesion and
excessive friction can cause devices to malfunction
or just not to work, Robbins said. "Hopefully,
this will help in the creation of new tools needed
to guide the design of nanotechnology"
he said.
The study was funded by the National Science Foundation.
Related Web site:
Mark Robbins' Web page: http://www.pha.jhu.edu/~mr/mr.html
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