Johns Hopkins Gazette: September 6, 1994

By Emil Venere 

Friction plays a key role in everyday life. Without it,
people wouldn't be able to walk, cars wouldn't stay parked on
hilly grades, and buildings would fall down.
    At the same time, friction is a serious handicap. It
consumes as much as 20 percent of the energy used by an
internal combustion engine and causes engine parts to wear
out. If this fundamental force can be mastered, scientists
might one day create substances that reduce or enhance the
effects of friction, resulting in superior materials and more
efficient machines.
    But researchers have a lot to learn about the workings
of friction before that day is here. They don't know, for
example, how all the energy consumed by cars gets converted
into heat by friction.
    "Our understanding of the molecular origins of this
important force is relatively primitive," Mark O. Robbins, a
Hopkins professor of physics, wrote in the Aug. 26 issue of
    Dr. Robbins worked with Hopkins graduate student
Elizabeth D. Smith and visiting associate professor Marek
Cieplak, from the Institute of Physics in the Polish Academy
of Science, to create a computer model that sheds some light
on the mechanics of friction at the atomic level. Their
simulation reproduced the surprising results of recent
experiments conducted at Northeastern University.
    The Northeastern scientists found that friction behaves
much differently at the microscopic level than it does in the
familiar, macroscopic world. They injected krypton, which is
a gas at room temperature, into a vacuum chamber. The
researchers then cooled the chamber until a single layer of
krypton only one atom thick stuck to the surface of a gold
wafer inside the vacuum chamber. They were able to change the
krypton from a liquid to a solid by regulating the number of
atoms in the chamber, and they used a quartz crystal like the
one in a wristwatch to measure the friction produced when the
krypton layer slid over the gold.
    Their experiments yielded a startling discovery: a solid
layer of krypton slides more easily than a liquid layer over
the gold surface.
    This is just the opposite of what happens in the
macroscopic realm of everyday existence, where fluids clearly
slide with more ease and with less friction than solids. 
    But a closeup view, via the computer simulation, enabled
the Hopkins scientists to actually follow the motion of
krypton atoms. To explain why solids slide more readily than
fluids over the gold surface, Dr. Robbins likens the gold to
a layer of pingpong balls on a table and the krypton atoms to
tennis balls. The tennis balls would not sit right on top of
a pingpong ball, but slide down into the spaces between
several pingpong balls. When the krypton is fluid, its atoms
(tennis balls) have more freedom to move into the spaces. But
when it is solid, the krypton atoms are pressed together so
tightly that they can't slide sideways enough to find the
nearest hole to fall into.
    Therefore, the gold doesn't grab onto the solid layer as
well as the fluid layer, so the solid layer slides with more
ease and requires less frictional force than the fluid layer,
Dr. Robbins said.
    The simulation helped scientists develop a simple
equation describing the amount of friction involved in the
experiment. One factor is how readily krypton atoms fall into
"holes" in the gold surface. When sliding krypton atoms fall
into the holes, vibrations or sound waves are excited in the
layer of krypton. The more the atoms fall into the holes, the
greater the vibrations. The second factor is how rapidly
these molecular vibrations decay into heat. A more rapid
decay leads to higher friction.
    The equation reproduces the results of the Hopkins
simulation and the Northeastern experiments and could be used
to predict how other materials might behave in the same
experiment, Dr. Robbins said.

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