CHAMPAIGN, Ill. -- Scientists long have known that surface roughness
plays a major role in the wear and tear of moving parts. Now, researchers
at the University of Illinois have found that randomness, not roughness,
is a major contributor to friction at the molecular level.
In a series of measurements, U. of I. materials scientist Steve Granick
and graduate student A. Levent Demirel linked the occurrence of stick-slip
friction -- in which sliding surfaces momentarily stick and then slide --
with a random fracturing of the liquid lubricant. While other researchers
had previously observed the stick-slip motion, the tiny fluctuations were
generally attributed to experimental irregularities or background noise
in the measurements. The team's findings, which appeared in the Nov. 18
issue of Physical Review Letters, offer a new interpretation of a sticky
subject.
"We tend to take for granted that friction is an exact number,"
Granick said. "Our textbooks tell us that solid surfaces will remain
at rest until a certain frictional force is overcome; thereafter they will
move smoothly. We found that it really isn't that simple."
To identify the true nature of the stick-slip phenomenon, Granick and Demirel
placed a droplet of the synthetic lubricant squalane between two smooth
plates of mica. (The resulting film was about 2 nanometers thick, corresponding
to a layer of lubricant only 4 molecules high.) The researchers then forced
one plate to slide over the other, and recorded the frictional response.
They repeated their experiment over millions of stick-slip cycles.
"The overall stick-slip process was preceded by jagged, microscopic
stick-slip events," Granick said. "The sliding surface moved several
nanometers, paused, then moved again. The sliding motion was neither smooth
nor periodic; it was random."
Granick attributes the random sticking and sliding to a fracturing and healing
process occurring within the lubricant. When the surfaces are in motion,
the lubricant breaks down and the surfaces stick. The lubricant then reforms
at rest; the surfaces begin to slide and the process starts over.
But the fracturing doesn't happen the same way each time, and that random
nature shows clearly in the measurements, Granick said. Although the liquid
does not break down in a chemical sense, the physical characteristics do
change. The way in which the fluid breaks down and then heals gives rise
to the stick-slip motion.
"When studying friction at the molecular level, we must consider the
random fracturing of the lubricant film itself, in addition to all the conventional
mechanical principles that people have thought about for a long time,"
Granick said. "A better understanding of how the lubricant fractures,
and how we might control it, may lead to significantly reduced wear and
tear on machinery."