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Friction and Superlubricity
July 2, 2015
Friction is one
physical principle that's important to the
common man for the simple reason that it costs him
money. When you sum all losses due to friction, such as increased
fuel use and
component replacement, there are estimates that friction accounts for losses equivalent to up to 10% of the US
gross domestic product.[1] I wrote about friction in several previous articles (
Tribology, April 27, 2015,
Van der Waals Friction, August 12, 2013, and
Friction, February 1, 2012).
We invest a lot of money in
lubricants and
lubricious coatings, such as
Teflon, but the best money spent is that on
research into how friction arises at the
atomic level. Knowledge of that can lead to
innovations in friction reduction. Friction is such an important
phenomenon that it captured the attention of
Leonardo da Vinci (1452-1519), who discovered that friction is a function of
load, only, and not a function of
contact area.
Guillaume Amontons (1663-1705) restated da Vinci's results, and he
theorized that friction arose form the
work needed to lift an object over
surface roughness.[3] After the discovery of the atomic nature of
matter, subsequent
conjectures on the fundamental processes for friction needed to await advances in
technology and
instrumentation, such as those that enabled recent
research at the
Massachusetts Institute of Technology (Cambridge, Massachusetts) just reported in
Science.[2-5]
A team of
physicists at MIT has performed an interesting
experiment in which they were able to adjust the difference in atomic spacing between two rubbing surfaces and measure their friction. Their
model system had one surface composed of up to six
ions moving across another "surface" simulated by an
optical lattice (see figure).
Atoms are represented as balls on springs in this representation of ions sliding along the corrugations of an optical lattice.
In the top case, the periodicity of the ions and the lattice are the same. In the lower case, they're different.
(Still Images from a YouTube video.)[5)]
When atomically smooth surfaces are involved, friction usually occurs by a
stick-slip process.[2] Friction is important at the
nanoscale, where greater frictional forces are involved that can destroy small, moving mechanisms much faster than their larger counterparts. Says
Vladan Vuletic, a
professor in the
MIT Physics Department,
"There's a big effort to understand friction and control it, because it's one of the limiting factors for nanomachines, but there has been relatively little progress in actually controlling friction at any scale... What is new in our system is, for the first time on the atomic scale, we can see this transition from friction to superlubricity.[4]
A principal part of the MIT experiments is an optical lattice. This is an
egg carton shaped region of
electric potential creating by
interfering two
laser beams traveling in opposite directions. Their
electric fields form a
sinusoidal pattern in one
dimension.[4] Atoms traveling across an optical lattice are drawn to regions of minimum
potential, so the arrangement is an
analog to having an object move on an irregular surface.[4] In the experiments, from one to six ions could be moved across the optical lattice.[3]
While the optical lattice acts as one surface in the friction couple, the other is an ion
crystal comprised of
ionized ytterbium atoms. These atoms, sourced from a
small heated oven, are ionized by one laser, and then cooled to near
absolute zero by another. They're then trapped on a
metallic surface where their charges cause a
Coulombic repulsion that keeps them arrayed into a
lattice on the metal surface.[2,4]
The lasers allowed the research team to stretch and squeeze the ion crystal and push and pull it across the trapped ytterbium atoms.[4] Maximum friction, in a slip-stick fashion, was found when the period of the optical lattice matched that of the ytterbium ion array.[2,4] Says Vuletic, "It's like an
earthquake... There's force building up, and then there's suddenly a
catastrophic release of
energy."[4]
When the optical lattice period is mismatched to the atom spacing, friction between the two surfaces vanishes. At that point, stick-slip vanishes, the atoms can move fluidly across the optical lattice, and "
superlubricity" is achieved.[4] In this arrangement, some atoms are in troughs of the electric potential, others are at its peaks, and still others are somewhere in between. When the atom array is pulled across, the sliding of one atom down a peak relieves some
stress, and this allows another atom to climb out of a trough.[4]
The MIT research was funded by the
National Science Foundation and the
National Sciences and Engineering Research Council of Canada.[4]
In more
conventional friction research, scientists from the
Jülich Research Center (Jülich, Germany), the
Hankook Tire Co. LTD.(Daejeon, South Korea) and
Multiscale Consulting (Jülich, Germany) have been investigating the
molecular scale mechanisms responsible for
rubber friction for
tire tread compounds on
asphalt road surfaces.[6-7] They used measurements of road surface
topographies by
atomic force microscopy and conventional
stylus instruments, and they measured the friction at different
temperatures and sliding
speeds.[6] The stylus measurements were limited to a speed less than a
meter per second to avoid
heating from friction.[6]
While road
asperities, the rough points of the road, cause
viscoelastic deformations of the rubber surface that lead to friction, the research team found that
mechanical shear, when rubber is dragged
parallel to the road surface, is also a factor.[5] It appears that the shear friction arises from
bonding-stretching-
debonding cycles in the rubber molecules as they repeatedly stick to the road, stretch, and then release.[6]
Bo Persson, a scientist at the Jülich Research Center in Germany, developed a
model of rubber friction that incorporated the viscoelasticity of the particular rubber compound and this shear effect. The
published experiments show a good
fit to Persson's
theory, which shows that temperature and speed are important
parameters.[6]
Says Bo Persson of the Jülich Research Center (Jülich, Germany), who has studied friction for twenty years, "Rubber friction is an extremely interesting topic and of extreme practical importance, for tires and very many other applications."[6]
(FZ Jülich photograph.)[6)]
These results, however, only apply to clean,
dry surfaces.
Wet surfaces would prevent the binding of the rubber molecules to the asphalt, so the shear contribution to friction is absent. In that case, a simplified model based on just viscoelasticity will give good results.[6]
References:
- Kenneth G. Budinski, "Friction, Wear, and Erosion Atlas," CRC Press, November 6, 2013, 309 pp. (via Google Books).
- Alexei Bylinskii, Dorian Gangloff, and Vladan Vuletić, "Tuning friction atom-by-atom in an ion-crystal simulator," Science, vol. 348, no. 6239 (June 5, 2015), pp. 1115-1118, DOI: 10.1126/science.1261422.
- Ernst Meyer, "Perspective, Physics - Controlling friction atom by atom," Science, vol. 348, no. 6239 (June 5, 2015), p. 1089, DOI: 10.1126/science.aab3539.
- Jennifer Chu, "Vanishing friction," MIT Press Release, June 4, 2015.
- Vanishing friction, Massachusetts Institute of Technology YouTube Video (Video produced and edited by Melanie Gonick, computer simulations courtesy of Alexei Bylinkskii), June 4, 2015.
- B. Lorenz, Y. R. Oh, S. K. Nam, S. H. Jeon, and B. N. J. Persson, "Rubber friction on road surfaces: Experiment and theory for low sliding speeds, "J. Chem. Phys., vol. 142, no. 19 (May 21, 2015), Document No. 194701, http://dx.doi.org/10.1063/1.4919221. A free PDF download of this paper is available at the link.
- Where the rubber meets the road, American Institute of Physics Press Release, May 15, 2015.
Permanent Link to this article
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