February 9, 2017
As we now routinely venture into the nanoscale realm, we find that nanoparticulate materials do not have the same properties as their bulk counterparts, and small machines do not behave as the larger versions. In essence, there's a limit to how much we can minimize familiar objects and materials and still obtain the properties we find for their "supersize" versions.
The Greek philosophers, Leucippus (fl. 5th cent. BC), and Democritus (c.460-c.370 BC) were the first to argue that there's a limit beyond which matter can't be reduced. This idea, called atomism from the Greek words meaning "not-cutable," is now an accepted part of modern science. It did, however, take 2,300 years to go from what was just an idea to an established scientific fact.
Leucippus is known only through the writings of Democritus, and much of what Democritus wrote is known to us only secondhand, such as mentions in the works of Aristotle. As were other philosophers of his time, Democritus was interested in many scholarly pursuits, including music, physics, cosmology, ethics, and mathematics. Democritus is said to have been the first to observe that a cone inscribed in a cylinder has one third the cylinder's volume.
As if progenitors of the expression, "making something out of nothing," Leucippus and Democritus theorized that the natural world was built not just from atoms, but from a void as well. The void separates the atoms; and, as a result, atoms can combine for a time, but they cannot fuse. The different things that we see are special arrangements of atoms. This explains change in the world despite the permanence of matter. Aristotle, who was critical of Democritus' ideas, still praised the logic of this world system.
While atoms have conveniently explained the properties of crystalline matter, we have just recently been examining how very small assemblages of atoms interact with each other on the nanoscale. The darling nanoscale material of the past few decades has been graphene, and its importance is underscored by the short span of time between its discovery and the award of a Nobel Prize to its discoverers. Andre Geim and Konstantin Novoselov published their first paper on graphene in 2004, and they received the Nobel Prize in Physics in 2010.
Graphite is used as a dry lubricant, and scientists are likewise interested in the tribology of graphene, which is a single layer of graphite. As I wrote in an earlier article (Friction, February 1, 2012), there have been some surprises. Scientists at the National Institute of Standards and Technology (NIST) have conducted friction experiments on graphene using atomic force microscopy (AFM) by rubbing AFM tips on single and multi-layer graphene sheets.[3-4]
The NIST experiments revealed that single layers of graphene have considerable friction, but the friction is reduced in graphene stacks. The hypothesized explanation for this behavior is that the top layer of a graphene stack deforms more when there are fewer layers beneath it. For a single layer, this warping creates a rolling resistance, so the friction is larger.
A large international team of scientists from the Xi'an Jiaotong University (Xi'an, China), the Massachusetts Institute of Technology (Cambridge, Massachusetts), the Karlsruhe Institute of Technology (Eggenstein-Leopoldshafen, Germany), Tsinghua University (Beijing, China), the University of Pennsylvania (Philadelphia, Pennsylvania), and the Fraunhofer Institute for Mechanics of Materials (IWM, Freiburg, Germany), have continued studies on graphene friction. They published their findings in an article in Nature at the end of 2016.[5-6] In this study, the research team looked at the minuscule variation in friction as atoms interact between the sliding surfaces.
This new study addresses not just the discovery that multi-layered graphene has less friction than a monolayer, but that the static friction diminishes after travel over a few atomic periods to then reach a constant value. This phenomenon has been seen, also, in the layered material, molybdenum disulfide, but it has not been reproduced in simulations.[5-6]
Contact area is known to be a factor in the interfacial friction between two rough bodies, and for graphene it would be defined by the number of atoms within the range of interatomic forces. For a given pair of materials, the nature of this contact can change during sliding to affect interfacial friction. Says Ju Li, a professor in the MIT departments of Nuclear Science and Engineering and Materials Science and Engineering,
"There is this broad notion in tribology that friction depends on the true contact area - that is, the area where two materials are really in contact, down to the atomic level. The true contact area is often substantially smaller than it would otherwise appear to be if observed at larger size scales. Determining the true contact area is important for understanding not only the degree of friction between the pieces, but also other characteristics such as the electrical conduction or heat transfer."
In the case of large objects, such as machine gears, the actual amount of material contacting one gear with another is smaller than it appears, since the gear teeth are rough and only the protruding points on the surfaces make contact. Polished gears would be flatter, their contact area would be larger, and the friction would likewise be larger. An accurate simulation would consider the true contact area.
Using atomistic simulations, the research team was able to reproduce these experimental observations on graphene of layer-dependent friction and transient frictional strengthening. The contact area was found to depend on how well atoms were aligned on the two contact surfaces, and on the synchrony of these alignments during motion. The atom-scale simulations revealed that the changes noted in static friction were a consequence of graphene's adjustment to the atomic forces made possible by its flexibility.
It was further found that pre-wrinkling the graphene sheets would change their tribology. Says Li, "We can use that to vary the friction by a factor of three, while the true contact area barely changes." It's supposed that these results will apply to other two-dimensional materials, such as boron nitride and molybdenum disulfide. This research was supported by the National Science Foundation.
- Democritus, Stanford Encyclopedia of Philosophy, The Metaphysics Research Lab, Center for the Study of Language and Information (CSLI), Stanford University, December 2, 2016.
- K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, "Electric Field Effect in Atomically Thin Carbon Films," Science, vol. 306, no. 5696 (October 22, 2004), pp. 666-669.
- Laura Ost, "Slippery When Stacked: NIST Theorists Quantify the Friction of Graphene," NIST Tech Beat, January 10, 2012.
- A. Smolyanitsky, J. P. Killgore and V. K. Tewary, "Effect of elastic deformation on frictional properties of few-layer graphene," Phys. Rev. B, vol. 85, no. 3 (2012), Document No. 035412 (6 pages).
- Suzhi Li, Qunyang Li, Robert W. Carpick. Peter Gumbsch, Xin Z. Liu, Xiangdong Ding, Jun Sun, and Ju Li, "The evolving quality of frictional contact with graphene," Nature. vol. 539, no. 7630 (November 24, 2016), pp. 541-545, doi:10.1038/nature20135
- David L. Chandler, "The science of friction on graphene," MIT Press Release, November 23, 2016.
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