Tikalon Blog by Dev Gualtieri

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).

Sliding friction model using an optical lattice and ions

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]

Friction and Superlubricity

References: