Tribology is the study of adhesion, friction, lubrication and wear of surfaces in relative motion. It remains as important today as it was in ancient times, arising in the fields of physics, chemistry, geology, biology and engineering. The more we learn about tribology the more complex it appears. Nevertheless, recent experiments coupled to theoretical modelling have made great advances in unifying apparently diverse phenomena and revealed many subtle and often non-intuitive aspects of matter in motion, which stem from the nonlinear nature of the problem.

The nonlinear nature of friction

The physics of sliding friction is gaining impulse from nanoscale and mesoscale experiments, simulations, and theoretical modeling This Colloquium reviews some recent developments in modeling and in atomistic simulation of friction, covering open-ended directions, unconventional nanofrictional systems, and unsolved problems

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Colloquium : Modeling friction: from nanoscale to mesoscale

As the size of electronic and mechanical devices shrinks to the nanometre regime, performance begins to be dominated by surface forces. For example, friction, wear and adhesion are known to be central challenges in the design of reliable micro- and nano-electromechanical systems (MEMS/NEMS). Because of the complexity of the physical and chemical mechanisms underlying atomic-level tribology, it is still not possible to accurately and reliably predict the response when two surfaces come into contact at the nanoscale. Fundamental scientific studies are the means by which these insights may be gained. We review recent advances in the experimental, theoretical and computational studies of nanotribology. In particular, we focus on the latest developments in atomic force microscopy and molecular dynamics simulations and their application to the study of single-asperity contact.

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Recent advances in single-asperity nanotribology

Stiction and wear are demanding problems in nanoelectromechanical devices, because of their large surface-to-volume ratios and the inapplicability of traditional liquid lubricants. An efficient way to switch friction on and off at the atomic scale is achieved by exciting the mechanical resonances of the sliding system perpendicular to the contact plane. The resulting variations of the interaction energy reduce friction below 10 piconewtons in a finite range of excitation and load, without any noticeable wear. Without actuation, atomic stick-slip motion, which leads to dissipation, is observed in the same range. Even if the normal oscillations require energy to actuate, our technique represents a valuable way to minimize energy dissipation in nanocontacts.

https://science.sciencemag.org/content/sci/313/5784/207.full.pdf

Atomic-Scale Control of Friction by Actuation of Nanometer-Sized Contacts

Dynamic force spectroscopy of single molecules is described by a model that predicts a distribution of rupture forces, the corresponding mean rupture force, and variance, which are all amenable to experimental tests. The distribution has a pronounced asymmetry, which has recently been observed experimentally. The mean rupture force follows a (lnV)2/3 dependence on the pulling velocity, V, and differs from earlier predictions. Interestingly, at low pulling velocities, a rebinding process is obtained whose signature is an intermittent behavior of the spring force, which delays the rupture. An extension to include conformational changes of the adhesion complex is proposed, which leads to the possibility of bimodal distributions of rupture forces.

https://www.pnas.org/content/pnas/100/20/11378.full.pdf

Beyond the conventional description of dynamic force spectroscopy of adhesion bonds

We model friction acting on the tip of an atomic force microscope as it is dragged across a surface at nonzero temperatures. We find that stick-slip motion occurs and that the average frictional force follows $|\mathrm{ln}v{|}^{2/3}$, where $v$ is the tip velocity. This compares well to recent experimental work, permitting the quantitative extraction of all microscopic parameters. We calculate the scaled form of the average frictional force's dependence on both temperature and tip speed as well as the form of the friction-force distribution function.

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Thermal effects on atomic friction.

Atomic stick-slip processes have been studied in detail by means of friction force microscopy with high spatial and temporal resolution. The influence of the tip-sample contact on the thermal fluctuations of the force sensor and on the dynamics of the stick-slip process are characterized. Results are compared with simulations based on an extended Tomlinson model including thermal fluctuations. A correlation between the duration of the atomic slip event and the atomic structure of the contact is established.

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Fluctuations and jump dynamics in atomic friction experiments

We study the dependence of friction on surface roughness, sliding velocity, and temperature. Expanding on the classic treatment of Greenwood and Williamson, we show that the fractal nature of a surface has little influence on the real area of contact and the static friction coefficient. A simple scaling argument shows that the static friction exhibits a weak anomaly mu ~ A(0)(-chi/4), where A0 is the apparent area and chi is the roughness exponent of the surface. We then develop a method to calculate atomic-scale friction between a microscopic asperity, such as the tip of a friction force microscope (FFM) and a solid substrate. This method, based on the thermal activation of the FFM tip, allows a quantitative extraction of all the relevant microscopic parameters and reveals a universal scaling behavior of atomic friction on velocity and temperature. This method is extended to include a soft atomic substrate in order to simulate FFM scans more realistically. The tip is connected with the support of the cantilever by an ideal spring and the substrate is simulated with a ball-spring model. The tip and substrate are coupled with repulsive potentials. Simulations are done at different temperatures and scanning velocities on substrates with different elastic moduli. Stick-slip motion of the tip is observed, and the numerical results of the friction force and distribution of force maxima match the theoretical framework.

https://www.physics.mcgill.ca/~grant/Papers/elastic.pdf

Yi Sang

Papers

Thermal effects on atomic friction.

Fluctuations and jump dynamics in atomic friction experiments

Dependence of friction on roughness, velocity, and temperature

Dependence of friction on roughness, velocity, and temperature