In the last 25 years, the global trend to miniaturization has raised interest in frictional processes on the micro and nano scale and facilitated appearance of nanotribology as a new branch of research. Soon it became clear that looking at tribological processes at micro- and nano-scale can be very fruitful also for macroscopic systems. Researchers recognized that these processes determine the macroscopic behavior via a number of mesoscopic levels. At first, this was perceived as a “gap” between macro and micro worlds. Over time, countless attempts were undertaken to “fill the gap” – both top-down and bottom-up.

Recently, the Journal Frontiers in mechanical Engineering | Tribology published an Open Access E-Book titled “Friction and Wear: From Elementary Mechanisms to Macroscopic Behavior”. The E-Book presents an Article Collection illustrating the current state of attempts to bring micro and macro together.

It is opened by a review “Controlling Friction With External Electric or Magnetic Fields: 25 Examples” by Jacqueline Krim. Both the title and the author are symbolic for the topic. In the late 1990s, her experimental works on interfacial energy dissipation made an important contribution to the emerging field of nanotribology and generally the physical understanding of interfacial processes. The review presents 25 examples of active control of friction from recent literature thus reminding us of the huge complexity of tribological phenomena. At the same time, it clearly shows that they can be understood and well controlled.

One of the most fundamental concepts discovered and analyzed in the last 25 years is the concept of superlubricity. It is classified in solid (or structural) superlubricity and liquid superlubricity, both of which are addressed in this article collection. In their paper “Macroscale Superlubricity Achieved With Various Liquid Molecules: A Review” Xiangyu Ge, Jinjin Li and Jianbin Luo discuss the advancements in liquid superlubricity at the macroscale.

Martin Müser combines the fundamental understanding of structural superlubricity with the advances of the contact mechanics of rough surfaces. He successfully links the scales and answers the question “Are There Limits to Superlubricity of Graphene in Hard, Rough Contacts?”

Any mechanism of friction considered at the atomic scale can be seen as the formation and destruction of intermolecular bonds, or small scale “adhesive contacts”. Therefore it is not surprising that the adhesive interaction of rough surfaces has long been of interest to friction physicists. Li, Pohrt and Popov employ a recent numerical technique to analyze both spreading and detaching of rough surfaces. Their paper “Adhesive Strength of Contacts of Rough Spheres” gives important implications: It is found that the short range adhesion is depending only on one single parameter introduced by Kenneth Johnson in 1995!

Surface roughness is the key factor leading to a scale-dependency of the “real contact area”. One of the central problems and challenges here is to not only determine the surface profile at one moment in time but to also follow its development during the relative sliding of bodies. The paper “Evolution of the True Contact Area of Laser Textured Tungsten Under Dry Sliding Conditions” by Lechthaler, Ochs, Mücklich, and Dienwiebel is devoted to an experimental investigation of the true contact area. In the work by Lechthaler, Ochs, Mücklich, and Dienwiebel, a combination of methods was used to estimate the development of the real contact surface, and the results were compared with the evolution of the coefficient of friction.

Yonggang Meng, Alexander Kovalev, Yazhao Zhang and Hui Cao present an efficient way of predicting wear evolution. They devote their paper “A concept of the effective surface profile to predict the roughness parameters of worn surface” to a theoretical study of the development of surface topography.

The paper “Atomistic Insights Into Lubricated Tungsten/Diamond Sliding Contacts” by Romero, Mayrhofer, Stoyanov, Merz, Kopnarski, Dienwiebel, and Moseler is an excellent illustration of the possibilities of both contemporary atomistic modeling and experimental analysis at the atomic scale. The authors reveal the chemical roots of the very low friction resistance in that system. It is the formation of a hydrocarbon film, preventing the partners from cold welding!

An intermediate scale between atomic and macroscopic is the scale of single asperities. Atomic force microscopy is the method of choice for experimental research at this scale. The authors of the paper “Role of Interfacial Water and Applied Potential on Friction at Au(111) surfaces” investigate how, in the presence of interfacial water, friction can be controlled by an electric field.

Friction in soft matter, including elastomers can be directly linked to rheological properties but such understanding is not extensive. The experimental study in the paper “Investigation on Dynamic Response of Rubber in Frictional Contact” shows that we are still far from understanding of how the dynamic response in frictional contacts should be described. A practical example in the form of a seal in machining tools is given in the last paper by researchers from Kumamoto University.

Tribologists will continue their quest to “bridge the gap” between nano and macro scale and the current E-Book is a representative landmark in this endeavor.

The complete E-Book can be downloaded here:

https://www.frontiersin.org/research-topics/8230/friction-and-wear-from-elementary-mechanisms-to-macroscopic-behavior

This article is based on the Editorial by R. Pohrt and V. Popov: https://doi.org/10.3389/fmech.2019.00047