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This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

We study the role of electronegativity in sliding friction for five different two dimensional(2D) monolayer systems using density functional theory (DFT) with van der Waals (vdW)corrections. We show that the friction between the commensurate 2D layered systems dependsstrongly on the electronegativity difference of the involved atoms. All the 2D layered structuresexhibit almost the same magnitude of friction force when sliding along the nonpolar path,independent of the material and the surface structures. In contrast, for sliding friction along thepolar path, the friction force obeys a universal linear scaling law as a function of theelectronegativity difference of its constituent atoms. Further analyses demonstrate that atomicdipoles in the 2D monolayers induced by the electronegativity difference enhance the corrugation ofcharge distribution and increase sliding barrier accordingly. Our studies revealthat electronegativity plays an important role in friction of low dimensional systems, and willprovide a strategy for designing nanoscale devices further.Keywords: Electronegativity; Friction; Two-dimensional (2D) monolayer; Density functionaltheory (DFT)

Most asteroids with a diameter larger than $\sim 300 \ {\rm m}$ are rubble piles i.e. consisting of more than one solid object. All asteroids are rotating but almost all asteroids larger than $\sim 300 \ {\rm m}$ rotate with a period longer than $2.3 \ {\rm hours}$, which is the critical period where the centrifugal force equals the gravitational force. This indicates that there are nearly no adhesive interaction forces between the asteroid fragments. We show that this is due to the surface roughness of the asteroid particles which reduces the van der Waals interaction between the particles by a factor of $100$ for micrometer sized particles and even more for larger particles. We show that surface roughness results in an interaction force which is independent of the size of the particles, in contrast to the linear size dependency expected for particles with smooth surfaces. Thus, two stone fragments of size $100 \ {\rm nm}$ attract each other with the same non-gravitational force as two fragments of size $10 \ {\rm m}$., 24 pages, 30 figures