What is the origin of macroscopic friction?

What is the origin of molecular friction, and how can macroscopic friction be explained in terms of molecular friction? To elucidate the origins of molecular and macroscopic friction, we conducted density functional theory calculations and double-direct shear tests at normal stresses ranging from 5 to 60 MPa for mica surfaces. Frictional forces between mica surfaces were theoretically predicted to oscillate periodically every 30° of sliding direction, in agreement with previous experimental findings. This result affirms that the potential energy roughness of mica under sliding is the origin of molecular friction, which depends on the normal stress and sliding direction. The discovered mechanism of molecular friction can quantitatively explain experimentally observed macroscopic friction of mica when the presence of wear particles is taken into consideration.

Studying physisorption processes and molecular friction of cycloparaphenylene molecules on graphene nano-sized flakes: role of π⋯π and CH⋯π interactions

We theoretically study, by means of dispersion-corrected and cost-effective methods, the strength of non-covalent interactions between cyclic organic nanorings and nano-sized graphene flakes acting as substrates.

Thermal Effects while Fatigue Cracking Growth under Bending

The paper presents the results of fatigue tests where temperature changes on specimen surfaces were registered. Some different materials were tested. A relation between the crack growth and temperature changes in the propagation place was found. The highest temperature gradients were measured on the crack growth path, and it was caused by molecular friction.

Boron and carbon: Antagonistic or complementary? Proposal for a simple prototype of a molecular clutch or molecular switch

Boron and carbon, either in elemental form or when combined, are structurally very different. They are indeed complementary, and the weaknesses of one can be complemented by the strengths of the other, and vice versa. The structural complementarity can be readily observed in the shape of [XnHn]y– (X = C or B) compounds. One visualization of this complementarity can be found by comparing the most popular carbon and boron organometallic sandwich molecules, [Fe(C5H5)2] and [3,3'-Co(1,2-C2B9H11)2]–. Both obey the 18e– rule, and in both the metal is η5 coordinated by two pentagonal faces. However, for [Fe(C5H5)2], the first ring of atoms outside the pentagonal face is coplanar with the coordinating face, whereas for [3,3'-Co(1,2-C2B9H11)2]– the substituents are out of the coordinating face featuring a canopy shading the metal. Taking advantage of this feature, [3,3'-Co(1,2-C2B9H11)2]– can be a well-performing molecular clutch electrochemically driven. When it is engaged, the beams of the upper [7,8-C2B9H11]2– ligand in [3,3'-Co(1,2-C2B9H11)2]– mesh the beams of the lower [7,8-C2B9H11]2–. This occurs when the molecular friction disk, the Co, is as Co3+. When Co3+ is reduced to Co2+, its radius is elongated, and both sets of beams are unmeshed allowing for a more free rotation, or molecular clutch disengagement.