Cylinder-flat contact mechanics during sliding

TL;DR

This study uses molecular dynamics to analyze how sliding speed affects contact mechanics in cylinder-flat contacts, revealing differences between commensurate and incommensurate surfaces and highlighting the role of frictional forces.

Contribution

It provides new insights into the velocity dependence of friction and contact area in molecular-scale cylinder-flat contacts, considering both commensurate and incommensurate surface interactions.

Findings

Incommensurate contact forces fluctuate around zero with sliding speed.

Commensurate contact exhibits higher, velocity-independent friction.

Contact width remains constant regardless of sliding speed.

Abstract

Using molecular dynamics (MD) we study the dependency of the contact mechanics on the sliding speed when an elastic block (cylinder) with a ${\rm cos} (q_0 x)$ surface height profile is sliding in adhesive contact on a rigid flat substrate.The atoms on the block interact with the substrate atoms by Lennard-Jones (LJ) potentials, and we consider both commensurate and(nearly) incommensurate contacts. For the incommensurate system the friction force fluctuates between positive and negative values, with an amplitude proportional to the sliding speed, but with the average close to zero. For the commensurate system the (time-averaged) friction force is much larger and nearly velocity independent. For both type of systems the width of the contact region is velocity independent even when, for the commensurate case, the frictional shear stress increases from zero (before sliding) to $\approx 0.1 \ {\rm MPa}$ during sliding. This frictional shear stress, and the elastic modulus used, are typical for Polydimethylsiloxan (PDMS) rubber sliding on a glass surface, and we conclude that the reduction in the contact area observed in some experiments when increasing the tangential force must be due to effects not included in our model study, such as viscoelasticity or elastic nonlinearity