Rack-and-pinion effects in molecular rolling friction

TL;DR

This paper investigates the physics of molecular rolling friction, revealing that matching the molecular 'pitch' with substrate periodicity can significantly reduce friction, akin to a rack-and-pinion system, and suggests experimental ways to optimize this matching.

Contribution

It introduces a novel analogy between molecular rolling friction and rack-and-pinion systems, showing how matching periodicities can minimize friction and proposing experimental methods to explore this effect.

Findings

Deep minima in molecular rolling friction occur when pitch matches substrate periodicity.

Poor matching leads to discontinuous, noisy rolling and higher energy dissipation.

The analogy suggests new experimental approaches to control nanoscale friction.

Abstract

Rolling lubrication with spherical molecules working as 'nanobearings' has failed experimentally so far, without a full understanding of the physics involved and of the reasons why. Past model simulations and common sense have shown that molecules can only roll when they are not too closely packed to jam. The same type of model simulations now shows in addition that molecular rolling friction can develop deep minima once the molecule's peripheral 'pitch' can match the substrate periodicity, much as ordinary cogwheels do in a rack-and-pinion system. When the pinion-rack matching is bad, the driven molecular rolling becomes discontinuous and noisy, whence energy is dissipated and friction is large. This suggests experiments to be conducted by varying the rack-and-pinion matching. That could be pursued not only by changing molecules and substrates, but also by applying different sliding directions within the same system, or by applying pressure, to change the effective matching.