Friction and Dissipation in Surfaces Functionalized by Organic Monomolecular Assemblies

Abstract

That organic monolayers provide protection to surfaces in contact and in relative motion is recognized in a wide range of applications; from artificial human joints to micromachines to automobile engines. The contact configurations and tractions in these applications vary over a span of many orders of length scales and pressures respectively. In this essay we first review past works to explore the validity of basic laws of friction over these spans of length scale and pressure and demonstrate this validity for two practical systems; silanes on aluminium and silicon, and fatty acids on steel. Such validity provides a means for design of microelectro-mechanical systems as much as it provides a rationale for design and selection of boundary lubricants in large machineries. Our present understanding of the genesis of friction in self assembled monolayers is reviewed next. Application of traction to these monolayers generates translational and rotational motions at the level of chemical bonds which disturb the crystallographic symmetries and generates disorder. On unloading these stored energies are dissipated. The more the avenues of energy dissipation higher is the frictional resistances. The frictional work equilibrates with the total energy that can be dissipated. We review the experimental and theoretical basis of this understanding and explore the dissipation mechanism experimentally. We conclude that translational motion, initiated almost instantaneously on contact, is purely elastic in character and consists of collective tilt and bending of molecular backbones accompanied by bond stretching. This is followed by viscoelastic rotational motions which generate gauche defects. The latter process is only partially reversible. The latter introduces a time element into the frictional response which we demonstrate is related to the ratio of contact intermittency which is a function of velocity and stroke length and relaxation time which is a function of the applied pressure and the molecular configuration of the monolayer.