LOW FRICTION IN BOUNDARY LUBRICATION - BY STRUCTURE OR DYNAMICS?

摘要

Low friction is an important and desirable attribute for a number of boundary lubricated systems. We ask the question what are the most promising strategies to attain low friction with nanometer thick lubricant films. Is it the structure or the dynamics of the boundary layers, or, maybe both? Using the extended surface forces apparatus we are investigating structural and dynamic effects on friction - in some cases at sub-Angstrom resolution. Results obtained with different molecular systems exhibiting low friction are presented. A water-soluble, PEG containing co-polymer architecture was used to construct a water-based lubrication system. A high-resolution measurement of the compression isotherm revealed film-thickness transitions under confinement that are due to a water-induced restriction of the conformational space of PEG. These findings are in accord with known solution properties of this polymer. The friction is found to be vanishingly small in a dilute aqueous solution of this co-polymer. Using an alternative approach, the active control of static and dynamic friction was demonstrated using sub-nanometer mechanical oscillations across a nanometer thick liquid layer. The dynamic frustration of molecular arrangements is responsible for this effect. Oscillations of just a few Angstrom are sufficient to reduce friction to a small fraction of the unperturbed system. Molecular film-thickness changes were monitored throughout the contact zone and the effect of the oscillations on the molecular dynamics was measured.