Theory of the Observed Ultra-Low Friction between Sliding Polyelectrolyte Brushes

TL;DR

This paper presents a theoretical explanation for ultra-low friction observed between sliding polyelectrolyte brushes, attributing it to osmotic pressure from counterions that supports load without polymer entanglement.

Contribution

It introduces a modified mean field theory to explain how counterion osmotic pressure reduces friction between polyelectrolyte brushes.

Findings

Osmotic pressure can support loads of about 10^6 Pa.

Polymer entanglement is avoided at sufficient brush separation.

The theory explains observed ultra-low friction in experiments.

Abstract

It is shown using a method based on a modified version of the mean field theory of Miklavic Marcelja that it should be possible for osmotic pressure due to the counterions associated with the two polyelectrolyte polymer brush coated surfaces to support a reasonable load (i.e., about $10^6 Pa$) with the brushes held sufficiently far apart to prevent entanglement of polymers belonging to the two brushes, thus avoiding what is believed to be the dominant mechanisms for static and dry friction.