Reduction of frictional forces between solid surfaces bearing polymer brushes

THE use of lubricants to reduce friction and wear between rubbing surfaces has been documented since antiquity1–3. Recent approaches have focused on boundary lubrication by surfactant-like species coating the surfaces, whereby the friction between them is replaced by the weaker forces required for shear of adhesive contacts between the surfactant layers3,4. An alternative approach is to tether polymer chains to the surfaces by one end which, when swollen by a solvent, then act as molecular ‘brushes’ that may facilitate sliding. The normal forces between sliding brush-bearing surfaces have been previously investigated5,6, but the lateral forces, which are the most important from the point of view of lubrication, are harder to measure. Here we report the measurement of lateral forces in such a system. We find a striking reduction in the effective friction coefficients μb between the surfaces to below our detection limit (μb < 0.001), for contact pressures of around 1 MPa and sliding velocities from zero to 450 nm s−1. We believe that this effect is due to the long-ranged repulsion, of entropic origin, between the brushes, which acts to keep the surfaces apart while maintaining a relatively fluid layer at the interface between them.