REDUCTION OF FRICTIONAL FORCES BETWEEN SOLID-SURFACES BEARING POLYMERBRUSHES

THE use of lubricants to reduce friction and wear between rubbing surf aces has been documented since antiquity(1-3). Recent approaches have focused on boundary lubrication by surfactantlike species coating the surfaces, whereby the friction between them is replaced by the weaker forces required for shear of adhesive contacts between the surfactant layers(3,4). An alternative approach is to tether polymer chains to th e surfaces by one end which, when swollen by a solvent, then act as mo lecular 'brushes' that may facilitate sliding. The normal forces betwe en sliding brush-bearing surfaces have been previously investigated(5, 6), but the lateral forces, which are the most important from the poin t of view of lubrication, are harder to measure. Here we report the me asurement of lateral forces in such a system. We find a striking reduc tion in the effective friction coefficients mu(b) between the surfaces to below our detection limit (mu(b) < 0.001), for contact pressures o f around 1 MPa and sliding velocities from zero to 450 nm s(-1). We be lieve that this effect is due to the long-ranged repulsion, of entropi c origin, between the brushes, which acts to keep the surfaces apart w hile maintaining a relatively fluid layer at the interface between the m.