Nature444, 191-194 (9 November 2006) | doi:10.1038/nature05196; Received 10 May 2006; Accepted 24 August 2006
Boundary lubrication under water
Wuge H. Briscoe1, Simon Titmuss1, Fredrik Tiberg1,3, Robert K. Thomas1, Duncan J. McGillivray1,3 and Jacob Klein1,2
Boundary lubrication, in which the rubbing surfaces are coated with molecular monolayers, has been studied extensively for over half a century1, 2, 3, 4, 5, 6, 7. Such monolayers generally consist of amphiphilic surfactants anchored by their polar headgroups; sliding occurs at the interface between the layers, greatly reducing friction and especially wear of the underlying substrates. This process, widespread in engineering applications, is also predicted to occur in biological lubrication via phospholipid films8, 9, though few systematic studies on friction between surfactant layers in aqueous environments have been carried out5, 10. Here we show that the frictional stress between two sliding surfaces bearing surfactant monolayers may decrease, when immersed in water, to as little as one per cent or less of its value in air (or oil). We attribute this to the shift of the slip plane from between the surfactant layers, to the surfactant/substrate interface. The low friction would then be due to the fluid hydration layers surrounding the polar head groups attached to the substrate. These results may have implications for future technological and biomedical applications
