Boundary lubrication under water

Classical boundary lubrication is widespread in engineering applications and is also thought to exist in biological systems. It minimizes friction and wear by ensuring that rubbing takes place between the 'boundary' layers of surfactant molecules that coat each surface, rather than between the substrates themselves. Experiments with boundary-lubricant coated sliding surfaces immersed in water now show that friction stress can be reduced by two orders of magnitude or more, relative to its value in dry air. The mechanism may involve the hydration of charged head groups that can then slide easily on the surfaces to which they are attached. This new type of boundary lubrication could be useful for developing better lubricated artificial implants and new medical treatments for joint problems. 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.