Boundary lubrication under water

Abstract

Boundary lubrication, in which the rubbing surfaces are coated with molecular monolayers, has been studied extensively for over half a century. 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 films, though few systematic studies on friction between surfactant layers in aqueous environments have been carried out. 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.

Origins of hydration lubrication

“…Earlier findings on lubricated sliding between hydrated surface layers including hydrated ions [3][4][5] , surfactants [6][7][8] , polymers [10][11][12] or liposomes 9 could not reveal the underlying process. This is because the measured friction was a convolution of the hydration lubrication with other dissipation pathways, such as the activated processes described above, or deformation [6][7][8][9] or entanglement [10][11][12] effects in the boundary layers themselves.…”

“…This is because the measured friction was a convolution of the hydration lubrication with other dissipation pathways, such as the activated processes described above, or deformation [6][7][8][9] or entanglement [10][11][12] effects in the boundary layers themselves. The present study elucidates this-for the simplest case of trapped, hydrated ions-by unravelling this convolution and separating the dissipation modes (and in future work it would be interesting to examine different aspects such as the case of different ions and of the transition between the regimes).…”

“…Thus, friction coefficients of order 10 À 3 , at pressures exceeding 100 atm, typical of articulating cartilage surfaces in healthy human joints 1 , cannot be reproduced by any synthetic surfaces, and its molecular-level understanding remains elusive 2 . Over the past decade, the concept of hydration lubrication has been invoked to account for the extremely low sliding friction observed at high pressures between charged surfaces in high-salt solution [3][4][5] , or when coated by surfactants [6][7][8] , liposomes 9 or hydrated polymer brushes [10][11][12] boundary layers resembling those at articular cartilage surfaces 12,13 . According to this, hydration shells formed by water molecules are tenaciously attached to the charges they surround, and so cannot be easily squeezed out on compression [14][15][16][17][18] , yet are labile and so respond to shear in a fluid manner.…”

Direct Measurement of Normal and Shear Forces between Surface-Grown Polyelectrolyte Layers

“…However, since surface-active biological polymers are typically electrically charged, biological lubrication would be better modeled by studying polyelectrolyte brushes. The presence of electrical charge could influence the friction by two mechanisms: firstly, electrical charge affects brush structure and may reduce the degree of interpenetration between opposing brushes 21 , and secondly it has been suggested that bound hydration layers around charged groups can have a lubricating effect 22,23 .…”

Sustained Frictional Instabilities on Nanodomed Surfaces: Stick–Slip Amplitude Coefficient

“…This amplitude parameter,σ f , is plotted against the applied load L and shown for the φ = 92 nm in Figure 10(a) at 2,5 and 100 µm s −1 velocities. It is clear from this plot that a linear relationship exists between the amplitude of the oscillations and the applied load.…”