The Role of Adsorbed Water on the Friction of a Layer of Submicron Particles

Abstract

Anomalously low values of friction observed in layers of submicron particles deformed in simple shear at high slip velocities are explained as the consequence of a one nanometer thick layer of water adsorbed on the particles. The observed transition from normal friction with an apparent coefficient near l = 0.6 at low slip speeds to a coefficient near l = 0.3 at higher slip speeds is attributed to competition between the time required to extrude the water layer from between neighboring particles in a force chain and the average lifetime of the chain. At low slip speeds the time required for extrusion is less than the average lifetime of a chain so the particles make contact and lock. As slip speed increases, the average lifetime of a chain decreases until it is less than the extrusion time and the particles in a force chain never come into direct contact. If the adsorbed water layer enables the otherwise rough particles to rotate, the coefficient of friction will drop to l = 0.3, appropriate for rotating spheres. At the highest slip speeds particle temperatures rise above 100°C, the water layer vaporizes, the particles contact and lock, and the coefficient of friction rises to l = 0.6. The observed onset of weakening at slip speeds near 0.001 m/s is consistent with the measured viscosity of a 1 nm thick layer of adsorbed water, with a minimum particle radius of approximately 20 nm, and with reasonable assumptions about the distribution of force chains guided by experimental observation. The reduction of friction and the range of velocities over which it occurs decrease with increasing normal stress, as predicted by the model. Moreover, the analysis predicts that this high-speed weakening mechanism should operate only for particles with radii smaller than approximately 1 lm. For larger particles the slip speed required for weakening is so large that frictional heating will evaporate the adsorbed water and weakening will not occur.

Frictional strength of wet and dry montmorillonite

“…At higher pressures (and temperatures) this water is expelled and its ability to aid particles in slipping past each other is reduced. This tendency is clearly demonstrated for wet chrysotile [ Moore et al ., ] and has been proposed as a mechanism for high deformation rate strengthening in granitic gouge layers [ Sammis et al ., ].…”

Friction Evolution of Granitic Faults: Heating Controlled Transition From Powder Lubrication to Frictional Melt

“…Under this rate, the friction coefficient may approach zero at seismic slip velocity of 1 m/s and above (Di Toro et al, ; Goldsby & Tullis, ). While this continuous velocity weakening fits experimental results of many rocks (green band in Figure ) (Di Toro et al, ), experimental granitic faults do not follow this trend (yellow band in Figure ) (Kuwano & Hatano, ; Liao & Reches, ; Reches & Lockner, ; Sammis et al, ). The granitic faults display three stages of friction evolution: velocity weakening stage at low slip velocity (<0.01 m/s), velocity strengthening stage at intermediate slip rate of 0.05–0.2 m/s, and a poorly documented stage of velocity weakening at high, seismic slip velocity.…”

Mechanisms for Friction of Rock at Earthquake Slip Rates

“…Another possible explanation for the weakening of submicron powders sheared at high velocities as observed by Reches and Lockner (2010) has been presented by Sammis et al (2011). This mechanism involves the behavior of a nanometer-thick layer of water surrounding the particles.…”