Moisture-induced ageing in granular media and the kinetics of capillary condensation

In 1773 Coulomb(1) recognized that the static properties of granular systems can be discussed in terms of the frictional properties between different layers(2), leading to his relationship between the angle of repose of a granular pile (theta(0)) and the coefficient of static friction mu(s): tan theta(0), = mu(s). Two centuries later, solid friction and granular media still present many puzzles. One such is that the coefficient of static friction depends on the time during which the solids remain in contact before the measurement. Here ive show that this ageing effect is manifested too in the angle of repose of granular media and originates from capillary condensation of water vapour between the packed particles, leading to the formation of water bridges. By assuming that the kinetics of this process are governed by the thermally activated nucleation of bridges, we can reproduce both the time- and humidity-dependence of the ageing behaviour. Our results also clarify the kinetics of adsorption in porous media more generally.