ar X iv : c on d - m at / 9 80 12 18 v 1 2 1 Ja n 19 98 Moisture induced Ageing in Granular Media

We present experiments showing that a granular system of small beads does exhibit ageing properties : its maximum stability angle is measured to increase logarithmically with resting time, i.e., the time elapsed before performing the measure. We show that humidity is the crucial ingredient responsible for this behaviour : while ageing effects are important at intermediate humidity, they disappear at vanishing humidity. On the basis of these experimental results, we propose a model based on the activated condensation of liquid bridges between the beads. Within this picture, we are able to reproduce both the waiting-time and humidity dependence of the ageing properties. In his pioneering treatise published in 1773 [1], Charles de Coulomb pointed out the peculiar properties of granular systems. His decisive remark was to recognize the links existing between the statics of granular systems and the friction properties of two solids at contact : the equilibrium and stability of granular edifices could be discussed in terms of the friction between the different part or layers of the system [2]. In particular, he deduced a relationship between the rest angle of the pile, θ 0 , and the static friction coefficient µ s of the medium, in the form tan θ 0 = µ s. One knows by now that this relation is only a good first approximation and that a wide variety of behaviours can be observed in granular systems [2]. The same is true for dry friction problems : while the subject remained poorly understood during more than 150 years, a lot of new results emerged in the last four decades.