Humidity Effects and Aging Behavior in Granular Media
- PDF / 1,273,611 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 119 Downloads / 218 Views
• Laboratoire de Physique, tcole Normale Supdrieure de Lyon, F-69364 Lyon Cedex 07, frestagn @physique.ens-lyon.fr ** Ddpartement de Physique des Mat~riaux, Universit6 Lyon I, 63 boulevard du 11 Novembre 1918, F-69622 Villeurbanne Cedex ABSTRACT We present a study of humidity effects on the maximum stability angle in granular media. We show that a granular medium of small glass beads exhibits aging properties : the first avalanche angle increases logarithmically with the resting time of the pile. This aging behavior is found to depend on the relative humidity of the surrounding atmosphere. A short interpretation of this effect, based on a model of activated capillary condensation, is proposed. INTRODUCTION Granular materials display variety of behaviors that in many ways differs from those of other substances [1]. As distinct from liquids, granular heaps are stationary as long as the slope of the surface is less than the "maximum angle of stability", 0m. When the slope is increased above 0m, grains begin to flow and an avalanche of particles occurs. Following this avalanche, the slope of the surface reaches the "angle of repose", 0,. Those different angles are allowed by the existence of different settlements of the balance between gravity and intergranular forces (contact, cohesion, friction etc.). The simplest model to explain the origin of the angle 0m was introduced as early as 1773 by Coulomb [2]. The equilibrium of a layer of the medium parallel to the free surface is obtained (if) when tan(Om) = g,, where /j, is the static friction coefficient between the layers. This relation is only an approximation which does not give the depth of the avalanche in the pile but it provides a simple argument for the stability at a non zero angle. A feature common to a wide range of materials is that the static friction coefficient /1, increases slowly with the contact time of the two solids [3,4]. Following Coulomb's analogy, we have studied the evolution of the first avalanche angle in a pile of glass beads as a function of the resting time of the heap. We report here experimental results showing : i) the existence of an aging behavior of the first avalanche angle and ii) the dependence of this aging on the humidity of the surrounding atmosphere. EXPERIMENTS The experimental setup is described in figure 1. The granular material was placed in a cylindrical drum of duraluminium with an inner diameter and a length of 10 cm. The lateral 363 Mat. Res. Soc. Symp. Proc. Vol. 543 ©1999 Materials Research Society
faces of the cylinder are made of glass with an opening at the center to allow an exchange with the outer atmosphere. The drum is half filled with glass beads, whose diameter ranges between 125 and 160 microns. In order to maintain the relative humidity of the atmosphere constant during the experiments, the setup is located in a sealed lucite box. The relative humidity H, i.e. the ratio of the partial pressure of water pvp to the saturated vapor pressure pm in the box, is measured by a capacitive hygrometer with a precis
Data Loading...
