Microscopic and macroscopic aspects of stick-slip motion in granular shear- art. no. 016413

An annular sheer cell has been used to investigate a number of factors know n to influence stick-slip motion in an assembly of near monosized, spherica l glass beads. In this paper, both the sample shear stress and volumetric s train were recorded, allowing new insights into the possible mechanics of s tick-slip motion in a granular body. Rather than the commonly presented mec hanism of sample dilation and fluidization accompanying the slip events, in the material studied here, sample dilation occurred during the preslip def ormations of the granular body, while the slip event was accompanied by ass embly contraction. Drive velocity and applied normal pressure were both fou nd to influence the magnitude of the slick-slip spikes in a manner analogou s to previous studies of stick-slip in assemblies of confined, near-spheric al lubricant systems, Finally, atmospheric relative humidity was found to h ave a marked effect on the magnitude of the stick-slip motion. To investiga te this mechanism more fully, the atomic force microscopy was employed to m easure the particle-particle interaction forces as a function of atmospheri c relative humidity. A water meniscus was found to form under all humiditie s, from less than 5% to greater than 95%. However, its influence on the adh esive forces varied by an order of magnitude. While most previous studies o f stick-slip phenomena have attempted to remove atmospheric humidity as a v ariable, here we present a useful link between the role of relative humidit y on particle-particle interaction forces and the macroscopic response of t he granular assembly.