Dynamics of a grain-scale intruder in a granular system with and without basal friction*

When a granular medium is sheared by a spring-driven mechanism, the system can exhibit stick-slip dynamics: The grains form a rigid solid as the spring loads (stick event) which plastically deforms when the spring load exceeds the yield stress of the stable configuration (slip event). We experimentally investigate the dynamics of a single grain-scale intruder driven by a spring through a 2D annular bed of photoelastic disks. We vary the grain packing fraction, the interparticle friction, and the basal friction between grains and the table surface, and we measure the azimuthal force on the intruder. For grains resting on a dry surface, where basal friction is present, the system undergoes aperiodic stick-slip motion; when the grains are floating on a fluid and basal friction is negligible, the intruder flows unimpeded through the medium but occasionally clogs. These qualitative behaviors do not seem to depend on packing fraction and interparticle friction coefficient. We report on measurements of the intruder force and speed distributions as well as particle displacements.