Stick-slip and intermittent flow dynamics of a single-grain intruder driven through a granular medium with and without basal friction*

We report on experiments in which a grain-sized intruder is pushed by a spring through a quasi-2D granular material in an annular geometry. We study intruder dynamics as a function of packing fraction for two types of supporting substrates: a frictional glass plate and a layer of water, which completely removes basal friction. In the presence of basal friction, we observe a novel crossover with increasing packing fraction from intermittent flow to stick-slip dynamics. In intermittent flow, the intruder only occasionally gets stuck by the medium; in stick-slip, the intruder advances via a sequence of distinct, rapid slip events. With lower interparticle friction, the crossover packing fraction shifts to higher values; when basal friction is removed, no crossover to stick-slip dynamics is observed. We characterize the dynamics using statistics of the intruder velocity, the force of the medium on the intruder, and the waiting times between sticking periods. Our results indicate the qualitative importance of basal friction and suggest a possible connection between intruder dynamics in a static material and clogging dynamics in granular flows.