  We report on a series of experiments in which a grain-sized intruder is
pushed by a spring through a 2D granular material comprised of photoelastic
disks in a Couette geometry. We study the intruder dynamics as a function of
packing fraction for two types of supporting substrates: a frictional glass
plate and a layer of water for which basal friction forces are negligible. We
observe two dynamical regimes: intermittent flow, in which the intruder moves
freely most of the time but occasionally gets stuck, and stick-slip dynamics,
in which the intruder advances via a sequence of distinct, rapid events. When
basal friction is present, we observe a smooth crossover between the two
regimes as a function of packing fraction, and we find that reducing the
interparticle friction coefficient causes the stick-slip regime to shift to
higher packing fractions. When basal friction is eliminated, we observe
intermittent flow at all accessible packing fractions. For all cases, we
present results for the statistics of stick events, the intruder velocity, and
the force exerted on the intruder by the grains. Our results indicate the
qualitative importance of basal friction at high packing fractions and suggest
a possible connection between intruder dynamics in a static material and
clogging dynamics in granular flows.

Soft Condensed Matter