  We study experimentally the motion of an intruder dragged into an amorphous
monolayer of horizontally vibrated grains at high packing fractions. This
motion exhibits two transitions. The first transition separates a continuous
motion regime at comparatively low packing fractions and large dragging force
from an intermittent motion one at high packing fraction and low dragging
force. Associated to these different motions, we observe a transition from a
linear rheology to a stiffer response. We thereby call "fluidisation" this
first transition. A second transition is observed within the intermittent
regime, when the intruder's motion is made of intermittent bursts separated by
long waiting times. We observe a peak in the relative fluctuations of the
intruder's displacements and a critical scaling of the burst amplitudes
distributions. This transition occurs at the jamming point characterized in a
previous study and defined as the point where the static pressure (i.e. the
pressure measured in the absence of vibration) vanishes. Investigating the
motion of the surrounding grains, we show that below the fluidisation
transition, there is a permanent wake of free volume behind the intruder. This
transition is marked by the evolution of the reorganization patterns around the
intruder, which evolve from compact aggregates in the flowing regime to
long-range branched shapes in the intermittent regime, suggesting an increasing
role of the stress fluctuations. Remarkably, the distributions of the kinetic
energy of these reorganization patterns also exhibits a critical scaling at the
jamming transition.
