  We experimentally study the unjamming dynamics of strongly compressed
particle rafts confined between two fixed walls and two movable barriers. The
back barrier is made of an elastic band, whose deflection indicates the local
stress. The front barrier is pierced by a gate, whose opening triggers local
unjamming. The rafts are compressed by moving only one of the two barriers in
the vicinity of which folds form. Using high speed imaging, we follow the
folded, jammed, and unjammed raft areas and measure the velocity fields inside
and outside of the initially confined domain. Two very different behaviors
develop. For rafts compressed by the back barrier, only partial unjamming
occurs. At the end of the process, many folds remain and the back stress does
not relax. The flow develops only along the compression axis and the particles
passing the gate form a dense raft whose width is the gate width. For rafts
compressed at the front, quasi-total unjamming is observed. No folds persist
and only minimal stress remains, if any. The particles flow along the
compression axis but also normally to it and form, after the gate, a rather
circular and not dense assembly. We attribute this difference to the opposite
orientation of the force chain network that builds up from the compressed side
and branchs. For rafts compressed at the gate side, keystone particles are
immediately removed which enhances local disentanglement and leads to large
scale unjamming. In contrast, for back compressed rafts, the force chain
network redirects the stress laterally forming arches around the gate and
resulting in a limited unjamming process.
