Unjamming of particle-laden interfaces: effects of geometry and history

The unjamming of uniaxially compressed particle rafts triggered by the opening of a finite orifice on the opposite side is experimentally studied. Using glass beads of about 100 μm, three main behaviors are identified. Minimal unjamming does not allow significant relaxation. Axial unjamming corresponds to the growth of the unjammed domain along the compression direction with an almost constant width. The resulting channel, possibly extends through the entire raft length and may lead to partial stress relaxation. Finally, after the completion of axial unjamming, lateral unjamming may occur according to an erosion process during which jammed blocks detach from the channel edges. This is associated with important stress relaxation. By using different raft geometries, i.e. various raft lengths, compression levels, and opening widths, we rationalize the occurrence of these behaviors, attributing them to the rupture of the force chain network against shear and elongation, respectively. Comparing results from equally densely packed rafts prepared with three different protocols demonstrates that these two thresholds are strongly affected by the raft's history.