Relaxation of particle-laden interfaces: geometric and preparation effects

The relaxation of uni-axially compressed particle rafts through a finite opening found at the opposite side is experimentally studied. Three main behaviours are identified. The lowest relaxation degree corresponds to local unjamming. The other extreme corresponds to full relaxation and is characterized by the unjamming of the entire raft. In between, intermediate relaxation is observed. The unjammed domain first grows along the compression direction with an almost constant width and possibly extends through the entire raft length. In this case, a second phase may start during which erosion enables the unjammed channel to develop normally to the compression direction. Employing different raft geometries, i.e. various length and compression levels, and openings of various widths, we rationalize the occurrence of these different behaviours, which we attribute to the mechanical robustness of the force chain network. The threshold for channel formation and erosion are interpreted as its rupture against excessive shear and elongation, respectively. By further comparing results obtained for rafts prepared according to three different mixing degrees, we evidence that these thresholds are strongly affected by the raft history and quantify these effects in terms of shift of the rupture limits.