Collapse of a Granular Raft

Granular rafts consisting of monodispersed glass particles on a fluid-fluid interface collapse when biaxially compressed. Contrary to intuition, small particles are preferentially expelled individually, whereas large particles form a collective crease. We experimentally show that the collective crease is enhanced when the particle weight per unit interface area is large, and is suppressed by a difference in density between the two fluids. We develop a one-dimensional continuum model for the shape of the interface and the concentration of particles along the interface that shows that the particle weight per unit area normalized by the density difference between the two fluids is the key parameter. When this value is large, the raft creases; when small, the raft cannot crease, so steric interactions force particles to be expelled individually.