Collapse of a granular raft: Transition from single particle falling to collective creasing

A granular raft - a two-dimensional (2D) particle layer floating at a fluid-fluid interface - collapses when losing its stability under compression. Although granular rafts are frequently encountered in various natural and engineering settings, how a raft fails under compression is still an open question. Here, by combining experiments with theoretical modeling, we examine the failure modes of granular rafts without free periphery under quasistatic biaxial compression. Different from granular rafts with open periphery, granular rafts in our study remain stable under finite compressive stresses. More surprisingly, under large compression, granular rafts made of small particles sink gradually by expelling individual particles, whereas rafts of large particles collapse catastrophically by forming large-scale creases. The collective creasing is enhanced by the 2D particle density and is suppressed by the density difference of the two fluids. We develop a one-dimensional continuum model for the shape of the granular rafts and the concentration of particles along the fluid-fluid interface, which provides quantitative explanations of our experimental findings.