Rifts in rafts

A particle raft floating on an expanding liquid substrate provides a macroscopic analog for studying material failure. The time scales in this system allow both particle-relaxation dynamics and rift formation to be resolved. In our experiments, a raft, an aggregate of particles, is stretched uniaxially by the expansion of the air-liquid interface on which it floats. Its failure morphology changes continuously with pulling velocity. This can be understood as a competition between two velocity scales: the speed of re-aggregation, in which particles relax towards a low-energy configuration determined by viscous and capillary forces, and the difference of velocity between neighboring particles caused by the expanding liquid surface area. This competition selects the cluster length, i.e. , the distance between adjacent rifts. A model based on this competition is consistent with the experimental failure patterns. Particle rafts floating on expanding liquid substrates exhibit velocity-dependent failure morphology and provide an accessible model system for studying cluster formation and material failure across many scales from the microscopic to the celestial.