Rifts in Rafts

Abstract

The failure of materials is often considered as a quasi-static process in classical fracture mechanics. No dynamics can be allowed because the speed of sound is much greater than any other velocity. In this paper, we introduce a macroscopic system where the dynamics of particle relaxation and rift formation can be resolved: a particle aggregate is stretched uniaxially by the expansion of the air-fluid interface on which it floats. The failure morphology of these rafts change continuously with pulling velocity. This can be understood by the competition between two velocities: the speed of particle relaxation towards the nearest low energy configuration (i.e., re-aggregation), which is determined by viscous and capillary forces, and the velocity difference between neighboring particles, which is caused by the expanding underlying fluid. This crossover behavior selects the cluster length scales, i.e., the distance between adjacent rifts, and is consistent with the the experimental failure patterns.