  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 fluid. 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.
