The collapse of a granular raft under bi-axial compression

Granular particles can self-assemble on a liquid interface via capillary attraction, even if they are denser than either liquid. Here, we show two distinct failure modes of granular rafts without a free periphery when they are subjected to a quasi-static bi-axial compression. For smaller particles, the rafts fail by expulsion of individual particles from the interface, whereas the failure is via collective creasing of the entire interface for larger particles. We performed systematic experiments with different particle wettability, diameter, liquid density, and surface tension and mapped a phase diagram of raft failure modes. We found that the creasing can be suppressed by either increasing the density difference between the liquids or decreasing the interfacial tension. This latter observation is counter-intuitive since the weight of the rafts in our system is balanced by the hydrostatic pressure due to liquid displacement rather than interfacial tension. A transient failure mode, intermediate to creasing and single-particle expulsion, is also observed during gradual addition of surfactant in the heavier liquid. Taken together, our study provides new insights on the unusual dynamics of fluid interfaces laden with a monolayer of granular particles.