Forced destabilization of a granular raft

Particles floating at an oil-water interface can aggregate via lateral capillary interactions to form granular rafts with unique mechanical properties. When destabilized---either spontaneously or through external forcing---these rafts can encapsulate oil within armored drops. Here, we investigate the forced destabilization of such rafts by indenting them with cylinders of varying size and at various speeds. We identify three distinct phases of destabilization: stable deformation, particle ejection, and oil encapsulation. The onset of particle ejection is explained by considering the cumulative weight of particles coating the indenter, with the particle ejection rate increasing with indentation depth. The encapsulation process consists of a dripping regime, where individual encapsulated oil drops grow and detach under gravity, and a jetting regime, where a continuous oil thread breaks into drops via capillary instability. In the dripping regime, drop size increases with the indenter size, and drop frequency increases with indentation depth. In contrast, the jetting regime produces smaller, more frequent drops with greater size variability due to the dynamic nature of the instability. After destabilization, a residual cap of particles remains adhered to the indenter whose size scales with an effective Bond number incorporating the total particle weight.