Gravity-induced double encapsulation of liquids using granular rafts

We experimentally investigate a millimetric armored droplet of a water-isopropyl alcohol solution sedimenting through oil and approaching a water layer at the bottom of the container. Upon reaching the oil-water interface, the droplet is shown to rupture and coalesce with the water either for low droplet densities (floating rupture) or for low oil viscosities (sinking rupture). By contrast, for sufficiently large drop density or oil viscosity, the oil covering the armored drop pinches off in the underlying water, as the armored drop continues to sink. This leads to the double encapsulation of an aqueous solution in water, which can be utilized to transport desired ingredients within a wet environment. We show that a simplified quasistatic model of a rigid sphere successfully captures the limit of the floating rupture behavior. We also rationalize the transition from the sinking rupture to oil pinch-off, by comparing the timescales of the film drainage versus sinking. Our results demonstrate that an effective Bond number and an effective Ohnesorge number are the two key dimensionless parameters that characterize the pinch-off threshold in good agreement with the experiments.