Drops Floating on Granular Rafts: A Tool for Liquid Transport and Delivery

Solid particles can modify the properties of liquid interfaces and are therefore widely used to coat drops, bubbles, and stabilize emulsions and foams. Here, we propose a new, easy, and affordable method to produce millimetric to centimetric water-in-water capsules using solid particles. We prevent the coalescence of a water drop at an oil-water interface using a monolayer of large, dense, and hydrophobic particles: a "granular raft". The capsule is then formed by a mechanical instability occurring when the interface collapses under the combined load of the floating drop and particle weight. During the destabilization, the water drop sinks into the water subphase through an oil-particle film which covers it to produce the armored capsule. By modeling the raft as a heavy membrane, we predict the floating drop shape, the raft deformation, its destabilization and highlight the complex dual nature (solid- and liquid-like) of the capsule shell. Because armored capsules' content is isolated, transportable, and easily releasable, they are great candidates for applications requiring transport of water-soluble compounds in aqueous systems such as green chemistry or cell biology.