Floating and sinking of self-assembled spheres on liquid-liquid interfaces: rafts versus stacks

The floating and sinking of objects on fluid-fluid interfaces occurs in nature, and has many important implications in technology. Here, we study the stability of floating self-assembled spheres on an oil-water interface, and how the sphere deposition geometry affects the size limits of the assemblies before they collapse and sink through the interface. Specifically, we compare the critical size of particle rafts to particle stacks. We show that, on liquid-liquid interfaces, monolayer rafts and stacked sphere exhibit different scaling of the critical number of spheres to the Bond number - the dimensionless ratio of buoyancy to interfacial tension effects. Our results indicate that particle stacks will sink with a lower threshold number of particles than particle rafts. This finding may have important implications to engineering applications where interfacial assemblies are not monolayers.