Avalanches and dynamics of granular aggregates made of oil micro-droplets spreading on a surface

Granular materials display unusual properties and structures compared to continuous materials. Common examples of such materials are sand piles with shapes and mechanical properties that are determined by a balance between gravity and inter-grain friction. In this study, monodisperse lightly attractive oil droplets (radius approximately 10 microns) are produced one-by-one in an aqueous solution. Droplets are buoyant and accumulate underneath a glass slide which acts as the top of a liquid cell, forming 3D aggregates. Droplets initially arrange to form crystals growing along the vertical direction. As a critical height is reached, the aggregate collapses and spreads on the glass slide, in an event analogous to avalanches in sand piles. This process repeats. We find that the geometry of such crystals is controlled by the balance between the adhesion strength and buoyancy which can be modified by changing the size of the droplets. Finally, crystallization can be prevented by using two different droplet sizes leading to glassy aggregates spreading more easily on the glass slide. This study has potential implications in the understanding of the spreading of cells on a surface, or the physics of avalanches in a novel system.