Spreading of a 2D granular analogue of a liquid puddle: Predicting structure through a “granular capillary length”

The structure of an accumulation of granular material, such as a pile of sand, is understood through the balance between gravity, which acts to limit the height, and inter-grain friction, which holds the particles together. In contrast, for the case of a continuous medium, the height of a puddle is dictated by the capillary length which balances gravity and surface tension. Here we develop an experimental model that allows us to probe the structure of a 2D pile of monodisperse microscopic oil droplets. The droplets are buoyant, adhesive, and friction is negligible. Oil droplets are deposited within a chamber and accumulate at a barrier. The droplets accumulate to a certain critical height at which point the pile collapses and spreads across the barrier. This process is reminiscent of the spreading of a liquid even though the pile is granular and 2D in nature. We define a parameter, analogous to the capillary length, which determines the height of the pile and is dictated by the balance between buoyant and adhesive forces. These parameters can be controlled experimentally in order to modify the height of the pile. We developed a simple model that can predict the shape of the pile based on the balance of adhesion and buoyancy.