High-speed tracking of rupture and clustering in freely falling granular streams

It is a common, well known occurrence for a thin liquid stream to break up into droplets due to the surface-tension driven Plateau-Rayleigh instability [1]. Surprisingly, this effect can also occur in granular materials, where an initially uniform stream of grains breaks up into discrete clusters, or droplets, of grains, even though granular materials are generally considered to lack surface tension. This thesis investigates this break up in a stream of grains freely falling out of a nozzle. We employ high-speed video imaging in the co-moving frame with the freely-falling granular stream, which allows us to track the onset of clustering and the subsequent cluster evolution in detail. By directly measuring grain-grain interactions with Atomic Force Microscopy (AFM) and carefully controlling the surface properties of the grains, we demonstrate that the cluster formation is driven by minute, nanoNewton cohesive forces due to a combination of van der Waals interactions and capillary bridges between nanoscale surface asperities. The shapes of these weakly cohesive, non-thermal clusters of macroscopic particles closely resemble droplets resulting from thermally induced rupture of liquid nanojets and ultra-low surface tension fluids. Like in the liquid break up, the size of the clusters is determined solely by the stream width and independent of particle-scale details though the range of selected aspect ratios differs from the fluid case. The scaling for the minimum neck diameter near break up is consistent with the scaling for an inviscid liquid, allowing us to extract an effect surface tension nearly four orders of magnitude lower than in normal liquids. These free falling streams are both an exquisite probe of the weak interactions between grains and a unique system to probe the ultra-low surface tension fluid regime in the absence of thermal fluctuations.