Thin streams of
liquid commonly break up into characteristic
droplet patterns owing to the surface-tension-driven
Plateau-Rayleigh instability. Very similar patterns are observed when initially uniform streams of dry
granular material break up into clusters of grains, even though
flows of macroscopic particles are considered to lack
surface tension. Recent studies on freely falling
granular streams tracked fluctuations in the stream profile, but the clustering mechanism remained unresolved because the full evolution of the
instability could not be observed. Here we demonstrate that the cluster formation is driven by minute, nanoNewton cohesive forces that arise from a combination of van der Waals interactions and
capillary bridges between nanometre-scale surface asperities. Our experiments involve high-speed video imaging of the
granular stream in the co-moving frame, control over the properties of the grain surfaces and the use of
atomic force microscopy to measure grain-grain interactions. The cohesive forces that we measure correspond to an equivalent
surface tension five orders of magnitude below that of ordinary
liquids. We find that the shapes of these weakly cohesive, non-thermal clusters of macroscopic particles closely resemble
droplets resulting from thermally induced rupture of
liquid nanojets.
