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

Abstract

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.

Droplet and cluster formation in freely falling granular streams

“…1, the smaller stream diameter to particle diameter ratio D 0 /d = 15 for Figs. 2-9 is computationally more efficient, but, as in the experiments [18], it leads to more scatter and less definition in the resulting stream features. Since the larger grain mass increases the collisional kinetic energy, the strength of cohesion required for droplet formation is also increased.…”

“…During vertical free fall, particle interactions lead to spatial inhomogeneities that can be detected downstream. This makes it possible to observe the effects of attractive forces as small as nanoNewtons between macroscopic grains [18]. Such forces constitute an effective 'surface tension' that can affect the bulk dynamics of the stream.…”

“…New possibilities to tackle this problem have emerged from recent experiments on freely falling granular streams [16][17][18]. In these systems, a dense stream of particles emerges from a small opening at the bottom of a reservoir and is accelerated downward by gravity.…”

Small-Scale Particle Interactions Are Having Significant Effects on Global Fluidized Bed Behavior

“…Possible mechanics for the formation of particle clusters include hydrodynamic interactions (drag minimization) (Geldart, 1987;Zelenko et al, 1996) , inelastic grain-grain collision (collisional cooling (Lu et al, 2005;Wang et al, 2009) and viscous dissipation (Subbarao, 1986;Shuyan et al, 2008)), electrostatic forces, capillary bridging or van der Waals forces (Israelachvili, 1992;Podczeck, 1998;Visser, 1989). Royer et al (2009) investigated some of these effects in granular streams of particles freely falling from a small opening at the bottom of a hopper. For particles in the 50-150 micron range they tracked cluster formation with a high-speed video camera falling alongside the stream.…”

“…How- ever, the work of Royer et al (2009) and Waitukaitis et al (2013) clearly shows that material type and surface morphology have a significant role in the level of particle clustering. Though, the BFE might be able to trend with a cluster level, that trend is most likely to be different for each material with respect to size, surface chemistry, morphology, etc.…”