Droplet and cluster formation in freely falling granular streams

Abstract

Particle beams are important tools for probing atomic and molecular interactions. Here we demonstrate that particle beams also offer a unique opportunity to investigate interactions in macroscopic systems, such as granular media. Motivated by recent experiments on streams of grains that exhibit liquid-like breakup into droplets, we use molecular dynamics simulations to investigate the evolution of a dense stream of macroscopic spheres accelerating out of an opening at the bottom of a reservoir. We show how nanoscale details associated with energy dissipation during collisions modify the stream's macroscopic behavior. We find that inelastic collisions collimate the stream, while the presence of short-range attractive interactions drives structure formation. Parameterizing the collision dynamics by the coefficient of restitution (i.e., the ratio of relative velocities before and after impact) and the strength of the cohesive interaction, we map out a spectrum of behaviors that ranges from gas-like jets in which all grains drift apart to liquid-like streams that break into large droplets containing hundreds of grains. We also find a new, intermediate regime in which small aggregates form by capture from the gas phase, similar to what can be observed in molecular beams. Our results show that nearly all aspects of stream behavior are closely related to the velocity gradient associated with vertical free fall. Led by this observation, we propose a simple energy balance model to explain the droplet formation process. The qualitative as well as many quantitative features of the simulations and the model compare well with available experimental data and provide a first quantitative measure of the role of attractions in freely cooling granular streams

Penetration in bimodal, polydisperse granular material

“…However, it is unclear how the rattlers affect the dynamical response of the granular material to impact, for example, or how they affect the interstitial forces between grains. Our study here indicates that the inclusion of fine grains into a bed of larger grains brings in the influence of ambient air and local fluidization, which has been observed previously for dense spheres impacting onto granular beds [23][24][25] and in falling granular streams [29,30]. The influence of ambient air only appears to be significant for finer grains (Geldart group A), such as the smallest particles used here with d p = 31 μm, and is typically not present for larger grains (Geldart group B, typically with mean diameter between 40 and 500 μm), such as the larger grains used here with d p = 178 and 520 μm, according to Geldart's classification [31].…”

Incompressible-compressible transition in falling granular jets

“…Indeed the jets continue to thin as in the presence of air even at 0.25 mbar. This may suggest either a weak influence of the surface tension previously reported [15,16] [18].…”

From nanoscale cohesion to macroscale entanglement: Opportunities for designing granular aggregate behavior by tailoring grain shape and interactions

“…Figure 1 shows the results of 3d DEM simulations (for details, see [6]). The range of the attractive force was set to 1/1000 of a grain diameter, mimicking van der Waals or capillary forces due to adsorbed molecular layers.…”

“…For short-ranged attractions, we recently showed [5,6] that the macroscopic behavior of dense granular streams can be directly linked to the strength of the interparticle force F coh . Figure 1 shows the results of 3d DEM simulations (for details, see [6]).…”

“…gravity driven flows of grains falling from an orifice. By conducting experiments in vacuum and imaging these flows with a co-falling high-speed camera, dynamic grain-grain interactions can be tracked in-situ with unprecedented detail [5,6].…”