Waitukaitis, Scott RIST Austria ; Grütjen, Helge; Royer, John; Jaeger, Heinrich
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 gaslike 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.
Physical Review E
This work was supported by the NSF through CBET-0933242. We acknowledge use of shared facilities provided by the Keck Facility for Ultrafast Imaging at the University of Chicago and by the Chicago MRSEC through NSF DMR-0820054. Software support by Itasca Consulting Group, Inc., under the Itasca Educational Partnership is gratefully acknowledged. H.G. thanks the German-American Fulbright Commission for fellowship support during his stay at the University of Chicago.
Waitukaitis SR, Grütjen H, Royer J, Jaeger H. Droplet and cluster formation in freely falling granular streams. Physical Review E. 2011;83(5). doi:10.1103/PhysRevE.83.051302
Waitukaitis, S. R., Grütjen, H., Royer, J., & Jaeger, H. (2011). Droplet and cluster formation in freely falling granular streams. Physical Review E. American Physical Society. https://doi.org/10.1103/PhysRevE.83.051302
Waitukaitis, Scott R, Helge Grütjen, John Royer, and Heinrich Jaeger. “Droplet and Cluster Formation in Freely Falling Granular Streams.” Physical Review E. American Physical Society, 2011. https://doi.org/10.1103/PhysRevE.83.051302.
S. R. Waitukaitis, H. Grütjen, J. Royer, and H. Jaeger, “Droplet and cluster formation in freely falling granular streams,” Physical Review E, vol. 83, no. 5. American Physical Society, 2011.
Waitukaitis SR, Grütjen H, Royer J, Jaeger H. 2011. Droplet and cluster formation in freely falling granular streams. Physical Review E. 83(5), 051302.
Waitukaitis, Scott R., et al. “Droplet and Cluster Formation in Freely Falling Granular Streams.” Physical Review E, vol. 83, no. 5, 051302, American Physical Society, 2011, doi:10.1103/PhysRevE.83.051302.