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Particle beams are important tools for probing atomic and molecularinteractions. Here we demonstrate that particle beams also offer a uniqueopportunity to investigate interactions in macroscopic systems, such asgranular media. Motivated by recent experiments on streams of grains thatexhibit liquid-like breakup into droplets, we use molecular dynamicssimulations to investigate the evolution of a dense stream of macroscopicspheres accelerating out of an opening at the bottom of a reservoir. We showhow nanoscale details associated with energy dissipation during collisionsmodify the stream's macroscopic behavior. We find that inelastic collisionscollimate the stream, while the presence of short-range attractive interactionsdrives structure formation. Parameterizing the collision dynamics by thecoefficient of restitution (i.e., the ratio of relative velocities before andafter impact) and the strength of the cohesive interaction, we map out aspectrum of behaviors that ranges from gas-like jets in which all grains driftapart to liquid-like streams that break into large droplets containing hundredsof grains. We also find a new, intermediate regime in which small aggregatesform by capture from the gas phase, similar to what can be observed inmolecular beams. Our results show that nearly all aspects of stream behaviorare closely related to the velocity gradient associated with vertical freefall. Led by this observation, we propose a simple energy balance model toexplain the droplet formation process. The qualitative as well as manyquantitative features of the simulations and the model compare well withavailable experimental data and provide a first quantitative measure of therole of attractions in freely cooling granular streams.

Droplet and cluster formation in freely falling granular streams