Children are taught that liquids are an intermediate state of matter: they flow easily, much as gases do, yet, like solids, hold themselves together in a condensed state due to inter-particle attractions. Later they may learn that liquids can be simulated without attractions if the density of the system is kept high by confinement. Unexpectedly, even with no attractions or confinement, a system of particles can collectively respond like a liquid a high-density jet of non-cohesive granular particles hitting a target ejects particles in a thin sheet similar to water bells created when water jets hit a target. This similar outcome in two dissimilar systems raises the possibility that highly collimated ejecta is a generic outcome of dense particulate stream impact, one independent of precise microscopic interactions. Here I describe results on granular jets from experiment, simulation and continuum modeling that show collimated ejecta emerging regardless of whether the internal state is static or flowing.
Our result implies that characterization techniques based on scattering of dilute particulate streams cannot be extended to high density. This calls into question recent efforts to interpret collimation in the scattering pattern generated by gold-ion collisions at the Relativistic Heavy Ion Collider as evidence for a liquid phase in the quark-gluon plasma. Second, an interior dead zone with highly collimated ejecta have also been observed in impact experiments using cohesive silica aggregates. This regime of impact sheds light on the formation of planetesimals. Thus our results may provide a framework for thinking about impact in the more complex circumstances that arise for impact in granular materials as well as foams, emulsions and dense suspensions.
