Perfect Fluid Flow from Granular Jet Impact

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 we 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 at high density can have the collective response of a liquid—a jet of non-cohesive granular particles hitting a target causes the particles to collimate into a thin ejecta sheet similar to water bells created by high-velocity water jets hitting a target. An analogous ejecta collimation was observed at the Relativistic Heavy Ion Collider (RHIC) and has been interpreted as evidence that a liquid phase of the quark-gluon was briefly created by high-speed collisions between gold granular jet impact generically produces a liquid-like ejecta, regardless of the granular jet's internal state. These results are not only relevant for RHIC, but also for ultracold chemistry and the aggregation of small dust projectiles to form planetesimals. All three processes hinge on the outcomes of collisions between systems of particles at high density and very low temperatures.