Spark and glow discharges in supersonic granular flows

Supersonic granular flows often host spark and glow discharges. In nature, such phenomena are exemplified by the lightning storms that accompany volcanic eruptions. Other types of explosive particulate flows are also capable of generating discharges. When and where discharges occur seem to depend strongly on the compressible hydrodynamics of the multiphase system. For instance, experimental work has found that discharges in granular jets are generally confined to the region of rarefaction between the source (nozzle or volcanic vent) and the Mach disc. Additionally, discharges are most readily observed when large overpressures (relative to ambient) exist at the source. Surprisingly, however, these conditions are insufficient to guarantee the production of electrostatic phenomena in supersonic jets. Specifically, the chemical composition and physical properties of the solid phase in these flows appear to play key roles in the genesis of discharges. Here, we provide a review of discharge processes in supersonic flows. We first discuss the various breakdown criteria that may operate in such systems. We also describe the radio frequency signals emitted by spark discharges and how to detect them. Such signals provide information about the structure of the flows, opening the way for novel instrumentation to diagnose explosive phenomena remotely. Lastly, recent experiments with various materials—ranging from diamond particles to ground coffee—provide insight into how conductivity, ductility, and polarizability tune the capacity of a jet to manifest discharges.