Formation mechanism of shock-induced particle jetting

Morphodynamics of a dense particulate medium under radial explosion

In this paper, we investigate the initiation and growth of instability patterns arising from the shock loaded internal surfaces of granular rings confined in a Hele-Shaw cell using both experimental and numerical approaches. A variety of patterns are formed in granular media consisting of grains with varying morphologies. When the particle shape becomes increasingly irregular, and/or the gap in the Hele-Shaw cell becomes narrower, it is increasingly hard for confined particles to fluidize. Consequently the emergent pattern transitions from a smooth circle with trivial undulation which grows in a self-similar manner to an unstable finger-like structure with significant tip-splitting. The distinct growth mode of the well-defined instability pattern is closely associated with its inception phase alongside the transmission of the compaction front. The runaway growth of the incipient perturbations gives rise to the unstable growth of the late-time finger-like instabilities. Conversely the minimal growth of the perturbations in the inception phase guarantees the ensuing self-similar growth of the instability patterns featuring insignificant corrugation. The grain-scale simulations reveal the fundamental role played by the heterogeneous non-linear force network inherent to granular media in the stable-to-unstable transition of the instability pattern. The present work reveals the correlation between the grain-scale physics underpinning the formation of surface instability upon shock loading granular media and the nature of the resulting macro-scale instability patterns. The macroscopic flowability of particles through the confined space is found to be the foremost indicator of the nature of the shock induced granular instability pattern.

Dual hierarchical particle jetting of a particle ring undergoing radial explosion

We study experimentally the formation of a dual hierarchical jetting pattern in dry dense particle media subjected to the radially divergent shock loadings in a radial Hele-Shaw cell. The distinct internal and external jetting patterns were formed on the internal and external surfaces of a ring at different times, respectively. The former features dozens of radially aligned fine filaments. By contrast, the latter consists of a large number of small spikes. Once the internal jets are fully developed, a novel proportionate growth is observed, in which distinguishable structures of the overall pattern all grow at the same rate until the interaction between the internal and external jets becomes significant, leading to the inversion of the internal jetting pattern. Although the external jetting is found to be an instability of a Rayleigh-Taylor (RT) type, the internal jetting exhibits a much delayed onset and slower early-stage growth compared with the RT instability, indicating different underlying physics.