Erin Nissen, University of Illinois at Urbana-Champaign
Nitromethane (NM), a flammable liquid, has been a model system for the shock-to-detonation transition in homogeneous condensed-phase explosives for more than 50 years, but we still do not understand the fast processes in the detonation front at the molecular scale. That is largely because prior studies triggered detonations in bomb-sized charges with input shock durations and times-to detonation that were typically microseconds, which made it impossible to observe the faster processes in real time. We studied NM shocked with 4-ns-duration input pulses using a tabletop apparatus with laser-launched flyer plates and arrays of tiny (submicroliter) disposable optical cuvettes, where the pressure and temperature were probed in real time (1 ns) with photon Doppler velocimetry, optical pyrometry, and high-speed high-resolution photography. We achieved the minimum time-to-detonation, about 15 ns, where the time-to-detonation is controlled by fundamental molecular processes. We demonstrated the reproducibility of our detonations and showed that they had the same properties as in bomb-sized charges. In addition, we obtained cellular patterns imprinted on the luminous shock font by the two-stage explosion in NM. These spatiotemporal patterns arise naturally as a result of the asymmetry produced by the moving shock front. In this way the shock front serves as a thin moving intrinsic optical gauge that reports and characterizes the two-stage NM explosion behind the front. Being able to trigger realistic reproducible detonations from a short pulse makes it possible to investigate molecular and fluid dynamics in the detonation by measuring transient responses in real time.
Abstract Author(s): Erin J. Nissen