Detonation on a tabletop: Nitromethane with high time and space resolution

Abstract

Nitromethane (NM), a flammable liquid, has been a model system for the shock-to-detonation transition in homogeneous condensed-phase explosives for over 50 years, but we do not understand the fast processes at the molecular scale 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 disposable optical cuvettes, where the pressure and temperature were probed in real time (1 ns) with photon Doppler velocimetry, optical pyrometry, and high-speed video. Using a 4 ns shock with an input pressure close to the von Neumann spike pressure of 19 GPa, we achieved the minimum time-to-detonation, about 12 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: our detonation velocity, von Neumann spike and Chapman-Jouguet pressures, temperatures, and reaction zone lengths were the same as in bomb-sized charges. 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. We found that it took 6 ns for the temperature to reach 3430 K. The high pressure was observed at about 8 ns, when there was a volume explosion to nearly twice the von Neumann spike pressure before settling down to a steady detonation.

Shock Initiation Microscopy with High Time and Space Resolution

“…Pyrometry measurements of a 40 μm thick nitromethane layer with a 4 km/s impact, which we previously showed runs to detonation , are shown in Figure . With an integration time of about 1 ns, our detection sensitivity limits us to temperatures of about 1800 K and above.…”

“…We have done a lot of work on formulations that are 80 % explosive by weight and 20 % PDMS binder. To distinguish our laboratory-formulated PBX versions from analogous commercial explosives, we use an "X-" prefix, e. g. X-PETN consists of 80 wt.% PETN and 20 wt.% PDMS, etc., where PETN denotes pentaerithritol tetranitrate, C(CH 2 ONO 2 ) 4 .…”

Introductory Chapter: Interferometry

“…The power of interferometry has made it an obvious tool to explore interdisciplinary research in modern times and has expanded to topics related to chemistry, mechanical engineering and material science related topics such as shock spectroscopy, surface profiling, and microfluidics [3][4][5]. In the recent gravitational wave discovery, it was an interferometer called "LIGO" that made it possible.…”

“…These techniques fall under the umbrella of "laser velocity interferometry" and consist of well-known and established methods such as VISAR, ORVIS, several Quartz-based stress gauges and so on [6]. A similar technique, termed as photon Doppler velocimetry (PDV) has been very useful to probe explosive chemistry in the short length scale with nanosecond time resolution [3]. These interferometric techniques can also accurately investigate shock response in solids.…”

“…The same experiment can be performed in two different geometries, the most common being one where we can record a velocity history for an incoming object. In shocked systems, one can thus obtain information regarding pressure and density evolutions with high time resolution [3,6]. In another geometry, a thin layer of reflective surface (typically a metal) is deposited at one side of the substrate being shocked, and the substrate is exposed to shock in such a way that the shock emerges through the reflective surface.…”