Shock compressing liquids on a tabletop microscope, with special focus on detonating nitromethane

In this dissertation the shock compression of liquid nitromethane (NM), one of the simplest homogeneous condensed-phase explosives, is explored using a tabletop microscope. The shock is produced using a spatially homogenized laser to launch flyer plates, 0.5 mm diameter, 18 – 37 µm thick disks of Al-1100 at velocities up to 4.5 km/s. Upon impact the flyer transmits a shock wave into the desired sample. A full description of the new sample array designed to contain microliters of liquid and enable ~ 100 independent experiments per day is described. A combination of high-speed diagnostics including optical pyrometry, velocimetry, and high-speed high-resolution imaging were used to monitor the reaction progress of pure NM and NM with additives. After accumulating data on hundreds of NM experiments, it was verified the tabletop shock compression microscope could produce detonations with properties that are consistent with bomb sized charges. This was not initially obvious, due to the multiple orders of magnitude difference in material amount and shorter input shock duration. Due to the short shock duration (4 ns), the time it takes the shock to transition into a detonation, the shock-to-detonation, was reduced by orders of magnitude, nearing an ultimate minimum. In this limit, the shock-to-detonation is dominated by the fundamental fluid mechanics and chemical dynamics in the reaction zone behind the shock front. Previous time-to-detonation experiments were dominated by the length of the input shock duration and not solely by the fundamental properties of the reaction. My work shows that the NM reaction falls into three specific regimes dependent on input shock strength: (1) around 14 GPa ignition begins and hot spots, seemingly randomly dispersed clusters of thermal emission, are seen, (2) at 17 GPa a transient sub-detonation occurs, wherein hot spots coalesce and ignite further downstream but eventually decay and die, and (3) during detonation, at and above 19 GPa, the hot spots coalesce and continue to ignite downstream until material runs out. Patterns can sometimes form on a shock front when oblique reaction waves race forward and collide with the front. The patterns are often referred to as cellular structures, due to their cell like appearance. Cellular structures have frequently been observed in homogeneous (gas or liquid) explosives, but most often by post mortem analysis via the imprint they etch on the walls of the confinement chamber. Although these structures have been speculated to exist in pure liquid NM, they have not been directly imaged until now because high spatial (2 µm) and temporal (5 ns) resolution was needed to capture these dynamic events. My results show that spherical explosions behind the planar shock front imprint unique cellular patterns on the front. These patterns can determine exactly when, where, and how many explosions occurred, which provides detailed kinetic information on the NM reaction. The patterns imaged on the detonation front are strikingly similar to patterns formed on the bottom or pools or lakes, made by the reflection/refraction of light from the rippling surface of water, referred to as caustics. Under high pressure and temperature, NM becomes significantly more acidic wherein initiation of detonation is hypothesized to occur via an intermolecular proton transfer reaction. This reaction can be catalyzed by reducing the energy barrier of proton transfer, with the addition of basic molecules like ethylenediamine (EDA). Prior to ignition, the mixture, 1 wt.% EDA in NM, absorbs twice as much of the input shock compared to pure NM, suggesting an increase in proton transfer reactions. This triggers a significant increase in compression and subsequent decrease in ignition and detonation threshold. The effect that a trace number of basic molecules has on the material properties is quite remarkable. Additional additives, such as acetic acid, acetone, and glass beads were also explored to inhibit the reaction. By changing the thickness of the flyer plate the shock duration changes. Our apparatus can successfully initiate NM with flyers 18 – 37 µm thick, with shock durations of 2 – 7 ns. The initiation threshold was discovered to be independent of shock duration, forming hot spots at an input of 14 GPa regardless of flyer thickness. The non-specific duration dependence suggests once NM reaches a certain density the reaction will proceed independent of the volume of reacting material. However, the threshold for detonation did show a slight shock duration dependence. The shortest input shock required the highest input pressure (23 GPa) to achieve detonation. As the shock duration increased, the threshold pressure decreased until plateauing at 19 GPa. Once the threshold for detonation is reached the shock-to-detonation transition remains constant with shock duration, referencing back to the fundamental limit achieved by our apparatus. Lastly, we investigated the shock compression of rhodamine 6G, a fluorescent dye, in liquid water. We developed an advanced spectroscopy system that employs a quasi-continuous laser excitation pulse and a spectrograph with streak camera to measure time-dependent photoemission spectra. The shocked dye shows an immediate loss of emission intensity due to enhanced intersystem crossing followed by a more gradual and long-lasting increase in emission intensity over about 150 ns, likely due to proton transfer from the shocked water to the dye molecules