The role of rapidly compressed gas pockets in the initiation of condensed explosives

The mechanism of initiation of explosion by the rapid compression of gas spaces has been studied by means of high-speed photography at framing rates up to 10$^{7}$ frames s$^{-1}$. Single crystals of silver azide, lead azide and PETN were mounted in a water tank and gas bubbles of chosen composition and size (diameter in the range 50$\mu $m to 1 mm) collapsed on to them by water shocks of strength about 0.1 GPa (1 kbar). The gas bubbles collapsed to minimum volumes in times of the order of 1$\mu $s (depending on the initial bubble size). Initiation of fast reaction occurred in the azides within $ca.5\times 10^{-8}$ s of the bubble reaching minimum volume provided the bubble made thermal contact with the explosive. During the collapse, the bubble involuted to form a jet of velocity of a few 100 m s$^{-1}$, and after reaching minimum volume, expanded giving an expansion shock. The importance of these phenomena in the initiation of explosion, as well as possible initiation by shock perturbation, was assessed in a series of experiments designed to separate the various possible mechanisms. The conclusion is that adiabatic heating of the gas in the bubble was the prime cause for initiation. Calculations, and experiments with gases such as argon and helium (high value of $\gamma $; the ratio of the specific heats) and butane (low$\gamma $) supported this conclusion. Finally, the relevance to other explosive situations is discussed.