Bubble Compression Research Articles

Shock tube measurement of explosive sensitivity

A shock tube test is described, in which fine dusts or sprays of explosive materials are ignited behind the reflected shock, and the dependence of the measured ignition delay on reflected shock temperature is determined. The test simulates the impact initiation of liquids and solids by bubble hot spots in a clean, controlled and well-defined fashion. From only a few tests, one can obtain (1) a valid measure of the sensitivity of the explosive, using only milligramme quantities of material, and (2) information about the physical and, in some cases, the chemical, mechanism of the initiation of the substance upon impact. The hot gaseous environment of the sample is well defined through normal shock relations and the nature of the channel gas may be varied to study environmental effects. Test data on a number of explosives are presented and are interpreted in terms of initiation mechanism. The experimental results suggest that hot spot temperatures required to initiate sensitive materials are between 500° and 800°C, and that bubble compression to form hot spots is decidedly non-adiabatic.

Lung area from surface tension effects.

The surface tension of pulmonary edema fluid was 5–10 dynes/cm as measured in aged, compressed bubbles; it was 40–50 dynes/cm as measured by capillary tube. Nasal mucus on aging and compression exhibits a fall of surface tension from 45–50 dynes/cm to 17 dynes cm with a minimum compressibility coefficient of 0.016 cm per dyne. Calculations of relative surface tension in lungs of rats, cats and dogs assuming 50 dynes cm for the upper limit showed a fall to 5–10 dynes/cm during deflation with minimum compressibility coefficient of 0.012 to 0.020 cm/dyne. The lung surface areas calculated for these species were proportional to body weight and extrapolated to 70 m2 for a 70 kg man.

Initiation of Explosions by Grit Particles

EARLIER work1,2 has shown that the initiation of liquid and solid explosives by impact or friction is due to the formation of localized hot spots. These hot spots may be generated by adiabatic compression of small entrapped bubbles, by friction and shear at solid surfaces, and, in extreme cases, by the viscous heating of rapidly flowing explosive. It was found that for nitroglycerine the minimum temperature of the momentary hot spot necessary for initiation by friction Was c. 400–500° C.