Coulomb explosion and ultra-fast hypergolic ignition of borohydride-rich ionic liquids with WFNA

In this work, the hypergolic ignition process of 8 recently synthesized ionic liquids (I.L.s) with [BH₃(CN)BH₂(CN)]⁻ anions with white fuming nitric acid is experimentally investigated by using droplet test method and long distance microscope-high speed photography techniques. Results show that the hypergolic ignition process of the present I.L.s is of a completely different three stage nature and ultra-fast hypergolic ignition is observed for this family of I.L.s (< 5 ms). Specifically, at the very first stage upon contact (within a fraction of millisecond), ‘coulomb explosion’ like behavior in terms of fast and vigorous ejection of liquid spikes that protrude from mixing layer is observed, and this phenomenon has never been reported in previous hypergolic test for I.L.s or conventional hydrazine based fuels. The ‘coulomb explosion’ delay time (CEDT) is found to be correlated with the side alkyl functional group, which indicates the I.L. structure (characterized by the side alkyl functional group and the heterocyclic core) effects on shifting the Rayleigh instability limit (E_coulomb/2E_surface> 1). Subsequently in the second stage, liquid spikes that are ejected into the oxidizer pool during CEDT significantly increase the reactive surface area underneath the liquid surface. As a consequence, local temperature increases, gas phase intermediate product and oxidizer/fuel vapor accumulate due to the continuous liquid phase reaction underneath the surface. When the local pressure overcomes the surface tension, disintegration of the surface in terms of larger ligaments and secondary droplets ejection was observed, together with shooting out of the vapor/smoke. Finally, in the third stage when the local temperature and the vapor concentration increase sufficiently, further gas phase reaction leads to ignition. Fine and stable flame with strong bright and green luminescence continuously evolve from the initial mixing layer. Some red and dark smokes are gradually generated above the burning bright and green flame until finally the flame dies out. The ignition delay time (IDT) decreases with the increase of unsaturation index of the heterocyclic core in the cation, and also the decrease of CEDT. The enthalpy of formation of the I.L.s for different cation structures are correlated with IDT, which represents both the fuel structure chemistry and the ‘coulomb explosion’ enhances mixing effect on the overall reactivity of this diffusive system.