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Ultrafast Chemistry under Nonequilibrium Conditions and the Shock to Deflagration Transition at the Nanoscale
journal contribution
posted on 2015-09-24, 00:00 authored by Mitchell
A. Wood, Mathew J. Cherukara, Edward M. Kober, Alejandro StrachanWe use molecular dynamics simulations
to describe the chemical
reactions following shock-induced collapse of cylindrical pores in
the high-energy density material RDX. For shocks with particle velocities
of 2 km/s we find that the collapse of a 40 nm diameter pore leads
to a deflagration wave. Molecular collisions during the collapse lead
to ultrafast, multistep chemical reactions that occur under nonequilibrium
conditions. Exothermic products formed during these first few picoseconds
prevent the nanoscale hotspot from quenching. Within 30 ps, a local
deflagration wave develops; it propagates at 0.25 km/s and consists
of an ultrathin reaction zone of only ∼5 nm, thus involving
large temperature and composition gradients. Contrary to the assumptions
in current models, a static thermal hotspot matching the dynamical
one in size and thermodynamic conditions fails to produce a deflagration
wave indicating the importance of nonequilibrium loading in the criticality
of nanoscale hot spots. These results provide insight into the initiation
of reactive decomposition.