Molecular dynamics (MD) simulations
were used to study shock wave
passage with normal incidence through the equilibrium interface between
(100)-oriented nitromethane and the melt. The simulations were performed
using the fully flexible, nonreactive SRT force field (Sorescu, D.
C.; Rice, B. M.; Thompson, D. L. The Journal of Physical Chemistry
B
2000, 104, 8406–8419).
The local kinetic energies (intermolecular, intramolecular, and total)
and stress states differ significantly in the liquid and crystal regions,
and depend on whether the shock is initiated in the crystal or liquid.
The number and spatial distributions of shock-induced molecular disorientations
in the crystal for shocks initiated in the crystal are similar to
those obtained for analogous simulations for a completely crystalline
sample; however, substantial differences in the extent and distribution
of shock-induced molecular disorientations in the crystal region were
observed when the shock was initiated in the liquid. All three measures
of kinetic temperature in the crystal region are higher when the shock
is initiated in the crystal than when it is initiated in the liquid.
Kinetic temperature profiles exhibit features in the vicinity of the
interface considerably different from those in either bulk phase.
The shock-induced local mechanical states (von Mises stress) indicate
that the crystal is less able to support shear stresses when the shock
is initiated in the crystal than when it is initiated in the liquid.
There is a strong reflection back into the liquid when the shock wave
passes through the liquid and encounters the interface with the crystal.
This causes a large increase in the potential energy of the liquid
and limits the amount of energy transmitted into the crystal, which
limits the molecular disorientations in the crystal. Thus, a shock
from liquid to crystal yields less inelastic deformation in the crystal.