Numerical analysis of nanograin collision by classical molecular dynamics

Abstract

Abstract. We have carried out atomistic simulations of grain-grain collisions for spherical grains of 1.4 and 4 nm radii, with relative velocities of 3.6-6.1 km/s and a number of impact parameters. Since the initial grains are crystallites without any pre-existing defects, grain shattering due to nucleation of cracks was not observed in our simulations. We find grain fusion in some events, but generally melting occurs, leading to nucleation, growth and linkage of voids in the melt, which then leads to production of small clusters. The size distribution does not obey a simple power law and can be considered as having four different regimes, where each regime can be fitted as a power law. IntroductionInterstellar dust grains [1] comprise only 1% of the mass in the molecular clouds of galaxies and yet catalyze the formation of many gas phase molecules, in particular H 2 [2], which allows for the cooling and collapse of these clouds and the formation of stars and planets. High-energy radiation and particles from hot stars, supernovae, or active black holes can alter the physical properties of dust grains and thereby affect their role in these processes.There is no experimental study on grain-grain collisions, for grain smaller than tens of microns, except for clusters with less than 100 atoms. Studies at the mm/cm scale can be roughly understood by continuum models, but these models might break down at the nanometer scale. There are many atomistic molecular dynamics (MD) simulations on the destruction of 3D droplets due to large temperature input [3], 2D solids [4,5], or collision of disks [6], but there are very few simulations on grain-grain collisions, never going beyond tens of atoms [7,8].Here we demonstrate how MD simulations of grain-grain collisions for grain with more than 100 atoms can be used to understand what happens for nanometer-sized grains, colliding at relatively low velocities.

Penetration scaling in atomistic simulations of hypervelocity impact

“…For f ¼ 2 and f ¼ 4 nm the early frames show void nucleation in a molten region, similar to the initial stages of spall in a liquid [16,17]. This mechanism is similar to that seen during the collision of nanograins by Ohnishi et al [18]. Note that this molten region covers the entire thickness of the film.…”

The elastic–plastic transition in nanoparticle collisions

“…1 gives an overview over the processes that occur upon the collision of 2 identical NPs. For certain events around v c , we observe production of transient SFs which do not reach the opposite surface, and can be reabsorbed leaving defect-free grains at the end of the simulation, as noted before [30,32]. The defects produced by the large stress in the contact area are partial dislocations [47].…”

“…Because of the simplicity of the setup, NP collisions have been frequently studied, and results on the restitution coefficient [25], NP bouncing off a surface or off other NPs [26,27], grain mixing [28], fragmentation [29,30], and the occurrence of inelastic processes [31] have been reported. Because of the simplicity of the setup, NP collisions have been frequently studied, and results on the restitution coefficient [25], NP bouncing off a surface or off other NPs [26,27], grain mixing [28], fragmentation [29,30], and the occurrence of inelastic processes [31] have been reported.…”

Collision fragmentation of aggregates. The role of the interaction potential between comprising particles

“…plates [33], or clusters obtained by ballistic cluster-cluster aggregation [34] were investigated. Although many studies reported a power-law for the fragment mass distribution, P(m) ∼ m −α , the exponent α was not universal [35,36,37]. Moreover, it depended on the impact velocity [36].…”