An atomistic study of sticking, bouncing, and aggregate destruction in collisions of grains with small aggregates.

Molecular dynamics simulations are used to study central collisions between spherical grains and between grains and small grain aggregates (up to 5 grains). For a model material (Lennard-Jones), grain-grain collisions are sticking when the relative velocity v is smaller than the so-called bouncing velocity and bouncing for higher velocities. We find a similar behavior for grain-aggregate collisions. The value of the bouncing velocity depends only negligibly on the aggregate size. However, it is by 35% larger than the separation velocity needed to break a contact; this is explained by energy dissipation processes during the collision. The separation velocity follows the predictions of the macroscopic Johnson-Kendall-Roberts theory of contacts. At even higher collision velocities, the aggregate is destroyed, first by the loss of a monomer grain and then by total disruption. In contrast to theoretical considerations, we do not find a proportionality of the collision energy needed for destruction and the number of bonds to be broken. Our study thus sheds novel light on the foundations of granular mechanics, namely the energy needed to separate two grains, the difference between grain-grain and grain-aggregate collisions, and the energy needed for aggregate destruction.