Strong plastic deformation and softening of fast colliding nanoparticles.

Nanoparticles, with sizes ranging between 1 and ∼102 nm, show dynamical properties distinctly different than those of bulk materials. Due to their large surface area to volume ratio, their properties often depend on length scales. We investigate the size and the collision velocity (vcoll) dependence of the coefficient of restitution (COR) for nanoparticles made of a face-centered cubic lattice of Lennard-Jones atoms via nonequilibrium molecular dynamics simulations. A sharp crossover between elastic collision and plastic collision occurs when vcoll=vY, where vY is the size-dependent yield velocity. For high-collision velocities the COR ∼vcoll-α, α∼1. This result is in agreement with recent small system simulations and with experiments and is distinct from the elasticity-theory-based result for COR for inelastic collisions which behaves as vcoll-α, with α=14. We find that the size-dependent critical vY approaches the theoretical constant value for macroscopic spheres as our particle sizes grow. Possible insights into the origins of α∼1 and the size dependence of the yield velocity are suggested. The work also suggests that sufficiently fast moving nanoparticles traveling through vacuum could be sticky and hence could be of potential interest in many applications.