  In this molecular dynamics study, we examine the local surface geometric
effects of the normal impact force between two approximately spherical
nanoparticles that collide in a vacuum. Three types of surface geometries,
facets, sharp crystal edges, and amorphous surfaces of nanoparticles with radii
R < 10 nm are considered, and the impact force is compared with its macroscopic
counterpart described by a nonlinear contact force, FN proportional to D with n
= 3/2 derived by Hertz (1881), where D is the overlap induced by elastic
compression. We study the surface geometry-dependent impact force. For
facet-facet impact, the mutual contact surface area does not expand due to the
large facet surface, and this in turn leads to a non-Hertz impact force, n <
3/2. A Hertz-like contact force, n = 1.5, is recovered in the edge contact and
in the amorphous surface contact, allowing expansion of the mutual contact
surface area. The results suggest that collisions of amorphous nanoparticles or
nanoparticles with sharp edges may maintain dynamic phenomena, such as
breathers and solitary waves, originating from the nonlinear contact force.
