  Since the discovery of the Amonton's law and with support of modern
tribological models, friction between surfaces of three-dimensional materials
is known to generally increase when the surfaces are in closer contact. Here,
using molecular dynamics simulations of friction force microscopy on suspended
graphene, we demonstrate an increase of friction when the scanning tip is
retracted away from the sample. We explain the observed behavior and address
why this phenomenon has not been observed for isotropic 3-D materials.
