  The charge inversion phenomenon is studied by molecular dynamics simulations,
focusing on size and valence asymmetric salts, and a threshold of surface
charge density for charge inversion. The charge inversion criteria by the
electrophoretic mobility and the radial distribution functions of ions coincide
except around the charge inversion threshold. The reversed electrophoretic
mobility increases with the ratio of coion to counterion radii, while it
decreases with the ratio of coion to counterion valences. The monovalent salt
enhances charge inversion of a strongly charged macroion at small ionic
strength, but it reduces reversed mobility otherwise. A cylindrical macroion is
more persistent to monovalent salt than a spherical macroion of the same radius
and surface charge density.
