Direct observation of charge inversion by multivalent ions as a   universal electrostatic phenomenon
  We have directly observed reversal of the polarity of charged surfaces in water upon the addition of tri- and quadrivalent ions using atomic force microscopy. The bulk concentration of multivalent ions at which charge inversion reversibly occurs depends only very weakly on the chemical composition, surface structure, size and lipophilicity of the ions, but is dominated by their valence. These results support the theoretical proposal that spatial correlations between ions are the driving mechanism behind charge inversion. 
K. Besteman, M. A. G. Zevenbergen, H. A. Heering, and S. G. Lemay