  Using an Atomic Force Microscope (AFM) we measure the interaction between two
identically charged silica surfaces in the presence of saline solution. For
pure NaCl the interaction is always repulsive. Upon addition of cobalt hexamine
ions, [Co(NH3)6]+3, the repulsion is gradually suppressed and a pronounced
attraction develops at distances much shorter than the screening length. Higher
concentrations of cobalt hexamine turn the attraction back into repulsion.
Measurements of surface charge renormalization by the three valent cations
provide their surface density and their association constant to the negatively
charged silica surface. These estimates exclude interaction between two
condensed Wigner crystals as an explanation for the attraction.
