Attraction between weakly charged silica spheres at a water-air interface induced by surface-charge heterogeneity.

We report an optical and atomic force microscopic (AFM) study of interactions between weakly charged silica spheres at a water-air interface. Attractive interactions are observed at intermediate interparticle distances and the amplitude of the attraction increases with the amount of salt (NaCl) added into the water phase. AFM images obtained in the salty water show the formation of patchy charge domains of size approximately 100 nm on the silica surface. The experiment suggests that surface heterogeneity produced during ionization plays an important role in the generation of attractions between like-charged particles at the interface.