Bound pairs: Direct evidence for long-range attraction between like-charged colloids

We report observations of stable bound pairs in very dilute deionized aqueous suspensions of highly charged polystyrene colloidal particles, with monovalent counterions, using a confocal laser scanning microscope. Through an analysis of several thousands of time series of confocal images recorded deep inside the bulk suspension, we find that the measured pair-potential, U ( r ) has a long-range attractive component with well depths larger than the thermal energy. These observations provide direct and unequivocal evidence for the existence of long-range attraction in U ( r ) of like-charged colloidal particles.