Force measurements between weakly attractive polystyrene particles

The forces between two 4.5 μm polystyrene latex particles in an aqueous suspension have been measured using "differential electrophoresis". The experiments were done in the presence of electrolyte (KCl), surfactant (SDS), silica nanoparticles, and polyelectrolyte (sodium polystyrene sulfonate, NaPSS). When the solution contained only KCl, colloidal doublets formed, and the forces between the particles were much larger than classical predictions; the particles in fact appeared to be adhered together irreversibly. In the presence of SDS-KCl solutions, similar results were obtained. Next, solutions with KCl and silica nanoparticles were made, and doublets again formed. While about 40% of the doublets showed piconewton-size attractive forces, about 60% again appeared to be adhered together irreversibly. Finally, NaPSS-KCl solutions were made; in this case, many colloidal doublets formed, and about 90% showed attractive forces of O(1 pN). Although the mechanism for this consistent but weak aggregation in the presence of NaPSS cannot be concluded with certainty, the data suggest that surface charge nonuniformity might play a role.