Nanoparticle halos: A new colloid stabilization mechanism

A new mechanism for regulating the stability of colloidal particles has bee n discovered. Negligibly charged colloidal microspheres, which flocculate w hen suspended alone in aqueous solution, undergo a remarkable stabilizing t ransition upon the addition of a critical volume fraction of highly charged nanoparticle species. Zeta potential analysis revealed that these microsph eres exhibited an effective charge buildup in the presence of such species. Scanning angle reflectometry measurements indicated, however, that these n anoparticle species did not adsorb on the microspheres under the experiment al conditions of interest. It is therefore proposed that highly charged nan oparticles segregate to regions near negligibly charged microspheres becaus e of their repulsive Coulombic interactions in solution. This type of nanop article haloing provides a previously unreported method for tailoring the b ehavior of complex fluids.