Nanoparticle engineering of complex fluid behavior

A new mechanism for regulating the stability of colloidal suspensions has been discovered, known as nanoparticle haloing.(1) Colloidal microspheres suspended under conditions near their isoelectric point were stabilized by the addition of highly charged nanoparticles. Such species segregated to regions near negligibly charged microspheres leading to their effective charge buildup. At even higher nanoparticle volume fraction, system stability was reversed due to attractive depletion interactions. By engineering the strength of energetic and entropic interactions via nanoparticle additions, we have assembled colloidal fluid, gel, and crystalline phases from binary mixtures of colloidal silica microspheres and hydrous zirconia nanoparticles whose structure and flow behavior vary dramatically.