A new mechanism for regulating the stability of colloidal suspensions has been discovered, known as
nanoparticle haloing. 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 fractions,
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.
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