PARTICULAR Below a critical nanoparticle concentration (top) or above it (bottom), microparticles coalesce because of attractive forces. But within a critical range (middle), uncharged colloid particles are stabilized. Images are not drawn to scale.

Colloidal dispersions play key roles in a number of industrial technologies. Inks, paints, and coatings, for example, are made from suspensions of microscopic particles. Colloids are also used in ceramics processing, photonic materials, and pharmaceutical applications.

Common to all of these applications is a requirement that the suspensions exhibit suitable colloidal stability. Too little stabilization and the particles coagulate as a result of ever-present long-range attractive forces, leaving the suspensions too viscous for some applications. Traditionally, scientists have stabilized the suspensions by designing colloid particles that repel one another via electrostatic, steric, or other interactions.

Now, researchers at the University of Illinois, Urbana-Champaign, have developed a technique in which uncharged or negligibly charged micrometer-sized particles are forced into stable arrangements through the interactions of a second component in the suspension--highly charged nanoparticles [Langmuir, 17, 8414 (2001)].

Jennifer A. Lewis, an associate professor of materials science and engineering who led the study, explains that the self-organizing technique works on the basis of Coulombic repulsions between the charged nanoparticles. In the absence of the nanoparticles, micrometer-sized silica spheres bearing almost no charge are attracted to other microspheres and ultimately coalesce. But as soon as charged nanoparticles are added to the solution, their mutual repulsions cause them to migrate to the charge-free regions around the microparticles.

The result is that the smaller particles decorate a region surrounding the larger spheres' surfaces, thereby mitigating long-range attractive van der Waals forces. Referred to as "nanoparticle haloing" by the Urbana-Champaign group, the process imparts an effective charge to the otherwise negligibly charged microparticles, preventing them from coalescing.

Working with graduate students Valeria Tohver and Angel Chan and others, Lewis finds that suspensions of silica spheres can be prepared as colloidal fluids, gels, and crystals by controlling the fraction of charged hydrous zirconia nanoparticles added to the suspensions. Below a critical nanoparticle volume, the microspheres flocculate and form a gel. Within a critical volume range, the suspensions are highly stable and remain fluid. Exceeding the critical nanoparticle volume range causes stable suspensions to revert to an unstable gel phase.

"Now people can use inorganic nanoparticles more or less as surfactants," Lewis says. She adds that, although the charged particles lack the hydrophilic and hydrophobic character that defines classic surfactants, the charged particles possess self-organizing properties that can be tailored and exploited to prepare custom colloidal inks and other materials.

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