MATERIALS SCIENCE
Nanoparticles 'tailor' complex fluids for photonics, ceramics applications

James E. Kloeppel, Physical Sciences Editor
(217) 244-1073; kloeppel@uiuc.edu

8/1/2001

Photo by Bill Wiegand Jennifer Lewis and her colleagues have devised a process that they call nanoparticle haloing. This self-organizing process imparts stability to otherwise attractive colloidal microspheres by decorating regions near their surface with highly charged nanoparticles.

CHAMPAIGN, Ill. — Researchers at the University of Illinois have discovered a fundamentally new approach for tailoring the stability of colloidal suspensions.

Colloidal suspensions are complex fluids utilized in numerous applications ranging from advanced materials to drug delivery. Controlling the stability of these fluids can influence such characteristics as flow behavior, structure and mechanical response, and may result in materials with improved optical and electrical properties.

As reported in the July 31 issue of the Proceedings of the National Academy of Sciences, Jennifer Lewis and her colleagues have devised a process that they call nanoparticle haloing. This
self-organizing process imparts stability to otherwise attractive colloidal microspheres by decorating regions near their surface with highly charged nanoparticles.

Negligibly charged colloidal microspheres (blue) aggregate in aqueous solution but undergo a stabilizing transition upon addital of highly charged nanoparticles (red).

"Using this nanoparticle haloing approach, we can control the phase behavior and structure of materials assembled from colloidal systems," said Lewis, a UI professor of materials science and engineering and of chemical engineering. "Our approach complements traditional stabilization techniques, such as electrostatic stabilization, by allowing systems of negligible charge or high ionic strength to be stabilized."

Tailoring the interactions between particles allows the researchers to engineer the desired degree of colloidal stability into the mixture.

"That means we can create designer colloidal fluids, gels and even crystals," Lewis said. "Our ability to control colloidal forces and phase behavior depends not only on the charge of the nanoparticles, but also on their size. Through nanoparticle engineering, we can assemble structures with properties that would not be possible through traditional stabilization routes."

For example, Lewis has teamed up with co-author Paul Braun, a UI professor of materials science and engineering, to explore the use of these nanoparticle-stabilized colloidal microsphere mixtures in assembling robust periodic templates for photonic band gap materials. The researchers recently were awarded funding by the National Science Foundation to pursue such efforts.

Lewis and her students are also studying the structure and flow behavior of colloidal fluids and gels assembled from these microsphere-nanoparticle mixtures. By compositionally modulating interparticle forces, the researchers can produce systems whose properties vary dramatically. Such studies provide the foundation of ongoing efforts in the area of colloidal processing of electrical ceramics.

In addition to Lewis and Braun, the research team included UI doctoral students Valeria Tohver and James Smay, and Carnegie Mellon University graduate student Alan Braem. The National Aeronautics and Space Administration Microgravity Research Program funded the work.