Self-assembly III: choreographing molecular dancers

Filed in archive Materials on January 29, 2006

A decade-old nanotech mystery has been solved by a Princeton research team, and their surprising conclusions may lead to new methods for the guided self-assembly of nanoparticles.

For more than ten years scientists have been puzzled about why minute clusters of soap bubbles called surfactant micelles tend to aggregate as low arched forms like Quonset huts when placed on a graphite surface.

The Princeton team used new atomic force microscope imaging techniques to study the micelles and found that they are not static structures at all, but a constantly evolving dance of ever-changing rod-like particles.

"We spent a year trying to describe why these rods orient themselves on the graphite surface," investigator Dudley Saville said. "But it turns out that we had imaged the dancers in freeze-frame. What we did not take into account in our original thinking was that micelles on the surface are in constant rotary motion."

Their discovery is significant in the search for self-assembly--the need for nanoparticles to organize themselves into larger structures according to predetermined "blueprints". Without self-assembly, there is virtually no way to produce useful large-scale products from innovative nanoparticles.

One obstacle to self-assembly is Brownian motion, the tendency for nanoparticles to move at random. The Princeton team found that Brownian motion may be overcome in the case of their dancing micelles by another atomic force called van der Waals force.

"When micelles appear on the graphite stage, they begin dancing to the music of a van der Waals orchestra," Saville said. Understanding these forces then makes it possible to choreograph the micelle dance, or organize their assembly.

The team hopes their discovery may lead to valuable technological applications like anti-corrosion coatings for metals and bio-medical applications involving plaque formation with proteins. Their report, co-authored by Saville and colleagues Ilhan Aksay and Roberto Car, appears in the January 13 issue of Physical Review Letters.