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High-fidelity patterns form spontaneously when solvent evaporates
James E.
Kloeppel, Physical Sciences Editor
217 244-1073; kloeppel@illinois.edu
2/28/2005
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CHAMPAIGN,
Ill. — Resembling neatly stacked rows of driftwood abandoned by
receding tides, particles left by a confined, evaporating droplet can
create beautiful and complex patterns. The natural, pattern-forming
process could find use in fields such as nanotechnology and optoelectronics.
“A lot of work in nanotechnology has been directed toward the
bottom-up imposition of patterns onto materials,” said Steve Granick,
a professor of materials science, chemistry and physics at the University of Illinois at Urbana-Champaign. “We found that
beautiful patterns of high fidelity and regularity could form naturally
and spontaneously, simply by allowing a drop to evaporate in a confined
geometry.”
Granick and former postdoctoral research associate Zhiqun Lin (now a
professor of materials science at Iowa State University) describe their
work in a paper that has been accepted for publication in the Journal
of the American Chemical Society, and posted on its Web site. Funding
was provided by the U.S. Department of Energy.
To produce the patterns, Granick and Lin began by gluing two small mica
sheets to cylindrical mounts. With the cylinders at right angles, a
droplet of volatile solution containing small polymer chains was inserted
between the curved mica sheets. The sheets were then brought into contact
and left undisturbed until evaporation was complete.
Because evaporation in this geometry is restricted to the edge of the
droplet, the process results in hundreds of concentric rings with regular
spacing, very much resembling a miniature archery target. Each ring
– composed of polymer chains abandoned as the solvent receded
– is several nanometers high and several microns wide.
The droplet evaporates in a jerky, stick-slip fashion, said Granick,
who also is a researcher at the Frederick
Seitz Materials Research Laboratory and at the Beckman
Institute for Advanced Science and Technology.
“While the droplet is sticking to the surface, a ring of polymer
is deposited,” he said. “As evaporation continues, tension
builds in the droplet. Eventually the droplet jerks to a new position,
the tension is temporarily relieved, and another ring is deposited.”
The simple evaporative process could be used to form patterns with many
other materials, such as electrically conducting polymers, nanoparticles
and proteins. Pattern formation could be controlled by altering the
size of the material, changing the solvent, or modifying the surfaces.
“The pattern emerges spontaneously from the geometry in which
we put the droplet,” Granick said. “This means we could
make other kinds of patterns by using different geometries or surfaces
with tailored wettability.”
