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Polarized particles join toolbox
for building unique structures
E. Kloeppel, Physical Sciences Editor
photo to enlarge
of Illinois Photo
Granick, a professor of materials science and
engineering, of chemistry and of physics, above,
Luijten, a professor of materials science and
created polarized, spherical particles that spontaneously
self-assemble into clusters with specific shapes
and distributions of electric charge.
CHAMPAIGN, Ill. —
Researchers at the University of Illinois at Urbana-Champaign have created
polarized, spherical particles that spontaneously self-assemble into
clusters with specific shapes and distributions of electric charge.
The polarized particles can be used in the directional self-assembly
of intricate shapes and unique structures.
“The world abounds with particles that have traditionally been
treated as geometrically symmetric, chemically isotropic and electrically
uniform,” said Steve Granick, a professor of materials
science and engineering, chemistry and physics. “We have
muddied the waters a bit by asking: ‘What happens when we build
clusters from particles that have an uneven distribution of electric
The polarized spheres are called Janus particles; Janus was the Roman
god of change, often portrayed with two faces gazing in opposite directions.
The spheres offer new opportunities in particle engineering for building
particular structures. The clusters may also prove useful as simple
systems in which to explore the role of charge interactions in determining
how proteins aggregate.
Granick and his collaborators describe their work in a paper accepted
for publication in the journal Nano Letters, and posted on its Web site.
To make their Janus particles, the researchers begin with negatively
charged beads one micron in diameter. Using electron beam deposition,
they coat one hemisphere of the beads with a gold film, which is then
made positively charged.
When placed in solution, the particles spontaneously self-assemble into
specific geometrical shapes depending on the number of particles. For
example, clusters of seven particles resemble a flywheel, which can
revolve around a polar axle.
The compact shapes differ fundamentally from the strings and rings formed
by magnetic particles, said Granick, who also is a researcher at the Frederick Seitz Materials Research
Laboratory and at the Beckman
Institute for Advanced Science and Technology.
“The observed shapes are in excellent agreement with computer
simulations,” said Erik Luijten, a professor of materials science
and engineering, and a corresponding author of the paper. “The
simulations not only show you the shapes, they also show you how the
particles are oriented in the cluster.”
Surprisingly, the charge distribution of the initial Janus particles
is preserved in the clusters. One half of each cluster tends to be positively
charged; the other half negatively charged. This uneven distribution
of surface charge could be utilized, perhaps, in the directional self-assembly
of particles into more elaborate and intricate shapes.
“Future work could consider particles whose shape is not just
spherical, but also rod-like or oblate,” Granick said. “This
is just the beginning of something that will catch a lot of people’s
Lead authors of the paper were graduate student Liang Hong and postdoctoral
research associate Angelo Cacciuto. The work was funded by the National
Science Foundation and the Petroleum Research Fund.
Editor’s note: To reach Steve Granick,
call 217-333-5720; e-mail: firstname.lastname@example.org.
To reach Erik Luijten, call 217-244-5622; e-mail: email@example.com.