Researchers led by Founder Professor of Engineering Steve Granick, right, have developed tiny spheres that attract water to form “supermolecule” structures. Team members, from left, Qian Chen, doctoral student in materials science and engineering; Sung Chul Bae, research scientist; and Jonathan Whitmer, doctoral student in physics. Credit: L. Brian Stauffer
Researchers at the Univ.
of Illinois and Northwestern Univ.
have demonstrated bio-inspired structures that self-assemble from simple
building blocks: spheres.
The helical “supermolecules” are made of tiny colloid balls
instead of atoms or molecules. Similar methods could be used to make new
materials with the functionality of complex colloidal molecules. The team will
publish its findings in Science.
“We can now make a whole new class of smart materials, which
opens the door to new functionality that we couldn’t imagine before,” said
Steve Granick, Founder Professor of Engineering at the Univ. of Illinois
and a professor of materials science and engineering, chemistry, and physics.
Granick’s team developed tiny latex spheres, dubbed “Janus
spheres,” which attract each other in water on one side, but repel each other
on the other side. The dual nature is what gives the spheres their ability to
form unusual structures, in a similar way to atoms and molecules.
In pure water, the particles disperse completely because
their charged sides repel one another. However, when salt is added to the
solution, the salt ions soften the repulsion so the spheres can approach
sufficiently closely for their hydrophobic ends to attract.
The attraction between those ends draws the spheres together
At low salt concentrations, small clusters of only a few
particles form. At higher levels, larger clusters form, eventually
self-assembling into chains with an intricate helical structure.
“Just like atoms growing into molecules, these particles can
grow into supracolloids,” Granick said. “Such pathways would be very
conventional if we were talking about atoms and molecules reacting with each
other chemically, but people haven’t realized that particles can behave in this
The team designed spheres with just the right amount of
attraction between their hydrophobic halves so that they would stick to one
another but still be dynamic enough to allow for motion, rearrangement, and
“The amount of stickiness really does matter a lot. You can
end up with something that’s disordered, just small clusters, or if the spheres
are too sticky, you end up with a globular mess instead of these beautiful
structures,” said graduate student Jonathan Whitmer, a co-author of the paper.
One of the advantages of the team’s supermolecules is that
they are large enough to observe in real time using a microscope. The
researchers were able to watch the Janus spheres come together and the clusters
grow—whether one sphere at a time or by merging with other small clusters—and
rearrange into different structural configurations the team calls isomers.
“We design these smart materials to fall into useful shapes
that nature wouldn’t choose,” Granick said.
Surprisingly, theoretical calculations and computer
simulations by Erik Luijten, Northwestern Univ. professor of materials science
and engineering and of engineering sciences and applied mathematics, and
Whitmer, a student in his group, showed that the most common helical structures
are not the most energetically favorable. Rather, the spheres come together in
a way that is the most kinetically favorable.
Next, the researchers hope to continue to explore the
colloid properties with a view toward engineering more unnatural structures.
Janus particles of differing sizes or shapes could open the door to building
other supermolecules and to greater control over their formation.
“These particular particles have preferred structures, but
now that we realize the general mechanism, we can apply it to other systems—smaller
particles, different interactions—and try to engineer clusters that switch in
shape,” Granick said.
The team also included Univ. of Illinois graduate students
Qian Chen and Shan Jiang and research scientist Sung Chul Bae. The U.S.
Department of Energy and the National Science Foundation supported this work.