![]() Churchill Collegiate Professor of Chemical Engineering, and a professor of materials science and engineering, macromolecular science and engineering, and physics.No, this is not a rare digestive disorder. Glotzer is also the John Werner Cahn Distinguished University Professor of Engineering, the Stuart W. This study was funded by the Department of Energy and Office of Naval Research, with computing resources provided by the National Science Foundation's Extreme Science and Engineering Discovery Environment and the University of Michigan. Thi Vo, a former postdoctoral researcher in chemical engineering at U-M and now an assistant professor of chemical and biomolecular engineering at the Johns Hopkins University, contributed to the study. Isn't that cool? I bet that happens in other systems too - not just clathrates," Glotzer said. What's more, when they removed the guest particles, the structure threw bipyramids that had been part of the networked cage structure into the cage interiors - it was more important to have spinning particles available to maximize the entropy than to have complete cages. ![]() Methane in water clathrates rotates too, the researchers say. This was the system with the most degrees of freedom that the truncated bipyramids could build in a limited space, but nearly all the freedom belonged to the guest particles. "But the guest particles are spinning around like crazy - like the system dumped all the entropy into the guest particles." The host particles move, but they all move together like a single, rigid object, which is exactly what happens with water clathrates," Glotzer said. "What's really fascinating, looking at the simulations, is that the host network is almost frozen. Instead, the structure was completely stabilized by entropy. Glotzer has ideas for how to create selectively sticky sides that would enable different materials to act as cage and guest particles, but in this case, there was no glue holding the bipyramids together. When a second shape was added, that shape became the trapped guest particle. The spaces filled in with more truncated bipyramids when they were the only particle in the system. He took the three central corners off each bipyramid and discovered the sweet spot where spaces appeared in the structure but the sides of the pyramids were still intact enough that they didn't start organizing in a different way. We reasoned that if we truncate the particle a little, it would create space in the cage made by the bipyramid particles," said Sangmin Lee, a recent doctoral graduate in chemical engineering and first author of the paper. "This time, we investigated what happens if we change the shape of the particle. Unlike water clathrates, earlier nanoparticle clathrate structures had no gaps to fill with other materials that might provide new and interesting possibilities for altering the structure's properties. Clathrate structures are under investigation for a range of applications, such as trapping and removing carbon dioxide from the atmosphere. This substance is found in abundance under the ocean floor and is an example of a clathrate. Fire ice is made of water molecules that form cages around methane, and it can burn and melt at the same time. Previously, Glotzer's team had shown that bipyramid particles - like two short, three-sided pyramids stuck together at their bases - will form structures resembling that of fire ice if you put them into a sufficiently small box. But if you put them in the right size box, they will naturally order themselves into a recognizable structure. Oxygen molecules don't huddle together in a corner - they spread out to fill a room. Often, this ends up as disorder in the colloquial sense. While entropy is often explained as disorder in a system, it more accurately reflects the system's tendency to maximize its possible states. "Entropic forces can stabilize even more complex crystals than we thought." Lembke Department Chair of Chemical Engineering, who led the study published today in Nature Chemistry. "We are developing new ways to structure matter across scales, discovering the possibilities and what forces we can use," said Sharon Glotzer, the Anthony C. It also showcased an unusual effect that the team is calling entropy compartmentalization. The finding could open new avenues for photonic materials that manipulate light in ways that natural crystals can't.
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