Strange Physical Phenomena to Control Light in New Ways

Researcher at University of Michigan have demonstrated how to fabricate cage structures with nanoparticles using computer simulations, which can demonstrate ways to fabricate nanostructured structures with heterogeneous materials.

The host tissue cage of the bipyramid particle is shown in blue on the left side, becoming increasingly transparent towards the right. The red bipyramid particles are guest particles, trapped in a caged clathrate structure. Image Credit: Sangmin Lee, Glotzer Group

This discovery could provide new opportunities for photonic materials that can control light differently than natural crystals. Moreover, it exhibits a curious result that the group calls entropy compartmentalization.

We’re developing new ways to structure material across scales, discovering what possibilities and strengths we can use. Entropic forces can stabilize crystals that are more complex than we think.

Sharon Glotzer, Study Leader, Anthony C. Lembke Department Chair, Chemical Engineering, University of Michigan

The tendency of a system to maximize its conceivable state is more accurately reflected by entropy, which is sometimes characterized as the disorder of the system.

This often resulted in chaos in the normal sense. Instead of clustering in a corner, the oxygen molecules spread out to occupy space. Correctly sized squares, however, will cause them to automatically arrange themselves into a recognizable shape.

The same is true for nanoparticles. Glotzer’s team had previously shown that bipyramid particles, two small three-sided pyramids attached to their base, would form formations that mimic fire ice formations if placed in a small enough box.

The water molecules surrounding the methane to create a cage form fire ice, which can melt and burn simultaneously. This material is an example of clathrate and is widely distributed beneath the ocean floor.

The clathrate structure is being studied for a variety of uses, including the collection and removal of carbon dioxide from the environment.

Previous clathrate nanoparticle structures, unlike water clathrate, had no gaps to fill with other materials which would present new and exciting opportunities to change the characteristics of the structure. The group is trying to change that.

This time, we investigate what happens when we change the shape of a particle. We reasoned that if we clipped the particles slightly, it would create space in the cage created by the bipyramidal particles.

Sangmin Lee, First Study Author, Postdoctoral Student, University of Michigan

He removed the three central corners of each bipyramid and found the sweet spot where openings occurred in the structure, but the sides were still intact enough to prevent the pyramids from reorganizing.

When they are the only particles present in the system, the voids are filled by additional truncated bipyramids. The shape inserted as the second shape eventually evolves into a trapped guest particle.

There’s no glue holding the bipyramid together in this case, although Glotzer has ideas on how to selectively make the adhesive sides so that various materials can serve as guest cages and particles. Instead, entropy completely preserves structure.

Glozer added, “What’s really interesting, looking at the simulation, is that the host’s network almost freezes. The host particles move, but they all move together like a single, rigid body, just as happens with water clathrate. But the guest particles are spinning around like crazy — like a system dumping all the entropy into the guest particles.

These are systems that can be constructed by truncated bipyramids in the smallest amount of space, but almost all of the flexibility belongs to the guest particles. According to the researchers, methane also swirls in water clathrate.

In addition, when guest particles are taken out, the structure throws a bipyramid that has become part of the network cage structure into the interior of the cage because it is more important to make rotating particles accessible than a full cage to increase entropy.

Glotzer further stated, “Entropy compartmentalization. Isn’t that cool? I’m sure it happens on other systems too—not just clathrate.

The study was assisted by Thi Vo, who is currently an assistant professor of chemical and biomolecular engineering at Johns Hopkins University and was a former postdoctoral researcher in chemical engineering at the University of Michigan.

Extreme Environment Discovery Science and Engineering from the National Science Foundation and the University of Michigan provided computing resources for this study, supported by the Department of Energy and the Office of Naval Research.

Glotzer is also a professor of materials science and engineering, macromolecular science and engineering, and physics, as well as the Distinguished University Professor of Engineering John Werner Cahn and the Stuart W. Churchill Collegiate Professor of Chemical Engineering.

Journal Reference:

Lee, S., et al. (2023) Entropy compartmentalization stabilizes open host-guest colloidal clathrate. Natural Chemistry. doi:10.1038/s41557-023-01200-6.


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