Utilizing computer simulations, researchers at the University of Michigan in Ann Arbor have demonstrated they can design a crystal and work backward as a way to create the particle shape. The discovery could lead to a new class of materials, such as crystal coatings that produce colors that never fade.
“These results turn materials design and our understanding of entropy on their heads,” says Sharon Glotzer, the Anthony C. Lembke department chair of chemical engineering
at the University of Michigan and senior author on the paper in Science Advances.
Materials with truly new properties are usually discovered by accident. It took an experiment with cellophane tape and a lump of graphite in 2004 to discover graphene, a Nobel-winning wonder material for its combination of strength, flexibility, transparency, and conductivity.
Rather than wait for another accidental discovery, materials scientists would like to come up with a wonder material and figure out how to make it. The inverse approach to designing materials, working backward from the desired properties, is what the team is calling “digital alchemy.”
“It really allows us to focus on the outcome and leverage what we know to find a starting point to building that material,” says Greg van Anders, a corresponding author on the paper and assistant professor of physics at Queen’s University in Kingston, Ontario. The research was done during his time at U-M.
Glotzer is a leader in studying how nanoparticles self-assemble through the mechanism of entropy. While entropy is commonly thought of as a measure of disorder, Glotzer’s team harnesses it to create ordered crystals from particles. The team is able to do this because entropy is a measure of how free the system is, not actual disorder. When particles have a lot of space, they distribute and orient randomly across the space.
In the systems Glotzer focuses on, particles do not possess much space. The system of particles is most free if the particles organize themselves into a crystal structure, and the particles work toward freedom.
Depending on particle shapes, Glotzer’s team and others have exhibited how one can get a variety of interesting crystals, some similar to salt crystals or the atomic lattices in metals and
some apparently new. This was previously done by choosing a particle’s shape and simulating the crystal it would make. Teams have spent years discovering the design rule that enable particles of specific shapes to build certain crystals.
With the new study, the team is studying how to predict particle shape based on the completed crystal instead of simulating the crystal based on particle shape. The team explored more than 100 million different particle shapes in the study.
“In a single day, on a regular computer, we were able to study more different kinds of particles than have been reported in the last decade,” says van Anders.
The team used software to identify particle shapes for building four common crystal lattices and two complex lattices. After proven successful, the team tried a lattice not known to nature, but was a design that was their own.
The team anticipates experimental nanoscientists will have the ability to make crystals by producing batches of particles in the right shape and adding them to a fluid. In the fluid, nanoparticles will assemble themselves. They will keep their structure as long as they remain confined.
The discovery could lead to advances in human-made structural color, similar to the way butterfly wings produce their brilliant hues through interactions with light. Unlike pigments, structural color does not fade. The color could also be turned on and off with a mechanism to confine the particles so they form the crystal or give them space so the crystal falls apart.
The research was reported in Science Advances in a paper titled “Engineering Entropy for the Inverse Design of Colloidal Crystals From Hard Shapes.”