Scientists create liquid crystals that closely resemble their solid counterparts.
A team at the University of Colorado Boulder has designed new types of liquid crystals that reflect the complex structures of some solid crystals, a major breakthrough in building material spills that match the colorful diversity of minerals and shapes seen in gems, topaz from lazulite.
The group’s findings in a journal published on February 10, 2021 Nature, may one day create new types of smart windows and TV or computer screens so that the light can be tilted and controlled more than ever.
The results have reached the property of solid crystals that will be known to many chemists and gemologists: symmetry.
Ivan Smalyukh, a professor in the Physics Department at CU Boulder, explained that all crystals known to scientists are classified into seven main classes, plus many more subclasses, based in part on the “symmetry operations” of internal atoms. In other words, how many ways can you put an imaginary glass mirror inside or rotate it and still see the same structure? Think of this classification system as Baskin-Robbin’s 32 flavors, but for minerals.
So far, however, scientists have not been able to create liquid crystals (emissive materials found in most modern display technologies) with many flavors.
“We know everything about all the possible symmetries of solid crystals we can make. There are 230 of them,” said Smalyukh, lead author of the new study. He is also a member of the CU Boulder Institute for Renewable and Sustainable Energy (RASEI). “As for the nematic liquid crystals that appear on most screens, we only have a few that have been proven so far.”
That is, so far.
In recent discoveries, Smalyukh and his colleagues created a way to design the first liquid crystals resembling monoclinic and orthorhombic crystals — two of these major classes of seven solid crystals. The findings, he said, are a little more orderly in the chaotic world of fluids.
“There are many types of liquid crystals, but so far very few have been found,” Smalyukh said. “That’s great news for students, because a lot more can be found.”
Symmetry in action
To understand the symmetry of crystals, first figure out your body. If you put a giant mirror in the middle of your face, you will see a (roughly) reflection that looks like the same person.
Solid crystals have similar properties. Cubic crystals, which contain diamonds and pyrite, for example, are made up of atoms arranged in the shape of a perfect cube. They have many symmetry operations.
“If you rotate these crystals 90 or 180 degrees around many special axes, for example, all the atoms stay in the right places,” Smalyukh said.
But there are other types of crystals as well. The inner atoms of monoclinic crystals, including gypsum or lazite, are arranged in the shape of inclined columns. Flip or rotate these crystals all you want, and they still have only two different symmetries: a plane mirror and a 180-degree axis of rotation, or the same symmetry and appearance you can see by rotating a crystal around an axis every 180 degrees. Scientists call the situation “low symmetry”.
Traditional liquid crystals, however, do not exhibit these complex structures. The most common liquid crystals, for example, are made up of small rod-shaped molecules. Under a microscope, they tend to line up like dry pasta thrown into a pot, Smalyukh said.
“When things can be put together they usually don’t show such low symmetries,” Smalyukh said.
In liquid order
He and his colleagues wanted to see if they were able to change that. To begin with, the group mixed two different types of liquid crystals. The first was a conventional class of rod-shaped molecules. The second consisted of ultrafine disk-like particles.
When the researchers came together, they noticed something strange: in the right conditions in the laboratory, these two types of crystals pushed and tightened each other, changing their orientation and organization. The end result was a nematetic liquid crystal with symmetry that closely resembled a solid monoclinic crystal. The internal molecules showed some symmetry, but only a mirror plane and a single 180-degree axis of rotation.
The group, in other words, created a material that had the mathematical properties of lazulite or gypsum crystal, but its own could be placed like a fluid.
“We’re asking a very basic question: what are the ways to combine order and fluidity in a single material?” Smalyukh said.
The group’s creations are dynamic: if you heat or cool liquid crystals, for example, you can turn them into rainbows of different structures, each with its own properties, said Haridas Mundoor, author of the new article. . That’s pretty useful for engineers.
“This provides different avenues that can change display technologies, which can improve energy efficiency in the operation of devices like smartphones,” said Mundoor, a post-doctoral research association at CU Boulder.
He and his colleagues are not yet making liquid crystals that can repeat the full spectrum of solid crystals. The new paper brings the good news closer than ever to lovers of bright things everywhere.
Reference: Haridas Mundoor, Jin-Sheng Wu, Henricus H. Wensink, and Ivan I. Smalyukh, “February 10, 2050,” monoclinic monoclinic colloidal fluid. Nature.
DOI: 10.1038 / s41586-021-03249-0
Other colleagues in the new article are Jin-Sheng (Jason) Wu, a graduate student at CU Boulder and Henricus Wensink of Paris-Saclay University.