Scientists create liquid crystals that look very much like 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 – an important step forward in building liquid materials that match the colorful variety of shapes seen in minerals and gemstones. , from lazulite to topaz.

The group’s findings, published today in the journal Nature, could one day lead to new types of smart windows and television or computer screens that can bend and control light like never before.

The results come down to a property of solid crystals known to many chemists and gemologists: symmetry.

Ivan Smalyukh, a professor in the Department of Physics at CU Boulder, explained that scientists categorize all known crystals into seven main classes, plus many more subclasses – based in part on the ‘symmetry operations’ of their internal atoms. In other words, how many ways can you insert or rotate an imaginary mirror inside a crystal, and still see the same structure? Consider this classification system as Baskin-Robbins’ 32 flavors, but for minerals.

To date, however, scientists have not been able to create liquid crystals – liquid materials found in most modern display technologies – that exist in the same taste.

“We know all about all the possible symmetries of solid crystals we can make. There are 230 of them,” said Smalyukh, senior author of the new study. He is also a Fellow of the Institute for Renewable and Sustainable Energy (RASEI) at the CU. Boulder. “When it comes to nematic liquid crystals, the kind that appear on most exhibits, there are only a few that have been shown so far.”

That is, until now.

In their latest findings, Smalyukh and his colleagues devised a way to design the first liquid crystals that look like monoclinic and orthorhombic crystals – two of the seven main classes of solid crystals. The findings bring a little more order to the chaotic world of liquids.

“There are many possible types of liquid crystals, but so far very few have been discovered,” Smalyukh said. “This is good news for students because there is so much more to find.”

Symmetry in action

First imagine your body understanding symmetry in crystals. If you place a giant mirror in the center of your face, you will see a reflection that (more or less) looks like the same person.

Solid crystals have similar properties. Cubic crystals, which include diamonds and pyrite, for example, consist of atoms arranged in the shape of a perfect cube. They have many symmetry operations.

“If you rotate those crystals 90 or 180 degrees about very special axes, all the atoms stay in the right places,” Smalyukh said.

But there are other types of crystals as well. The atoms in monoclinic crystals, which include gypsum or lazulite, are arranged in a shape resembling an oblique column. Rotate or rotate these crystals all you want, and they still have only two clear symmetries – one mirror plane and one axis of 180 degrees, or the symmetry that you can see through a crystal around an axis and notice that it the the every 180 degrees the same. Scientists call it a ‘low-symmetry’ condition.

However, traditional liquid crystals do not exhibit such complex structures. The most common liquid crystals, for example, consist of small rod-shaped molecules. Under the microscope, they take the tendency to stand like dry pasta noodles in a pot, Smalyukh said.

“If things can flow, they usually do not exhibit such low symmetries,” Smalyukh said.

Order in liquids

He and his colleagues wanted to see if they could change that. To begin with, the team mixes two different types of liquid crystals. The first was the ordinary class consisting of rod-shaped molecules. The second consists of particles that have formed like ultra-thin disks.

When the researchers brought them together, they noticed something strange: under the right conditions in the laboratory, the two types of crystals pressed and pressed each other and changed their orientation and arrangement. The end result was a nematic liquid crystal liquid with symmetry very similar to that of solid monoclinic crystals. The molecules inside show some symmetry, but only one mirror plane and one axis rotate 180 degrees.

The group, in other words, created a material with the mathematical properties of a lazulite or gypsum crystal – but it can flow like a liquid.

“We ask a very fundamental question: in what ways can you combine order and fluidity into one material?” Smalyukh said.

And the team’s creations are dynamic: heating or cooling the liquid crystals, for example, can turn them into a rainbow of different structures, each with its own characteristics, said Haridas Mundoor, lead author of the new article. . It’s pretty handy for engineers.

“It offers different ways to adapt display technologies, which can improve the energy efficiency in the performance of devices such as smartphones,” said Mundoor, a postdoctoral research fellow at CU Boulder.

He and his colleagues still do not make nearly liquid crystals that can repeat the full spectrum of solid crystals. But the new newspaper brings them closer than ever before – good news for lovers of shiny things everywhere.