Discovering new defects in liquid crystals

Sometimes, exciting discoveries are made when scientists are looking for something else entirely.

While developing a mathematical model for the orientation of liquid crystals at the interface with another substance or surface, researchers with the Institute for Molecular Engineering at the University of Chicago discovered an entirely new kind of defect, and a new transition between two types of defects.

The results, published recently in Nature Communications, offer new insights into these materials while expanding the frontier of possible applications.

“This discovery of a new defect that we didn’t know existed shows that our understanding of these materials continues to be incomplete,” said Juan de Pablo, Liew Family Professor in Molecular Engineering at the University of Chicago and co-author of the research. “It could have interesting consequences for assembly of micro or nanoparticles in liquid crystals.”

Understanding defects at an angle

Liquid crystals, the basis of liquid crystal displays (LCDs), offer exciting possibilities for technological applications. They typically have a uniform molecular structure and orientation, but when defects among this order occur—such as when the liquid crystals interface with another immiscible substance or surface—they can be used to develop new kinds of optical technologies.  

When de Pablo and his team read about the discovery of a new defect, called a hexadecapole, in research done by University of Colorado Boulder scientists, they realized that current mathematical models could not fully explain how the defect arose.

Current models explain defects that develop when the molecular orientation of the interface is perpendicular or parallel. The interface that results in the hexadecapole has an orientation that’s tilted at an angle to the surface, a mode called “degenerate conic anchoring.”

To better understand the development of this defect, de Pablo and his team created a new model that took into account the tilted orientation and made it equally probable that the molecules at the interface could be oriented at any angle with respect to the main axis.

Predicting a new kind of defect

Surprisingly, this new model also predicted the formation of another defect, the elastic dipole, and predicted how the dipole transforms into the hexadecapole.

The team asked the University of Colorado Boulder researchers to examine their experimental data to look for the existence of this new type of defect. Sure enough, they found both the elastic dipole and its transition to the hexadecapole.

“It’s always exciting to predict something that no one knew existed,” de Pablo said. “It worked out very beautifully.”

Next, the researchers hope to explore how particles with these defects self-organize and to consider how to create arrays of particles using these new classes of defects to assemble crystals of nanoparticles. Such an array could be used in photonic crystals for a variety of optical technologies.

“These new defects could have a whole host of interesting properties,” de Pablo said.

Other authors on the paper include graduate student Ye Zhou and postdoctoral researcher Rui Zhang of the University of Chicago, and Bohdan Senyuk and Ivan Smalyukh of the University of Colorado Boulder.

Citation: Degenerate conic anchoring and colloidal elastic dipole-hexadecapole transformations Ye Zhou et al., Nature Communications, March 1, 2019. doi: 10.1038/s41467-019-08645-9

Funding: Department of Energy