Already the basis for display technologies, liquid crystals also have the potential to be used in advanced photonic devices, such as lasers or sensors.
But while standard liquid crystals have uniform molecular orientations, scientists are also studying the effects of different molecular patterns within the material—patterns that create defects in the molecular order that can be exploited for new optical responses.
In novel simulations conducted at UChicago’s Pritzker School of Molecular Engineering (PME) in the group of Juan de Pablo, Liew Family Professor of Molecular Engineering, researchers have created new morphologies within chiral liquid crystal assemblies that could ultimately form the basis for new technologies that rely on precise optical control, such as optical guides, or biosensors.
The results were recently published in the journal Soft Matter. Viviana Palacio-Betancur, a senior PhD student within the group, is first author of the paper.
Exploiting liquid crystals’ “handedness”
The simulations involved chiral liquid crystals, which have twists and an asymmetrical “handedness”—like right-handedness or left-handedness—that allows them to have more interesting optical behaviors, including fast response times.
When packed together, these crystals tend to tilt to one side, and together they naturally follow a helical shape. But Palacio-Betancur wanted to better understand how to manipulate this natural twist and control the material structure.
Using the robust computational methods that she and her coauthors developed, the simulations explore what would happen if chiral liquid crystals were placed in a cylinder and then in a torus (a doughnut shape). By precisely matching the size of the cylinder to the inherent twist of the chiral liquid crystal, Palacio-Betancur found that she could disrupt their natural twisting tendency to create new structures. When the material was confined into a torus, an additional deformation was imposed on the material, resulting in the formation of novel structures.
Ultimately, Palacio-Betancur and the team used these simulated confinements combined with the natural twist of the material to create several new structural shapes, including chiral ribbons with double helices. The simulations also ultimately created helical blue phase crystals, ultra-sensitive liquid crystals that selectively reflect visible light.
“We wanted to see what would happen as the material twisted increasingly and how it competed with a curved boundary, in the middle we found this region that allowed us to tune the morphology,” Palacio-Betancur said. Potential uses for these structures include optical waveguides and materials that can drive the assembly of nanoparticles into arrays.
Though scientists have previously manipulated the material using lasers and electricity, Palacio-Betancur says their technique shows a simpler, sophisticated method to create new structures.
“We show that there are basic methods that get you very surprising morphologies,” she said. “It has helped us discover new effects.”
Other authors on the paper include Julio C. Armas-Pérez of Universidad de Guanajuato and Juan P. Hernández-Ortiz of Universidad Nacional de Colombia.
Citation: “Curvature and confinement effects on chiral liquid crystal morphologies,” Palacio-Betancur et al, Soft Matter, June 8, 2023. DOI: 10.1039/D3SM00437F
Funding: Department of Energy, Fulbright Colombia and COLCIENCIAS