Kent State Physicist Shines a Light On Complex
In June 2017, an article published in the journal Nature described a significant breakthrough in liquid crystal technology by an international team of researchers. Dr. Robin Selinger, professor of the Chemical Physics Interdisciplinary Program in the Kent State College of Arts and Sciences was co-author of “Making waves in a photoactive polymer film,” along with her graduate students Michael Varga and Andrew Konya. The work was conducted in collaboration with the team of Prof. Dick Broer of the Eindhoven University of Technology.
“We studied a photo-active liquid crystal polymer film that oscillates with continuous mechanical wave motion when illuminated with ultraviolet (UV) light,” Selinger said.
Liquid Crystal, an advanced material made from organic molecules comprised of hydrogen, nitrogen, oxygen, and carbon, has already changed the world\ by giving us liquid crystal display screens. If you’re reading this on a Mac or a smart-phone, you’re using technology created here at Kent State.
Scientists have for many years now been seeking liquid crystal solutions and applications for a wide variety of problems.
Selinger’s involvement in this project began when Broer’s PhD student, Anne Helene Gelebart, told her about puzzling experimental results: a thin ribbon of polymer film wiggled and oscillated when light shone from an oblique angle, but stopped, motionless, when light was applied from directly above the sample. Even more peculiar was the direction of wave motion. Depending on the orientation of liquid crystal molecules in the polymer film, the lightinduced mechanical waves traveled either toward the light source or away from it. The Eindhoven team needed help to understand these perplexing but exciting results.
Selinger and her students built a theoretical model and computer simulation to explain the mechanism driving the material’s unique behavior. “The material flexes when illuminated, then relaxes when light is removed,” Selinger explained, “and the interplay of mechanical motion and shadowing drives the oscillation. When one part of the sample flexes, it blocks light from reaching the rest of the material behind it. This self-shadowing generates wave motion that regenerates every few seconds.” The simulation demonstrated this oscillatory behavior and explained how molecular orientation in the liquid crystal polymer controls the direction of wave propagation.
Most scientists and engineers who perform finite element simulations use commercial software packages, but Selinger’s team took a different approach. They developed their own simulation code, using graphics-processing-unit (GPU)-accelerated computing nodes at the Ohio Supercomputer Center to run their simulations. Selinger presented these results at the International Liquid Crystal Elastomers Conference last October.
Gelebart took the project one step further by tethering the light-responsive polymer film to a thin rectangular frame. When she switched on the UV lamp, the polymer film began oscillating and the device crawled across the table away from the light source. When she flipped the device over, its direction reversed and it crawled toward the light source.
“Gelebart demonstrated that soft robots can be both powered and controlled entirely by light,” Selinger explained. “Our simulation explained how the device’s microstructure and direction of illumination control its mechanical motion.”
Another potential application of these materials is in self-cleaning surfaces. Broer’s team in Eindhoven has already demonstrated this possibility. In one experiment, researchers scattered sand on the ribbon.
“When the UV lamp was switched on,” Selinger explained, “the material flexed and wiggled to shake the sand off.”
Liquid crystal technology is still best known for creating the display screens used in mobile phones, tablets, computers, and televisions. However, as scientists at Kent State’s LCI and institutions around the world continue to learn more about these unusual phases of matter, innovative new concepts for potential uses continue to emerge. Liquid crystal polymers can be patterned to undergo a specific deformation trajectory under a stimulus such as a change of temperature or, as the Kent State-Eindhoven experiment shows, illumination.
Last year, Selinger — along with her husband, Jonathan Selinger, professor of Physics and Ohio Eminent Scholar at the LCI — joined Kent State Ph.D. graduates Vianney Gimenez-Pinto and Badel Mbanga, and former KSU post-doctoral student Fangfu Ye, as co-authors of “Modeling out-of-plane actuation in thin-film nematic polymer networks: From chiral ribbons to autoorigami boxes via twist and topology.” The article discusses research on using liquid crystal elastomers to create programmable solids that deform autonomously when heated.
“By imprinting a specific molecular orientation in different parts of the polymer sample, we can encode the way its shape evolves in response to changes in its environment,” Selinger said. “This kind of material is known as a programmable solid. If it folds on its own into a complex shape, we call the process ‘auto-origami.’”
By controlling the orientation of the molecules’ alignment, stimuli may cause them to curl, fold, or bend. Selinger said the materials “remember” their original shape, and with a reverse stimulus will revert to that shape. The technology offers potential solutions for problems in the medical field and advanced robotics.
Another potential application of programmable solids is to inhibit the growth of harmful bacteria. Selinger recently received a grant from the National Science Foundation, together with colleagues Taylor Ware of the University of Texas at Dallas and Ravi Shankar of Univ of Pittsburgh. The project is entitled, “Collaborative Research: Microfabrication and Self-Assembly of Shape-Changing Hydrogels with Chromonic Liquid Crystalline Order.” Its goal is to design and test coatings of soft active hydrogel cilia that flex mechanically under biologically-benign changes in temperature to disrupt biofilm formation. The project brings together three research groups with complementary expertise in hydrogel chemistry (Ware), mechanical design (Shankar) and materials modeling (Selinger).
Selinger was named a fellow of the American Physical Society in 2017. She is also deeply engaged in STEM outreach activities. She organizes a hands-on research experience for high school students dual-enrolled at Kent State through College Credit Plus; hosts Kent State’s annual science fair for grades 4-12; serves as co-leader of two sponsored STEM scholarship programs; and serves as faculty advisor to Kent State’s Scientista clubs for undergraduate and graduate women in STEM fields.