A groundbreaking study published today in Physical Review X, titled "Frank-Read Mechanism in Nematic Liquid Crystals," sheds light on a fundamental connection in the behavior of nematic liquid crystals and crystalline solids. Led by a team of researchers from Kent State University, the study reveals how defects in liquid crystals can be continuously generated under deformation, following the same process that drives plasticity in metals.
"Nematics, while structurally quite different from crystalline solids, exhibit similar behavior at the mesoscopic level," explains Robin L.B. Selinger, Ph.D., professor in the Department of Physics at Kent State and one of the lead authors of the study. “A nematic phase is a fluid of rod-shaped molecules that spontaneously align, while a crystal is made of atoms arranged in a repeating pattern. It’s really surprising that defects in such vastly different materials can be generated via the same mechanism.”
Discovered by Sir Frederick Charles Frank and Thornton Read in 1950, the Frank-Read mechanism describes how a pinned defect line segment in a crystalline solid repeatedly bows outward under stress and detaches, generating a series of concentric defect loops which expand outward. This process is fundamental to the ductility of metals. Surprisingly, the Kent State research team found that this phenomenon also occurs in nematic liquid crystals, materials widely used in display screens for electronic devices, even though the two classes of materials are completely different and the nature of the line defects is likewise distinct.
"Experiments by our co-author, Ohio Research Scholar Hiroshi Yokoyama, Ph.D., professor (Kent State Department of Physics), with Joseph Angelo, Ph.D., and Christopher Culbreath, Ph.D., show that a pinned defect line in a nematic liquid crystal can likewise bow out and snap off a new loop under a twist deformation, mirroring the Frank-Read mechanism observed in crystals," Selinger explained. “I recognized the characteristic images at Joe Angelo’s dissertation defense and was so excited. It took us several years to explore the connection and explain how it works.”
Besides Robin Selinger, theorists on the team include lead author Cheng Long, Ph.D., now a postdoctoral fellow at Harvard University; Matthew Deutsch, a materials science graduate student and now a student intern at Los Alamos National Laboratory; and Ohio Eminent Scholar Jonathan Selinger, Ph.D., also a professor in Kent State’s Department of Physics.
This discovery suggests new avenues for engineering liquid crystal devices with control of defect generation. Unlike in metals where Frank-Read sources form randomly, in nematics, researchers can create pinning points on confining walls, controlling precisely where defects will form.
Besides his work on plasticity in metals, Sir Frederick Charles Frank (1911-1998) also made major contributions to the study of liquid crystals, where he formulated a widely used theory of elastic behavior in nematics in 1958.
“This project brings together and builds on Frank’s fundamental contributions in both areas,” Robin Selinger explained. “Our work celebrates his enduring impact in the field of materials science.”
The project was funded by a grant from the National Science Foundation.
The research team and co-authors:
Hiroshi Yokoyama, Jonathan V. Selinger, and Robin L. B. Selinger are all Kent State faculty members affiliated with both the Department of Physics and the Advanced Materials and Liquid Crystal Institute (AMLCI) at Kent State. Cheng Long completed his Ph.D. in Physics at Kent State and is currently a postdoctoral fellow at Harvard University. Matthew J. Deutsch is a current Kent State Ph.D. student in Materials Science and also serves as a graduate research assistant at the Los Alamos National Laboratory. Joseph Angelo completed his Ph.D. in Chemical Physics at Kent State and currently works at AlphaMicron in Kent, Ohio. Christopher Culbreath completed his Ph.D. in Chemical Physics at Kent State and is currently a Lecturer at Cal Poly San Luis Obispo.
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Media Contacts:
Robin Selinger, 330-672-1582, rselinge@kent.edu
Jim Maxwell, 330-672-8028, JMAXWEL2@kent.edu