In a crystalline solid under mechanical stress, a Frank-Read source is a pinned dislocation segment that repeatedly bows outward and detaches, generating a series of concentric dislocation loops during plastic deformation. We demonstrate that in nematic liquid crystals, an analogous Frank-Read mechanism can generate concentric disclination loops. We consider a disclination segment pinned between topological point defects in the anchoring pattern on one substrate in a nematic cell, while the other substrate has uniform planar anchoring. When we increase the twist angle between the two substrates, the pinned segment bows out laterally. When the imposed twist angle exceeds a threshold value, a new disclination loop snaps off and expands, leaving the original pinned segment intact. Loop emission repeats for each additional 180◦ of applied twist between the two substrates. We present a combination of experiment, theory, and simulations to demonstrate this mechanism, and find that the critical stress for disclination loop emission scales as the inverse of segment length. Simulation studies suggest that the critical stress also changes as a function of strain rate and temperature. Frank-Read sources form at random in crystalline solids whenever dislocations pin via interactions with other defects; but in a nematic, they can be engineered in specific locations by inscribing surface anchoring patterns containing point defects. We discuss potential implications for control of microstructural evolution in both active and passive nematic liquid crystals. 
 

Robin Selinger completed undergraduate and graduate studies in physics at Harvard University. After postdoctoral work at UCLA, University of Maryland, and NIST, she joined the physics faculty at the Catholic University of America in 1995. In 2005, she joined the faculty at Kent State University. She is a fellow of the American Physical Society (APS), and was elected to the APS Council of Representatives for 2019-2022, also serving as Speaker of the Council (2022) and as a member of the APS Board of Directors (2020-2022.)

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