In a crystalline solid under mechanical stress, a Frank-Read source is a pinned dislocation segment that repeatedly bows and detaches, generating concentric dislocation loops. We demonstrate that in nematic liquid crystals, an analogous Frank-Read mechanism generates concentric disclination loops. Using experiment, simulation, and theory, we study a disclination segment pinned between surface defects on one substrate in a nematic cell. Under applied twist of the nematic director, the pinned segment bows and emits a new disclination loop which expands, leaving the original segment intact; loop emission repeats for each additional 180 degrees of applied twist. We present experimental micrographs showing loop expansion and snap-off, numerical simulations of loop emission under both quasistatic and dynamic loading, and theoretical analysis considering both free energy minimization and the balance of competing forces. Lastly, we discuss potential use of Frank-Read sources to modify microstructural evolution in both passive and active nematics.
We have developed a new device called the dynamic cell, whereby the twist angle and cell thickness of a liquid crystal cell can be dynamically varied. For a twist angle larger than 90°, there occurs a breaking of anchoring at a certain threshold cell thickness. This structural transition converts the super-twisted state to a normal twisted state by flipping the surface director by 180°. A disclination loop separates the normal twisted region from the super-twisted region. By controlling the twist and thickness, we can stabilize this loop and calculate its line tension.
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