Twist And Snap: Heterogeneous Defect Nucleation via Frank-Read Sources in Nematic Liquid Crystals

Topological defects can nucleate heterogeneously, as when a pinned dislocation segment in a crystal bows out under stress and snaps off repeatedly, emitting concentric dislocation loops, known as a Frank-Read source. A similar mechanism can arise in a nematic liquid crystal containing a pinned disclination. We consider a disclination half-loop pinned to a planar anchoring layer [1]. When director twist is imposed on the top substrate, the half-loop expands and snaps off new disclination loops leaving the original half-loop intact [2,3]. We model this mechanism via a 3D Lebwohl-Lasher rotor model, representing a uniaxial nematic with equal Frank constants. We study the Frank-Read mechanism and explore the effects of temperature and twist rate. Using a materials-by-design approach, we propose that a liquid crystal cell with a patterned array of Frank-Read sources will demonstrate a rheological response that depends on disclination half-loop sizes, density, orientation, and pattern. Defect half-loops pinned on colloids may also serve as sources. We discuss the importance of this mechanism in rheology of both passive and active nematics. [1] Guo et al doi:10.1002/adom.202100181, [2] Long et al doi:10.1039/d0sm01899f, [3] Angelo http://tiny.cc/jangelo.