Fig. 10.

Mechanisms of defect propulsion and annihilation: In the microtubule, active nematic defect propulsion can occur through a combination of thinning and fracture mechanisms. (A) Constituent fibers reorient and fracture, allowing the $+1/2$ defect core to move forward. The fracture-driven motility does not require material transport. (B) In the regime where constituents cannot easily reorient, ordered fibers lying in front of the $+1/2$ defect prevent its forward movement. Extensile active transport within the nematic along director lines (red arrows) causes elongation and thinning that both propels the defect pair forward and brings them closer together. Their annihilation rate is limited by the rate at which the extensile flows can displace the materials in front of the $+1/2$ defect. Dashed line indicates the initial defect position.