Spontaneous flow driven by +1/2 and -1/2 defect pairs
Active nematics — such as cytoskeletal filaments driven by molecular motors — sustain spontaneous chaotic flows via creation and annihilation of topological defects. Half-integer defects (+1/2 comet-shaped, -1/2 trefoil) are the elementary excitations; activity drives +1/2 defects to self-propel, creating turbulent-like dynamics at low Reynolds number.