Active Nematics

Active nematics: apolar self-propelled rods align head-to-tail (nematic, not polar). The key signature is ±½ topological defects — the +½ defect is a comet that self-propels; the −½ is a stationary trefoil. These spontaneously form, annihilate in pairs, and drive large-scale active turbulence. Examples: vibrated granular rods, microtubule-kinesin gels, bacterial monolayers.