The importance of being discrete: fluctuations, defects, and density-orientation coupling in agent-based active nematics

Abstract We propose an agent-based model of active flexible rods. Inspired by cytoskeletal flows, we introduce activity by an internal flow that contributes to the dissipative forces. The active force between our agents is central and reciprocal, ensuring linear and angular momentum conservation. For nematic activity, we find spontaneous, thresholdless flows and stochastic flow-reorientation, which is accompanied by the formation of topological defects. Defects appear and vanish with activity-dependent rates, and + 1 / 2 -defects self-propel. These hallmarks of active turbulence are present even on the scale of individual agents. The particle-based interactions lead to an emergent coupling between density and orientation that generates density dipoles around + 1 / 2 -defects. Finally, we highlight the versatility of our agent-based model by presenting spontaneous flows in three dimensions and tissue growth. Our framework opens the way for an integrated description of living materials, including several forms of activity in the same system.