Spontaneous self-constraint in active nematic flows

Active processes drive and guide biological dynamics across scales -- from subcellular cytoskeletal remodelling, through tissue development in embryogenesis, to population-level bacterial colonies expansion. In each of these, biological functionality requires collective flows to occur while self-organized structures are protected; however, the mechanisms by which active flows can spontaneously constrain their dynamics to preserve structure have not previously been explained. By studying collective flows and defect dynamics in active nematic films, we demonstrate the existence of a self-constraint -- a two-way, spontaneously arising relationship between activity-driven isosurfaces of flow boundaries and mesoscale nematic structures. Our results show that self-motile defects are tightly constrained to viscometric surfaces -- contours along which vorticity and strain-rate balance. This in turn reveals that self-motile defects break mirror symmetry when they move along a single viscometric surface, in contrast with expectations. This is explained by an interdependence between viscometric surfaces and bend walls -- elongated narrow kinks in the orientation field. Although we focus on extensile nematic films, numerical results show the constraint holds whenever activity leads to motile half-charge defects. This mesoscale cross-field self-constraint offers a new framework for tackling complex 3D active turbulence, designing dynamic control into biomimetic materials, and understanding how biological systems can employ active stress for dynamic self-organization.