  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.
