Spontaneous Self-Constraint in Actively Kinky Situations

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 all these situations, biological functionality requires collective flows occur simultaneous to autonomous protection of self-organized structures. However, the mechanisms by which active flows can spontaneously constrain their dynamics has not previously been explained. In this talk, I'll explore how non-linear coupling between flow and orientational fields in active nematics leads to a strong, two-way, spontaneous self-constraint. On the one hand, self-motile topological defects are tightly constrained to specific contours of coherent flow structures, while, on the other hand, the contours are driven by mesoscale defect-associated nematic deformations. Specifically, our results demonstrate that self-motile half-integer defects are only found on the interface of the fluid coherent structures identified as viscometric surfaces—contours where vorticity and strain-rate balance. Identifying this spontaneous self-constraint revealed that the ideal picture of solitary self-motile defects generating a pair of mirror-symmetric vortices is not always true in practice. While we will focus on 2D extensile active nematic turbulence, our conclusions hold whenever activity leads to motile half-integer topological defects. Ultimately spontaneous constraints in active materials could have far-reaching implications as a framework for understanding how biological systems employ active stresses for simultaneous dynamics and restraint.