Active dislocations and topological traps govern dynamics of spiraling filamentous cyanobacteria

Abstract

Activity can organize matter in unique configurations inaccessible to equilibrium systems, including a sundry of spiraling shapes seen in nature that range from galaxies to living tissues to fossilized stromatolites. How these dynamic yet stable patterns form in motile active systems that span a range of length and time scales remains an open question. Here we study the collective gliding dynamics of ultra-long filamentous cyanobacteria confined in two dimensions and present the discovery of an emergent pattern we call ``active spirals". Individual filaments in the spiral bulk remain confluent due to adhesion forces and exhibit reversible gliding motility. Thus individual filaments undergo bidirectional movement and the spiral object as a whole has no fixed vorticity. Using single filament tracking, we discover that spirals permit the radial flux of material as filaments shear past one another. We demonstrate that these rearrangements can be entirely described by topological rules of interaction between filaments tips. We thus reduce the dynamics of a spiral to a set of active dislocations (corresponding to the filament tips) on a polar coordinate lattice and show that we can reproduce and predict the material flux in the system. Finally, we present a discovery of a novel topological trap present in these spirals, and is induced purely by the geometric chirality of long winding filaments with winding number greater than zero. A topological trap creates boundaries in the spiral across which material cannot flow, leading to persistent structures that are topologically locked for the lifetime of the system. The emergent mechanics of active spirals presented here sheds light on the critical role of adhesion forces, activity and geometry in the formation of long-term, stable, yet dynamic active patterns.