Active Topology

Topological defects – singular tears of the order parameter field that cannot be removed by smooth deformations - are often formed in quenches from the disordered state or when order is frustrated by curvature, external fields or boundary conditions. In equilibrium two-dimensional systems, such as thin films of superfluids, crystals, liquid crystals and magnets, order-disorder transitions are controlled by defect unbinding described via the Berezinskii- Kosterlitz-Thouless mapping of the statistical physics of defects onto a Coulomb gas. In active liquid crystals, topological defects become motile particles and proliferate spontaneously upon the transition from laminar flow to self-sustained turbulent-like motion ubiquitously observed in these systems. Additionally, experiments have suggested the possible biological relevance of defects in active nematics as loci of cell extrusion and death in epithelia and organizing centers of morphogenetic processes. In this talk I will outline a framework for formulating the statistical physics of defects in active nematics as quasiparticles and show that by viewing the active nematic as a collection of swarming and interacting active defects, the onset of active turbulence can be described as an activity-driven defect unbinding transition. A hydrodynamic theory of the gas of unbound defects additionally captures states of hierarchically organized active matter and the role of activity gradients for confining defects and harnessing active flows.

Part of the NSCS webinar series https://softmatterisrael.wixsite.com/nscs