Active nematics and topological defects

Collaborators: Mark Bowick (UC Santa Barbara), Sriram Ramaswamy Indian Institute of Science), Suraj Shankar (Harvard)

Active nematics are composed of elongated active units that organize in ordered states with nematic symmetry. Although active, these fluids, unlike their polar counterparts, go nowhere on average. Activity, however, drives spontaneous flows and a spatiotemporal chaotic dynamics that has been named ``active turbulence''. The number of experimental realizations continues to grows, spanning inert systems such as vibrated layers of granular rods, in vitro suspensions of cytoskeletal filaments and motor proteins, and living systems such as monolayers of epithelial cells. Experiments have shows that the onset of active turbulence is accompanied by the proliferation of topological defects that drive the motion. What is new is that in active systems defects become dynamical objects, self-propelled (the +1/2 disclinations in active nematics ) or self-rotating (the vortices in active polar fluids) entities that can themselves exhibit emergent behavior.  Our group has shown that defects in active nematic films on a substrate can be described as a mixture of motile (+1/2’s) and passive (-1/2’s) quasi-particles interacting via Coulomb forces. This model quantifies the onset of spatio-temporal chaos as an activity-driven Berezinkii-Kosterlitz Thouless transition and predicts new emergent states of defect order. Open questions include understanding the role of flows and employing tunable activity gradients to control and guide active defects and active flows.