Leo P. Kadanoff Prize Talk: Complex flows and topology in active matter*

Active matter is the named coined to describe collections of interacting self-driven entities that spontaneously organize in active fluids and solids, with nonequilibrium transitions between ordered and disordered states. Examples include subcellular structures, groups of motile organisms, and chemical and mechanical analogues with life-like properties. The distinguishing property of active systems is that they are driven out of equilibrium by a forcing that acts independently on each unit, breaking detailed balance at the micro-scale and resulting in behaviors that challenge our intuition. For instance, active fluids flow with no externally applied forces and active gases do not fill their container. After highlighting some of these behaviors, I will focus in this talk on the self-sustained flows of active nematics where the proliferation of motile topological defects mediates the transition to spatiotemporal chaotic dynamics. I will conclude by identifying some of the challenges that lie ahead.