Abstract

Topological defects play a prominent role in the physics of two-dimensional materials. When driven out of equilibrium in active nematics, disclinations can acquire spontaneous self-propulsion and proliferate to drive self-sustained flows. In this talk I will describe the derivation of an interacting particle description of defects that includes active torques and show that activity drives a non-equilibrium variant of the Berezinskii Kosterlitz-Thouless defect-unbinding transition. On scales larger than the defect separation, the dynamics of the gas of unbound defects can be described by hydrodynamic equations derived by coarse-graining the defects microdynamics. At high activity, nonequilibrium torques combined with many-body screening cause the active disclinations to spontaneously break rotational symmetry, forming a collectively moving defect ordered polar liquid. Finally, I will show that the hydrodynamic theory provides an excellent framework for treating inhomogeneous activity that can be used to pattern and control the dynamics of defects in active fluids.