Higgs Hour: 'Driven Active Nematics: External stresses drive topological transitions in spontaneously flowing fluids'

While traditional fluids only flow when acted upon, a remarkable class of biomaterials spontaneously flow by means of their own internal energy. These “active fluids” comprise a wide range of biological systems that bridge between biological and condensed matter systems. This Brown Bag talk will focus on the example of microtubule-based extensile films modelled through active nematohydrodynamic simulations. We’ll beparticularly interestedin the continuous creation and annihilation of topologicaldefects, singularities in the orientation field that lie at the heart of many phenomena in active nematics. While these defectsplay a pivotal role ingeneratingcomplex spatiotemporaldynamics, they typically come in pairs of the lowest topological charge. We locally manipulate active nematic films through hydrodynamic stresses created by magnetically actuated rotation of disk-shaped colloids in proximity to the films to drive the formation of higher energy topological vortices through the merger of two lower energy defects. The directed motion of the defects is accompanied by ordering of the vorticity and velocity of the active flows within the film that is qualitatively unlike the response of passive viscous films. We discuss how these ideas can be used to sculpt flowing topological states without directly confining active nematics and inspire future designs of hybrid bio-mechanical devices to work in conjunction with active biological fluids.