Broken symmetries at active interfaces

In living matter, specialized active flows drive shape asymmetry to accomplish asymmetric tasks, including endocytosis and gastrulation. To better understand the generic pathways through which flows drive shape asymmetry, we study the deformations of an interface between a passive fluid and an active fluid composed of extensile bundles of cytoskeletal polymers. Combining experiments and theory, we show active flows pull harder than they push, causing the interface to break height inversion symmetry, consistent with previous results in the literature. 1) First, we demonstrate at a flat interface that the onset of asymmetry is a nonlinear effect caused by vortex sheet dynamics. 2) Second, asymmetry persists in the fully nonlinear regime through one-sided defect nucleation along the interface, which injects intermittent bursts in the curvature fluctuations. 3) Finally, we show droplets active inside versus outside are eversion (inside/out) asymmetric because they nucleate defects on their interface at different rates. Together, our results connect the rich dynamics of active flows to the emergent broken symmetries of biointerfaces.