Probing 3D active system with dynamic phase separation

We study 3D active fluids which merge passive liquid-liquid phase separation with a microtubule-based active fluid. Phase separation breaks the spatial symmetry of a homogeneous active material by confining the activity to one of the two coexisting phases. The resulting mixture exhibits an interplay between the activities of the microtubules in the bulk and the fluctuations of the soft and easily-deformable phase interfaces. To determine the influence of dimensionalities we study the structure and dynamics of bulk-separated interfaces in three-dimensional samples, as well as the shape of 3D droplets on a 2D substrate. Our work demonstrates how active stresses generated in the bulk internally driven fluids can generate interfacial structures that are not accessible in equilibrium systems. In turn, the soft deformable interfaces can be used as elastic probes to measure active stress which is a defining property of active fluids.