Interfacial dynamics of active microtubule nematics*

Phase-separated systems minimize their interfacial area by producing rounded droplets that coalesce and eventually bulk separate. Non-equilibrium driving can fundamentally alter such dynamics. Conventional non-equilibrium systems rely on the input of energy through macroscopic boundaries, which cause shear flows that deform and elongate the interface. In contrast, in active matter energy is injected at the microscopic scale, and from there it propagates upward to create large-scale turbulent-like flows. To study coupling of active matter with soft interfaces, we have developed an experimental phase-separated active system by merging a microtubule-based isotropic active fluid with phase-separating polymers. The system spontaneously partitions into an active phase containing the microtubules and kinesin motors, and a passive phase. The two phases are separated by an interface with an ultralow surface tension that is deformed by active flows, resulting in large fluctuations. Sufficiently strong active stresses destabilize interfaces altogether, resulting in droplet break-up. We quantify the fluctuations of a single interface, examine how activity fundamentally alters the course of phase separation and produces surprising emergent states like spontaneous wetting.