Biphasic Active Nematics

There are many instances of active matter coexisting with a passive fluid. These include active droplets, biofilms, bacterial colonies and carpets, and layers of active fluid on a substrate such as in wound-healing assays. In this work we perform dynamical simulations of a two-dimensional active nematic in coexistence with a passive isotropic fluid. In such a system the phase-ordering tendency of the surface tension is balanced by the stirring tendency of the activity, leading to a dynamic steady state with domains of finite size.

We show that active forces at the nematic-isotropic interface act to elongate nematic domains in the direction parallel (perpendicular) to the director field for extensile (contractile) systems. As a result, the system exhibits different length-scales parallel and perpendicular to the nematic director. Furthermore, this elongation effect, acting together with tangential flow fields generated at the interface, produces an effective anchoring of the nematic orientation at the interface.

In contrast to bulk active nematics, where point defects of topological charge +1/2 and -1/2 are always created and annihilated in pairs, we find that defect formation is dominated by the ejection of singular point defects with topological charge +1/2 from the interface into the nematic. This leaves the interface itself with a diffuse negative topological charge. We explain this topological asymmetry in terms of the active forces acting at a curved interface.