Asymmetry in Active-Passive Phase Separation

Passive immiscible liquids phase separate through spinodal decomposition, initially forming bicontinuous structures. Over time, the bicontinuous structures break up and the minority phase forms droplets within a continuous majority phase. The droplets then coarsen, eventually leading to macroscopic phase separation into two bulk phases. In mixtures of an active nematic and a passive isotropic component, activity can arrest coarsening resulting in a dynamical emulsion or micro-phase separated state, and even completely suppress phase separation. Here, we study how activity influences the morphology of microphase separation. We discover that in this dynamical steady state, the minority active phase can be the continuous phase with elongated domains, while the majority passive phase forms isotropic droplets. We also find that nematic elasticity can independently control this asymmetry, allowing for the formation of a highly perforated but percolating active phase. Supporting experiments with microtubule-kinesin based systems demonstrate the formation of such a perforated active phase populated by the droplets of a passive polymer fluid. The insights derived from our work can potentially lead to a way to harness the turbulent flows within active fluids to create soft metamaterials with tunable microstructures.