Mechanical deformations enable line tension measurements in phase-separated active liquids

Phase coexistence plays a critical role in biological systems, from macromolecular condensates that regulate cellular functions to phase separation driven by cell motility in multicellular structures. These phase-separated states are regulated by interfacial tension, which controls key processes such as droplet merging and dissolution. Variations in interfacial tension can lead to distinct biological outcomes, including states linked to dysfunction and disease, making it crucial to understand how different configurations of line tension influence these processes. In recent years, particle-based models have shed light on these phenomena, particularly in active systems where motility—rather than attraction—drives phase separation. However, measuring interfacial tension in such systems remains challenging, as traditional equilibrium approaches fall short. In this talk, we present a mechanical method for measuring line tension, applicable to both passive and active systems, by analyzing the work done during biaxial deformations in two-dimensional systems. We first demonstrate this method on a binary Lennard-Jones mixture, then extend it to active systems where self-propulsion leads to motility-induced phase separation. Through this work, we clarify the effect of non-conservative driving forces on interface fluctuations in highly fluctuating phase-separated systems. Our method for measuring line tension not only provides a new perspective on the mechanical properties of active liquids but also offers a practical framework for experimental setups to probe the role of non-conservative forces, such as motility, in phase-coexisting active liquids.