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 like droplet merging and dissolution. Variations in interfacial tension can lead to distinct biological states, including those 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—not attraction—drives phase separation. However, measuring interfacial tension in such systems remains challenging, as traditional equilibrium approaches fail. In this talk, I will 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 dimensions. I will first demonstrate this method on a binary Lennard-Jones mixture and then extend it to active systems where self-propulsion leads to motility-induced phase separation. This approach not only bridges theory and experiment but also provides a practical framework that can be directly applied in experimental setups to probe the role of motility in biological phase separation
