Internal stresses regulate phenotypes of active phase separation

Phase coexistence is found in several living and non-living systems: from the formation of chiral crystals in starfish embryos to the accumulation of light-activated colloids on fixed obstacles. A theoretical framework for phase coexistence in the presence of non-conservative forces is Motility-Induced Phase Separation (MIPS). Here, large fluctuations in the density field lead to the formation of nucleation sites that expand and form dense liquid droplets that coexist with a dilute vapor. While the dynamics of MIPS and the resulting steady state have been elucidated by drawing comparisons with phase separation in passive fluids, a theory describing stiffness and mobility of the liquid-vapor interface has been challenged by the presence of the non-conservative work performed by active forces. In this work, we detect the liquid and vapor phase with a classification algorithm based on the Radical Delaunay Triangulation and distinguish different phase separation phenotypes for a two-dimensional system of self-propelled particles. We measure the effective stiffness and mobility of the liquid-vapor interface and conduct a similar analysis for a Lennard-Jones fluid in order to establish a robust comparison between active and passive phase coexistence. Our results highlight the connection between the morphology and dynamics of active droplets and the local stresses they are subject to, relating macroscopic properties of active systems to the underlying mechanical driving.