Athermal Phase Separation of Self-Propelled Particles with no Alignment

We study numerically and analytically a model of self-propelled polar disks on a substrate in two dimensions. The particles interact via isotropic repulsive forces and are subject to rotational noise, but there is no aligning interaction. As a result, the system does not exhibit an ordered state. The isotropic fluid phase separates well below close packing and exhibits the large number fluctuations and clustering found ubiquitously in active systems. Our work shows that this behavior is a generic property of systems that are driven out of equilibrium locally, as for instance by self propulsion.

💡 Research Summary

The paper investigates a minimal model of self‑propelled polar disks moving on a two‑dimensional substrate, where particles interact only through isotropic short‑range repulsion and experience rotational diffusion, but have no aligning torque. Because of the absence of any alignment interaction, the global polar order parameter remains zero and the system never develops a long‑range ordered phase. Despite this, extensive numerical simulations and analytical arguments reveal that the system undergoes a robust phase separation well below close packing, a phenomenon commonly referred to as motility‑induced phase separation (MIPS).

In the model each particle moves with a constant speed (v_{0}) along its orientation (\theta_{i}). The translational dynamics obey
\