  Phase separation in a low-density gas-like phase and a high-density
liquid-like one is a common trait of biological and synthetic self-propelling
particles' systems. The competition between motility and stochastic forces is
assumed to fix the boundary between the homogeneous and the phase-separated
phase. Here we demonstrate that motility does also promote the homogeneous
phase allowing particles to resolve their collisions. This new understanding
allows quantitatively predicting the spinodal-line of hard self-propelling
Brownian particles, the prototypical model exhibiting a motility induced phase
separation. Furthermore, we demonstrate that frictional forces control the
physical process by which motility promotes the homogeneous phase. Hence,
friction emerges as an experimentally variable parameter to control the
motility induced phase diagram.
