Abstract
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.
Highlights
Many biological and synthetic systems of self-propelled particles exhibit a transition from a homogeneous state to one in which a gas- and a liquid-like phase coexist [1,2,3]
While not representing any experimental system faithfully, active Brownian particles emerged as a prototypical model exhibiting a motility induced phase separation, and are the standard benchmark for statistical physics of active matter theories
In active Brownian particle (ABP), motility promotes phase separation, as clarified by the motility-induced phase separation (MIPS) [4] or, equivalently, by a mechanistic approach [11]: due to the presence of motility, colliding particles are much slower than the others, so that collisions induce a local pressure drop which seeds phase separation
Summary
Many biological and synthetic systems of self-propelled particles exhibit a transition from a homogeneous state to one in which a gas- and a liquid-like phase coexist [1,2,3]. The rotational diffusion plays a role because, before a collision seeds the growth of a cluster, the two colliding particles may change their self-propelling direction and swim away [4,7,14,15,16] This process gives rise to a flux of particles from the dense to the less dense phase promoting the homogeneous phase. The other possibility is that the translational rather than the rotational noise promotes the homogeneous phase This scenario is suggested by a continuum equation for the evolution of the coarse-grained density and polarization fields [1,8,17,18,19], which is formally related to a thermodynamic approach aiming to map active Brownian particles into an equilibrium system [11]. We demonstrate that friction tunes the features of the motility-induced phase diagram, as it controls the instability mechanisms we have uncovered
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