Abstract
As a result of the competition between self-propulsion and excluded volume interactions, purely repulsive self-propelled spherical particles undergo a motility-induced phase separation (MIPS). We carry out a systematic computational study, considering several interaction potentials, systems confined by hard walls or with periodic boundary conditions, and different initial conditions. This approach allows us to identify that, despite its non-equilibrium nature, the equations of state of Active Brownian Particles (ABP) across MIPS verify the characteristic properties of first-order liquid-gas phase transitions, meaning, equality of pressure of the coexisting phases once a nucleation barrier has been overcome and, in the opposite case, hysteresis around the transition as long as the system remains in the metastable region. Our results show that the equations of state of ABPs account for their phase behaviour, providing a firm basis to describe MIPS as an equilibrium-like phase transition.
Highlights
In spite of the above-mentioned efforts, the equality of pressure at coexistence imposed by the very existence of an equation of state (EoS) for spherical Active Brownian Particles (ABP),[17] has not been properly confirmed neither by experiments nor simulations.[15,17,18,19,20] Several numerical studies have measured the pressure in ABP, but its interpretation has raised several conceptual problems
We find that the EoS of an open system quenched from ‘above’ approaches the behaviour of the system under confinement, showing that the difficulties in interpreting the pressure data are due to the presence of a large nucleation barrier that can be bypassed by including a nucleation core
As we show, motility-induced phase separation (MIPS) can be stabilized at much lower densities than those reported to date, and the phase diagram can be understood from the EoS despite the absence of a Maxwell construction
Summary
In spite of the above-mentioned efforts, the equality of pressure at coexistence imposed by the very existence of an equation of state (EoS) for spherical ABP,[17] has not been properly confirmed neither by experiments nor simulations.[15,17,18,19,20] Several numerical studies have measured the pressure in ABP, but its interpretation has raised several conceptual problems. The pressure drop near MIPS disappears as the system is quenched from ‘above’, and we recover instead a flatter EoS between the binodals, showing the equality of pressure at coexistence and allowing to estimate the location of the low-density binodal from the extrapolation of the pressure measurements above the spinodal We analyze both open and confined systems in order to establish the role played by the presence of a nucleation barrier in the pressure anomalies reported in previous numerical works, and the inherent difficulty of sampling the thermodynamic behaviour of the EoS of ABPs at coexistence.
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