Equations of state of active Brownian particles
Note - room change to 5327
Title: Equations of state of Active Brownian Particles.
Active matter systems made of self-propelled particles can be found both in living and synthetic man-made systems: typical examples are bacteria suspensions, animal groups or artificial colloidal swimmers. The fundamental difference between these active systems and standard ’passive’ matter made of thermally agitated constituents, is that the microscopic dissipative dynamics of the former breaks detailed balance and, as such, evolves out-of-equilibrium. The out-of-equilibrium nature of active matter manifests strikingly in the presence of interactions. It is now quite well established that self-propelled active particles accumulate in regions of space whenever their velocity decreases as a consequence of the competition between persistence and interactions. At high enough densities and activities a purely Motility-Induced Phase Separation (MIPS) takes place, leading to the coexistence of an active low density gas with a high density, solid-like phase solely sustained by activity.
I will present here the phase behavior of a canonical model system in this context- i.e. Active Brownian Disks - and discuss to what extent is MIPS analog to an equilibrium liquid-gas transition. It is tempting to try to extend the thermodynamic description of first order phase transitions in terms of, for instance, equations of state, to ABPs. However, this poses several fundamental difficulties, that I will briefly review and discuss, since no thermodynamic variable is, in principle, well defined in this context. I will focus on the concept of pressure and analyze the resulting equations of state across MIPS. I will show that the system easily falls into metastable states when activity is turned on, such that the equations of state show hysteresis, as usual in first order phase transitions. One can then characterize the equations of state and corresponding phase diagram of Active Brownian disks and understand the role played by the the particles’ softness.
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