From effective interactions to the interfacial phase behavior and pressure of active particles

We employ classical density functional theory to study the self-organization in active systems.
Using a first-principles approach, we map the self-propulsion onto an effective
pair interaction potential, which has been shown to account for the motility-induced
phase separation (MIPS) observed for active Brownian particles. We further introduce an
effective external potential and define a free energy to investigate inhomogeneous
situations. Solely as a result of their activity, we predict that active (Brownian) particles
undergo a variety of interfacial phase transitions, e.g., wetting and capillary condensation
in purely repulsive systems or drying and capillary evaporation of attractive colloids.

We also explain why the effective thermodynamic pressure and interfacial tension do not satisfy
the mechanical force-balance condition derived for the present effective equilibrium
model and show how to define more appropriate (rescaled) expressions for
the mechanical pressure and interfacial tension. Finally, we demonstrate that
our theory can also be applied to situations with a non-vanishing particle current
and to mixtures of particles with different activities.