  Suspensions of Active Brownian Particles (ABP) undergo motility induced phase
separation (MIPS) over a wide range of mean density and activity strength [1],
even in the absence of an explicit attraction. Negative values of the
mechanical surface tension have been reported, from the total forces across the
interface, while the stable fluctuations of the interfacial line would be
interpreted as a positive capillary surface tension [2], while in equilibrium
liquid surfaces these two magnitudes are equal. We present here the analysis of
2D-ABP interfaces in terms of the intrinsic density and force profiles,
calculated with the particle distance to the instantaneous interfacial line.
Our results provide a new insight in the origin of the MIPS from the local
rectification of the random active force on the particles near the interface.
As it had been pointed, that effect acts as an external potential [3] that
produces a pressure gradient across the interface, so that the mechanical
surface tension of the MIPS cannot be described as that of equilibrium
coexisting phases; but our analysis shows that most of that effect comes from
the tightly caged particles at the dense (inner) side of the MIPS interface,
rather than from the free moving particles at the outer side that collide with
the dense cluster. Moreover, a clear correlation appears between the decay of
the hexatic order parameter at the dense slab and the end of the MIPS as the
strength of the active force is lowered. We test that with the strong active
forces required for MIPS, the interfacial structure and properties are very
similar for ABP with purely repulsive (WCA-LJ model truncated at its minimum)
and when the interaction includes a range of the LJ attractive force.
