Intrinsic structure perspective for MIPS interfaces in two-dimensional systems of active Brownian particles

Area of Science:

• Soft Matter Physics
• Statistical Mechanics
• Non-equilibrium Systems

Background:

• Active Brownian particles (ABPs) exhibit motility-induced phase separation (MIPS), a phenomenon where particles spontaneously aggregate due to persistent motion, even without attractive forces.
• MIPS shares similarities but also shows qualitative differences compared to equilibrium liquid-gas phase coexistence, particularly concerning surface tension.
• Reported negative mechanical surface tension and positive capillary surface tension in MIPS contrast with equilibrium systems where these values are equal.

Purpose of the Study:

• To analyze the intrinsic density and force profiles of 2D-ABP interfaces using particle distance to the interfacial line.
• To gain new insights into the origin of MIPS from local rectification of active forces near the interface.
• To investigate the role of caged particles versus free-moving particles in MIPS surface tension.

Main Methods:

• Analysis of 2D-ABP interfaces using intrinsic density and force profiles.
• Calculation of particle distance to the instantaneous interfacial line.
• Comparison of MIPS properties for purely repulsive interactions (WCA-LJ model) versus those including attractive forces (LJ model).

Main Results:

• MIPS surface tension is not accurately described by equilibrium phase coexistence due to an external potential created by local force rectification near the interface.
• The primary contribution to MIPS surface tension arises from caged particles on the dense side of the interface.
• A correlation exists between the decay of the hexatic order parameter and the cessation of MIPS as active force strength decreases.
• Interfacial structure and properties are similar for ABPs with purely repulsive or attractive interactions when strong active forces drive MIPS.

Conclusions:

• The mechanical surface tension of MIPS differs from equilibrium systems due to local force rectification and the behavior of caged particles.
• MIPS is strongly influenced by the interplay between particle activity, interactions, and resulting interfacial properties.
• The study highlights the unique characteristics of non-equilibrium phase separation in active matter systems.