Phase coexistence of active Brownian particles

Area of Science:

• Soft Matter Physics
• Statistical Mechanics
• Active Matter Physics

Background:

• Active Brownian particles (ABPs) are self-propelled entities exhibiting complex collective behaviors.
• Motility-induced phase separation (MIPS) is a key phenomenon in active matter, leading to spontaneous clustering.
• Understanding the fundamental forces driving MIPS is crucial for predicting active matter systems.

Purpose of the Study:

• To develop an analytical theory for nonequilibrium phase coexistence and interfacial structure in ABPs.
• To identify and quantify the internal force fields governing MIPS.
• To validate theoretical predictions against computational simulations.

Main Methods:

• Application of power functional concepts to derive an analytical theory.
• Brownian dynamics computer simulations to model ABP behavior.
• Analysis of internal one-body force fields, including drag, pressure gradients, and gradient forces.

Main Results:

• Identified four nonequilibrium contributions to the internal one-body force field: isotropic drag, interfacial drag, superadiabatic spherical pressure gradient, and quiet life gradient force.
• Demonstrated that intrinsic spherical pressure is balanced by swim pressure from interface polarization.
• Showed that the balance of quiet life and adiabatic forces determines bulk coexistence, independent of interfacial contributions.

Conclusions:

• The phase transition in ABPs originates from nonequilibrium repulsion, with the gas phase being more repulsive than the liquid phase.
• Internal force fields are kinematic functionals dependent on density and current, consistent with power functional theory.
• The developed theory provides a robust framework for understanding MIPS in active matter systems.