Thermodynamic phases in two-dimensional active matter

TL;DR

This study maps out the phase diagram of 2D active matter, showing it retains equilibrium phases with activity shifting phase transitions and revealing a gas-liquid MIPS region at high activity, using a minimal particle model.

Contribution

It demonstrates that 2D active matter preserves equilibrium phases and introduces a kinetic Monte Carlo method for comprehensive phase diagram mapping.

Findings

Active systems retain equilibrium phases with shifted transition densities.

A gas-liquid MIPS region emerges at high activity levels.

Two-step melting persists despite activity.

Abstract

Active matter has been intensely studied for its wealth of intriguing properties such as collective motion, motility-induced phase separation (MIPS), and giant fluctuations away from criticality. However, the precise connection of active materials with their equilibrium counterparts has remained unclear. For two-dimensional (2D) systems, this is also because the experimental and theoretical understanding of the liquid, hexatic, and solid equilibrium phases and their phase transitions is very recent. Here, we use self-propelled particles with inverse-power-law repulsions (but without alignment interactions) as a minimal model for 2D active materials. A kinetic Monte Carlo (MC) algorithm allows us to map out the complete quantitative phase diagram. We demonstrate that the active system preserves all equilibrium phases, and that phase transitions are shifted to higher densities as a function of activity. The two-step melting scenario is maintained. At high activity, a critical point opens up a gas-liquid MIPS region. We expect that the independent appearance of two-step melting and of MIPS is generic for a large class of two-dimensional active systems.