# Re-entrant phase separation in nematically aligning active polar particles

## TL;DR

This paper numerically investigates the complex phase behavior of active polar particles with nematic alignment, revealing re-entrant phase separation and transitions driven by activity and noise levels.

## Contribution

It introduces a detailed numerical analysis of phase transitions, including re-entrant behaviors, in active polar particles with nematic alignment, highlighting novel phase phenomena.

## Key findings

- Re-entrant fluid-phase separation observed at varying activity levels.
- Distinct mechanisms for phase separation and remelting identified.
- Flocking and jamming behaviors influence phase transitions.

## Abstract

We present a numerical study of the phase behavior of repulsively interacting active polar particles that align their active velocities nematically. The amplitude of the active velocity, and the noise in its orientational alignment control the active nature of the system. At high values of orientational noise, the structural fluid undergoes a continuous nematic-isotropic transition in active orientation. This transition is well separated from an active phase separation, characterized by the formation of high density hexatic clusters, observed at lower noise strengths. With increasing activity, the system undergoes a re-entrant fluid-phase separation-fluid transition. The phase coexistence at low activity can be understood in terms of motility induced phase separation. In contrast, the re-melting of hexatic clusters, and the collective motion at low orientational noise are dominated by flocking behavior. At high activity, sliding and jamming of polar sub-clusters, formation of grain boundaries, lane formation, and subsequent fragmentation of the polar patches mediate remelting.