Phase Transition of Hard Disk Systems with Vicsek-type Interactions

TL;DR

This study explores the complex phase behavior of self-propelled hard disks with Vicsek-type interactions, revealing how competing order parameters influence phase transitions and how microscopic configurational parameters relate to macroscopic phenomena.

Contribution

It introduces a detailed analysis of phase transitions in active hard disk systems with Vicsek interactions, highlighting the interplay of different order parameters and microscopic configurational factors.

Findings

Incompressibility suppresses motility-induced phase separation at high densities.

Distinct fluctuations occur near transition points, with shifts influenced by noise and packing fraction.

Local configurational parameters help explain microscopic origins of phase transition shifts.

Abstract

The phase diagram of self-propelled hard disk systems with Vicsek-type alignment interactions was investigated by event-driven molecular dynamics simulations. The model incorporates two competing order parameters: the polar order-disorder transition associated with collective velocity alignment (Vicsek model) and the orientational order arising from solid-fluid transitions (Alder transition) induced by excluded volume effects. The incompressibility of hard disks suppresses motility-induced phase separation at high packing fractions. Distinctive fluctuations were observed near the transition point, accompanied by anomalous shifts in the transition point as functions of noise intensity and packing fraction. Analysis of local configurational parameters -- specifically, orientational order and circularity of free volume -- provides insight into the microscopic origins of these anomalous phase transition shifts.