Self-propelled particles with selective attraction-repulsion interaction - From microscopic dynamics to coarse-grained theories

TL;DR

This paper develops coarse-grained models for self-propelled particles with selective attraction-repulsion, deriving kinetic and hydrodynamic equations, and compares predictions with simulations revealing a new dense, nematically aligned phase.

Contribution

It introduces a kinetic and hydrodynamic framework for particles with state-dependent interactions, extending existing theories to include new phases.

Findings

Good agreement between theory and simulations at high densities.

Identification of a new dense, nematically aligned phase.

Discrepancies at low repulsion levels suggest limitations of current hydrodynamic models.

Abstract

In this work we derive and analyze coarse-grained descriptions of self-propelled particles with selective attraction-repulsion interaction, where individuals may respond differently to their neighbours depending on their relative state of motion (approach versus movement away). Based on the formulation of a nonlinear Fokker-Planck equation, we derive a kinetic description of the system dynamics in terms of equations for the Fourier modes of a one-particle density function. This approach allows effective numerical investigation of the stability of possible solutions of the system. The detailed analysis of the interaction integrals entering the equations demonstrates that divergences at small wavelengths can appear at arbitrary expansion orders. Further on, we also derive a hydrodynamic theory by performing a closure at the level of the second Fourier mode of the one-particle density function. We show that the general form of equations is in agreement with the theory formulated by Toner and Tu. Finally, we compare our analytical predictions on the stability of the disordered homogeneous solution with results of individual-based simulations. They show good agreement for sufficiently large densities and non-negligible short-ranged repulsion. Disagreements of numerical results and the hydrodynamic theory for weak short-ranged repulsion reveal the existence of a previously unknown phase of the model consisting of dense, nematically aligned filaments, which cannot be accounted for by the present Toner and Tu type theory of polar active matter.