Spiral and never-settling patterns in active suspensions

TL;DR

This paper investigates pattern formation in active suspensions, revealing novel never-settling and spiral patterns through combined numerical and analytical methods, with implications for understanding bacterial colonies and actomyosin gels.

Contribution

It introduces a new model incorporating density-dependent motility and logistic growth, explaining complex pattern dynamics in active suspensions.

Findings

Disordered phase shows stable and blinking patterns due to motility suppression and growth.

Dense systems develop internal orientational order, forming asters or spirals.

Ordered phase exhibits unique never-settling patterns related to actin dynamics.

Abstract

We present a combined numerical and analytical study of pattern formation in an active system where particles align, possess a density-dependent motility, and are subject to a logistic reaction. This is a model for suspensions of reproducing bacteria, but it can also represent, in the ordered phase, actomyosin gels in vitro or in vivo. In the disordered phase, we find that motility suppression and growth compete to yield stable or blinking patterns, which, when dense enough, acquire internal orientational ordering, to yield asters or spirals. In the ordered phase, the reaction term leads to previously unobserved never-settling patterns which can provide a simple framework to understand the formation of motile and spiral patterns in actin.