Minimal continuum theories of structure formation in dense active fluids

TL;DR

This paper introduces minimal continuum models using higher-order PDEs to describe the universal large-scale behaviors of dense active fluids, such as microbial suspensions and animal groups.

Contribution

It proposes new scalar and vector continuum models that capture structural transitions and collective motion in dense active matter systems.

Findings

Scalar Swift-Hohenberg-type model reproduces pattern formation.

Vector model extends Toner-Tu theory for dense bacterial suspensions.

Models exhibit Landau-type transitions in Fourier space.

Abstract

Self-sustained dynamical phases of living matter can exhibit remarkable similarities over a wide range of scales, from mesoscopic vortex structures in microbial suspensions and motility assays of biopolymers to turbulent large-scale instabilities in flocks of birds or schools of fish. Here, we argue that, in many cases, the phenomenology of such active states can be efficiently described in terms of fourth- and higher-order partial differential equations. Structural transitions in these models can be interpreted as Landau-type kinematic transitions in Fourier (wavenumber) space, suggesting that microscopically different biological systems can share universal long-wavelength features. This general idea is illustrated through numerical simulations for two classes of continuum models for incompressible active fluids: a Swift-Hohenberg-type scalar field theory, and a minimal vector model that extends the classical Toner-Tu theory and appears to be a promising candidate for the quantitive description of dense bacterial suspensions. We also discuss briefly how microscopic symmetry-breaking mechanisms can enter macroscopic continuum descriptions of collective microbial motion near surfaces and conclude by outlining future applications.