Inertial Effects on the Stress Generation of Active Fluids

TL;DR

This paper investigates how particle inertia influences the stress generation and collective behavior of self-propelled bodies across different scales, revealing that total stress remains invariant despite individual stress contributions varying with inertia.

Contribution

It introduces the concept that both swim and Reynolds stresses are affected by inertia, but their sum remains constant, highlighting a key consideration for simulations of active matter systems.

Findings

Swimmers generate both swim and Reynolds stresses affecting collective motion.

Particle inertia reduces swim stress and alters phase behavior.

Total stress remains independent of particle inertia.

Abstract

Suspensions of self-propelled bodies generate a unique mechanical stress owing to their motility that impacts their large-scale collective behavior. For microswimmers suspended in a fluid with negligible particle inertia, we have shown that the virial `swim stress' is a useful quantity to understand the rheology and nonequilibrium behaviors of active soft matter systems. For larger self-propelled organisms like fish, it is unclear how particle inertia impacts their stress generation and collective movement. Here, we analyze the effects of finite particle inertia on the mechanical pressure (or stress) generated by a suspension of self-propelled bodies. We find that swimmers of all scales generate a unique `swim stress' and `Reynolds stress' that impacts their collective motion. We discover that particle inertia plays a similar role as confinement in overdamped active Brownian systems, where the reduced run length of the swimmers decreases the swim stress and affects the phase behavior. Although the swim and Reynolds stresses vary individually with the magnitude of particle inertia, the sum of the two contributions is independent of particle inertia. This points to an important concept when computing stresses in computer simulations of nonequilibrium systems---the Reynolds and the virial stresses must both be calculated to obtain the overall stress generated by a system.