Confinement-induced Self-Pumping in 3D Active Fluids

TL;DR

This paper explains the counterintuitive transition from turbulence to coherent flow in 3D active fluids as the confinement height increases, using a hydrodynamic model and simulations to identify key design principles.

Contribution

It provides a theoretical and numerical framework for understanding confinement-induced self-pumping in 3D active fluids, highlighting the role of aspect ratio and offering unified design principles.

Findings

Coherent flows emerge when the confinement height matches the width.

Aspect ratio critically influences the transition to coherence.

The mechanism applies broadly across different symmetries and propulsion methods.

Abstract

Two dimensional active fluids display a transition from turbulent to coherent flow upon decreasing the size of the confining geometry. A recent experiment suggests that the behavior in three dimensions is remarkably different; emergent flows transition from turbulence to coherence upon \emph{increasing} the confinement height to match the width. Using a simple hydrodynamic model of a suspension of extensile rod-like units, we provide the theoretical explanation for this puzzling behavior. Furthermore, using extensive numerical simulations supported by theoretical arguments, we map out the conditions that lead to coherent flows and elucidate the critical role played by the aspect ratio of the confining channel. The mechanism that we identify applies to a large class of symmetries and propulsion mechanisms, leading to a unified set of design principles for self-pumping 3D active fluids.