Hydrodynamics of confined active fluids

TL;DR

This paper provides a theoretical framework for understanding the hydrodynamics of confined active fluids, revealing how shape influences interactions and predicting collective behaviors like swarming and clustering.

Contribution

It introduces a shape-dependent hydrodynamic interaction model for confined swimmers and analyzes their large-scale collective dynamics and phase behavior.

Findings

Hydrodynamic interactions depend only on swimmer shape, not swimming mechanism.

Confined swimmers do not reorient due to flow gradients, only flow fields.

Different swimmer geometries lead to distinct collective behaviors, including swarming and clustering.

Abstract

We theoretically describe the dynamics of swimmer populations confined in thin liquid films. We first demonstrate that hydrodynamic interactions between confined swimmers only depend on their shape and are independent of their specific swimming mechanism. We also show that due to friction with the walls, confined swimmers do not reorient due to flow gradients but the flow field itself. We then quantify the consequences of these microscopic interaction rules on the large-scale hydrodynamics of isotropic populations. We investigate in details their stability and the resulting phase behavior, highlighting the differences with conventional active, three-dimensional suspensions. Two classes of polar swimmers are distinguished depending on their geometrical polarity. The first class gives rise to coherent directed motion at all scales whereas for the second class we predict the spontaneous formation of coherent clusters (swarms).