Optimal Turbulent Transport in Microswimmer Suspensions

TL;DR

This paper investigates how microswimmer suspensions create complex flow patterns affecting passive tracer transport, revealing different regimes of flow dynamics and identifying an optimal transport condition at a specific turbulence transition point.

Contribution

It introduces a continuum model to analyze microswimmer-induced flow regimes and identifies a regime of optimal transport at the turbulence transition.

Findings

Slow flow regime causes trapping effects due to vortex structures.

Fast turbulent regime leads to transport dominated by temporal correlations.

Maximum diffusion coefficient occurs at the turbulence transition point.

Abstract

Microswimmer suspensions self-organize into complex spatio-temporal flow patterns, including vortex lattices and mesoscale turbulence. Here we explore the consequences for the motion of passive tracers, based on a continuum model for the microswimmer velocity field. We observe two qualitatively different regimes distinguished via the dimensionless Kubo number $K$. At advection strengths right above the transition to turbulence, the flow field evolves very slowly ($K \gg 1$) and the spatial vortex structures lead to dominant trapping effects. In contrast, deep in the turbulent state, much faster dynamics ($K \ll 1$) consistent with the so-called sweeping hypothesis leads to transport properties completely determined by the temporal correlations. In between ($K \approx 1$), we observe a regime of optimal transport, signaled by a maximum of the diffusion coefficient.