Enhanced dispersion of active microswimmers in confined flows

TL;DR

This study investigates how active microswimmers like microalgae disperse in shear flows within microfluidic channels, demonstrating that their effective diffusion increases with flow amplitude and confirming the applicability of Taylor-Aris law to active particles.

Contribution

The paper extends Taylor-Aris dispersion theory to active microswimmers in shear flows, providing experimental validation using microalgae trajectories.

Findings

Velocity fluctuations increase with flow amplitude.

Dispersion coefficient grows as flow amplitude increases.

Taylor-Aris law applies to active microswimmers.

Abstract

In the presence of a laminar shear flow, the diffusion of passive colloidal particles is enhanced in the direction parallel to the flow. This classical phenomenon is known as Taylor-Aris dispersion. Besides, microorganisms, such as active microswimmers, exhibit an effective diffusive behavior at long times. Combining the two ingredients above, a natural question then emerges on how the effective diffusion of active microswimmers is altered in shear flows -- a widespread situation in natural environments with practical implications, \textit{e.g.} regarding biofilm formation. In this Letter, we investigate the motility and dispersion of \textit{Chlamydomonas reinhardtii} microalgae, within a rectangular microfluidic channel subjected to a sinusoidal Poiseuille flow. Using high-resolution optical microscopy and a particle-tracking algorithm, we reconstruct individual trajectories in various flow conditions and statistically analyze them through moment theory and sliding windowed demodulation. We find that the velocity fluctuations and the dispersion coefficient increase as the flow amplitude is increased, with only weak dependencies on the flow periodicity. Importantly, our results demonstrate that the generalization of Taylor-Aris law to active particles is valid.