Gyrotactic swimmer dispersion in pipe flow: testing the theory

TL;DR

This study combines theoretical modeling, numerical simulations, and preliminary experiments to investigate how gyrotactic microalgae disperse in pipe flow, revealing distinct behaviors from passive particles and implications for bioreactor design.

Contribution

The paper introduces new solutions to axial dispersion theory for gyrotactic swimmers, incorporating buoyancy and nonlinear shear responses, and presents a novel experimental method for measuring their dispersion.

Findings

Predicted nonzero drift of gyrotactic swimmers in flow.

Validated theoretical predictions with preliminary experimental data.

Identified non-monotonic effective diffusivity with Péclet number.

Abstract

Suspensions of microswimmers are a rich source of fascinating new fluid mechanics. Recently we predicted the active pipe flow dispersion of gyrotactic microalgae, whose orientation is biased by gravity and flow shear. Analytical theory predicts that these active swimmers disperse in a markedly distinct manner from passive tracers (Taylor dispersion). Dispersing swimmers display nonzero drift and effective diffusivity that is non-monotonic with P$\'e$clet number. Such predictions agree with numerical simulations, but hitherto have not been tested experimentally. Here, to facilitate comparison, we obtain new solutions of the axial dispersion theory accounting both for swimmer negative buoyancy and a local nonlinear response of swimmers to shear, provided by two alternative microscopic stochastic descriptions. We obtain new predictions for suspensions of the model swimming alga $\it Dunaliella\,salina$, whose motility and buoyant mass we parametrise using tracking video microscopy. We then present a new experimental method to measure gyrotactic dispersion using fluorescently stained $\it D. salina$ and provide a preliminary comparison with predictions of a nonzero drift above the mean flow for each microscopic stochastic description. Finally, we propose further experiments for a full experimental characterisation of gyrotactic dispersion measures and discuss implications of our results for algal dispersion in industrial photobioreactors.