Photofocusing: Light and flow of phototactic microswimmer suspension

TL;DR

This paper investigates photofocusing, where flow vorticity and biased swimming of microalgae towards light cause suspension focusing, supported by experiments and models that explain observed scalings.

Contribution

It introduces a combined experimental and theoretical study of photofocusing, revealing the mechanisms and scaling laws governing microswimmer suspension focusing.

Findings

Focal region width scales with flow velocity.

Establishment length depends on flow parameters.

Experimental results align with theoretical predictions.

Abstract

We explore in this paper the phenomenon of photofocusing: a coupling between flow vorticity and biased swimming of microalgae toward a light source that produces a focusing of the microswimmer suspension. We combine experiments that investigate the stationary state of this phenomenon as well as the transition regime with analytical and numerical modeling. We show that the experimentally observed scalings on the width of the focalized region and the establishment length as a function of the flow velocity are well described by a simple theoretical model.