# Light-Control of Localised Photo-Bio-Convection

## TL;DR

This paper demonstrates how light can be used to precisely control the movement and collective behavior of microalgae, enabling real-time, reconfigurable bioconvective flows for bio-mixing applications.

## Contribution

It introduces a novel method of controlling microalgal motility and bioconvection using light, with a simple model explaining the dynamics.

## Key findings

- Light-induced control of microalgal accumulation and bioconvection.
- Real-time reconfigurable bio-mixing demonstrated.
- A continuum model explains the phototactic response and flow dynamics.

## Abstract

Microorganismal motility is often characterised by complex responses to environmental physico-chemical stimuli. Although the biological basis of these responses is often not well understood, their exploitation already promises novel avenues to directly control the motion of living active matter at both the individual and collective level. Here we leverage the phototactic ability of the model microalga {\it Chlamydomonas reinhardtii} to precisely control the timing and position of localised cell photo-accumulation, leading to the controlled development of isolated bioconvective plumes. This novel form of photo-bio-convection allows a precise, fast and reconfigurable control of the spatio-temporal dynamics of the instability and the ensuing global recirculation, which can be activated and stopped in real time. A simple continuum model accounts for the phototactic response of the suspension and demonstrates how the spatio-temporal dynamics of the illumination field can be used as a simple external switch to produce efficient bio-mixing.