# Controlling the Collective Transport of Large Passive Particles With Suspensions of Microorganisms

## TL;DR

This paper shows how light can control microalgae to transport large particles, which could be useful for drug delivery and decontamination.

## Contribution

The study introduces a method to control the collective transport of hundreds of large passive particles using phototactic microalgae.

## Key findings

- Directional light triggers bioconvection rolls in microalgae suspensions.
- Large passive particles are transported based on their density relative to the bioconvection rolls.
- The method could be applied to targeted drug delivery and decontamination.

## Abstract

A promising approach to transport cargo at the microscale lies within the use of self‐propelled microorganisms, whose motion entrains that of passive particles. However, most applications remain limited to just a few passive particles of similar size as the microorganisms, since the transport mechanism relies on the interaction between individual swimmers and single particles. Here, we demonstrate how to control the collective transport of hundreds of large passive particles with phototactic microalgae. Using directional light stimuli in suspensions of Chlamydomonas reinhardtii, we trigger bioconvection rolls capable of macroscale transport. Passive particles an order of magnitude larger than the microalgae are either attracted or repelled by the rolls, depending on their density. Using experiments and simulations, we rationalize these bioconvective flows and describe how to harness them for cargo transport, with future applications in targeted drug delivery and decontamination.

Directional light stimuli induce the accumulation of microalgae in suspensions of Chlamydomonas reinhardtii. This accumulation triggers large scale bioconvection rolls capable of macroscale transport. Exploiting these bioconvective flows enables to collectively transport of hundreds of large passive particles, paving the way for future applications in decontamination and drug delivery using phototactic microalgae.