In the manuscript, we model these bioconvection rolls with simulations of phototactic advection-diffusion and demonstrate how to harness them for particle transport.

Depending on the particles’ density, they are either attracted to or repelled by the dense algae region, allowing various modes of micromanipulation.

But since these algae are slightly denser than water, concentrating them generates bioconvection rolls. These flows act on a much larger scale than individual algae and can thus efficiently transport large particles over millimetric distances!

Chlamydomonas reinhardtii is a phototactic alga. When exposed to a strong light stimulus, it swims away from it. This allows us to locally concentrate them with a light stimulus.

Did you know you can manipulate hundreds of microparticles using phototactic algae?
This is what we show in our last preprint, led by T. Laroussi and J. Bouvard: arxiv.org/abs/2509.08133
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Coupling the two gives a reduced order fluid-structure interaction model that quantitatively reproduces the experiments. It also suggests that the system can be described by a single dimensionless parameter combining elastic, viscous and geometrical properties.

To rationalize it, I model the hair array as a deformable porous media whose size is dictated by the bending of individual hairs under fluid loading.

This yields a non-linear hydraulic resistance that I explore experimentally and theoretically for laminar flows.

When confined in a channel of size comparable to the hairs themselves and subject to a pressure driven flow, they strongly perturb the flow and if soft enough they bend and change the channel geometry significantly.

New preprint: arxiv.org/abs/2501.01875
"Dense array of elastic hairs obstructing a fluidic channel"
I guess it was time I post something here! Explanations below 🧵👇