MSC Seminar. 10/02/2025. Julien Bouvard (Living Systems Institute, University of Exeter, UK): ” Transport of passive beads by random and directed motion of swimming microorganisms”

The swimming motion of microorganisms has recently seen an increasing interest from physicists, utilizing the recent progresses in microscopy and the development of microfluidics to study this interdisciplinary subject. In particular, the immersion of passive particles in active baths of microswimmers have led to the discovery of diverse rich phenomena. In this seminar, I will present how passive beads are moved by randomly swimming bacteria and directionally swimming microalgae.
First, in a uniform environment, we highlight the aggregation dynamics of the beads in a bacterial bath of Burkholderia contaminans. Unexpectedly, the passive beads display a dynamic clustering similar to Ostwald rippening: clusters are slowly growing in time as a t^1/3 power-law, dynamically as beads are constantly getting exchanged from one cluster to another [1] .
Second, the experiment is brought closer to a natural environment by adding biases to the swimming motion of the microorganisms. This time, we use microalgae Chlamydomonas reinhardtii considering their quick reaction to light. As their local concentration grows, the algae generate local convection rolls [2,3]. Those rolls are able to attrack or repel the beads depending on theire relative density with the fluid. By controlling the light
sources, the microspheres can be directed towards precise locations. Such directed motion of microparticles opens up exciting perspectives in microplastics decontamination for instance.

Co-authors: Frédéric Moisy, Harold Auradou, Taha Laroussi, Mojtaba Jarrahi et Gabriel Amselem.