Mouvements collectifs et contrôle externe dans des gouttes actives. -(Gabriel Ramos / LPT / Seminar). – 21/10/2025, 14H

Séminaire LPT

Gabriel Ramos (Softmat Toulouse)

Seminar LPT, 21/10/2025, 14H, 3R4, salle de conférence

Summary
The collective motions performed by self propelled particles, such as flocks of birds or schools of fish, are studied in physics within the domain of active matter. Using the precision and low cost of microfluidics, living and synthetic microparticles have become an ideal playground to study these active matter systems.

In this talk, we will present two experiments using bacterial suspensions to produce active droplets. The objective of these experiments is to understand how the local-scale microactivity (the collective motions resulting from bacterial-self-organization) is able to perform macromovements at the scale of the confining droplet, which is about 100 times the size of a single bacterium. In other words, we study how we can extract useful work from these confined suspensions.

First, we will show that the confinement of a suspension of Escherichia coli (E. coli) in a water-in-oil emulsion leads to bacterially propelled droplets that perform a persistent random walk. Secondly, we will show that under the action of a constant magnetic field, magnetotactic bacteria (MTB) confined in a water-in-oil emulsion self-assemble into a rotary motor that applies a torque on the external oily phase. A vortex collective motion is formed at the scale of the confining droplet size, which is perpendicular to the magnetic field and is reversible when inverting the magnetic field direction. Finally, we will briefly introduce a synthetic system of active droplets using liquid crystals, showing their ability to reproduce living features such as chemotaxis, light sensing and collective motions.