Mixing and unmixing induced by active camphor particles

Clément Gouiller (1) , Florence Raynal (2) , Laurent Maquet (1) , Mickaël Bourgoin (3) , Cécile Cottin-Bizonne (1) , Romain Volk (3) , Christophe Ybert (1)

1 iLM - L&I - iLM - Liquides et interfaces
2 LMFA - Laboratoire de Mecanique des Fluides et d'Acoustique
3 Phys-ENS - Laboratoire de Physique de l'ENS Lyon

In this experimental study, we report on the mixing properties of interfacial colloidal floaters (glass bubbles) by chemical and hydrodynamical currents generated by self-propelled camphor disks swimming at the air-water interface. Despite reaching a statistically stationary state for the glass bubbles distribution, those floaters always remain only partially mixed. This intermediate state results from a competition between (i) the mixing induced by the disordered motion of many camphor swimmers and (ii) the unmixing promoted by the chemical cloud attached to each individual self-propelled disk. Mixing/unmixing is characterized globally using the standard deviation of concentration and spectra, but also more locally by averaging the concentration field around a swimmer. Besides the demixing process, the system develops a "turbulent-like" concentration spectra, with a large-scale region, an inertial regime and a Batchelor region. We show that unmixing is due to the Marangoni flow around the camphor swimmers, and is associated to compressible effects.

Domaines: Matière Condensée [cond-mat]; Mécanique des fluides [physics.class-ph]

Clément Gouiller, Florence Raynal, Laurent Maquet, Mickaël Bourgoin, Cécile Cottin-Bizonne, et al.. Mixing and unmixing induced by active camphor particles. Physical Review Fluids, 2021, 6 (1), pp.014501. ⟨10.1103/PhysRevFluids.6.014501⟩. ⟨hal-03092757v2⟩