Pantheon SEMPARIS Le serveur des séminaires parisiens Paris

Statut Confirmé
Date Vendredi 20 Mai 2022
Heure 13:00
Institut LPENS
Salle Salle Favard, IBENS
Nom de l'orateur Brunet
Prenom de l'orateur Philippe
Addresse email de l'orateur
Institution de l'orateur Laboratoire Matière et Systèmes Complexes, U. Paris 7
Titre Nonlinear phototaxis and instabilities in suspensions of light-seeking algae
Résumé The mechanism by which living organisms seek optimal light conditions, phototaxis, is a fundamental process for motile photosynthetic microbes. It is involved in a broad array of natural processes and applications from bloom formation to the production of high-value chemicals in photobioreactors. Our experiments with model motile micro-algae Chlamydomonas Reinardthii investigate the response of a dilute or semi-dilute suspensions to local illumination via a laser beam. The denser micro-algae concentrate around the laser spot which results in an increase of the local fluid density. In turn, this can generate bio-convection cells, which spatial range is far larger than the spot width. These bio-convective flows appear in a range of dimensionless Rayleigh number (which quantifies the relative importance of convection over diffusion) far below the critical one required for spontaneous (without light excitation) bio-convection. When initial concentration and suspension thickness are small enough (low Rayleigh number), the stationary concentration profile results in an equilibrium between diffusion and phototactic flux. In this situation, the very good agreement between experiments and numerics also enables to measure the diffusion and phototactic motility coefficients of Chlamydomonas Reinardthii [1]. More recently, we have shown that a population of the model alga Chlamydomonas reinhardtii exhibits a highly sensitive nonlinear response to light [2]. At higher Rayleigh number, various dynamical regimes are observed : waves of concentration propagate radially with well-defined velocity, a fingering pattern with a well-defined orthoradial wavelength or directional budding with an unique finger of high-concentration. Our experimental results compare well with numerical simulations of a relatively simple model of bio- convection. This allowed us to identify the mechanisms of the instability that generates the pattern of waves. More precisely, it is gyrotaxis (i.e. the ability of flagellated microbes to self-orient their swimming direction in a shear flow) that induces the focusing of algae in a thin layer, which eventually destabilizes under gravity. These bioconvective flows enable the continuous mixing of the fluid by a simple light beam, which is especially interesting in confined geometries or when algae concentration is low. [1] J. Dervaux, M. Capellazzi Resta and P. Brunet. Light-controlled flows in active fluids. Nature Physics 13, 306-312 (2017). [2] A. Ramamonjy, J. Dervaux and P. Brunet. Nonlinear phototaxis and instabilities in suspensions of light-seeking algae. Physical Review Letters (In press)
Numéro de preprint arXiv
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