|
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) |
