Résumé |
A massive formation of stable sea foam is regularly observed on certain shores.
These foams, of natural origin, are concomitant with a loss of phytoplankton
biomass and biodiversity. Besides, liquid foams are known to act as filters for
solid particles, due to the complex network of internal channels through which the
liquid flows. We therefore hypothesise that a relevant part of the phytoplankton,
advected in the foam during the foam formation, could be trapped in the foam.
Among phytoplanktonic organisms, many are flagellated and therefore motile. In
this talk, I will present experiments performed in the laboratory on a model
system to investigate the sedimentation of microswimmers in a liquid foam: the
unicellular bi-flagellate alga Chlamydomonas reinhardtii (CR) was incorporated
into a liquid foam stabilized with biocompatible proteins. Over time, the liquid
contained in the foam flows downward by gravity drainage, advecting the solid
particles suspended in the liquid, which then escape from the foam and reach the
underlying liquid. We measured the dynamics of escape of CR cells from the foam,
and compared the case of living and of dead cells. While the dead cells are
totally advected by the liquid flow, as expected for passive solid particles of
similar size, the living cells sediment much more slowly, and a significant amount
remains trapped in the foam at long times. I will ultimately discuss the
microscopic mechanisms that can lead to this trapping. |