Active trapping of microswimmers in a foam

Inspired by the consequences of aquatic foams on planktonic ecosystems, we have studied the sedimentation of a microswimmer in a liquid foam. The model unicellular bi-flagellated \textit{Chlamydomonas reinhardtii} (CR) algae was incorporated in a biocompatible foam, and the dynamics of cell sedimentation out of the foam was measured.\footnote{Roveillo et.al., 2020. J. R. Soc. Interface 17 : 20200077.} Due to gravity, the liquid in a freshly formed foam flows downwards, advecting solid particles. The cells eventually reach the underlying liquid, escaping the foam. Comparing the dynamics of living and dead CR cells in a draining foam, we found that dead cells were totally advected by the flow, as expected for passive solid particles of this size (10 $\mu$m). In contrast, living motile CR cells escape the foam significantly much slowly: two hours after the foam formation, a significant fraction of cells remained blocked in the foam, whereas 95\% of the volume of liquid initially contained in the foam the liquid was released. Microscopic observation of the swimming CR cells in a chamber mimicking the cross-section of a foam channel revealed that the microswimmers accumulate near channels corners, potentially increasing their retention in the foam.