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Active trapping of microswimmers in a foam

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The formation of massive persistent foam at the surface of seawater may be observed in various coastlines, often concomitant with a loss of biodiversity in the seawater. In many cases, the foam has a natural origin and the presence of chemical pollutants is excluded. In this context, the hypothesis that the phytoplankton biomass is mechanically retained in the foam has been suggested. The capture or sedimentation of solid particles in a liquid foam has been well studied. Many phytoplankton species, however, are flagellated, hence motile.

Inspired by this problematic, we have studied, in the laboratory, the impact of the motility of flagellated algae on their sedimentation dynamics within a liquid controlled foam. The model unicellular motile algae Clamydomonas rheinardtii (CR) was incorporated in a bio-compatible foam, and the number of cells escaping the foam at the bottom was measured in time. Comparing the escape dynamics of living and dead CR cells, we found that living motile CR cells escape from the foam much at a much slower rate than dead cells: after two hours, up to 60 of the injected cells may remain blocked in the foam, while 95 of the initial liquid volume in the foam has been drained out of the foam. Microscopic observation of the swimming CR cells in a chamber mimicking the cross-section of foam internal channels revealed that swimming CR cells accumulate near channels corners. A theoretical analysis based on the probability density measurements in the micro chambers have shown that this trapping at the microscopic scale contributes to explain the macroscopic retention of the microswimmers in the foam.

This talk is part of the DAMTP BioLunch series.

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