Massive foam formation in aquatic environments is a seasonal event that has a significant impact on the stability of marine ecosystems. Liquid foams are known to filter passive solid particles, with large particles remaining trapped by confinement in the network of liquid channels and small particles being freely advected by the gravity-driven flow. By contrast, the potential role of a similar retention effect on biologically active particles such as phytoplankton cells is still relatively unknown. To assess if phytoplankton cells are passively advected through a foam, the model unicellular motile alga Chlamydomonas reinhardtii (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 dead cells are totally advected by the liquid flow towards the bottom of the foam, as expected since the diameter of CR remains smaller than the typical foam channel diameter. By contrast, living motile CR cells escape the foam at a significantly lower rate: after 2 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 has shown that this trapping at the microscopic scale contributes to explain the macroscopic retention of the microswimmers in the foam. At the crossroads of distinct fields including marine ecology of planktonic organisms, fluid dynamics of active particles in a confined environment and the physics of foam, this work represents a significant step in the fundamental understanding of the ecological consequences of aquatic foams in water bodies.

中文翻译：

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在泡沫中捕获游泳微藻

水生环境中大量泡沫的形成是一个季节性事件，对海洋生态系统的稳定性有重大影响。已知液体泡沫可以过滤被动固体颗粒，大颗粒仍被限制在液体通道网络中，而小颗粒则由重力驱动的流动自由平流。相比之下，类似的保留效应对浮游植物细胞等生物活性颗粒的潜在作用仍然相对未知。为了评估浮游植物细胞是否通过泡沫被动平流，模型单细胞运动藻莱茵衣藻 (CR) 被纳入生物相容性泡沫，并及时测量从泡沫底部逃逸的细胞数量。比较活的和死的 CR 细胞的逃逸动力学，我们发现死细胞完全被液体流向泡沫底部平流，正如预期的那样，因为 CR 的直径仍然小于典型的泡沫通道直径。相比之下，活动的 CR 细胞以显着较低的速度逃离泡沫：2 小时后，多达 60% 的注入细胞可能仍然阻塞在泡沫中，而泡沫中 95% 的初始液体体积已被排出的泡沫。在模拟泡沫内部通道横截面的腔室中对游泳 CR 细胞的显微镜观察表明，游泳 CR 细胞聚集在通道角落附近。基于微室中概率密度测量的理论分析表明，这种微观尺度上的捕获有助于解释泡沫中微型游泳者的宏观滞留。