在水道中游动的变形虫
Amoeboid swimming in a channel
摘要：
一些微生物，如细菌，藻类或精子，使用鞭毛或纤毛在流体中游泳，而许多其他微生物则使用大量的形状变形，称为变形虫，通过在基质上爬行或游泳来推动自己。许多真核细胞被认为需要通过膜变形（如起泡或产生片状伪足）的底层来迁移（爬行），但现在越来越多的证据表明，各种细胞（包括免疫系统的细胞）可以在没有粘着斑的帮助下迁移，使它们能够像爬行一样有效地游泳。本文详细分析了变形虫游泳在封闭的流体中的游泳者作为一个不可扩展的膜部署当地的积极力量建模。游泳者表现出丰富的行为：它可能会在通道中停留在直线轨迹上，或者根据其限制从一个壁航行到另一个壁。游泳者的性质也被发现受到限制：游泳者可以表现，平均超过一个游泳周期，作为一个推动者在低限制，并成为一个拉在更高的限制。因此，游泳者的本性并不是一种内在的属性。详细分析了游泳者速度V与力幅值A的比例关系，表明在足够小的A下，$V\sim A^2/η^2$，而在足够大的A下，V与力无关，仅由划水频率和游泳者体型决定。这一发现与目前已知的游泳模型的结果形成鲜明对比，在游泳模型中，运动是基于鞭毛或纤毛活动，其中$V\sim A/η$。最后，对文献中提出的两种效率定义进行了分析，得出了不同的结果。我们发现，一种类型的效率有一个最佳的限制的函数，而其他没有。未来的前景进行了概述。
英文摘要：
Several micro-organisms, such as bacteria, algae, or spermatozoa, use flagella or cilia to swim in a fluid, while many other micro-organisms instead use ample shape deformation, described as amoeboid, to propel themselves by either crawling on a substrate or swimming. Many eukaryotic cells were believed to require an underlying substratum to migrate (crawl) by using membrane deformation (like blebbing or generation of lamellipodia) but there is now increasing evidence that a large variety of cells (including those of the immune system) can migrate without the assistance of focal adhesion, allowing them to swim as efficiently as they can crawl. This paper details the analysis of amoeboid swimming in a confined fluid by modeling the swimmer as an inextensible membrane deploying local active forces. The swimmer displays a rich behavior: it may settle into a straight trajectory in the channel or navigate from one wall to the other depending on its confinement. The nature of the swimmer is also found to be affected by confinement: the swimmer can behave, on the average over one swimming cycle, as a pusher at low confinement, and becomes a puller at higher confinement. The swimmer's nature is thus not an intrinsic property. The scaling of the swimmer velocity V with the force amplitude A is analyzed in detail showing that at small enough A, $V\sim A^2/η^2$, whereas at large enough A, V is independent of the force and is determined solely by the stroke frequency and swimmer size. This finding starkly contrasts with currently known results found from swimming models where motion is based on flagellar or ciliary activity, where $V\sim A/η$. To conclude, two definitions of efficiency as put forward in the literature are analyzed with distinct outcomes. We find that one type of efficiency has an optimum as a function of confinement while the other does not. Future perspectives are outlined.
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