Several URLanisms, such as bacteria, algae, or spermatozoa, use flagellum or cilium activity to swim in a fluid . Many URLanisms use rather ample shape deformation, described as amoeboid, to propel themselves , either crawling on a substrate or swimming. Many eukaryotic cells were believed to require an underlying substratum to migrate (crawl) by using ample membrane deformation (like blebbing ). There is now an increasing evidence that a large variety of cells (including those of the immune system) can migrate without the assistance of focal adhesion, and can perform swimming as efficiently as crawling . This paper deals with a detailed analysis of amoeboid swimming in a confined fluid , by modeling the swimmer as an inextensible membrane deploying local active forces. The swimmer exhibits a rich behavior: it can settle into a straight trajectory in the channel , or can navigate from one wall to the other , depending on confinement. Furthermore, the nature of the swimmer is  found to be affected by the confinement: the swimmer can behave  as a pusher at low confinement, and becomes a puller at higher confinement , or vice versa; this shows that the swimmernature is not an intrinsic property. The scaling of the swimmer velocity V with the force amplitude A is analyzed in details and this shows that at small enough A, V\sim A^2/\eta^2, whereas at large enough A, V is independent of the force and is only fixed by the stroke cycle frequency and the swimmer size. This finding markedly contrasts with results known for swimming models referring to motion based on cilium and flagellum activity where V\sim A/\eta. Two types of efficiencies, put forward in the literature , are analyzed and it is found that the outcomes from each definition are quite distinct . We find that one type of efficiency has an optimum at a given confinement while the other has none . Future perspectives are outlined.
Several URLanisms, such as bacteria, algae, or spermatozoa, use flagella or cilia to swim in a fluid , while many other URLanisms 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/\eta^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/\eta. 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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