The behaviours of micro-organisms at interfaces play important roles in various biological, medical and engineering phenomena. Despite its widely recognized importance, our understanding of swimming micro-organisms at interfaces is limited. Ferracci et al. ( PLoS One , vol. 8, 2013, e75238) reported that the ciliate, Tetrahymena , was entrapped at a water–air interface, while it escaped from a solid wall. Although the entrapment was speculated to be induced by physical processes, the mechanism is still unclear. To clarify the entrapment phenomenon, we focus on cell shape and numerically investigate the behaviour of a swimming micro-organism at interfaces from a hydrodynamic point of view. The model cell is assumed to propel itself by generating homogeneous tangential stress above the cell body. The results reveal that two major shape parameters, i.e. fore-and-aft asymmetry and a constriction, are dominant in the entrapment phenomenon. The mechanism can be explained by the balance of two opposite rotational velocities: repelling velocity due to the ciliary beat and attracting velocity due to the collision at the interface. In other words, the mechanism can be understood by hydrodynamic and steric effects. Moreover, cells tend to be entrapped more by the water–air interface than by the solid wall, which agrees with experimental observations reported previously (Ferracci et al. 2013). Finally, we experimentally observe Tetrahymena thermophila entrapped on the surface of an air bubble, and qualitatively discuss the shape of entrapped cells. The knowledge obtained provides a basis for understanding the behaviours of swimming micro-organisms at various interfaces, both in nature and in industrial applications.

中文翻译：

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形状很重要：在界面处截留模型纤毛虫

界面处微生物的行为在各种生物、医学和工程现象中起着重要作用。尽管其重要性得到广泛认可，但我们对界面处游动微生物的理解是有限的。费拉奇等人。( PLoS One , vol. 8, 2013, e75238) 报道称，纤毛虫四膜虫被困在水-空气界面，而它从固体壁中逃脱。虽然推测诱捕是由物理过程引起的，但其机制仍不清楚。为了阐明截留现象，我们关注细胞形状，并从流体动力学的角度对界面处游泳微生物的行为进行了数值研究。假设模型单元通过在单元体上方产生均匀的切向应力来推动自身。结果表明，两个主要的形状参数，i。e. 前后不对称和收缩在诱捕现象中占主导地位。该机制可以通过两个相反旋转速度的平衡来解释：纤毛搏动引起的排斥速度和界面碰撞引起的吸引速度。换句话说，该机制可以通过流体动力学和空间效应来理解。此外，细胞更容易被水-空气界面捕获，而不是被固体壁捕获，这与之前报道的实验观察结果一致（Ferracci 等人，2013 年）。最后，我们通过实验观察了被困在气泡表面的嗜热四膜虫，并定性地讨论了被困细胞的形状。获得的知识为理解游泳微生物在各种界面的行为提供了基础，