到达表面：表面重力波中的球状微泳者
Reaching for the surface: Spheroidal microswimmers in surface gravity waves
摘要：
微泳者（浮游微生物或人工活性颗粒）浸入流体中与周围的流动相互作用，改变它们的轨迹。在表面重力波中，微泳者的共同目标是垂直迁移（例如，以到达自由表面或潜入更深的深度）。通过模拟微游泳者作为球体的内在游泳速度，补充平流和重新定向的流量，我们调查形状和游泳如何影响垂直运输的微游泳者在波浪中。我们发现，这是可能的microswimmer最初是向下游泳，但恢复和头部回到表面，反之亦然。这是因为游泳和流动诱导的重定向之间的耦合在垂直运输中引入了形状依赖性。从一个波平均分析的microswimmer轨迹，我们表明，每个轨迹的位置-取向相空间中，只依赖于形状的临界平面的边界。我们也给出了明确的解决方案，这些轨迹，并确定部分的microswimmer开始内的水柱，并最终到达表面。对于最初随机定向的微泳者，到达表面的分数随着起始深度的减小而单调增加，正如预期的那样，但也随形状和游泳速度而变化。在小的游泳速度的限制，高度长形的microswimmer到达表面的分数是0.5，这表明这些游泳者将能够选择的垂直运输方向与游泳行为的小变化。
英文摘要：
Microswimmers (planktonic microorganisms or artificial active particles) immersed in a fluid interact with the ambient flow, altering their trajectories. In surface gravity waves, a common goal for microswimmers is vertical migration (e.g., to reach the free surface or to dive to deeper depths). By modelling microswimmers as spheroidal bodies with an intrinsic swimming velocity that supplements advection and reorientation by the flow, we investigate how shape and swimming affect vertical transport of microswimmers in waves. We find that it is possible for microswimmers to be initially swimming downwards, but to recover and head back to the surface, and vice versa. This is because the coupling between swimming and flow-induced reorientations introduces a shape dependency in the vertical transport. From a wave-averaged analysis of microswimmer trajectories, we show that each trajectory is bounded by critical planes in the position-orientation phase space that depend only on the shape. We also give explicit solutions to these trajectories and determine the fraction of microswimmers that begin within the water column and eventually reach the surface. For microswimmers that are initially randomly oriented, the fraction that reach the surface increases monotonically as the starting depth decreases, as expected, but also varies with shape and swimming speed. In the limit of small swimming speed, the fraction of highly prolate microswimmers reaching the surface is 0.5, suggesting that these swimmers would be able to choose direction of vertical transport with small changes in swimming behaviour.
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