Rheotaxis of spherical active particles near a planar wall.
Rheotaxis of spherical active particles near a planar wall.
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DOI：
10.1039/c5sm01088h
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发表时间：
2015-02
期刊：
影响因子：
3.4
通讯作者：
W. Uspal;W. Uspal;M. N. Popescu;M. N. Popescu;S. Dietrich;S. Dietrich;M. Tasinkevych;M. Tasinkevych
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文献类型：
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作者：
W. Uspal;W. Uspal;M. N. Popescu;M. N. Popescu;S. Dietrich;S. Dietrich;M. Tasinkevych;M. Tasinkevych
对于活性颗粒，自生流体动力流和外部剪切流之间的相互作用，特别是在边界表面附近，可以导致颗粒的丰富行为，而从单独考虑主动和外部驱动机制中不容易预见到这些行为。例如，在某些条件下，颗粒表现出“流变性”，即它们将其运动方向与由流动方向和边界表面法线跨越的剪切平面对齐，并顺流或逆流移动。迄今为止，流变性的研究主要集中在细长颗粒（例如精子）上，其流变性可以根据“风向标”机制直观地理解。在这里，我们研究了球形活性颗粒（由于形状的对称性而排除了“风向标”机制）仍然可能表现出流变性的可能性。结合分析和数值计算，我们表明，对于一大类球形活性颗粒，流变行为可能通过一种机制出现，该机制涉及由于颗粒的主动推进而在硬壁附近“自陷”，结合它们的旋转、对齐和运动方向“锁定”到剪切平面中。在这种状态下，颗粒以稳定的高度和方向仅向上或向下游移动。
For active particles the interplay between the self-generated hydrodynamic flow and an external shear flow, especially near bounding surfaces, can result in a rich behavior of the particles not easily foreseen from the consideration of the active and external driving mechanisms in isolation. For instance, under certain conditions, the particles exhibit "rheotaxis", i.e., they align their direction of motion with the plane of shear spanned by the direction of the flow and the normal of the bounding surface and move with or against the flow. To date, studies of rheotaxis have focused on elongated particles (e.g., spermatozoa), for which rheotaxis can be understood intuitively in terms of a "weather vane" mechanism. Here we investigate the possibility that spherical active particles, for which the "weather vane" mechanism is excluded due to the symmetry of the shape, may nevertheless exhibit rheotaxis. Combining analytical and numerical calculations, we show that, for a broad class of spherical active particles, rheotactic behavior may emerge via a mechanism which involves "self-trapping" near a hard wall owing to the active propulsion of the particles, combined with their rotation, alignment, and "locking" of the direction of motion into the shear plane. In this state, the particles move solely up- or downstream at a steady height and orientation.