Institute for Environmental Engineering, Department of Civil, Environmental and Geomatic Engineering, ETH Zurich, 8093 Zurich, Switzerland.
Institute for Environmental Engineering, Department of Civil, Environmental and Geomatic Engineering, ETH Zurich, 8093 Zurich, Switzerland
Proc Natl Acad Sci U S A. 2020 Feb 18;117(7):3372-3374. doi: 10.1073/pnas.1917163117. Epub 2020 Feb 3.
Rods settling under gravity in a quiescent fluid can overcome the bottleneck associated with aggregation of equal-size spheres because they collide by virtue of their orientation-dependent settling velocity. We find the corresponding collision kernel [Formula: see text], where l, A, and [Formula: see text] are the rods' length, aspect ratio (length divided by width), and volume, respectively, [Formula: see text] is the density difference between rods and fluid, μ is the fluid's dynamic viscosity, g is the gravitational acceleration, and [Formula: see text] is a geometrical parameter. We apply this formula to marine snow formation following a phytoplankton bloom. Over a broad range of aspect ratios, the formula predicts a similar or higher encounter rate between rods as compared to the encounter rate between (equal volume) spheres aggregating either by differential settling or due to turbulence. Since many phytoplankton species are elongated, these results suggest that collisions induced by the orientation-dependent settling velocity can contribute significantly to marine snow formation, and that marine snow composed of elongated phytoplankton cells can form at high rates also in the absence of turbulence.
在静止流体中，由于其沉降速度取决于取向，因此处于重力作用下的棒状物可以克服与等大球体聚集相关的瓶颈。我们发现了相应的碰撞核[公式：见正文]，其中 l、A 和[公式：见正文]分别为棒状物的长度、长径比（长度除以宽度）和体积，[公式：见正文]是棒状物和流体之间的密度差，μ是流体的动力粘度，g 是重力加速度，[公式：见正文]是一个几何参数。我们将此公式应用于浮游植物大量繁殖后的海洋雪的形成。在很宽的长径比范围内，该公式预测棒状物之间的碰撞率与由于沉降差异或湍流引起的（等体积）球体之间的碰撞率相似或更高。由于许多浮游植物种类呈长形，这些结果表明，由取向依赖性沉降速度引起的碰撞可以显著促进海洋雪的形成，并且在没有湍流的情况下，由长形浮游植物细胞组成的海洋雪也可以以较高的速率形成。
