Diffusion-limited retention of porous particles at density interfaces
Diffusion-limited retention of porous particles at density interfaces
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DOI：
10.1073/pnas.1012319108
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发表时间：
2010-12-21
影响因子：
11.1
通讯作者：
Stocker, Roman
中科院分区：
文献类型：
--
作者：
Kindler, Kolja;Khalili, Arzhang;Stocker, Roman
海洋中向下的碳通量很大程度上是由颗粒沉降控制的。大多数海洋颗粒在低雷诺数下沉降，并且具有高度多孔性，但这种状态的流体动力学仍未被探索。本文介绍了在雷诺数为0.1和1之间时多孔粒子通过密度界面沉降的实验研究结果。我们跟踪了100到500 μ m的水凝胶球，孔隙度为95.5%，渗透率可以忽略不计。我们发现，相对于较低（密度较大）的流体层，一个小的负初始超额密度δ rho(p)，在海洋中是一种常见的情况，导致颗粒在界面上停留时间较长。我们假设，滞留时间是由分层剂在间隙和环境流体之间的扩散交换决定的，这增加了滞留在界面上的颗粒的过量密度，使它们能够恢复沉降。观察结果证实了这一假设，表明保留时间与粒径呈二次依赖关系，与扩散交换一致。这些结果表明，孔隙度可以控制颗粒在密度界面的滞留时间，从而控制颗粒在密度界面的积聚，这一机制可能支持在海洋洋斜中经常观察到的颗粒层的形成。我们估计海洋颗粒的特征尺寸范围的保留时间为3分钟至3.3天。这种滞留时间的增加可以通过增加微生物定植和颗粒利用以及溶解有机物的释放来影响碳转化。观察到的保留时间的尺寸依赖性可以进一步有助于提高垂直碳通量的量化。
Downward carbon flux in the ocean is largely governed by particle settling. Most marine particles settle at low Reynolds numbers and are highly porous, yet the fluid dynamics of this regime have remained unexplored. We present results of an experimental investigation of porous particles settling through a density interface at Reynolds numbers between 0.1 and 1. We tracked 100 to 500 mu m hydrogel spheres with 95.5% porosity and negligible permeability. We found that a small negative initial excess density Delta rho(p) relative to the lower (denser) fluid layer, a common scenario in the ocean, results in long retention times of particles at the interface. We hypothesized that the retention time was determined by the diffusive exchange of the stratifying agent between interstitial and ambient fluid, which increases excess density of particles that have stalled at the interface, enabling their settling to resume. This hypothesis was confirmed by observations, which revealed a quadratic dependence of retention time on particle size, consistent with diffusive exchange. These results demonstrate that porosity can control retention times and therefore accumulation of particles at density interfaces, a mechanism that could underpin the formation of particle layers frequently observed at pycnoclines in the ocean. We estimate retention times of 3 min to 3.3 d for the characteristic size range of marine particles. This enhancement in retention time can affect carbon transformation through increased microbial colonization and utilization of particles and release of dissolved organics. The observed size dependence of the retention time could further contribute to improve quantifications of vertical carbon flux.