Mixing-induced reactions play a key role in a large range of biogeochemical and contaminant transport processes in the subsurface. Fluid flow through porous media was recently shown to exhibit chaotic mixing dynamics at the pore scale, enhancing microscale concentration gradients and controlling mixing rates. While this phenomenon is likely ubiquitous in environmental systems, it is not known how it affects chemical reactions. Here, we use refractive index matching and laser-induced fluorescence imaging of a bimolecular redox reaction to investigate the consequence of pore scale chaotic mixing on the reaction rates. The overestimation of measured reaction rates by the classical macrodispersion model highlights the persistence of incomplete mixing on the pore scale. We show that the reaction product formation is controlled by microscale chaotic mixing, which induces an exponential increase of the mixing interface and of the reaction rates. We derive a reactive transport model that captures experimental results and predicts that chaotic mixing has a first order control on reaction rates across a large range of time scales and Péclet and Damköhler numbers. These findings provide a new framework for understanding, assessing, and predicting mixing-induced reactions and their role on the fate and mobility of environmental compounds in natural porous media.

中文翻译：

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微尺度混沌混合作为多孔介质中化学反应的驱动因素


混合引起的反应在地下的大量生物地球化学和污染物传输过程中发挥着关键作用。最近表明，流经多孔介质的流体在孔隙尺度上表现出混沌混合动力学，增强了微观浓度梯度并控制了混合速率。虽然这种现象在环境系统中可能普遍存在，但尚不清楚它如何影响化学反应。在这里，我们使用双分子氧化还原反应的折射率匹配和激光诱导荧光成像来研究孔隙尺度混沌混合对反应速率的影响。经典宏观分散模型对测量反应速率的高估凸显了孔隙尺度上不完全混合的持续存在。我们表明，反应产物的形成是由微尺度混沌混合控制的，这会导致混合界面和反应速率呈指数增长。我们推导了一个反应输运模型，该模型捕获实验结果并预测混沌混合对大范围时间尺度以及佩克莱特和达姆克勒数的反应速率具有一阶控制。这些发现为理解、评估和预测混合引起的反应及其对天然多孔介质中环境化合物的命运和流动性的作用提供了一个新的框架。