Open the article here: Dynamic multiphase flow triggers chaotic mixing in porous media | PNAS
Authors: Gaute Linga, Kevin Pierce, Marcel Moura, Joachim Mathiesen, François Renard and Tanguy Le Borgne
Significance
Chemical and biological processes in soils, rocks, and industrial porous media are often controlled by the mixing of chemicals by fluid flow. When a single fluid phase carries dissolved chemicals through a porous medium, the mixing process is fairly well understood. However, mixing when two or more fluids move together has remained largely unexplored, despite its widespread importance. We conducted experiments and extensive computer simulations to investigate this process. Our findings reveal that moving fluid–fluid interfaces can generate chaotic mixing, characterized by solute blobs stretching exponentially over time. Hence, mixing in unsteady two-phase flows is qualitatively different and much more efficient than in steady single or two-phase flows, potentially leading to faster mixing and chemical reactions than previously thought.
Abstract
Solute mixing plays a pivotal role in a broad spectrum of chemical and biological processes across natural and engineered porous media. However, current understanding of mixing dynamics remains largely constrained to steady flows in fully or partially water-saturated environments. Multiphase flow systems are generally unsteady, with moving fluid interfaces and flow paths that change in time. Despite the widespread occurrence of dynamic multiphase flows, their impacts on solute mixing are largely unknown. Here, we use experiments and numerical simulations to investigate the effect of dynamic two-phase flow on the stretching and folding of fluid elements, a fundamental mechanism driving solute mixing and reactions in porous media. We find that dynamic two-phase flows induce chaotic mixing, characterized by exponential stretching of fluid elements, leading to strongly enhanced mixing compared to steady single-phase flows. By extensive numerical multiphase flow simulations, we establish dynamic steady states where we reliably measure the mean fluid stretching rate as a function of flow rate. We show that stretching is maximized at an optimum flow rate which balances fluid shear deformation against the frequency of flow reorientation by the intermittent motion of the fluid interface. The findings are rationalized by a mechanistic model linking basic multiphase flow characteristics to the stretching rate, opening perspectives to understand and control mixing and reactions in a wide range of multiphase flow systems.