Dynamic multiphase flow triggers chaotic mixing in porous media

TL;DR

This study demonstrates that dynamic multiphase flows in porous media induce chaotic mixing, significantly enhancing solute mixing and reactions compared to steady flows, with optimal flow rates maximizing stretching.

Contribution

The paper reveals how unsteady two-phase flows cause chaotic mixing through exponential stretching, providing a mechanistic model linking flow dynamics to mixing efficiency.

Findings

Dynamic two-phase flows induce exponential stretching of fluid elements.

Chaotic mixing is maximized at an optimal flow rate balancing shear and reorientation.

A mechanistic model links multiphase flow characteristics to mixing rates.

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 new perspectives to understand and control mixing and reactions in a wide range of multiphase flow systems.