Enhanced mixing in porous media by dispersed two-phase flow

Efficient solute mixing in porous media is essential for a wide range of natural processes and industrial applications, including nutrient transport in biological systems, groundwater bioremediation, and carbon dioxide geological sequestration. The extent of mixing directly controls the rates of associated biological and chemical reactions. Although turbulence is widely employed to promote mixing due to its transient and chaotic nature, its effectiveness in porous media is severely limited by the presence of extensive solid boundaries that suppress turbulent fluctuations. In contrast, dispersed two-phase flows—characterized by inherently transient flow features—offer a promising alternative for enhancing mixing efficiency.

Despite substantial research on dispersion and mixing in two-phase flow systems, the majority of existing studies assume static phase interfaces [1]. However, dispersed two-phase flows are intrinsically associated with dynamic and evolving phase interfaces. While recent studies [2, 3] have begun to explore this issue, the pore-scale mechanisms that control solute transport and mixing in porous media under dispersed two-phase flow remain poorly understood.

In this study, we examine transverse solute mixing in porous media under dispersed two-phase flow and steady single-phase flow conditions using microfluidic experiments. The results show that, at a Péclet number of 1000, dispersed two-phase flow leads to a substantial enhancement of transverse mixing relative to single-phase flow. Mixing efficiency, quantified by the dilution index, is approximately doubled under dispersed two-phase flow compared with single-phase flow at identical injection rates. Complementary direct numerical simulations indicate that this improvement originates from transient flow structures, including vortex formation induced by dynamically evolving phase interfaces, which are absent in steady single-phase flow. Together, these findings offer new pore-scale mechanistic insights into solute mixing in porous media and highlight flow-regime modulation as an effective strategy for enhancing mixing performance.

Keywords: mixing; porous media; dispersed two-phase flow.

Reference

[1] J. Jiménez-Martínez, P.d. Anna, H. Tabuteau, R. Turuban, T.L. Borgne, Y. Méheust, Pore-scale mechanisms for the enhancement of mixing in unsaturated porous media and implications for chemical reactions, Geophys. Res. Lett. 42 (13) (2015) 5316-5324.

[2] J. Mathiesen, G. Linga, M. Misztal, F. Renard, T. Le Borgne, Dynamic Fluid Connectivity Controls Solute Dispersion in Multiphase Porous Media Flow, Geophys. Res. Lett. 50 (16) (2023) e2023GL105233.

[3] X. Zhang, Z. Dou, M. Hamada, P. de Anna, J. Jimenez-Martinez, Enhanced Reaction Kinetics in Stationary Two-Phase Flow through Porous Media, Environ. Sci. Technol. 59 (2) (2025) 1334-1343.