Ke Xu’s group from the PKU Department of Energy and Resources Engineering makes important progress on the thermodynamic stability of multiphase systems in porous media. This work is published in Proceedings of the National Academy of Sciences of the United States of America (PNAS, https://doi.org/10.1073/pnas.2024069118).

Bubbles/droplets/ganglia commonly emerge in subsurface porous media. Although endued with very high surface free energy, such multiphase dispersed fluid systems can keep stable for thousands of years without coarsening. It is completely contrary to the fact in open space that the dispersed fluid clusters (e.g. bubble population shown in Figure. a) spontaneously coarsen and merge to reduce total surface free energy. A lack of explanation on this counter-intuitive observation has greatly hindered the in-depth understanding of the long-term evolution of fluid systems in porous media.

To reveal the origin of bubble population’s thermodynamic stability in porous media, Ke Xu’s group proposes a conceptual model of a bubble’s capillary equilibrium associated with free energy inside a porous medium (Figure. c). The model accounts for all metastable bubble configurations that are regulated by the pore-throat geometry. Based on this model, the researchers find that the specific surface area of bubbles in porous media fluctuates within a narrow range regardless of their volume, which is fundamentally different from the spherical bubble in open space. This approximately constant specific surface area results in an approximately linear correlation between the surface free energy and bubble volume (Figure. d). Therefore, coalescence of multiple bubbles in porous media brings no benefit of free energy reduction, and thus, the coarsening of the bubble population in porous media cannot occur spontaneously. So far, this study has successfully provided a compact explanation of bubble population’s thermodynamic stability in porous systems.

This work provides a perspective for understanding dispersed fluids in porous media which is relevant to CO₂ sequestration, petroleum recovery, and fuel cells, among other applications. It is also a demonstration of porous structure’s fundamental modification on multiphase fluid systems.

This work is a follow-up of earlier approaches by Ke Xu’s group on Physical Review Letters (https://dx.doi.org/10.1103/PhysRevLett.119.264502) and Geophysical Research Letters (https://dx.doi.org/10.1029/2019gl085175).

First-year Ph.D. student Chuanxi Wang is the first author of this work. Assistant professor Ke Xu is the corresponding author. Assistant professor Yashar Mehmani from Pennsylvania State University also involved in this work.

Figure. (a) Free bubble population in open space. (b) A trapped bubble in porous media. (c) Bubble population trapped in a two-dimensional homogeneous porous medium. (d) Correlation between the surface free energy and the bubble volume for a bubble in porous media.