Plain-tailed wrens coordinate with each other to sing duets by inhibiting motor circuits in the brain.
Image credit: Melissa J. Coleman.
Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved March 21, 2021 (received for review November 25, 2020)
Significance
We show how pore geometry modulates the capillary equilibrium states of a trapped bubble inside a porous medium using a simple conceptual model. The model explains recently published data on bubbles trapped in porous rock samples. We explain the counterintuitive observation that bubbles with large surface areas in the subsurface can remain thermodynamically stable for geologically long times. The stability is the result of a modified relationship between a bubble’s surface free energy and volume due to geometric confinement. The implications are relevant to applications ranging from petroleum recovery, to CO2 sequestration, to groundwater oxygen supply, and to fuel-cell water management.
Abstract
In geologic, biologic, and engineering porous media, bubbles (or droplets, ganglia) emerge in the aftermath of flow, phase change, or chemical reactions, where capillary equilibrium of bubbles significantly impacts the hydraulic, transport, and reactive processes. There has previously been great progress in general understanding of capillarity in porous media, but specific investigation into bubbles is lacking. Here, we propose a conceptual model of a bubble’s capillary equilibrium associated with free energy inside a porous medium. We quantify the multistability and hysteretic behaviors of a bubble induced by multiple state variables and study the impacts of pore geometry and wettability. Surprisingly, our model provides a compact explanation of counterintuitive observations that bubble populations within porous media can be thermodynamically stable despite their large specific area by analyzing the relationship between free energy and bubble volume. This work provides a perspective for understanding dispersed fluids in porous media that is relevant to CO2 sequestration, petroleum recovery, and fuel cells, among other applications.
Footnotes
- ↵1To whom correspondence may be addressed. Email: kexu1989{at}pku.edu.cn.
Author contributions: K.X. designed research; C.W. performed research; Y.M. contributed new reagents/analytic tools; C.W. and K.X. analyzed data; and C.W., Y.M., and K.X. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2024069118/-/DCSupplemental.
Data Availability
All study data are included in the article and/or supporting information.
Change History
May 03, 2021: Equation 1 has been updated.
Published under the PNAS license.
References
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Capillary equilibrium of bubbles in porous media
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- Physical Sciences
- Applied Physical Sciences
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