Bubbles in highly porous media: Clogging and unclogging at constrictions

TL;DR

This study combines modeling, simulations, and experiments to analyze how gas bubbles move through porous layers in electrochemical devices, identifying key clogging and unclogging mechanisms.

Contribution

It introduces analytical expressions and simulation results that reveal new pathways for bubble clogging and unclogging in porous media, validated by experiments.

Findings

Critical Bond number predicts passage vs. clogging in capillaries.

Hydrodynamic unclogging occurs via pressure buildup in interbubble films.

Distinct regimes of bubble dynamics are mapped based on confinement and Bond number.

Abstract

Gas bubble transport through highly porous transport layers (PTLs) is a key process in electrochemical devices such as proton exchange membrane water electrolyzers, where bubbles generated at catalyst surfaces must migrate through complex porous networks. To understand this process, we focus on model systems, namely the motion of single, paired and multiple bubbles in capillaries and study these by combining analytical modeling, three-dimensional color-gradient lattice Boltzmann simulations, and X-ray radiography. For single bubbles, we derive an analytical expression for the critical Bond number separating passage from clogging and show that, in the low deformation regime, it accurately predicts this transition in circular capillaries. Extending the study to bubble pairs, we uncover additional clogging and unclogging pathways, including hydrodynamic unclogging driven by pressure buildup in the interbubble film, and coalescence-induced clogging and unclogging. By mapping our results as functions of confinement ratio and Bond number, we define distinct dynamical regimes that control bubble passage. Experiments on bubble chains rising through highly porous nickel foams confirm the predicted clogging and unclogging mechanisms.