Formation and collapse of gas cavities in a soft porous medium

Gas bubbles can form and grow in otherwise liquid-saturated granular media due to various physical processes, such as corrosion or the microbial decomposition of organic matter. The gas bubbles are typically non-wetting to the solid grains; as such, it is energetically costly for the gas to invade the narrow pore throats between grains. If the solid skeleton is sufficiently soft, the gas can instead displace the solid grains to open macroscopic cavities. An increase in external confining stress can suppress the formation of these cavities and even trigger the collapse of existing cavities, forcing the gas into the pore space. Here, we investigate this problem experimentally using a packing of hydrogel beads as a model soft porous medium. We vary the external confining stress to study the formation and collapse of gas cavities within this system. We complement our experimental observations with a phase-field model informed by large-deformation poromechanics. We study the formation and collapse of gas cavities within a 1D setting, identifying the confining stress at which cavities collapse, with gas invading the pore space. We also study the impact of the rate of compression as well as the reversibility of cavity formation and collapse under fluctuating confining stress.