Formation and collapse of gas cavities in a soft porous medium #

Oliver Paulin, Liam Morrow, Matthew Hennessy, Christopher MacMinn

11:30 Monday in 2Q48.

Part of the Mechanics of hydrogels and poroelastic media session.

Abstract #

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 and/or the confining stress is sufficiently low, the gas can instead displace the solid grains to open macroscopic cavities. These gas cavities form in a variety of soft porous media, including seabed sediments, industrial waste ponds, and peatlands. An increase in the confining stress can suppress the formation of cavities and even trigger the collapse of existing cavities, forcing the gas into the pore space. A quantitative understanding of the total volume of gas cavities, their size distribution, and their formation and collapse are important for the macroscopic mechanics of this three-phase system, and for predicting the rate of gas venting to the surrounding environment. Here, we consider this problem via a novel phase-field model, in which two immiscible fluids interact with a poroelastic solid skeleton. Our model captures the competing effects of elasticity and gas-liquid-solid interactions (capillarity). We complement our theory with a series of simple experiments, using a packing of hydrogel beads as an example soft porous medium. We study the formation and collapse of gas cavities in a 1D setting, identifying the confining stress at which cavities collapse and investigating the reversibility of cavity formation and collapse under fluctuating confining stress.