Formation and collapse of non-wetting cavities in soft porous media

Thesis

Various biological and chemical processes lead to the nucleation and growth of non-wetting fluid bubbles within the pore space of a soft porous medium, such as the formation of gas bubbles in liquid-saturated sea-bed sediments, or biomolecular condensates in the cellular matrix. The non-wetting nature of these bubbles makes it energetically costly for them to invade the narrow pore throats between solid. If the solid skeleton is sufficiently soft and/or the confining stress is sufficiently low, non-wetting bubbles can instead displace solid to form macroscopic (open) cavities. An increase in the confining stress can suppress the formation of cavities and even trigger the collapse of existing cavities, forcing the non-wetting fluid into the pore space. A quantitative understanding of the total volume of non-wetting cavities, their size distribution, and their formation and collapse are important for the macroscopic mechanics of this three-phase system, and its interactions with the surrounding environment. Here, we consider this process through the lens of phase separation, where thermomechanics govern the separation of the non-wetting phase from a fluid--fluid--solid mixture. We construct a novel phase-field model informed by large-deformation poromechanics, in which two immiscible fluids interact with a poroelastic solid skeleton. Our model captures the competing effects of elasticity and fluid--fluid--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. As model problems, we consider the spontaneous formation of multiple cavities from an initial distribution of non-wetting fluid in the pore space, and the deformation-driven collapse of cavities due to the application of a fluid-permeable piston. We study our model in a 1D setting, via numerical simulation, linear stability analysis, and analytical solution of a reduced model. We identify the key parameters that control cavity formation, and the characteristic size of resulting cavities. We also investigate the confining stress required to trigger cavity collapse, and explore the reversibility of cavity formation and collapse under fluctuating confining stress. We complement our theory with a series of simple experiments, using a packing of hydrogel beads as an example soft porous medium.

Authors: Paulin, OW

• DOI: 10.5287/ora-xqpj8emxm
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: capillarity; phase separation; soft porous media; phase-field method
• Subjects: Fluid dynamics; Soft condensed matter