Liquid foam: Fundamentals, rheology, and applications of foam displacement in porous structures

Liquid foams, as colloidal systems comprising a dispersed gas phase within a continuous liquid medium, exhibit unique structural and rheological properties beneficial for various industrial and environmental applications. This review synthesizes current knowledge on the fundamentals, stability mechanisms, and practical applications of liquid foams. We first discuss foam structures, transitioning from ball to wet and dry foams, influenced by the liquid fraction and surfactant presence, which also influence the foam's mechanical and stability properties. We further describe the mechanisms of foam generation (for confined foams), stability, and decay, highlighting the roles of snap-off, lamellae division, and leave-behind in foam formation and the adverse effects of coarsening, gravity drainage, and collapse on foam stability. Additionally, the review covers the rheological behavior of foams under shear stress, illustrating their complex viscoelastic or viscoplastic nature. Finally, we review recent studies of foam injection and displacement in porous structures, utilizing Hele-Shaw cells and microfluidics.