Foam coarsening in granular packings

Foam coarsening refers to the process by which gas diffuses between bubbles, leading to an increase in their average size and a corresponding evolution of the foam's physical properties. When foam is confined within a porous medium, this process can continue, albeit in a modified manner. Anticipating the rate of coarsening under such conditions is therefore crucial. In this study, we examine the coarsening dynamics of liquid foam confined within a packing of spherical grains, systematically varying both the grain size and the foam's liquid fraction, in the range of 7% to 25%. Our key finding is that, under confinement, the coarsening rate is much less sensitive to the liquid fraction than in bulk foams. Instead, the dynamics are more strongly governed by the ratio of bubble size to grain size. This behavior is well captured by a model based on the effective liquid fraction-a measure that accounts for the substantial amount of liquid retained at grain surfaces and contacts in the form of liquid bridges. Additionally, we observe numerous small, persistent bubbles located at the periphery of the liquid bridges. These structures act as hosts, facilitating the accumulation of such bubbles, particularly at higher liquid fractions. Depending on the context, this phenomenon may be either advantageous or detrimental to the intended application.