Neutron radiography of liquid foam structure near a vertical wall

At a solid boundary, the structural formation of bubbles is different from that in the bulk of a liquid foam. The presence of a solid boundary imposes additional constraints, resulting in a crystalline arrangement of the bubbles. For dry and monodisperse foam, the Kelvin and Fejes-Tóth structure is expected in the vicinity of the wall, while a random ordering should occur in the bulk. In this study, we investigate the transition from a crystalline to a random structure near a vertical wall located in the middle of a flat foam cell. The corresponding layering of the liquid was quantified by measuring the distribution of liquid fraction within the cell using neutron radiography. The amplitude of the liquid fraction distribution and its decay with distance from the solid boundary were correlated with the foam bubble size and polydispersity. Furthermore, by applying forced drainage, we measured the corresponding permeability and wetting front velocity near the vertical wall. We found that the crystalline sorting reduces the permeability and wetting front velocity compared to a randomly packed foam.