Boosting the self-production of roaming bubbles to hierarchize the microstructure of foamed materials

Controlling the bubble size distribution in liquid foams is essential for tuning their physical properties for targeted applications. A particularly promising example lies in the design of hierarchically porous materials, which hold great potential across various fields. Here, we show that temporarily storing a foam in a column packed with monodisperse spherical grains promotes coarsening in a way that generates a substantial number of bubbles significantly smaller than the average size. These bubbles, reminiscent of the recently identified "roaming bubbles" [Galvani <i>et al.</i>, <i>Proc. Natl. Acad. Sci. U. S. A.</i>, 2023, <b>120</b>, e2306551120], are hosted within the abundant liquid bridges that naturally form at grain contacts. By monitoring the evolution of bubble sizes at the column wall, we systematically investigated the roles of grain size and foam liquid fraction. We show that, by appropriately tuning these parameters along with the storage duration, a significant and reproducible population of roaming bubbles can be obtained. Moreover, we demonstrate that this roaming-bubble-enriched foam can be easily extracted from the column after storage. This simple and versatile method may offer new pathways for the controlled production of foam-based materials with tailored microstructures.