Mechanics of Active Foam: Local Energy Injection in an Addressable 2D Foam*

The study of foam has inspired many insights into cellularized materials, including biological tissue. While cellular sheets superficially resemble passive foam structures, cells incorporate many layers of internal activity and feedback. To the best of our knowledge, no abiotic “active” foam experimental systems currently exist. Such a material is developed here, where activity of a single voxel is driven by volume oscillations in an addressable foam. The platform allows air to be periodically injected and removed from individual voxels in a 2D soap foam in a Hele-Shaw cell. This cyclic energy injection leads to fascinating dynamics involving neighbor-swapping events (T1) including cascades and global dynamics. We quantify this response of a single active voxel as a function of energy injected, symmetry and time course of evolution. Next, we study interactions between multiple active voxels with cyclic perturbations (in-phase and out-of-phase). Stroboscopic analysis is used when studying cyclic forcing. Just like our understanding of active fluids and active solids has brought new light to a multitude of problems - active foam provides a new platform to ask questions both in foam physics and, by analogy, in tissue mechanics.