Emergent Dynamics in Vibrating Active Foams

Foams are complex materials composed of numerous bubbles that intricately adapt their shapes to large-scale geometric constraints, yet are prone to coalescence. Existing stabilization techniques with additives often obscure the foam's intrinsic physics while keeping the system largely passive. Here, we demonstrate that vertical vibration can not only stabilize a densely packed assembly of bubbles but also transform it into an active foam that exhibits rich collective dynamics. In our experiments, a layer of capillary-sized bubbles is held by buoyancy against the top wall of a vertically vibrating fluid chamber. Owing to parametric forcing, each bubble undergoes shape oscillations and, in some regimes, exhibits self-propulsion. Within the assembly, bubbles dynamically deform and rearrange in response to interactions with neighbors. A sweep of bubble size and driving parameters reveals a variety of collective behaviors, including labyrinthine pattern formation, unjamming transitions, and active turbulence. We rationalize the onset of collective motion in terms of the excitation of two oscillation modes: a lateral breathing-like mode that opens gaps, and quadrupolar elongations that promote neighbor exchange. More broadly, our active foam offers a controllable platform to study collective behavior of soft, shape-shifting agents, enabling comparison with biological systems such as cells to reveal universal principles across inert and living deformable active systems.