Free-foaming: harnessing the free surface of expanding polyurethane foams*

Expanding polyurethane foams are used widely in industry as lightweight, cost-effective fillers with exceptional insulation properties. Despite their appearance in everyday products such as building insulation, sporting equipment, and furniture, they are typically hidden from view – often sandwiched between sheathing layers and intended to take the shape of their container. Contrarily, our study examines the rich potential for harnessing this material's free surface. We probe the behavior of puddles, droplets, and droplet collisions to build a predictive understanding of the material through the lens of familiar physical laws applicable to simpler fluids. Our findings enable us to sculpt the foam's free surface into target shapes by controlling the reservoir geometry from which it grows, presenting a novel manufacturing technique for self-forming 3D structures.