Light‐driven reversible and repeatable switching between ultrastable and unstable liquid foam

Liquid foams with tunable and photoresponsive stabilities and mechanical properties are highly desired in many domains, including the chemical and environmental protection industries. Here, we constructed photoresponsive liquid foams by structuring the interfacial adsorption layers and nanoparticle‐embedded Plateau borders of the foam with biodegradable components. These foams exhibited ultrahigh foam stability but were easily destroyed by light, leading to a clean recovery of the liquid phase. In the absence of light, the hydroxypropyl cellulose (HPC) coils in the foam formed mechanically strong liquid films or “cohesive states.” Under irradiation, the ultrathin black phosphorus nanosheets induced changes in the packing parameters of the HPC assemblies within the Plateau borders and led to coil‐to‐globule transitions of the HPC and formed unstable liquid films with a “mobile state.” The two interfacial states were reversibly and repeatedly switched by turning the light on and off, which caused rapid bubble coalescence and foam collapse, and we also proved that this destabilizing mechanism was inhibited by cellulose nanocrystals. This work provides an environmentally friendly approach to controlling foam stability, and the proposed strategy can be expanded to the production of multiresponsive fully liquid objects in theory.