Critical role of nanocomposites at air–water interface: From aqueous foams to foam-based lightweight functional materials

The study of aqueous foams has long been an interest of the scientific community. A developing area is the use of foams to prepare lightweight, functional solid foam materials. To design such porous materials, abundant foams should first be generated (good foamability), followed by foam destabilization processes being ceased or significantly retarded (excellent foam stability) to avoid the foam decay upon drying, which can be achieved by finely controlling interfacial behaviour of the foaming agents. The current review considers the interfacial behaviour and foaming potential of emerging nanocomposites such as: surfactant-nanoparticle composites; polymer-nanoparticle composites; and surfactant-graphene oxide (GO) composites. Compared with using only surfactants, polymers or nanoparticles, composite stabilizers are of great interest and importance since they enrich the interfacial behaviour of the bubble-stabilizing film (e.g. mechanical and structural properties), ensuring excellent foamability and long-term foam stability, and novel applications of the solid foam materials. Preparation mechanisms, properties and applications of solid foams prepared via direct foaming methods using the nanocomposite stabilizers are presented and highlighted. For the surfactant-NPs composites, the influences of the property of the foamed suspension and the bubble-stabilizing interface, on the quality of the wet and solid foams, is still elusive. As to the polymer-NPs composites, functional solid foams preparation using their stimuli-responsive performance remains an interesting topic for future research. The fundamental colloidal properties of surfactant-GO composites at the air–water interface and in stabilizing foams remain incompletely understood. Future studies are desired, combining experiments at various scales: interfacial films, Plateau borders, bulk liquid, bubbles, and foams, etc.