Long-Lived and Thermoresponsive Emulsion Foams Stabilized by Self-Assembled Saponin Nanofibrils and Fibrillar Network

Nanofibrils from the self-assembly of the naturally occurring saponin glycyrrhizic acid (GA) can be used to produce an oil-in-water emulsion foam with a long-term stability. Through homogenization and aeration followed by rapid cooling, stable emulsion foams can be produced from the mixtures of sunflower oil and saponin nanofibrils. At high temperatures, the GA fibrils form a multilayer assembly at the interface, creating an interfacial fibrillar network to stabilize the oil droplets and air bubbles generated during homogenization. A subsequent rapid cooling can trigger the self-assembly of free GA fibrils in the continuous phase, forming a fibrillar hydrogel and thus trapping the oil droplets and air bubbles. The viscoelastic bulk hydrogel showed a high yield stress and storage modulus, which lead to a complete arrest of the liquid drainage and a strong slowdown of the bubble coarsening in emulsion foams. The jamming of the emulsion droplets in the liquid channels as well as around the bubbles was also found to be able to enhance the foam stability. We show that such stable foam systems can be destroyed rapidly and on demand by heating because of the melting of the bulk hydrogel. The reversible gel-sol phase transition of the GA hydrogel leads to thermoresponsive emulsion foams, for which the foam stability can be switched from stable to unstable states by simply raising the temperature. The emulsion foams can be further developed to be photoresponsive by incorporating internal heat sources such as carbon black particles, which can absorb UV irradiation and convert the absorbed light energy into heat. This new class of smart responsive emulsion foams stabilized by the natural, sustainable saponin nanofibrils has potential applications in the food, pharmaceutical, and personal care industries.