Cellulose nanofiber-created air barrier enabling closed-cell foams prepared via oven-drying

Cellulose foams are renewable and biodegradable materials that are promising substitutes for plastic foams . However, the scale-up fabrication of cellulose foams is severely hindered by technological complexity and cost- and time-consuming drying processes. Here, we developed a facile and robust method to fabricate cellulose foams via oven-drying following surfactant-assisted mechanical foaming of cellulose nanofibers (CNFs). CNFs in the air-water interface reduced the surface tension to stabilize bubbles in the wet foams, and generated densely arranged crystal barriers to seal air in the bubbles while oven-drying to prevent bubbles from collapsing. The optimal CNF foam has an ultra-low density of 12.10 mg/cm 3 , an ultra-high porosity of 99.14 %, and a low thermal conductivity of 34.87 mW/m/K, allowing it to act as an excellent thermal insulation material. Moreover, CNF foams can be easily integrated with diverse advanced properties such as flame retardancy, ultra-high mechanical strength , hydrophobicity , and magnetic responsiveness by incorporating functional components. The study paves the way for CNF foams to move toward practical applications.