Ultra-stable high-internal-phase emulsions fabricated solely by hydrophilic cellulose nanofiber via inter-droplet jamming

High-internal-phase emulsions (HIPEs) has been increasing interest owing to their large surface area and controllable flow behavior, however, their heavy addition of emulsifiers, e.g., surfactants and surface-active particles, leads to high cost and emulsifier abuse. To solve these issues, we report a new strategy to prepare HIPEs by solely adding a surface-inactive, hydrophilic cellulose nanofiber (CNF). Compared to conventional HIPEs, the highly flocculated CNF is closely packed in the liquid films between oil droplets instead of anchoring at the oil-water interfaces, as referred to the inter-droplet jamming. An efficient jamming behavior is observed at a low CNF concentration (as low as 0.075 wt%) and at very high volume fractions of oil (near 0.85), maintaining the HIPEs over timescales of two years, without the liquid drainage or phase separation. This unexpected stability is mainly attributed to the increase by dozen orders of magnitude in the viscoelasticity of the jammed films as compared with that in the bulk phase, generating a significantly spatial barrier to prevent the depletion attraction between droplets. This simple approach should offer a new pathway to achieve nature-inspired, pure and controlled materials.