Adhesive Protein Nanoparticles Control Emulsion Properties via Tunable Interdroplet Linking

The mechanical properties of emulsions are governed by the ability of droplets to resist shear, making interdroplet connections crucial for tuning emulsion behavior. Here, we elucidate the mechanism by which dispersed oil droplets can be "glued" together using pea-protein nanoparticles that self-assemble through electrostatic interactions. We experimentally demonstrate the adhesive action of these nanoparticles across multiple length scales, using microfluidics, the thin film balance, confocal microscopy, and rheology, and explain the physical mechanism using theoretical models. Our results show that the nanoparticles simultaneously bind to two opposing droplet interfaces, forming electrostatically driven bridges that link droplets together. This adhesion dramatically transforms the mechanical response of the emulsion from a liquid-like material to a plastic-like, 3D-printable system, raising the storage modulus of emulsions from $<$ 0.1 to 3.2 kPa. These insights highlight the potential of biosourced nanoparticles as a powerful tool for engineering emulsion mechanics via electrostatic forces, enabling applications that require materials with controlled mechanical properties, such as fat tissue mimetics or soft microelectronics.