Pickering emulsions stabilized by microgels: Three regimes of surface coverage

Oil-in-water Pickering emulsions can be stabilized by poly-N-Isopropylacrylamide based microgels that adsorb, deform and entangle at the droplets interface. The surface coverage Γ_emulsion, defined as the mass of microgels per unit interfacial area, likely plays a key role in the emulsion properties as stability and responsiveness. The objective of the present study is to link Γ_emulsion to the concentration of microgels used during the emulsification process. Γ_emulsion was monitored by combining droplet size analysis, UV-visible quantification of non-adsorbed microgels and Cryo-SEM visualization of the droplets interface, for a microgel concentration range over almost two decades. We demonstrate the existence of three regimes. At low microgel concentration, in the particle-poor regime, the well-known "Limited Coalescence" process takes place. All the microgels adsorb, the droplet size distribution is narrow and the mean droplet size is inversely proportional to the microgel concentration: a constant minimum value of Γ_emulsion characterizes this domain. For higher microgel concentrations, microgels partition between the interface and the bulk continuous phase. In this intermediate "Excess" regime, both adsorbed and non-adsorbed microgel increase, proving that microgels compress at the interface to maximize their adsorption. At higher microgel concentrations, a third regime named "Saturation" regime is observed for the first time. Γ_emulsion then reaches a constant high plateau value while the drop size distribution becomes polydisperse, showing that the fragmentation process dominates over the coalescence. These findings should open new perspectives to better tailor emulsion properties using deformable particles as stabilizers.