Pickering Emulsions: Role of Particle Wettability and Adhesive Force on Droplet Bridging

This study demonstrates the engineering of bridged Pickering emulsion (PE) gels by tuning the particle position at the interface and adhesive forces. This is achieved through controlled surface modification of hematite particles using oleic acid in a water-decane system. Microscopy observations revealed that the droplets are stabilized through a bridging mechanism, where oil droplets are connected by a shared monolayer of particles, with an intervening water layer between them. The experimental observations reveal that the concentration of oleic acid affects both the position of particles with respect to the interface (wettability) and adhesive forces, leading to the formation of emulsions with bridged droplets at specific oleic acid concentration ranges. To investigate this, the particle position at the interface and the strength of adhesive force are measured as a function of oleic acid concentration by direct visualization and droplet stretching technique, respectively. These studies confirm that at low oleic acid concentrations, the particle position favors the bridging, as particles are preferentially wettable by the continuous phase (water) but adhesive forces are not strong. Thus, this condition promotes the formation of oil-in-water emulsions without bridging. While, at higher oleic acid concentrations, the position of particles with respect to the interface hinders bridging, despite sufficient adhesive forces, because the particle surface becomes preferentially wettable by dispersed phase (oil), thereby supporting the inversion of emulsions. Therefore, a precise amount of oleic acid is necessary to achieve stable bridging with both factors contributing to the bridge formation. Further, the versatility of the process is illustrated by using different types of oil and particle surface modifiers. In all of the cases, stable emulsions are obtained by droplet bridging at a precise concentration of the modifier. The effect of the particle concentration and water-to-decane volume ratio on the stability of these emulsions is also studied. These emulsions show remarkable stability under undisturbed conditions due to gel-like nature despite the droplets being partially covered with particles. Moreover, after such emulsions are destabilized by external stimuli, emulsions with similar features can be readily and reversibly formed.