Effect of phase change on the rheology and stability of paraffin wax-in-water Pickering emulsions

Pickering emulsions are widely found in nature and industry including food, pharmaceuticals, and oil industries. Often, Pickering emulsion studied have a Newtonian dispersed phase. However, the dispersed phase can be non-Newtonian such as one that can be subjected to a phase change under certain experimental conditions. This work examines how changing the physical state of dispersed phase alters the shear stability and bulk viscoelasticity of o/w emulsions. Model silica-stabilized, paraffin wax-in-water emulsions are synthesized. The wax, with a melting temperature of about 55 C-ay is subjected to a phase change by changing the temperature between 15 and 80 C-ay. At lower temperatures (< 55 C-ay), the droplet deformability and particle mobility at the interface are significantly restricted while at higher temperatures (> 55 C-ay), the wax melts and expands, making the emulsion droplets deformable and the particles more relaxed. These directly affect bulk emulsion rheology. Flow curves and oscillatory shear experiments indicate that emulsion droplets are flocculated and the emulsions behave as elastic solids. These properties are directly influenced by temperature, which alters the state of aggregation and network-structure of the emulsion droplets. The effect of emulsion concentration is also analyzed. Three different concentrations are tried-15, 30, and 45 vol% (as measured at 25 C-ay when the wax is solid). At a given temperature, the rheological properties seem to scale with concentration. Further, we show that the emulsions are sensitive to destabilization (gelation) under flow with the sensitivity directly varying with temperature and magnitude of shear fields (steady shear) applied.