Tribology and rheology of water-in-water emulsions stabilized by whey protein microgels

This study aimed to investigate the microstructural, tribological and rheological properties of water-in-water (W/W) emulsion droplets with or without stabilization by proteinaceous microgel particles. The W/W emulsions were prepared from mixtures of gelatinized corn starch (GS) and κ-carrageenan (κC) in the two-phase regime and when particle-stabilized whey protein microgel particles (WPM) were used. The W/W emulsions were shear thinning liquids. The viscosity values of the emulsions were higher than the corresponding weight average values, calculated on individual samples irrespective of shear rates. Tribological results revealed that the frictional properties of W/W emulsions formed from 1.0 wt% GS + 0.1 w% κC were dominated by the κC properties alone in the mixed and hydrodynamic regimes, even though the starch played an essential role in decreasing friction coefficient (μ) in the boundary regime. Unlike the corresponding solutions of GS and κC, W/W emulsions containing higher concentrations of the biopolymers (3.0 wt% starch + 0.3 wt% κC) decreased μ in the mixed and boundary regimes, probably due to water droplets becoming entrained and forming a hydration film in the contact region. In the case of W/W emulsions containing WPM, confocal and cryo-scanning electron microscopy confirmed the presence of WPM at the interface and hence a Pickering-like stabilization. The WPM-stabilized W/W emulsions showed higher apparent viscosity (than those without WPM) and lower μ in the boundary and mixed regimes. Stabilization of W/W emulsions via microgel particles therefore seems to be a useful tool to improve the lubrication performance of such systems.