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

Abstract

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 ( κ C) 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% κ C were dominated by the κ C 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 κ C, W/W emulsions containing higher concentrations of the biopolymers (3.0 wt% starch + 0.3 wt% κ C) 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. • Water-in-water (W/W) emulsion was designed using starch (GS) and κ -carrageenan ( κ C). • Whey protein microgel (WPM) was used to stabilize W/W emulsion droplets. • WPM at the W–W interface was observed using microscopy across length scales. • The viscosity of W/W droplets was enhanced in presence of WPM. • WPM improved lubricity of W/W droplets in the boundary and mixed regimes.