Rheology and Catastrophic Phase Inversion of Emulsions in the Presence of Starch Nanoparticles

摘要

Emulsionsstabilizedbysolidnanoparticles, referredtoasPickeringemulsions, are becomingincreasingly important in applications as they are free of surfactants. However, the bulk propertiesand stability of Pickering emulsions are far from being well understood. In this work, the rheologicalbehavior and catastrophic phase inversion of emulsions in the presence of starch nanoparticles werestudied using in-situ measurements of viscosity and electrical conductivity. The aqueous phaseconsisting of starch nanoparticles was added sequentially in increments of 5% vol. to the oil phaseunderagitationconditiontopreparetheemulsions. Theemulsionswerewater-in-oil(W/O)typeatlowto moderate concentrations of aqueous phase. At a certain critical volume fraction of aqueous phase,catastrophic phase inversion of W/O emulsion to oil-in-water (O/W) emulsion took place accompanieda sharp jump in the electrical conductivity and a sharp drop in the emulsion viscosity. The W/Oemulsions were nearly Newtonian at low concentrations of aqueous phase. At high concentrations ofaqueous phase, prior to phase inversion, the W/O emulsions exhibited a shear-thickening behavior.The O/W emulsions produced after phase inversion were shear-thinning in nature. The comparison ofthe experimental viscosity data with the predictions of emulsion viscosity model revealed only partialcoverage of droplet surfaces with nanoparticles. With the increase in the concentration of starchnanoparticles (SNPs) in the aqueous phase of the emulsions, the phase inversion of W/O emulsion toO/W emulsion was delayed to higher volume fraction of aqueous phase. Thus SNPs imparted somestability to W/O emulsions against coalescence and phase inversion.