Abstract
A new model is proposed for the viscosity of Pickering emulsions at low shear rates. The model takes into consideration the increase in the effective volume fraction of droplets, due to the presence of an interfacial layer of solid nanoparticles at the oil-water interface. The model also considers aggregation of droplets and eventual jamming of Pickering emulsion at high volume fraction of dispersed phase. According to the proposed model, the relative viscosity of a Pickering emulsion at low shear rates is dependent on three factors: contact angle, ratio of bare droplet radius to solid nanoparticle radius, and the volume fraction of bare droplets. For a given radius of nanoparticles, the relative viscosity of a Pickering emulsion increases with the decrease in bare droplet radius. For O/W Pickering emulsions, the relative viscosity decreases with the increase in contact angle. The W/O Pickering emulsion exhibits an opposite behavior in that the relative viscosity increases with the increase in contact angle. The proposed model describes the experimental viscosity data for Pickering emulsions reasonably well.
Highlights
The relative desorption energy needed to desorb a nanoparticle from the oil-water interface to the oil phase decreases with the increase in contact angle
The experimental studies that are reported in the literature on the rheology of Pickering emulsions often deal with complicated systems where the non-Newtonian behavior is caused by factors, such as: network formation of droplets and nanoparticles; network of films of matrix phase in highly concentrated emulsion gels; and incorporation of thickeners in the matrix phase
Pickering emulsions can be calculated from the Einstein viscosity equation: ηr = η/ηc = [1 + 2.5φS]
Summary
The relative desorption energy needed to desorb a nanoparticle from the oil-water interface to the oil phase decreases with the increase in contact angle. Several authors [29,30,31,32,33] have developed viscosity equations for non-dilute dispersed systems taking into account the packing limits of particles and droplets. Pal [30] has extended the Taylor emulsion viscosity equation for dilute systems to concentrated emulsions using the differential effective medium approach and taking into consideration the packing limits of droplets. He proposed the following equation for the viscosity of concentrated emulsions at low shear rates: ηr. This equation is found to adequately describe a large pool of available viscosity data on unstable (without interfacial additives) and surfactant-stabilized emulsions
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