Role of particles in the rheology of solid-stabilized high internal phase emulsions

Hypothesis: The presence of colloidal particles on fluid interfaces can have a significant impact on the rheology of solid-stabilized high internal phase emulsions (HIPEs). Experiments: Using dynamic oscillatory rheometry and confocal microscopy, we investigate a broad array of solid-stabilized HIPEs formulated along four different compositional trajectories in their ternary state diagram, using particles of three different sizes and two different surface chemistries. Findings: We unveil three important consequences of the use of particles, in lieu of surfactants, on the rheology of HIPEs. First, particle excluded volume interactions take a pronounced role in the transition to solid -like rheology due to crowding. An effective dispersed phase volume fraction, taking into account the particle three-phase contact angle, must be defined to account for the dependence of the mixture's rheology on its composition. Second, weak, chemistry-dependent attractive colloidal interactions through the continuous phase result in a finite elastic modulus at low effective dispersed phase volume fractions. Third, we observe a secondary rise in the mixture's elastic modulus at increasingly high dispersed-to-continuous-phase volumetric ratios. We postulate these interactions stem from attractive lateral capillary interactions between the particles, due to thinning of the continuous fluid film between faceted droplets. (C) 2019 Elsevier Inc. All rights reserved.