(406j) Microstructural Origins of Gel-like Rheology in Solid-Stabilized Emulsions

Pickering-Ramsden emulsions rely on solid particles to stabilize fluid-fluid interfaces and are commonly found in many applications including pharmaceuticals, petroleum refining and various consumer products. Significant to many current and emerging technological applications of Pickering-Ramsden emulsions are their resulting microstructures and rheological behavior, which can influence processing, functionality, and stability of end-use products. Depending on the processing and physicochemical conditions, these multiphase mixtures can exhibit rich rheological behavior including liquid-like viscoelasticity, creep deformation, gelation, and yielding. The purpose of this study is to investigate the microstructural mechanisms that lead to gelation in solid-stabilized emulsions, and how the emergent morphology governs the mechanical behavior of the resultant mixture. We utilize two fluid systems to create four solid-stabilized emulsion gels with differing internal microstructures. These include bicontinuous fluid domains separated by a jammed colloidal monolayer (bijels); tenuous, percolating droplet networks bridged together by colloidal particles (Pickering emulsion gels); regular, concentrated Pickering-Ramsden emulsions; and tightly packed arrangements of faceted solid-stabilized droplets (biliquid foams). The microstructures and mechanical properties of the mixtures are characterized by 3D confocal imaging and oscillatory rheology, and the correlations between the two are investigated. We observe nontrivial dependence of the zero-shear elastic modulus and yield stress on colloid volume fraction in these systems, including non-monotonic dependence and the emergence of a maximum strength in bridging systems. Our observations will be discussed in the context of the emulsion microstructure. In fact, access to these four classes of solid-stabilized emulsions with markedly different morphologies suitably positions us to investigate these questions in a systematic fashion. Our findings have important implications for a wide range of different industries that utilize mixtures of fine particles and immiscible fluids to create new food formulations, cosmetic products, and composite materials.