Effect of Particles on the Microstructure and Rheology of Solid-Stabilized Emulsions

Emulsions are mixtures of immiscible fluids, which typically exhibit a thermodynamically unstable droplets dispersed within a continuous body of fluid. Though surfactants have traditionally been used to achieve kinetically stable emulsions, colloids are increasingly being utilized. Today, solid-stabilized emulsions, or Pickering emulsions, are ubiquitous within the consumer market, often found in food formulations and many personal care products. However, to expand their utilization in many emerging technological applications, such as drug delivery, oil recovery and composite materials, a greater fundamental understanding of the role particles play in mediating the microstructure and mechanical behavior of Pickering emulsions is needed. To this end, I have studied a variety of solid-stabilized emulsions spanning a wide range of droplet concentrations. First, the microstructure of high internal phase emulsions is examined utilizing confocal microscopy. These concentrated systems are composed of highly faceted droplets separated by thin films of the continuous fluid. By varying the size of the stabilizing particles used to create a HIPE, it is revealed that larger particles provide greater stability against droplet coalescence, as they more readily adopt a bridged particle (particle which resides on two droplet interfaces) monolayer configuration within the thin films, in lieu of particle bilayers. Second, combining confocal microscopy with rheological measurements I have explored the connection between the microstructure and rheology of solid-stabilized emulsions, and the influence particles have on these properties. In concentrated emulsions, particle excluded volume is shown to strongly impact the microstructural transition from spherical to highly faceted packed droplets, which is coupled to variations in sample rigidity. Pickering emulsion gels are dilute emulsions that feature a tenuous network of faceted droplets, held together by bridged monolayers of particles. Here, particle loading governs the degree of particle bridging within the droplet network, yielding a non-monotonic dependence of the zero-shear elastic modulus. Lastly, I have begun counter-rotation shear experiments, monitored with confocal microscopy to allow direct observation of the impact particles have on droplets deformation and rupture under shear. My fundamental results provide greater insight into the effect particles have on the microstructure and rheology of Pickering emulsions at various compositions and different process conditions.