UBC Theses and Dissertations
Shear-triggered coalescence Mashayekhi, Alireza
Emulsions occur in various industries such as food, pharmaceutical, agricultural, and oil and gas. Regardless of the application, controlling emulsion stability is crucial for increasing customer satisfaction, reducing manufacturing costs, and preserving the environment. Previously, it has been shown that emulsion droplets coalesce under shear, which finally results in emulsion phase separation. However, the underlying mechanism that causes droplet coalescence under shear has not been explained thoroughly. In this thesis, we first employed a Cantilevered Capillary Force Apparatus (CCFA) to capture a pair of droplets and collide them in two configurations: head-on and shearing. The aim is to find an emulsion composition that enables us to trigger coalescence under the shearing droplet collision scenario. We used surfactants and a range of particles, spherical and Janus silica, plate-like kaolin, and rod-shaped cellulose nanocrystals (CNCs), to stabilize droplets against coalescence. We showed that for all surfactant types, silica, and kaolin, shear-triggered coalescence is not possible. However, particle rolling on the interface results in a slightly higher coalescence probability under shear for droplets stabilized by Janus silica particles. A significant increase in the coalescence probability (more than two times higher) under shear occurs for droplets stabilized by CNC particles, which is due to particle reorientation under shearing flow. Without any salt in the CNC suspension, the repulsive force between individual CNC particles prevents them from forming a tight network at the interface, resulting in an unstable emulsion. With 10 mM salt in the suspension, the CNC network at the interface is strong enough to prevent coalescence in the head-on collision, but under shear, particles are oriented in the flow direction, leaving some fresh interface for coalescence. Finally, we showed that the increase in the coalescence probability causes a faster phase separation in model emulsions. When CNCs with 10 mM NaCl were used to stabilize emulsions, full phase separation occurred within two days when the emulsion bottle is placed on a roller. However, bottles on the shelf underwent no phase separation even after 4 months.
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