Colloidal particles at fluid-fluid interfaces: Emulsion stability and multiphase flow

The objective of this work is to investigate the mechanisms by which colloidal particles at oil-water interfaces emulsify and stabilize emulsions. It had been well documented that finely divided insoluble solid particles that adhere to fluid-fluid interfaces are capable of effectively stabilizing emulsions. However, the mechanisms by which these particles stabilize emulsions were not fully understood.; Experimental work is presented to show that colloidal particles stabilize emulsions by providing steric hindrance to drop-drop coalescence and by modifying the rheological properties of the interfacial layers. It is clearly shown in this dissertation that steric hindrance is related to the energy required to displace particles laterally along the interface away from the region of contact between droplets. The effectiveness of colloidal particles to stabilize emulsions, therefore, depends on the formation of a sufficiently dense layer of particles at the oil-water interface, and on the corresponding rheological properties of the colloid-laden interface.; A model is presented to describe the rheological properties of fluid-fluid interfaces containing colloidal particles. It is shown that at sufficiently high concentrations of particles, fluid-fluid interfaces exhibit viscoelastic behavior. This viscoelasticity accounts in part for the ability of colloidal particles to stabilize emulsions.; A model is presented that simulates for the first time the drainage of thin liquid films bounded by viscoelastic interfaces. The model allows us to relate droplet coalescence times to the viscoelastic properties of the interface. Results show that film thinning depends strongly on the dilational elasticity and the dilational viscosity of the interfacial region.; A method has been developed to measure the dilational rheological properties of fluid-fluid interfaces. Results are presented to show that colloid/surfactant-laden oil-water interfaces are viscoelastic. The method has been used successfully to show that effective demulsifiers exhibit short relaxation times (high surface activity) and must decrease the dilational elasticity of oil-water interfacial layers significantly.