Droplet-Based Approaches to Probe Complex Behavior in Colloidal Fluids with High Composition Resolution
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In this work, microfluidic and millifluidic droplets are utilized to study and control complex fluid behavior with high composition resolution. Different techniques are used on two length scales to create unique approaches towards the same goal of merging droplet-based experiments with classical colloidal characterization experiments. First, a microfluidic dehydrating droplet device is characterized and a procedure established by concentrating a phase separating organic-inorganic system on chip and using geometric calculations to determine composition. The device is then expanded to a more complex, particle-polymer system to investigate suspension stability and interparticle behavior. A model system containing silica particles and PEO polymer is found to transition from a bridging flocculation mechanism to polymer-coated particle jamming based on the mass ratio of polymer to particle. Lastly, a phase separating particle-polymer system consisting of polystyrene particles and hydroxyethyl cellulose is concentrated on-chip. Interparticle interactions are controlled by varying particle size, polymer size, and polymer type and the effects on phase behavior are examined. Droplet experiments are scaled-up to millifluidic droplets and concentration gradients are used to produce high composition resolution in place of time, used in the dehydrating microfluidic experiments. A novel, millifluidic containment device is created to study aggregation and sedimentation in droplets containing carbon black and OLOA surfactant suspended in dodecane. A slow increase in stabilization behavior is observed as opposed to the previously observed sharp “on-off” effect. The droplet production technique is then improved to achieve more complex composition paths and the device is expanded for a small angle neutron scattering (SANS) application. SANS is performed on flowing droplets with varying concentration to map interparticle interactions and phase behavior of complex particulate systems. Feasibility of device is demonstrated and preliminary model systems of silica particles and polymer, salt, and surfactant are analyzed and characterized.