Chemical and Biomolecular Engineering - Theses
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ItemDroplet interactions in structured fluids and charged colloidal systems( 2016)Emulsion systems are widespread in many industries but a full understanding of how the bulk properties of emulsions are influenced by the structure and components within an emulsion is still incomplete. The focus of thesis is to investigate droplet interactions with a specific focus on investigating various surface forces with unique, poorly understood, or unknown characteristics. This was achieved through both AFM experiments to directly measure the forces between drops and through microfluidic platforms to observe the behaviour and collisions of drops during bulk flow. The thesis can be separated into three main sections. The first section contains an investigation into surfactant free high concentration salt solutions. Measurements were taken using atomic force microscopy (AFM) to investigate the interactions between drop pairs. The specific focus was on a previously observed pull off phenomenon that is not expected based on the current understanding of surface and intermolecular forces. AFM measurements between two drops were taken in solutions of 50 mM NaNO3, 500 mM NaNO3, and 50 mM NaClO4. The measurements were in agreement with previous findings that the magnitude of the pull off force is primarily determined by the contact time between drops but also demonstrated the possibility that additional factors such as maximum compressive force or a force limit for very slow pull offs may also be important. The next section presents research of high concentration surfactant systems and the influence of nanocolloid shape on structural forces. Measurements were taken using atomic force microscopy (AFM) to investigate the interactions between drop-drop and particle-plate systems. Solutions of sodium dodecyl sulphate (SDS) and sodium bromide (NaBr) as well as solutions of hexadecyltrimethylammonium bromide (C16TAB) and sodium salicylate (NaSal) were used to generate micelles of varying profiles. Although the SDS and NaBr micelles were too similar in shape and too different in solution ionic strength, changes in behaviour explicitly from differing micellar profiles of the CTAB and NaSal micelles were successfully demonstrated. It was observed that the surface force behaviour was not sensitive to small changes to the micelle aspect ratio, however, once the micelles were elongated further the long range forces changed from oscillatory to that of a single attractive force well. The final section includes an inquiry into the influence of surface forces on droplet behaviour within and upon exiting a microfluidic device. Many different arrangements were tested using a variety of components including hexadecyltrimethylammonium chloride (C16TAC), sodium dodecyl sulphate (SDS), and polyvinylpyrrolidone (PVP) with the oils tetradecane, bromodecane, and perfluorooctane. One arrangement investigated the influence of close range attractive forces between droplets on droplet behaviour and breakup when flowing into an external stream. It was found that the behaviour of the drops was influenced by not only the attractive forces but also longer range repulsive forces that may prevent drops entering an attractive region. Another arrangement probed the collision between droplets with different interfacial coatings; a phenomenon not able to be investigated using bulk solutions and techniques. Overall it was found that investigating surface forces with microfluidics allows for new insights into colloidal solutions and properties.