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.
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ItemInterfacial tension study and rheological characterisation of water-in-oil emulsion explosives( 2007)Highly concentrated emulsion explosives are complex colloidal systems that remain robust for very many years after production. In this work a poly-isobutyl succinic anhydride (PIBSA) derived emulsions have been studied to understand the extraordinary nature of the stability and flow of these emulsions. In this study a model sodium oleate emulsion system is developed to further confirmation of the nature emulsion explosives. Two PIBSA based emulsifiers were used to prepare the emulsions. The main emulsifier of interest was comprised of an amide, ester and imide head group attached to a PIBSA chain, the second was entirely comprised of an imide head group attached to a PIBSA chain. The pendant drop method was used to obtain dynamic interfacial tension measurements for an interface between of paraffin oil/PIBSA-imide and ammonium nitrate, and dodecane/sodium oleate and sodium nitrate. The IsoFit-WardTordai computer package that incorporated Langmuir, Frumkin, Reorientation and Aggregation adsorption isotherms was used to calculate diffusion coefficients for total time adsorption. Diffusion coefficients for short time adsorption with Langmuir and Frumkin isotherms were determined and long time adsorption with the Langmuir isotherm were calculated. All of the diffusion coefficients results suggested that direct diffusion controlled adsorption was not the mechanism these surface active agents employed, rather, the results suggested that an activation barrier controlled adsorption process dominated the kinetics of the interfacial adsorption. There is much evidence to suggest that rheological properties of highly concentrated emulsions have not been characterised correctly for some time. This is mainly due to wall depletion or wall slip effect phenomenon inherent to rheological investigations performed on smooth stainless steel surfaces. The effect of wall slip was eliminated through physical means by performing rheological investigations with a sand blasted cone and plate, and the use of the vane geometry as a rheomter. Another method to eliminate wall slip effects in steady shear for highly concentrated emulsions was formulated and established in this investigation. A two-stage Tikhonov regularisation procedure that converts steady shear data into rheological property functions has been developed. The two-stage method is able to obtain not only the shear-stress shear-rate function but also the slip velocity as a function of wall shear- stress. The method is such that it obtains the rheological functions over the maximum range of shear-rate covered by the data. The results obtained using the new method were compared to those obtained using the sand blasted cone and plate and vane geometry with good agreement being observed. Over a 17 month period, a PIBSA based emulsion was observed in order to determine if any dramatic rheological changes were taking place with time, the results obtained in both shear and dynamic analysis effectively concluded that the PIBSA emulsion explosive was extremely stable and the minor changes observed were within experimental uncertainty of the measurements. Various microscopy techniques were used to determine the droplet structure and size distribution within various emulsions. Optical microscopy, Qualitative Phase Imaging, Differential Interference Contrast imaging were employed, and it was found that the PIBSA and PIBSA-imide emulsions had a spherical shape rather than a hexagonal close packed structure that was previously assumed for these emulsion explosives. Images of the sodium oleate emulsion indicated that there were obvious destabilising mechanisms at work, affecting the longevity of the emulsion.