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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ItemDepletion and structural force interactions of bubbles in aqueous polyelectrolyte solutions( 2016)The examination of several different experimental systems presented in this thesis work are linked through their singular aim of investigating the interactions of deformable interfaces, in this case air bubbles, in aqueous polyelectrolyte solutions. Direct force measurements were used to probe depletion and structural force interactions between colliding air bubbles at equilibrium timeframes. In addition, air bubbles were generated without the presence of interface stabilising molecules within a block-and-break microfluidic device to probe the interactions from generation to equilibrium. Direct force measurements were conducted between interacting bubble pairs in aqueous solutions of polyvinylpyrrolidone (PVP), a neutrally charged polymer, and sodium poly(styrene sulfonate) (NaPSS), a negatively charged polyelectrolyte. The selection of these molecules allowed a comparison between neutral and charged polymers and their influence on the measured force interactions. It was found that for measurements conducted in the presence of PVP, neither a structural force nor a depletion interaction was able to be measured. This was largely due to an increase in solution viscosity with increasing concentration of PVP, which results in an increase in the hydrodynamic fluid flow that subsequently overwhelms any potential presence of a depletion interaction. Also, the polydispersity of the molar mass of the PVP would appear to be responsible for the non-observance of structural forces in this system. This conclusion is based on the results of force measurements conducted in aqueous solutions containing monodisperse NaPSS. Uncharged polymers have also been shown to have a lower osmotic pressure when compared to a charged polyelectrolyte, which decreases the magnitude of the depletion interaction. The measurements conducted in the presence of NaPSS with the deformable interface of bubbles were shown to be more sensitive to the presence of the polyelectrolytes when compared to similar measurements using rigid interfaces. The study involved the use of both polydisperse and monodisperse molar mass distributions and the experimental factors that were examined were NaPSS concentration, bubble collision velocity and polyelectrolyte molar mass. Structural forces were measured with the use of a monodisperse sample but only a depletion interaction when the polydisperse molar mass distribution was present. This demonstrates that polydispersity in molar mass results in the structural forces to be smoothed. The dispersity of the various molar mass distributions was manipulated to further investigate the role they play on the observed interactions. The polydisperse samples were dialysed, which removed the low molar mass molecules and the monodisperse samples were blended to create a bidisperse mixture. When the dispersity was decreased through dialysis, structural forces were observed and the bidisperse mixture only allowed the presence of a depletion interaction to be measured. These measurements further highlight the role that molar mass dispersity plays on the observed interactions and shows how these interactions can be manipulated. These measurements were then compared with an analytical model based on polyelectrolyte scaling theory (depletion interaction) or an empirical model (structural forces) in order to explain the effects of concentration and bubble deformation on the interaction between bubbles. The modelling highlighted that these interactions can be accounted for by polyelectrolyte scaling theory taking into account the structural properties of the polymer in solution in the dilute and semi-dilute polymer concentration regimes. It was also shown in all measurements that depletion and structural forces were overwhelmed by hydrodynamic fluid flow at increase bubble collision velocities. Bubbles were generated for the first time within a block-and-break microfluidic device. The original design allowed water droplets in oil to be generated and design changes were required to ensure air bubbles could be formed. Block-and-break devices offer the advantage, that the generated bubble sizes are flowrate independent instead of other design types where the bubble size varies with flowrate. Air bubbles were able to be generated in solutions of pure water, NaPSS and sodium dodecyl sulfate (SDS) and their size was shown to be flowrate independent. The microfluidic device designs were modified to allow staged amphiphilic addition of SDS and NaPSS after the bubble was generated in pure water and this has the potential to allow complexity to be increased stepwise throughout an experiment. The combination of both direct force measurements and microfluidic studies allowed bubbles in the presence of aqueous solutions of NaPSS to be studied from generation through to their equilibrium form.