Chemical and Biomolecular Engineering - Theses
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ItemMicroscopic inter-particle friction and shear rheology of particulate suspensions( 2017)The study of the shear rheology of strongly-flocculated particulate suspensions is important for a range of industrial applications, such as start-up, pipeline flow and slumping. Examples include the improvement of industrial processes, equipment design and product development, such as gels and polymers. It can be achieved by thorough understanding of suspension behaviour and constructing a rheological model to enable an accurate prediction of the behaviour. Above a critical solid concentration, known as the gel point, these suspensions are able to withstand an applied force and show a solid-like characteristic before they yield and flow. Previous studies modelled this behaviour as viscoplastic, which described the sharp transition of no-flow and flow behaviour at a critical point termed the yield stress. Many experimental studies however, contradicted this idea and showed the non-linear elasticity below yielding, time-dependent yield and rate-dependent yielding. Understanding these phenomena requires assimilation of the bulk rheology to the microscopic features of the suspensions. The work presented in this project aims to correlate the micro- and macro- features of suspension behaviour. Constant stress (creep), stepped stress and constant rate experiments were performed on two strongly particulate suspensions, alumina and calcite, at different solids volume fractions using controlled-stress and controlled-strain rheometers with a vane in a large cup technique. In addition, to better understand the stress transfer between particles in the suspension network during shear, a novel experimental Atomic Force Microscope technique for frictional study was developed. This technique allows one to measure friction force between two microsphere particles sliding over each other. The shear rheological data for alumina and calcite were obtained from creep (constant stress), stepped stress and constant rate experiments. The suspensions exhibited non-linear viscoelasticity prior to yielding, non-monotonic shear softening flow, and time- and rate-dependent yielding phenomena. The non-linear viscoelasticity prior to yielding was studied from the creep testing data. The instantaneous and steady-state moduli were extracted. They exhibited softening behaviour with increasing strain and modelled using a modified Cross model. This model indicated that the moduli were constant at very low strain and then eventually softens to a power-law. In addition, the effects of solids concentrations were investigated. Using power-law scaled with the gel point model, the increasing magnitude of the properties from the gel point was predicted. From the stepped stress and constant rate tests, the non-monotonic behaviour of both suspensions was investigated. The steady state stress value from the constant rate test and the data from the stepped stress tests at intermediate shear rate were used to construct the non-monotonic flow curves of both suspensions. The data were modelled using the modified Herschel-Bulkey model. The curves showed shear softening of solids network at lower rates and the increasing of viscous stress with higher shear rates. As the solids concentration increased, the curve shifted up, indicating the increase of solids network and viscous stress of the suspensions. The solids concentration dependency of the extracted properties was also studied. The data showed an exponential increase with higher solids concentrations. The time- and rate-dependent yielding phenomena were studied using the creep testing and constant rate data. From the creep test, the breakage time of suspension at different stress values can be extracted. The decreasing trend with higher stresses in breakage time suggests the time-dependent yielding of particulate suspensions. The rate-dependent yielding was observed from the peak stresses at various rates from the constant rate experiment. The peak stress data, modelled using the modified Herschel-Bulkey model, showed decreasing trend at lower rates and eventually increased with higher rates. The extracted properties were also found to be solids concentration dependence and could be predicted with an exponential function. The study of particle-particle interactions in this work focused on the friction force between two spherical micrometer-sized particles. A novel experimental setup for friction force measurements using Atomic Force Microscope (AFM) was developed. A particle probe attached on the cantilever was pushed against an immobilized particle on a microscope slide. The topography of the scanned particle was obtained while measuring the normal and lateral deflections of the cantilever. Given calibration constants, these deflections were converted to forces and analysed. The results show a non-linear friction behaviour and the breakdown of Amontons’ law, which suggests a linear correlation between normal and friction forces. The friction results implied that friction between two particles are more complicated than a constant sliding friction. This work extends our fundamental understanding of the relationship between particle-particle forces and shear rheology of particulate suspensions. The results from AFM study suggest a more complicated mechanism of friction between two microsphere particles. The non-linear behaviour of friction between two particles was one of the underlying causes of the shear rheological phenomena in particulate suspension.
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ItemFormation and morphological properties of nanobubbles and nanodroplets at solid-liquid interfaces( 2017)The surface nanobubble has great potential for novel uses such as floatation in the water treatment and coal preparation, reducing boundary slip in micro-and nano-fluidic systems and antifouling. Many approaches have been used in recent years to produce nanobubbles, such as surface perturbation, gas-trapping, electrochemical or photochemical methods. In the meanwhile, the most popular used way to produce nanobubbles is the solvent exchange protocol. Although many research groups have frequently applied solvent exchange to produce nanobubbles, the nucleation mechanism behind the solvent exchange is still unclear, and the exact conditions for the nanobubble formation remain empirical. Various factors influence the nanobubble nucleation, for example, the solvent temperature, the supersaturation level of the dissolved gases, the physical and chemical properties of the substrate, or the flow pattern during mixing. In the first part of this Thesis will report the effects of temperature and dissolved gases on the formation of surface nanobubbles by solvent exchange. The temperature of the substrate and the first solvent has significant effects on the nucleation probability and size of the nanobubbles produced by the solvent exchange procedure. The formation of ordered structures was observed at the interface of HOPG and water when the sample exposed to the ambient air. However, no ordered structures were found when the sample was put in pure nitrogen and argon atmosphere for a relatively long time. Nanodroplets have comparable properties of the nanobubbles, e.g. the pinned contact line, the low contact angle and the long-time life at the surface. The study of nanodroplets can shed light on the research of nanobubbles. The second part of Thesis will focus on the collective interactions among surface nanodroplets produced by solvent exchange and the effect of the self-assembled monolayer on the morphology of nanodroplet. We show that there is a surface coverage maximum of nanodroplets produced by solvent exchange, regardless of the flow rate, channel height or solution concentration. We also found that the number density and morphology of surface nanodroplets change with the detailed molecular structure of the surface.
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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.
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ItemInhibitors of amyloid fibril formation( 2013)Despite significant recent advances in medical technology, there is still no commercially available treatment for the amyloid diseases that were first observed by Nicolaus Fontanus in 1639. These amyloids are fibrillar aggregates of misfolded proteins that form plaques in various organs and are the hallmarks of a number of incurable diseases such as Alzheimer’s disease and Parkinson’s disease among others. Alzheimer’s disease is the most well-known amyloid disease and was first described by Alois Alzheimer in 1906. It is widely believed that the amyloids or their precursors are responsible for the tissue damage that eventually leads to these diseases, though there is debate in the literature regarding the poor relationship between amount of fibrils and disease progression. Nonetheless, there is a general consensus that the fibrils are related to the progression of these amyloid diseases. Several strategies to prevent or cure amyloid diseases include reducing the amount of amyloid beta (Aβ) produced and the removal of amyloid plaques. Inhibitors that prevent the formation of amyloid fibrils can prevent amyloid diseases from occurring. Hence, this thesis is concerned with the design and testing of amyloid inhibitors as possible therapeutics for these diseases. In order to achieve this objective, the key universal physical properties of amyloid fibrils involved in their self-assembly have been used to design a generic class of fibril inhibitors. A design of an amphiphilic polymer with a hydrophobic backbone and hydrophilic side chains was proposed as a generic amyloid inhibitor and several compounds with this design were obtained. 3 model amyloid forming proteins: bovine insulin (BI), hen egg white lysozyme (HEWL) and Aβ were used to study the effect of these compounds on amyloid fibril formation. Suitable amyloid-forming conditions for these proteins were identified and fibril formation was monitored using Thioflavin T (ThT) fluorescence and other techniques. A naturally occurring compound that fits the proposed inhibitor design was identified and found to be effective at inhibiting amyloid fibril formation in all 3 protein systems. Unusually large fibrils were formed when incubating BI and HEWL with this natural compound and this has potential nanotechnological applications as nanowire templates. To further test the proposed structure, synthetic polymers based on the proposed structure with different chemical groups were produced and one of them, FA-diacid (“FA” was a designation used by the polymer science group of the University of Melbourne for divinylcyclopentane polymers), showed promising inhibitory capabilities. The FA-diacid was improved with the addition of larger hydrophilic side chains to produce the more effective inhibitors named PNGA and PNGE. Finally, glycoproteins based on the structure of AGP were produced and tested using the 3 proteins and were found to have some inhibitory ability. However, they were not as effective as PNGA or PNGE. The results show that compounds with the proposed inhibitor structure can be effective as a generic amyloid inhibitor, but further modification of the current compounds is needed to improve the effectiveness of these compounds as drugs. Further development on this class of chemicals can lead to the production of a new class of generic amyloid inhibitor that can be used to prevent and halt the progression of presently incurable amyloid diseases such as Alzheimer’s disease.
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ItemThe mechanical characterisation of nanostructured biomaterials( 2013)Hydrogel materials have demonstrated unique potential for biomedical application in areas ranging from macroscopic tissue engineering scaffolds to targeted nanoparticles for in vivo therapeutic delivery. Such propensity for biological application is largely due to the inherently high degree of hydration and low rigidity of hydrogel networks, which, being similar to those of natural tissue, often lead to good biocompatibility. The mechanics and nanostructure of such materials have been reported to have a significant effect on biological processes; from cellular interaction and fenestration clearance, to capillary flow and dynamics during circulation. This thesis examines novel methods forthe characterisation of both nanostructured planar and particulate hydrogel systems, using atomic force microscopy (AFM) force spectroscopy techniques in physiological buffer. The mechanical properties and material parameters for soft nanostructured biomaterials (thiol-modified poly(methacrylic acid) and poly(L-glutamic acid)) in various architectures (planar film, core-shell particle, free-standing capsule, and nanoporous particle) are herein investigated. It was found that a wide variety of soft structures could be characterised mechanically, and the corresponding results interpreted according to established theories and models for compressive deformation. As such, evaluation of the Young’s modulus for the hydrogel systems investigated in this thesis demonstrate the crucial role that system architecture and network density play in the resilience of soft structures to applied force. Compressive forces which occur in the biological domain, such as for cellular internalisation and soft-tissue cell retention, were subsequently linked to conclusions drawn from AFM measurements. Such preliminary investigations showed that both intrinsic and extrinsic properties of nanostructured hydrogels influenced cellular interaction; thereby forming the basis for further mechanobiological studies, allowing for the future rational design of nanostructured hydrogel biomaterial systems for in vivo biomedical applications.
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ItemProtein stability in shear flow( 2010)Given that globular proteins show a strong conformation-function relationship, the stability of a native protein structure is essential for its function. Aberrant proteins, resulting from structural instabilities in native protein conformations, and consequent aggregation, evolve a gain-of-function pathogenesis which has serious implications in industry and medicine. Therefore, it is important to appreciate the key factors that perturb the solution conformation of protein systems leading to aggregation. Considering the fact that proteins generally function in solution form, and those solutions have an inherent tendency to flow, knowledge of the stability of protein solutions in shear flow is essential. This thesis employs a combination of spectroscopic and microscopic techniques to study the conformational dynamics and morphological transformations of bulk peptide/polypeptide solutions in both uniform and heterogeneous velocity gradients. Preliminary studies in this thesis demonstrate that protein denaturation and subsequent aggregation can be probed using intrinsic protein fluorescence. The induction of protein aggregation was found to be greatly enhanced in heterogeneous flow regimes. Studies in a well defined flow field, Couette flow, revealed that the hydrodynamic stress generated in such flow regimes induce the unfolding of the helical segments of natively folded insulin; a prerequisite for aggregation and amyloid fibril formation. Further analysis of the shear-effect on α-helical conformations was performed using the homopolypeptide poly-L-lysine as a model protein system. The results reveal that the shear-induced unfolding of α-helical segments depends on both the shear rate and the duration of its application. An assessment of the chain-length-dependence of this phenomenon revealed that, contrary to classical theory, the strain in a given flow field varies inversely with the chain-length of α-helical poly-L-lysine. Collectively, the results provide new insight into existing theories in polymer physics. More importantly, it provides quantitative information on the conformational dynamics of peptide/polypeptide solutions in shear flow. This report is relevant to quality control measures during the commercial isolation and purification of protein products, and might help explain the role of shear stress, originating from pulsatile blood flow, in protein misfolding diseases and vascular disorders.