Chemical and Biomolecular Engineering - Theses

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    Hierarchical structure function relationships in biopolymer systems
    Homer, Stephen Henry ( 2023-03)
    Biopolymer networks are assemblies of biopolymer chains that form into a coherent self-supporting structure and on the macro-scale appear as gels. These biopolymers often assemble in a very specific manner exhibiting various structures at different length scales. Thus, a structural hierarchy exists. This work investigates whey protein isolate as a model biopolymer system to establish the relationship between the 3D microstructure and the mechanics of the network. The molecular, aggregate, and micro through to macroscopic assemblies of these materials leads to their complex physical and rheological properties. The structure at multiple length scales has been examined using a range of techniques including but not limited to x-ray scattering, circular dichroism, and microscopy. The rheological properties of the gels and aggregate suspensions resulting from preparation methods incorporating heating, pH adjustment and shear is reported. Particular attention focuses on the application of shear forces during gelation and the effects on the microstructure, aggregation behaviour, particle sizes, and rheological properties of the resulting protein suspensions and gels produced during heating has been investigated. Models have been proposed to explain the results. A key finding from this work relates to the role of effective concentration, resulting from an interplay between nominal concentration and pH, in determining the outcome during biopolymer microparticulation. The use of small-scale perturbations to augment gel rheology was also examined with effects such as strain hardening being introduced into otherwise non-strain hardening gels. This research brings new insights to structural design principles and opens avenues to control the mechanics of gelled systems and the sizes of aggregates resulting from microparticulation.
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    The behaviour of almond proteins in purified, minimally processed and complex food systems
    Devnani, Bhanu ( 2021)
    A growth in consumer preferences for plant-based diets has triggered the utilisation of plant ingredients in a variety of food and beverage applications, including substitutes to dairy products. The majority of commercial plant-based dairy alternative products, however, have reduced protein content compared to dairy products. Such protein plays an important role in dairy products, including yoghurt and cheese, where it imparts functionality, such as gelation and texture. These differences in protein content and properties in plant-based systems, compared to dairy products, may be responsible for the commonly encountered technological challenges, such as the replication of the texture and mouthfeel. With an attempt to fill these gaps, this thesis aimed to explore the potential of almond as an ingredient to develop novel high protein products. First, commercial almond-based yoghurt alternatives were studied to understand the role of almond proteins in these complex food systems and establish the difference between almond and dairy systems. Next, almond proteins were derived in the form of minimally processed extracts or purified isolates to develop almond milk and almond protein isolate. The effect of processing parameters, including temperature and pH, on the fundamental and functional properties of these protein extracts was then studied, with the objective of gaining deeper mechanistic insights into almond protein gelation. First, a range of objective instrumental techniques were used to assess whether commercial almond-based yoghurt alternatives behaved differently to soy and dairy yoghurts. All almond-based yoghurt alternatives contained added stabilisers and were lower in protein compared to dairy yoghurt (less than 2.7 wt percent in almond vs 5 wt percent in dairy). The interconnected protein network, which is known to structure dairy yoghurt, was absent in almond based yoghurts. Instead, these systems appeared to be flocculated and contained swollen starch granules, protein and fat particles/ aggregates. As a result, the almond yoghurts had lower colloidal and structural stability, in comparison to dairy yoghurt and also differed in their rheological and tribological properties. This study highlights the areas that require attention to further optimise almond yoghurts if product developers wish to mimic the properties of dairy yoghurt. Second, the effect of thermal treatment (45–95 degree Celsius for 30 min) on the structure of almond proteins extracted in minimally processed almond milk was assessed, as the unfolding and association of these proteins in response to heat, and its impact on colloidal stability and gelation of almond milk was not well understood. This temperature range was chosen based on the wide range of denaturation temperatures reported previously for almond proteins (45–115 degree Celsius). Above 55 degree Celsius, protein surface hydrophobicity and particle size increased, while alpha helical structure decreased, reducing the stability of skim or full fat almond milk. Fractal protein clusters were observed at 65–75 degree Celsius and weakly flocculated gels with a continuous protein network occurred at 85–95 degree Celsius, resulting in gels with high water holding capacity (approximately 70 percent) and a strength similar to dairy gels at similar protein concentrations of approximately 4 wt percent. The presence of almond fat increased the gel strength measured but led to a more heterogenous microstructure. The elasticity of almond gels could also be increased approximately 25 times with a threefold increase in protein concentration (i.e. from 3.6 percent in skim almond milk to 10.8 percent post concentration). This study provided a better understanding of the heat sensitivity of almond milk proteins and revealed temperatures that are critical to almond milk processing for a variety of applications, including heat treatment to induce denaturation prior to almond yoghurt formation or heat-induced gelation to form products like almond tofu/cheese. Third, the behaviour of purified almond protein isolate, containing predominantly amandin protein, was examined under conditions of neutral and acidic pH (pH 7 and 4). The isolate was highly soluble (70-80 percent) at either pH. An increase in acidity led to protein unfolding, an increase in random coil structure and the appearance of lower molecular weight proteins, potentially due to acidic and/or proteolytic hydrolysis. These structural changes at pH 4 increased the capacity for foam formation and foam stability, increased viscosity and led to concentration and age dependent thickening. Gels, similar in strength but with distinct microstructures and properties, were obtained following heating. At pH 7, a particulate type gel with an interconnected protein network was formed, while the gel produced at pH 4 had a dense continuous protein matrix. The gels differed in their susceptibility to chemical disruption, suggesting different underlying molecular interactions. This study illustrates the potential utility of almond proteins in foaming, thickening and gel formation and how almond preparations can be tuned by varying pH and temperature to obtain a range of tailored products with desired properties. Overall, these studies have increased our understanding of the response of almond proteins to process variables that are essential for both product and process development, performance and stability. Interconnected protein-based gel networks could be developed using either minimally processed or purified almond protein systems where the protein concentrations were equal to or greater than 4 wt percent. The formation of such gels and their pH tuneable properties may assist the formulation of novel almond based gelled vegan products, that are at par with dairy both in terms of protein content, functional performance and consumer acceptability.
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    Quantification of wastewater dewaterability for understanding sludge dewatering and fouling during membrane bioreactor operation
    Skinner, Samuel ( 2018)
    This thesis investigated the dewatering properties of wastewater treatment sludges, and biofouling layers in membrane bioreactors. An experimental and data analysis methodology was developed that unified existing procedures for lab-scale filtration, centrifugation and gravity settling tests. This unified dewaterability characterisation methodology was used to quantitatively compare fifteen wastewater sludge samples. The comparison highlighted a correlation between lower volatile suspended solids and improved filterability. Further modelling of the extreme compressibility of biofouling layers has provided insights into optimisation of water recycling.
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    Microscopic inter-particle friction and shear rheology of particulate suspensions
    Kusuma, Tiara Enggar ( 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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    Characterisation of buffalo milk, yoghurt and cheese
    Nguyen, Hanh Thi Hong ( 2014)
    Buffalo provide the second largest source of milk in the world after bovine animals. Knowledge and past research on buffalo milk and its product properties, however, is limited, particularly research relevant to Australian manufacturing conditions. This thesis asked four key questions: whether the chemical composition and physicochemical properties of buffalo milk, particularly the microstructure and proteomics of the milk fat globule membrane, significantly differ to bovine milk; whether these differences in milk are translated into the differences in the properties of the resulting yoghurt and cheese products; how to improve the quality of buffalo products; and how the properties of the buffalo cheese change over storage time and vary when produced by different producers. Techniques employed in this thesis included: confocal laser scanning microscopy (CLSM) and cryo-scanning electron microscopy (cryo-SEM) for microstructural investigation, controlled-strain and controlled-stress rheometers for rheological examination, sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and liquid chromatography combined with tandem mass spectrometry (LC-MS/MS) for mass and protein identification. Other techniques such as the inductively coupled plasma atomic emission spectroscopy (ICP-OES), high performance liquid chromatography (HPLC), light scattering and colorimetric methods were also used for the characterisation of chemical composition and physicochemical properties of buffalo milk and buffalo milk products under conditions relevant to an Australian manufacturing setting. This work showed that Australian buffalo milk had a richer composition than Australian bovine milk, including a higher concentration of fat, protein, total solids and calcium, consistent with other international studies. High performance liquid chromatography analysis showed that the organic acid profile was significantly different between the two milk types, with a lower concentration of orotic and uric acids in buffalo milk of particular note. Buffalo milk had larger fat globules with a broader size distribution. Confocal laser scanning microscopy observation showed a heterogeneous distribution of phospholipids with the occurrence of non-fluorescent domains that were hypothesized to be rich in sphingomyelin. These domains had various sizes and shapes at room temperature. The domains became larger and more irregular at 4oC and were smaller and more circular at 40oC or 60oC. Using a combination of both in gel digestion and in solution digestion methods, followed by liquid chromatography combined with tandem mass spectrometry (LC-MS/MS) analysis, 184 proteins were identified within the buffalo milk fat globule membrane. This is the largest profile of buffalo milk fat globule membrane proteomics to date. Further quantitative comparisons revealed that the buffalo milk fat globule membrane contained more xanthine dehydrogenase, platelet glycoprotein 4, heat shock cognate and cacineurin B homologous protein but less lactadherin and polymeric immunoglobulin receptor than the bovine counterpart. These differences in the properties of the two milk types affect the processing required during manufacturing and properties of the resulting products. The production of buffalo yoghurt does not require the addition of milk powder or thickener, as occurs for bovine yoghurt, due to the initial higher total solids content of buffalo milk. Yoghurt produced from buffalo milk, however, exhibited a significantly higher level of syneresis (17-20% w/w vs. 1-3% w/w) and poorer rheological properties compared to bovine yoghurt. Buffalo yoghurt was more susceptible to deformation and less able to recover the original network structure after deformation. These properties could be linked to the porous microstructure consisting of large fat globules that tended to disrupt the protein network. An optimisation of the process parameters was therefore performed to improve the quality of buffalo yoghurt, especially to reduce the syneresis. This optimisation initially considered the effect of varying fermentation temperature. Buffalo yoghurt was fermented at three different temperatures: 37oC, 40oC and 43oC. Buffalo yoghurt fermented at 37oC or 40oC required a longer fermentation and gelation time than at 43oC but exhibited a less porous microstructure with reduced syneresis (from 17-20% w/w to 14-16% w/w). The storage modulus was higher at lower temperatures but other rheological properties including the thixotropy, flow behaviour index and consistent coefficient were not improved by decreasing the fermentation temperature. The limited improvement in syneresis and rheological properties of buffalo yoghurt at different fermentation temperatures indicated that further optimisation of buffalo yoghurt production was required. Homogenisation has been reported to improve the quality of bovine yoghurt, including syneresis, texture and rheological properties. The effect of homogenisation on the properties of buffalo yoghurt, however, has not been explored systematically, despite the significantly higher fat content and larger fat globules in buffalo milk. In this experiment, buffalo yoghurt was produced from either milk homogenised at 80 bar or 160 bar. It was shown that homogenised buffalo yoghurt exhibited an improved microstructure consisting of a highly interconnected protein network with thick protein strands and small embedded fat globules. These structural changes resulted in a significant decrease in syneresis and thixotropy and led to a considerable increase in the storage modulus, gel firmness and flow behavior index. While both homogenisation pressures were effective, a higher homogenisation pressure of 160 bar resulted in a lower gel firmness and storage modulus, possibly due to the presence of bigger fat-protein clusters within the milk. These results suggest that a homogenisation pressure of 80 bar could be optimal for improving the quality of buffalo yoghurt and reducing syneresis. Traditional or high moisture buffalo Mozzarella cheese has long been produced but most of the studies in the literature to date have focused on low moisture bovine Mozzarella cheese. In this project, the microstructure and functional properties of laboratory prepared and commercially purchased high moisture buffalo Mozzarella cheeses were studied and compared to commercially purchased high moisture bovine Mozzarella cheeses. Laboratory cheeses were produced at an average yield of approximately 19% and the quality was stable during seven days at cold storage. The whey collected during buffalo cheese production was rich in calcium, lactose and contained an unidentified trisaccharide. Buffalo and bovine Mozzarella cheeses obtained from different producers were found to be significantly different in their chemical composition, organic acid profile and microstructure but had similar hardness and meltability. The buffalo cheeses exhibited a significantly higher ratio of fat/protein and larger fat patches with a less dense protein network within the microstructure compared to the bovine cheeses. These results reflect the effects of processing conditions and the milk types employed by different producers on the resulting cheese properties. They also demonstrate the potential application of buffalo cheese whey as a good source of prebiotics, sugars and minerals. These differences in the microstructure and chemical composition could also be used to identify the milk species of origin in commercial cheese products. In summary, buffalo milk exhibited significant differences from bovine milk, which in turn affected the properties of yoghurt and cheese. Buffalo yoghurt, prepared using the current industrial standard approach, exhibited a high degree of syneresis, a porous microstructure and poorer rheological properties than bovine yoghurt. Lowering the fermentation temperature and the utilisation of homogenisation lowered the syneresis and improved the microstructure and rheological properties of buffalo yoghurt. Buffalo Mozzarella cheese showed large variations and significant differences in microstructure and physicochemical properties compared to bovine Mozzarella cheeses. These results answer the four key questions posed in this thesis. The results presented are useful for buffalo farmers and manufacturers seeking to better understand and control buffalo milk quality and the properties of milk products, as well as the broader community of dairy researchers.
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    Effects of shear on the stability and the release properties of water-in-oil-in-water (W/O/W) emulsions
    Goh, Agustina ( 2014)
    Water-oil-water (W/O/W) emulsions are of great interest with many potential applications in different fields such as pharmaceutical products, cosmetics and food. W/O/W emulsions offer the possibility of reduction of the fat content, encapsulation of active species (e.g., vitamins, minerals) and controlled release of those active species. However, the inherent thermodynamic instability of W/O/W emulsions restricts their applications. Furthermore, emulsions are subjected to environmental stresses such as shear forces during their fabrication and use. An excessive shear force may lead to a breakdown of the primary emulsion, causing a decrease in the encapsulation efficiency of the W/O/W emulsion. The objective of this study was to investigate the shear stability of W/O/W emulsions as a function of parameters such as interfacial properties, viscosity and phase volume fractions. Mechanisms controlling the release of the internal aqueous phase during shear were identified and direct methods to measure the release of the internal aqueous phase were developed. W/O/W emulsions are fabricated with a two-step homogenisation method followed by an osmotic swelling process, i.e., a process of water transport from the external aqueous phase to the internal aqueous phase that swells the internal aqueous droplets. This method allows the fabrication of W/O/W emulsions containing different phase volume fractions while maintaining a low and constant amount of oil phase. In this research, the W/O/W emulsions were prepared using sunflower oil (polyglycerol polyricinoleate [PGPR]) as the emulsifier for the primary water-oil (W/O) emulsions and gum arabic as the emulsifier for the secondary W/O/W emulsions. In order to obtain different rheological properties of the W/O/W emulsions, different thickeners – xanthan gum, locust bean gum and hydroxyethyl cellulose (HEC) – were added into the external aqueous phase. Initially, experiments were carried out to determine the formulation space for the primary W/O emulsions. Stable primary W/O emulsions can be produced using as low as 1.00 wt% PGPR when D-glucose is contained in the internal aqueous phase. The presence of D-glucose decreases the chemical potential of water efficiently and thus induces an osmotic pressure opposing the Laplace pressure. As a result, it reduces the rate of Ostwald ripening in W/O emulsions. These simple emulsions were then processed to fabricate W/O/W emulsions. The concentration of PGPR had a significant effect on the stability of the W/O/W emulsions. As the concentration of PGPR was increased from 1.00 to 3.00 wt%, the overall zero viscosity of the W/O/W emulsions also increased. This indicates that zero or insignificant release of the internal aqueous phase during the fabrication process. In addition to its stabilising role at the inner (W/O) interface, PGPR was able to move to the second interface and was co-adsorbed with gum arabic to form interfacial complexes to provide an enhanced viscoelastic barrier against droplet coalescence and the loss of encapsulated materials from the internal aqueous phase. The shear stability of the W/O/W emulsions was tested by subjecting them to shear stress sweep using a stress-controlled rheometer. The shear stability was investigated, first as a function of globule volume fractions. Initially, the shear experiments were conducted on W/O/W emulsions containing 3 wt% PGPR and xanthan gum (XG) as a thickener in the external aqueous phase. No release of the internal aqueous phase or breakup of the globules was observed for all systems. The enhanced stability of the W/O/W emulsions against shear was attributed to a sufficient amount of PGPR to stabilise both the inner and the outer interfaces, thereby increasing the stability of the globules against deformation and breakup during the shear treatment. The shear thinning behaviour of the XG may contribute to the shear stability of W/O/W emulsions. As shear force is applied, the viscosity of the W/O/W emulsions containing XG decreases drastically. This leads to an insufficient shear stress to overcome the Laplace pressure of the globules, thus making it difficult to deform and break up the globules. In order to further study the shear stability of the W/O/W emulsions as a function of globule volume fractions and to investigate the mechanisms leading to the globule breakup and release of the internal aqueous phase, the thickener XG was replaced by locust bean gum (LBG) and the concentration of the PGPR was decreased from 3.00 to 1.00 wt%. Results from the shear experiment indicated that the globules did not undergo fragmentation; however, the release of the internal aqueous phase was observed. This indicates that the release of the internal aqueous phase occurs mainly through coalescence, which is due to the thinning of the thin liquid films separating the internal aqueous droplets and the globules’ surface, which eventually ruptures. The results also indicate that globule volume fraction, ϕg, does not significantly influence the release of the internal aqueous phase when the internal droplet volume fraction is constant. However, the release of the internal aqueous phase increases as a function of the internal droplet volume fraction, ϕi, and the size of the internal aqueous droplets. In the final set of experiments, the shear stability of the W/O/W emulsions was tested as a function of the internal aqueous droplet volume fractions. The role of globule breakup on the release of the internal aqueous phase was also studied. Hydroxyethyl cellulose (HEC) was used to replace LBG as the thickener due to solubility issues associate with LBG, especially at a high concentration. The presence of the internal aqueous droplets was shown to alter the rheological properties of the globules. As the internal droplet volume fraction increases, the viscoelastic properties of the globule also increase. As a result, higher shear stress is required to deform and fragment the globules. The shear experiments on the W/O/W emulsions as a function of the internal droplet volume fraction were initially conducted at low thickener concentration. The results showed that the viscous stress was insufficient to fragment the globules. However, the release of the internal aqueous phase was detected for the most concentrated globules, showing that breakup is not a necessary condition for release to occur under shear. In the second part of the study, the concentration of the thickener was increased to induce globule fragmentation. The results showed that the extent of fragmentation decreases with an increase in the internal droplet volume fraction; however, the extent of release was significantly higher for emulsions with an initial droplet volume fraction above the random close packing fraction. It should be noted that the extent of release is still higher than that of the W/O/W emulsions whose globules did not undergo globule breakup. Therefore, it can be deduced that although globule breakup may lead to the release of the internal aqueous phase, the effect is less significant than the release of the internal aqueous phase through coalescence. Thus, depending on the application, the behaviour of W/O/W emulsions under shear may be controlled by varying the formulation parameters of the concentration of emulsifiers, the viscosity and the phase volume fractions of the W/O/W emulsions.
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    Interfacial tension study and rheological characterisation of water-in-oil emulsion explosives
    ZAHIROVIC, SABINA ( 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.
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    A new paradigm in near-net-shape advanced ceramic components processing: development of the novel processing technique
    TANURDJAJA, STEPHEN ( 2011)
    There are two general ways of processing ceramic materials namely dry and colloidal (wet) processing technique. The colloidal processing route has been widely studied and known to produce better ceramic articles. However, the process of removing the solvents through drying by heating them at high temperature is highly energy intensive (especially if the solvent is water). This thesis will discuss an alternative route using high vapour pressure organic solvents to speed up the solvent removal process. This route involves the reduction in pressure (by applying vacuum) to promote drying as oppose to heating. The surface of alumina which is inherently hydrophilic was made hydrophobic through alkylation reaction with alcohols (chem-adsorbed suspensions) or addition of polymers (phys-adsorbed suspensions) such as Hypermer A70 or Solsperse 3000. The purpose of manipulating the surface chemistry was to enable the dispersion of alumina in the organic solvents such as dodecane, cyclohexane or pentane. The viscosity of suspensions was found to decrease with the increase in the length of the stabilising molecules. This was consistent with the calculated reduction in the inter-particles attraction as the extent of the stabilising molecules increases. A high solid loading of alumina (50%vol) in dodecane using Hypermer A70 (3%wt) was achieved while maintaining a low viscosity (0.25 Pa.s at 100s-1). The observed rheological behaviour fitted the Quemada viscosity model quite well when the effective volume of the polymer was accounted for, which indicated the absence of depletion attraction due to the unadsorbed polymer. The optimum concentration of Hypermer A70 was determined using UV-Visible Spectroscopy technique and was found to be 2.8%wt. The vacuum moulding process produced an interesting and intricate formation of macroscopic voids or channels instead of cracks inside the green body upon the removal of pentane. The fraction of the void space was found to decrease linearly with the increase in the initial solids concentration. By extrapolating this relationship, it was shown that the void space should disappear when the initial solid concentration of the suspension was 62.3% vol. Density measurements performed on the denser sections of the green body showed that the density values were very close to the maximum close packing of spheres (on average 60.7% of the theoretical density). This result is also consistent with the data from the air-driven filtration where the volume fraction of the cake (in essence, the green body) was found to be 64.1%, when the suspension was consolidated by the maximum capillary pressure. The close proximity of these values suggests that they are self-consistent and the dense part is consolidated by the capillary pressure at the same time as the formation of channels. The formation of these voids or channels was thought to be a drying-rate dependent process. It was possible to achieve the green and sintered densities of up to 65.7% and 97.1% theoretical density, respectively, when the rate of removal was slowed down.
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    Characterisation of xanthan based, polymer solutions, physical gels and permanent networks
    Rodd, Andrew B. ( 2001)
    Gels and the gel transition are topics that have been the subject of extensive and widespread academic and industrial interest. Polymer gels and particularly those involving biopolymers are extensively applied in the food, pharmaceutical, agricultural, photographic, oil recovery and paper industries. Such widespread commercial application is responsible for the academic and industrial interest in gaining a greater understanding of the intrinsic physics governing the behaviour of these systems. Rheological analysis of biopolymer based gelling systems is an invaluable tool for investigating fundamental properties as well as replicating processing and application conditions. Through combination of careful rheological analysis with techniques capable of probing molecular structure and dynamics, such as static and dynamic light scattering (LS), it is possible to develop structure-function relations that are considered critical in understanding and controlling gels and gelation of biopolymer systems. This thesis utilises the biopolymer xanthan gum, to investigate both rheologically and optically, polymer solutions, physical gels and permanent networks. A physical gel is one in which the interactions between molecules, responsible for the gel like properties, are not permanent, that is they have a finite timescale. Subsequently, on long enough time scales, a physical gel will flow. Alternatively, a permanent network, as the name suggests, is one in which the interactions (or crosslinks) are thermodynamically stable, and the system will therefore never flow. Aqueous xanthan solutions in the presence of mono or divalent cations will produce a solution with 'weak-gel' physical properties. A 'weak-gel' is a term commonly applied to structured fluids that on short time-scales possess properties allowing them to appear more 'gel like', however on longer timescales, they will flow. Conversely, on the addition of trivalent metal ions (and for the purposes of this work, aluminum ions Al(III)) , xanthan will form a strong thermally stable permanent network. Using rheological and light scattering (LS) techniques, this thesis, will consider the similarities and differences of xanthan based physical gels and permanent networks. (From Abstract)
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    Rheological characterisation of nickel laterite slurry in processing environments
    Fisher, Daniel Thomas ( 2006)
    With China’s continuing economic boom, the demand for nickel has seen unprecedented growth over the past 10 years. Most of the world’s nickel is present in nickel laterite deposits. These high volume, low grade deposits are now being exploited and processed. An understanding of nickel laterite rheology and the ability to obtain meaningful rheological data is essential to process intensification and stability. The properties and physical characteristics of 8 industrial nickel laterite slurries as well as two alumina slurries were examined using various rheological techniques. The samples chosen covered a wide range of physical conditions such as differing pH, particle size distributions, solids densities and mineralogy as well as country and deposit of origin. The rheological parameters investigated were the yield stress and shear stress vs. shear rate of the particulate slurries. Considerable attention was focused on the techniques used in shear stress vs. shear rate characterisation, including capillary rheometry, smooth and roughened cup and bob rheometry and the vane in infinite medium technique. This work confirmed the finding of previous works, showing nickel laterite slurry rheological behaviour ranging from time independent to thixotropic to rheopectic. It found the vane in infinite medium technique highly suitable for testing nickel laterites at process relevant yield stresses. This technique gave data that correlated well with vane yield stresses and capillary rheometry data. Cup and bob tests showed significant slip at lower shear rates. In a number of cases, the cup and bob techniques also showed erroneously high stresses at higher shear rates. The vane yield stress was found to be a fast and accurate method for monitoring nickel laterite sample aging and the samples tested exhibited 100 Pa yield stresses at solids fractions ranging from 0.389 to 0.524. Blending of nickel laterites was found to be nonlinear, and confirmed that characterisation at various blend ratios is necessary if blending is to be utilised during production.