41 results for Doctoral, Use commercially

  • Improving Clustering Methods By Exploiting Richness Of Text Data

    Wahid, Abdul (2016)

    Doctoral thesis
    Victoria University of Wellington

    Clustering is an unsupervised machine learning technique, which involves discovering different clusters (groups) of similar objects in unlabeled data and is generally considered to be a NP hard problem. Clustering methods are widely used in a verity of disciplines for analyzing different types of data, and a small improvement in clustering method can cause a ripple effect in advancing research of multiple fields. Clustering any type of data is challenging and there are many open research questions. The clustering problem is exacerbated in the case of text data because of the additional challenges such as issues in capturing semantics of a document, handling rich features of text data and dealing with the well known problem of the curse of dimensionality. In this thesis, we investigate the limitations of existing text clustering methods and address these limitations by providing five new text clustering methods--Query Sense Clustering (QSC), Dirichlet Weighted K-means (DWKM), Multi-View Multi-Objective Evolutionary Algorithm (MMOEA), Multi-objective Document Clustering (MDC) and Multi-Objective Multi-View Ensemble Clustering (MOMVEC). These five new clustering methods showed that the use of rich features in text clustering methods could outperform the existing state-of-the-art text clustering methods. The first new text clustering method QSC exploits user queries (one of the rich features in text data) to generate better quality clusters and cluster labels. The second text clustering method DWKM uses probability based weighting scheme to formulate a semantically weighted distance measure to improve the clustering results. The third text clustering method MMOEA is based on a multi-objective evolutionary algorithm. MMOEA exploits rich features to generate a diverse set of candidate clustering solutions, and forms a better clustering solution using a cluster-oriented approach. The fourth and the fifth text clustering method MDC and MOMVEC address the limitations of MMOEA. MDC and MOMVEC differ in terms of the implementation of their multi-objective evolutionary approaches. All five methods are compared with existing state-of-the-art methods. The results of the comparisons show that the newly developed text clustering methods out-perform existing methods by achieving up to 16\% improvement for some comparisons. In general, almost all newly developed clustering algorithms showed statistically significant improvements over other existing methods. The key ideas of the thesis highlight that exploiting user queries improves Search Result Clustering(SRC); utilizing rich features in weighting schemes and distance measures improves soft subspace clustering; utilizing multiple views and a multi-objective cluster oriented method improves clustering ensemble methods; and better evolutionary operators and objective functions improve multi-objective evolutionary clustering ensemble methods. The new text clustering methods introduced in this thesis can be widely applied in various domains that involve analysis of text data. The contributions of this thesis which include five new text clustering methods, will not only help researchers in the data mining field but also to help a wide range of researchers in other fields.

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  • Tama Samoa Stories: Experiences and Perceptions of Identity, Belonging and Future Aspirations at Secondary School

    Rimoni, Fuapepe (2016)

    Doctoral thesis
    Victoria University of Wellington

    This thesis investigates the experiences of twelve strong, articulate and thoughtful tama Samoa (Samoan boys) through their participation in secondary schools and lives outside the classroom and through the stories of others. The study looked at how the students enacted their identities as Samoans, as learners and as young men who are anticipating the future. The study is premised on the view that Pacific identities are fluid, diverse, multi-dimensional and include a range of different perspectives relating to social class, ethnicity, culture and gender. Such a view of identity as complex is not generally taken into consideration in the literature on educational success and achievement of Pacific students in New Zealand. The study employed a phenomenological qualitative design, using focus groups and semi-structured interviews by talanoa (conversations). As the study involved a group of indigenous tama Samoa, the Samoan fa’afaletui method was used. Participants were a group of twelve tama Samoa in three Wellington secondary schools and their twelve nominated persons. The study found that there are key aspects to making the experiences of tama Samoa positive and successful within the secondary school. These include acknowledging tama Samoa and their multiple identities while attending secondary school; supporting the development of a sense of belonging through everyday interactions with peers and teachers, and affirming the belief by tama Samoa that secondary school socialisation serves to help them make future decisions. This study argues that the experiences of tama Samoa are deeply embedded within wider social, economic and political trends. Indeed, their “voices” are shaped in part by these broader forces that construct and represent them as being historically “disadvantaged” and socio-economically “underserved.” Further, this study advocates for the diverse voices of tama Samoa, along with their experiences, stories, hopes, aspirations and dreams to be brought to light and placed alongside the official accounts of Pacific “disadvantage” to enable more balanced critical discourses taking place. It is hoped that this study will offer further insights into the experiences of tama Samoa in the New Zealand secondary school context, from which valuable knowledge is derived to inform and support schools in improving the New Zealand secondary school experiences of Samoan adolescent boys.

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  • Street perceptions: A study of visual preferences for New Zealand streetscapes

    Gjerde, Morten (2015)

    Doctoral thesis
    Victoria University of Wellington

    City leaders often make reference to their built and natural environments when they compete domestically or internationally for financial investment, tourism spending and high quality workers. These leaders are aware that people in the workforce, investors and tourists would prefer to be associated with vibrant and attractive places. Research has confirmed the important role the appearance of the built environment plays in people’s physical, financial and psychological wellbeing, not the least of which is helping to foster a sense of individual and community pride. However, there is also literature critical of the appearance of many individual buildings and urban streetscapes, particularly those that have arisen through the well-intentioned but uncoordinated efforts of those involved in the development of individual sites. Recognising that wider public interests have not always been well-served by private development, governments and local authorities become involved to control development outcomes. One aspect of development control is design review, which aims to improve the quality of urban places by influencing the design of individual buildings. However, given that design review is administered by professional experts and that design guidance is based on normative expressions of what good design should be, what assurances are there that urban transformation meets with public expectations? The research reported in this thesis addresses this question. This research seeks to identify those streetscape design characteristics that are best liked by people and those that they dislike. A methodology based on mixed research methods was developed. An initial study sought people’s preferences for six different urban streetscapes, as depicted photographically. Analysis of nearly 200 responses to the survey questionnaire identified several building and streetscape characteristics that were consistently liked and disliked. To explore these and other responses from another perspective, a second study was designed that would examine people’s preferences in more detail and on the basis of their actual experience of the streetscapes. Study Two was developed around three separate case studies and two focus group discussions. Demographic information about the 156 survey respondents was collected, along with their aesthetic perceptions about individual buildings, relationships between buildings and overall streetscapes. This enabled comparisons to be made on the bases of gender, age and occupational background. Of particular interest was to understand the streetscape preferences of lay members of the public, those whose interests design review aims to ensure, and change professionals, who make the design and planning decisions. Two focus group discussions were convened, one for change professionals and the other for lay people, to explore findings from the survey in more detail. The results indicate that people prefer older buildings whose façade designs are based on more traditional composition patterns, and that the activities with which a building is associated have considerable influence on perceptions. These are two matters about which design control of new building development is not interested. In general, people prefer moderate variations in height between buildings along the length of a street and change professionals seem to tolerate/prefer bigger variations than others. Abrupt differences in height between adjoining buildings were viewed negatively by lay people, in large part because blank walls on internal boundaries become evident. On the other hand, change professionals were less sensitive to such differences, in part because they understood the nature of change and anticipated that future change would help heal such conflicts. In addition to exploring people’s perceptions of New Zealand streetscapes the thesis discusses several of the best-liked and the least-liked buildings in the context of design control processes in order to speculate about which methods might hold the greatest promise for creating well-liked urban streetscapes. While this discussion is relevant it sits outside the main thrust of the project and is necessarily brief. In anticipation that this discussion will continue the thesis concludes with a summary of the matters around which design control could, and perhaps should, be interested, based on the collected evidence. The findings of the research help us to better understand how people perceive urban streetscapes and therefore these become a platform for future work, one aspect of which could explore how people’s preferences can be better integrated with development control.

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  • Fostering incidental vocabulary uptake from audio-visual materials: The role of text comprehension

    Nguyễn, Chí Đức (2017)

    Doctoral thesis
    Victoria University of Wellington

    This research project explores various factors that may influence the rate of incidental foreign/second language (L2) vocabulary acquisition from audio-visual materials, with a special focus on procedures that enhance learners’ comprehension of these input materials. Informed by relevant theories and research findings in the fields of L2 listening comprehension and incidental vocabulary acquisition, I investigate the effects of having learners (a) view a TED Talks video twice rather than once, (b) sum up the content of the video before watching it a second time, (c) watch TED Talks videos on the same subject in order to increase familiarity with that subject, and (d) exchange summaries of TED Talks videos with peers so as to assist each other’s subsequent processing of those videos. As these interventions are all deemed to facilitate L2 listening comprehension, they are also expected to create favourable conditions for incidental vocabulary uptake to occur. The effects on incidental vocabulary acquisition of the above interventions were gauged in a series of classroom experiments with Vietnamese EFL learners. Although vocabulary uptake was generally far from spectacular, all of the tested procedures were found to result in statistically significant vocabulary gains. The insertion of the output tasks (i.e., the summary activities) was particularly useful. First, they helped to enhance the learners’ text comprehension. Second, they created opportunities for the learners to use newly met words and thus consolidate their knowledge of these lexical items. A thread through the experimental data is the strong association between the learners’ vocabulary uptake and their comprehension of the input content. The findings from this research project are consistent with several established notions, models and theories in the fields, including Ausubel’s Advance Organizer (1978), Hulstijn and Laufer’s Involvement Load Hypothesis (2001), Krashen’s Input Hypothesis (1985), Nation’s Vocabulary Generation (2013), Swain’s Output Hypothesis (2005), and Wittrock’s Model of Generative Teaching of Comprehension (1991). However, there are also findings that go beyond the core tenets of these, and that can further our understanding of how learners process new lexical items in meaning-focused input and output tasks. Regarding pedagogical implications, this research project confirms that fostering L2 listening comprehension creates favourable conditions for incidental vocabulary acquisition to happen, and that the aforementioned classroom procedures are facilitative in this regard, albeit to different degrees.

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  • Discriminative Stimulus Properties of MDMA: The Role of Serotonin and Dopamine

    Webster, Jeremy (2016)

    Doctoral thesis
    Victoria University of Wellington

    Rationale: ±3,4-methylenedioxymethamphetamine (MDMA, “ecstasy”) produces unique and complex subjective effects which distinguish it from other recreationally used drugs. An understanding of the neurochemical mechanisms that underlie these effects is important in order to assess the potential for MDMA abuse and to inform researchers exploring of the drug’s therapeutic potential. The present thesis investigated the neurochemical mechanisms underlying the subjective effects of MDMA using drug discrimination procedures in laboratory animals. Despite evidence that training dose can markedly impact the results of drug discrimination studies, the impact of training dose on the discriminative stimulus properties of MDMA has been largely overlooked. The broad aims of these experiments were 1) to test the ability of two different doses of MDMA to support drug discrimination learning, and 2) to determine the role of serotonin (5-HT) and dopamine (DA) neurotransmitter systems in producing the discriminative stimulus effects of each MDMA training dose. Methods: Groups of rats were trained to discriminate MDMA (1.5 or 3.0 mg/kg) from saline or to discriminate MDMA (1.5 or 3.0 mg/kg) from amphetamine (0.5 mg/kg) and saline, using two- or three-lever, food-reinforced drug discrimination procedures. The first experiments determined the impact of training dose on the acquisition of the MDMA discrimination. Reliability of the discrimination was assessed by measuring the impact of changes in acquisition criteria. Once the discrimination had been acquired, generalisation tests were carried out in two-lever experiments with the SSRIs, fluoxetine and clomipramine, the 5-HT2 agonists, mCPP and DOI, and the 5-HT1 agonists, 8-OH-DPAT and RU-24969, to investigate the role of 5-HT in the discriminative stimulus effects of 1.5 mg/kg vs 3.0 mg/kg MDMA. Next, the role of DA was investigated in further generalisation test sessions with the DA releasing stimulant, AMPH, the non-selective D1/D2 agonist, apomorphine, the D1 agonist, SKF38393, and the D2 agonist, quinpirole. Finally, experiments were carried out in which the ability of the 5-HT2A antagonist, ketanserin, the 5-HT1B/1D antagonist, GR-127935, the 5-HT1A antagonist, WAY100635, the D1 antagonist, SCH23390, and the D2 antagonist, eticlopride, to attenuate the discriminative stimulus effects of 1.5 mg/kg vs 3.0 mg/kg MDMA was assessed. Results: A higher training dose of MDMA was associated with a more rapid acquisition of drug discrimination in both the two- and three-lever tasks, and significant differences were observed with respect to the ability of each dose of MDMA to maintain consistently accurate discrimination across both tasks. All of the serotonin agonists that were tested generalised to the discriminative stimulus effects of 1.5 mg/kg MDMA in a two-lever discrimination task. In contrast, only agonists for 5-HT1A or 5-HT2A receptors generalised to the discriminative stimulus effects of 3.0 mg/kg MDMA. Non-selective dopamine agonists generalised to the discriminative stimulus effects of 3.0 mg/kg but not 1.5 mg/kg MDMA, whereas selective D1 and D2 agonists failed to generalise to the discriminative stimulus effects of either training dose. None of the DA or 5-HT antagonists tested had a marked impact of the discrimination of 1.5 mg/kg MDMA whereas administration of a D2 antagonist produced a small but significant attenuation on the discriminative stimulus effects of 3.0 mg/kg MDMA. Conclusions: The results of the present thesis suggest that the discriminative stimulus effects of MDMA may change both quantitatively and qualitatively as a function of dose. The subjective effects produced by lower doses appear to be mediated primarily via serotonergic mechanisms, whereas higher doses may involve the additional recruitment of dopaminergic mechanisms. These findings have implications for our understanding of MDMA in terms of the drug’s potential for dependence and abuse.

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  • An investigation of foreign exchange risk management by exporting small and medium sized enterprises

    Dang, Vu (2015)

    Doctoral thesis
    Victoria University of Wellington

    Previous studies on foreign exchange (forex) risk management have tended to focus on multinational enterprises; while how SMEs manage their forex risk is still largely unexplored. As small and medium sized enterprises (SMEs) are increasingly involved in international markets, they have become a new research setting on forex risk management. Given that SMEs have limited access to resources, skills and capabilities, internal hedging techniques could be favoured by SMEs. There is limited research on this matter, and the extant literature on forex management generally considers derivatives as major hedging techniques for large firms. This thesis primarily investigates how exporting SMEs manage forex risk. In addition, approaches to forex management could be changed as a firm becomes more experienced internationally. Following the basic principles of internationalisation theory, the thesis also examines the impact of the internationalisation degree of the firm on forex management decisions. This thesis sheds new light on SMEs’ hedging practices by providing a better understanding of SMEs’ choices of forex risk management. Three research questions have been raised: (1) what determinants influence SMEs’ choice to hedge as a way of managing forex risk; (2) what strategies do SMEs use when they choose hedging to manage forex exposure; and (3) how does the degree of internationalisation impact the choice of forex management. The thesis draws on two theoretical perspectives to help address these overarching questions. It extends the use of the resource-based view (RBV), and combines this with internationalisation theory. The setting of SMEs is a context for using the RBV. New Zealand and Australian exporting SMEs provide the sample for testing the hypotheses. The contributions of this thesis are twofold. Firstly, the thesis identifies four determinants of forex risk strategy by exporting SMEs, i.e. degree of internationalisation (specifically, export ratio), forex exposure, perceived forex risk, and resources. Secondly, it extends the use of the RBV and the internationalisation theory in forex risk management of SMEs. In addition, the thesis uses a research approach combining an exploratory qualitative study and a main quantitative study.

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  • Self-improvement books: A genre analysis

    Koay, Dong Liang (2015)

    Doctoral thesis
    Victoria University of Wellington

    The aim of the thesis is to explore the characteristics of self-improvement books as a genre. Although self-improvement books are a widely read genre, particularly in the Western world, none to my knowledge has examined the linguistic features of this genre in detail. The thesis draws on the three main schools of genre theory: English for Specific Purposes, Systemic Functional Linguistics, and the New Rhetoric, and begins by investigating the sections (e.g., acknowledgement, introduction chapter) in self-improvement books and the typicality of the sections. Focusing on three sections: introduction chapters, body chapters, and about the author sections, the thesis examines how authors structure the sections by analysing the moves and steps. This study also examines the stories in self-improvement books by analysing the purpose of the stories and their structure. Stories were chosen because they seemed to be a feature of self-improvement books based on my observation and as suggested by interview data. To analyse self-improvement books at a register level, the thesis examines the most unambiguous aspects of engagement: personal pronouns focusing on you, imperative clauses, and questions. It also examines the lexicogrammatical features of self-improvement book titles and compares them to the titles of historical biographies, showing that imperative clauses and ing-clause are found only in self-improvement book titles. Drawing on interview data and literature on the American Dream, American individualism, Neoliberalism, and New Age beliefs, the thesis explains how the linguistic characteristics of the genre of self-improvement books reflect these ideologies. The dataset for the study is 40 self-improvement books, selected on the basis of a set of criteria that I developed. Subsets were selected from the main dataset for specific analyses. The text analysis part of the study is supplemented by interview data from specialist informants, who come from three categories: readers of the genre, non-readers of the genre, and authors of the genre. Move analysis identifies obligatory rhetorical moves and indicates that the main purposes of introduction chapters and about the author sections are persuading readers to read the book, and establishing credibility, respectively. Authors always persuade readers to read their books by listing reasons to read them. To demonstrate authors’ credibility, they refer to their areas of expertise. Unlike the introduction chapters and about the author sections, the body chapters have more than one obligatory rhetorical move. The body chapters present the problem that readers potentially experience, present the authors’ message, recommend practical applications, and encourage readers to apply them. From a genre perspective, the purpose of all the stories in my analysis is to illustrate the authors’ message. Register analysis, and drawing on interview data, suggests that authors use the personal pronoun you, imperative clauses, and questions to engage readers. The abundance of the personal pronoun you suggests that self-improvement books are a reader-oriented genre. The analysis of the imperative clauses using Halliday’s process types suggests that the main way to improve our lives, the authors recommend, is to change how we think. Finally, my thesis shows that the social purpose of self-improvement books is to help potential readers improve their lives, and the approach of improving one’s life has an individualistic orientation.

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  • A Critical Realist Study of Political Identity in Aotearoa New Zealand: Materiality, Discourse and Context

    Woodhams, Jay (2015)

    Doctoral thesis
    Victoria University of Wellington

    Political identity is a complex phenomenon that is generated within a rich sociocultural context. This thesis examines political identity in informal talk which is situated within a relatively under-explored context, New Zealand’s capital city and political centre, Wellington. Grounding the study within the critical realist model of stratified reality provides the philosophical motivation to explore multi-layered discourses alongside the extra-discursive referents that underpin them. The analysis centres on a model of identity, contra postmodernism, which shows that identities, while socially recognised in discourse, are articulated in reference to physical and social structures. I adopt a comprehensive multi-layered approach to discourse by examining the macro sociocultural influences that appear to pattern interaction across the country, the meso-level subnational discourses that influence dialogue at a more situated level and the micro-level interactional stances taken up in everyday communication. Discourse at all levels is implicated in the identities I examine in this thesis and it is against this backdrop that I unpack political identity into its indexed discourses and constitutive stance acts. Framed by my ethnographic immersion in the study context and drawing on in-depth semi-structured interviews with twenty-six individuals, I explore the way in which discourse and stancetaking are implicated in the genesis of the participants’ political selves. I first consider the extra-discursive context, including the geographical, economic and cultural structures that underlie New Zealand discourses. This is followed by detailed analysis of sociocultural discourse as it appears in talk. I identify egalitarianism and tall poppy as two related discourses which are embedded within the historical context of the country. I also explore four subnational discourses relating to Wellington city, including the political town, left-wing and small town discourses, which occur alongside a discourse of contrast. These sociocultural and subnational discourses influence much of the talk that occurs in reference to politics in Wellington and are thus implicated in political identity as it is generated in moment-by-moment interaction. To explore this in further detail I examine the micro-level of interactional discourse, more specifically the processes of stancetaking, in two detailed case studies. The two focus participants demonstrate prominent stance processes which I argue are central to much identity work: intersubjectivity, in which the stances of all those involved in the discussion interact in complex ways; and multiplicity, when participants take numerous stance directions that appear to contribute to different aspects of their identities. The intensive focus on the case studies, alongside analysis of the full discursive and extra-discursive context, provides a multi-layered and philosophically anchored approach that seeks to contribute to current understandings of and approaches to the study of discourse and identity.

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  • First Principles Study of Ga₍₂₀₋x₎Alx⁺ Nanoalloys: Structure, Thermodynamics and Phase Diagram

    Ojha, Udbhav (2015)

    Doctoral thesis
    Victoria University of Wellington

    Nanoalloys (a finite framework of two or more metal atoms) represent a rapidly growing field owing to the possibilities of tuning its properties as desired for various applications. Their properties are size, shape, composition, chemical ordering, and temperature dependent, thereby offering a large playground for varied research motivations. This thesis documents the investigations on how the addition of aluminium affects the cationic gallium clusters, both in terms of geometric & electronic structure and thermodynamics, which have been observed to show greater-than-bulk melting behaviour for small sizes. A specific cluster size of 20 atoms is selected, Ga₍₂₀₋x₎Alx⁺, with the overall intention of creating a phase diagram which is the most reliable way to predict the phase changes in the system. All the first principles (density functional theory) based Born-Oppenheimer molecular dynamics calculations have been performed in the microcanonical ensemble. Melting behaviour is first studied in the pure Al₂₀⁺ clusters and then in three representative clusters of Ga₍₂₀₋x₎Alx⁺ series: Ga₁₉Al⁺, Ga₁₁Al₉⁺ and Ga₃Al₁₇⁺ clusters. We observe that all the three nanoalloy compositions show greater-than-bulk melting behaviour behaviour as well and in Ga₁₉Al⁺, specifically, Al prefers the internal sites, contrary to the previous arguments. We go on to complete the solid-liquid-like melting phase diagram using the calculated information and further propose a model of these greater-than-bulk melting clusters to be components of the corresponding bulk phases, whether metals or alloys, with additional size-dependent contributions added to it.

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  • Human inspired robotic path planning and heterogeneous robotic mapping

    Williams, Henry (2016)

    Doctoral thesis
    Victoria University of Wellington

    One of the biggest challenges facing robotics is the ability for a robot to autonomously navigate real-world unknown environments and is considered by many to be a key prerequisite of truly autonomous robots. Autonomous navigation is a complex problem that requires a robot to solve the three problems of navigation: localisation, goal recognition, and path-planning. Conventional approaches to these problems rely on computational techniques that are inherently rigid and brittle. That is, the underlying models cannot adapt to novel input, nor can they account for all potential external conditions, which could result in erroneous or misleading decision making. In contrast, humans are capable of learning from their prior experiences and adapting to novel situations. Humans are also capable of sharing their experiences and knowledge with other humans to bootstrap their learning. This is widely thought to underlie the success of humanity by allowing high-fidelity transmission of information and skills between individuals, facilitating cumulative knowledge gain. Furthermore, human cognition is influenced by internal emotion states. Historically considered to be a detriment to a person's cognitive process, recent research is regarding emotions as a beneficial mechanism in the decision making process by facilitating the communication of simple, but high-impact information. Human created control approaches are inherently rigid and cannot account for the complexity of behaviours required for autonomous navigation. The proposed thesis is that cognitive inspired mechanisms can address limitations in current robotic navigation techniques by allowing robots to autonomously learn beneficial behaviours from interacting with its environment. The first objective is to enable the sharing of navigation information between heterogeneous robotic platforms. The second objective is to add flexibility to rigid path-planning approaches by utilising emotions as low-level but high-impact behavioural responses. Inspired by cognitive sciences, a novel cognitive mapping approach is presented that functions in conjunction with current localisation techniques. The cognitive mapping stage utilises an Anticipatory Classifier System (ACS) to learn the novel Cognitive Action Map (CAM) of decision points, areas in which a robot must determine its next action (direction of travel). These physical actions provide a shared means of understanding the environment to allow for communicating learned navigation information. The presented cognitive mapping approach has been trained and evaluated on real-world robotic platforms. The results show the successful sharing of navigation information between two heterogeneous robotic platforms with different sensing capabilities. The results have also demonstrated the novel contribution of autonomously sharing navigation information between a range-based (GMapping) and vision-based (RatSLAM) localisation approach for the first time. The advantage of sharing information between localisation techniques allows an individual robotic platform to utilise the best fit localisation approach for its sensors while still being able to provide useful navigation information for robots with different sensor types. Inspired by theories on natural emotions, this work presents a novel emotion model designed to improve a robot's navigation performance through learning to adapt a rigid path-planning approach. The model is based on the concept of a bow-tie structure, linking emotional reinforcers and behavioural modifiers through intermediary emotion states. An important function of the emotions in the model is to provide a compact set of high-impact behaviour adaptations, reducing an otherwise tangled web of stimulus-response patterns. Crucially, the system learns these emotional responses with no human pre-specifying the behaviour of the robot, hence avoiding human bias. The results of training the emotion model demonstrate that it is capable of learning up to three emotion states for robotic navigation without human bias: fear, apprehension, and happiness. The fear and apprehension responses slow the robot's speed and drive the robot away from obstacles when the robot experiences pain, or is uncertain of its current position. The happiness response increases the speed of the robot and reduces the safety margins around obstacles when pain is absent, allowing the robot to drive closer to obstacles. These learned emotion responses have improved the navigation performance of the robot by reducing collisions and navigation times, in both simulated and real-world experiments. The two emotion model (fear and happiness) improved performance the most, indicating that a robot may only require two emotion states (fear and happiness) for navigation in common, static domains.

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  • Spectroscopic investigation of excitonic and charge photogeneration processes in organic photovoltaic cells

    Gallaher, Joseph (2016)

    Doctoral thesis
    Victoria University of Wellington

    Organic photovoltaic (OPV) cells show significant promise as a renewable energy resource capable of meeting the world’s large and growing energy needs. Increasing device efficiency is central to achieving an economically viable option for widespread applications. To this end, a better understanding of the structure and dynamics of the electronic excited states is needed. In particular, the mechanism by which excitons (electron-hole pairs) escape their Coulombic attraction and generate photocurrent is yet to be established. In this thesis ultrafast laser spectroscopy, in particular transient absorption and time-resolved photoluminescence, are used to study: exciton relaxation, morphological effects on charge separation, and the pathway leading to triplet exciton states. In Chapter 3, a series of oligothiophenes are synthesised with well-defined conjugation lengths to act as molecular models of polymer backbone sub-units, and thereby probe exciton relaxation processes. Time-resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopy measurements presented in Chapter 4 reveal emission signatures evolve from a mirror image of absorption - which lacks vibronic structure - towards a spectrally narrower and vibronically structured species on the hundreds of femtosecond to early picosecond timescale. Analysis of this spectral evolution shows that a broad distribution of torsional conformers is driven to rapidly planarize in the excited state, including in solid films. This provides evidence that both torsional relaxation and energy migration could contribute to the non-mirror image absorption-emission spectra observed in polymer thin films. Recently, long lived TA signatures have been attributed to triplet excited states with the suggested formation pathway being similar to organic light emitting diodes, whereby non-geminate (bimolecular) charge recombination leads to the formation of both singlet and triplet states. Isolated oligothiophenes in solution provide an ideal model system to investigate the role of structural relaxation on triplet exciton formation. Through analysis of TA spectral dynamics in Chapter 5, singlet and triplet exciton populations were tracked. Restriction of the torsional relaxation increased triplet yield suggesting vibrational hot states could drive triplet formation. This model could aid in understanding triplet exciton formation in polymer-based solar cells via spin-mixing instead of non-geminate recombination. In a series of polymer:fullerene blends, the link between the nature of polymerfullerene intermixing and charge generation pathways was investigated. It is shown in Chapter 6 that free charge generation is most efficient in a 3-phase morphology that features intimately mixed polymer:fullerene regions amongst neat polymer and fullerene phases. Distinct spectroscopic signatures made it possible to determine whether holes occupy disordered or crystalline polymer chains. TA spectral dynamics reveal the migration of holes from intermixed to pure olymer regions in 3-phase morphology blends, which contrasted with observations in 2-phase blends. The energy gradient between the intermixed and phase-pure regions may be sufficient to drive efficient separation of charge pairs initially generated in intermixed regions, with free charges subsequently percolating through these phase-pure domains. The photophysics of a high performance polymer:polymer blend is studied in Chapter 7 in an effort to elucidate how these blends can rival their polymer:fullerene counterparts. Optical spectroscopy reveals incomplete exciton dissociation and rapid geminate recombination in the blends. This is shown to result from a largely phase-separated morphology with domains greater than the exciton diffusion length. Significant loss of charge carriers on early timescales highlights increasing polymer: polymer solar cell efficiency requires optimizing blend morphology to realise facile charge separation. Taken together, this thesis presents a valuable spectroscopic insight into the pathway of efficient charge separation and the importance of both blend morphology and polymer structure.

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  • Seismic investigations of the lithosphere in an amagmatic back-arc region: North Island, New Zealand

    Dimech, Jesse-Lee (2016)

    Doctoral thesis
    Victoria University of Wellington

    New seismic constraints on crustal and upper mantle structures, kinematics, and lithospheric rheology are reported from an amagmatic back-arc region: the southwest North Island of New Zealand. Robust earthquake locations reveal a hypocentre 'downwarp' beneath the east-west trending Taranaki–Ruapehu Line. These earthquakes occur in the uppermost mantle, at depths of 30–50 km, and are distinct from shallower 8–25 km-deep earthquakes near Mt. Ruapehu in terms of focal mechanisms and principal stress directions. A receiver function CCP stack shows that the mantle earthquakes occur beneath a large change in crustal thickness, where the Moho 'steps' from 28 to 35 km-deep and the steepest part of that step has a 20–50° dip. The mantle earthquakes are dominated by strike-slip fault movement and have a maximum compressive stress direction of NE–SW. The existence of mantle earthquakes beneath a steeply-dipping Moho step implies some sort of dynamic modication is occurring in the mantle lithosphere. One possibility to explain these features is the convective removal of the mantle lithosphere due to a Rayleigh–Taylor-type instability. South of the Taranaki–Ruapehu Line, the Moho conversion weakens on both the receiver function CCP stack, and marine seismic reflection data under most of the Wanganui Basin (SAHKE02 and GD100 seismic lines). However, localised bright reflections at Moho depths can be seen in both near-vertical and wide-angle seismic data. Attribute analysis of near-vertical seismic reflections suggests that the rocks beneath the reflectivity are strongly-attenuating (Q ~20) with a negative velocity contrast relative to the lower crust. These observations are interpreted to be related to the presence of serpentinite (antigorite) and/or high pore fluid pressures in the mantle wedge. The links between hydration of amagmatic back-arcs, serpentinisation and/or high pore fluid pressures, rock viscosity, and mantle instabilities are documented here for the southwest North Island of New Zealand. These associations may be applicable to other amagmatic back-arcs around the world.

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  • Essays on Disaster Risk and Economic Development

    Karim, Azreen (2016)

    Doctoral thesis
    Victoria University of Wellington

    This thesis consists of four self-contained papers in the areas of disaster risk and economic development. Chapter One provides a qualitative survey of the empirical literature on the nexus among poverty, inequality and natural disasters. The last few years have seen an explosion of economic research on the consequences of natural disasters. This new interest is attributable first and foremost to a growing awareness of the potentially catastrophic nature of these events, but also a result of the increasing awareness that natural disasters are social and economic events. Here, we survey the literature that examines the direct and indirect impact of natural disaster events specifically on the poor and their impact on the distribution of income within affected communities and societies. With a meta-regression analysis of the existing literature on the impacts of disasters on households in Chapter Two, we observe several general patterns. Incomes are clearly impacted adversely, with the impact observed specifically in per-capita measures. Consumption is also reduced, but to a lesser extent than incomes. Poor households appear to smooth their food consumption by reducing the consumption of non-food items; in particular health and education, and this suggests potentially long-term adverse consequences. Given the limits of our methodology and the paucity of research, we find no consistent patterns in long-term outcomes. We place disaster risk to the poor within the context of sustainable development and future climatic change. Our objective In Chapter Three is to identify all of the directly observable determinants’ of publicly allocated and realized spending for disaster risk reduction (DRR) at the local government (sub-district) level in Bangladesh. We employ the Heckman two-stage selection model with detailed public finance and other data from 483 sub-districts (Upazilas) across the country. While some of our results conform with our priors, our estimations surprisingly find that government does not respond to the sub-district’s risk exposure as a factor affecting the DRR financing mechanism. This variable is consistently counter-intuitively statistically insignificant. The DRR regional allocations do not seem to be determined by risk and exposure, only weakly by vulnerability, nor even by more transparent political economy motivations. In Chapter Four, we examine the short-run economic impacts of recurrent flooding on Bangladeshi households surveyed in 2000, 2005 and 2010. In 2010 Household Income and Expenditure Survey (HIES), households answered a set of questions’ on whether they were affected by flood and its likely impacts. We identify two treatment (affected) groups by using the self-reported data and historical rainfall data based flood risk index. We estimate a difference-in-difference (DID) model to quantify the impacts on income, expenditure, asset and labour market outcomes and further extend our analysis to different income and expenditure brackets. Overall, we find robust evidence of negative impacts on agricultural income and expenditure. Intriguingly, the extreme poor (i.e. the bottom 15th quintile) experience significant positive impacts on agricultural income in the self-reported treatment case.

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  • Effects of Corporate Governance Guidelines on Investors: An Experimental Examination of Bangladeshi Investors’ Decisions

    Tariquzzaman, ATM (2017)

    Doctoral thesis
    Victoria University of Wellington

    The main purpose of the study is to examine whether investors assign importance to corporate governance in making investment decisions. The study involves a 2x2x2 between-participant experiment on real investors that examines the effects of corporate governance structure, financial condition and insider trading on individual investor decisions. The findings of this study extend the literature on corporate board practices and investor perceptions by providing evidence from this emerging economy that strong corporate governance has a positive impact on investor decisions. The study also confirms the findings of prior literature that financial condition of a company positively influences investor decisions. Hence, the results provide insights into the effects of strengthening corporate governance guidelines and of variation in financial condition on investor decisions. The study provides evidence that the common occurrence of illegal insider trading in the emerging market of Bangladesh does not appear to impact on investor decision making, unlike in developed countries. The results of this study also contribute to understanding of how the quality of corporate governance impacts on decision making. It appears that governance directly impacts the perceived reliability of financial reports and trust in the board and management and that these factors fully mediate the impact on investor decision making. The theoretical model and instrument developed for this study will be useful for further studies to explore the impact of other corporate governance factors on investor decisions. Furthermore, the theoretical model and instrument will also be useful for further studies in other developed and developing countries, particularly where insider trading is regarded by investors as being a concern and to investigate the impact of other corporate governance factors on investors and financial analysts.

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  • The effect of clozapine and MIS416 on demyelination, remyelination and immunomodulation in the murine cuprizone model

    Templeton, Nicola (2017)

    Doctoral thesis
    Victoria University of Wellington

    Multiple Sclerosis (MS) is a disorder of the central nervous system that affects approximately 2.5 million people worldwide. Due to the heterogeneous nature of the disease, and the want of an identified cause, treatment of MS remains difficult. Treatments are available, however these are limited in efficacy and are not suitable for all forms of MS. Disease pathology is characterised by the formation of demyelinating lesions in the central nervous system (CNS) which lead to cognitive and motor impairments associated with the disease. These CNS lesions can be classified as those with immune cell involvement or those without immune cell infiltrate, which are more commonly seen in progressive forms of MS, and currently, there are no treatments available for progressive MS. Due to the limited options available for treating progressive MS, this thesis aims to identify the therapeutic effect provided by the immunomodulatory compounds, MIS416 and clozapine, in a non-immune mediated model of MS, which is believed to more closely resemble progressive disease. Both of these compounds have been shown previously to reduce disease burden in an immune-driven animal model of MS. To investigate the effect of immune-modulating therapies on lesions without immune cell infiltrate, the cuprizone model of non-immune demyelination was used. In summary, the work presented in this thesis found that treatment with MIS416 and clozapine led to improved performance on behavioural assays, although neither agent inhibited cuprizone-induced demyelination or enhanced remyelination. The cellular mechanism behind the observed behavioural improvement is yet to be confirmed. MIS416 was able to maintain its previously identified immunomodulatory properties when administered in this novel setting. Moreover, novel changes to serum growth factors were identified that could provide unexpected benefit to MS patients administered MIS416. In addition to reversing cuprizone-induced behavioural deficits, clozapine reduced LPS-driven inflammatory cytokine production by microglia, indicating that clozapine has the ability to directly reduce inflammation, which may benefit progressive MS patients. Protective effects provided by either of these compounds could aid in the development of unique combination therapies to target both the inflammatory immune component and the cellular components seen at different stages of MS. MIS416 induced changes to serum cytokines and growth factors in the periphery could be harnessed to treat not just MS but other auto-immune diseases characterised by a similar cytokine profile.

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  • Quantitative analysis of New Zealand-Antarctica plate motions during the Paleogene and Late Cretaceous

    Chambord, Amandine (2017)

    Doctoral thesis
    Victoria University of Wellington

    Quantifying past motions of tectonic plates in the southwest Pacific is important because the Pacific-Antarctic ridge is the only non-destructive boundary between the Pacific plate and other major plates. However, formation of sea-ice near Antarctica impairs the collection of magnetic anomaly data that are necessary to calculate plate rotations. A detailed analysis of all ship-track magnetic data available in the southwest Pacific (61 cruises, 153 profiles, including several cruises collected after 1995) is presented here. Four different sources of uncertainty are quantified: (1) confidence of magnetic anomaly identification, (2) magnetic reversal location picking precision, (3) ship navigation precision, and (4) magnetic data quality. Finite plate rotations are calculated for the southwest Pacific (42.5 to 79 Ma) using the resulting magnetic anomaly database (1528 magnetic reversal data). Finite rotations were calculated using the Hellinger criterion, or by a new method presented here that assumes orthogonality between fracture zones and ridge segments. The new method requires less parameters and is hence able better estimate rotations in cases with an uneven distribution of sparse magnetic data. Rotations and formal uncertainties are calculated for thirty-one chrons (c20y to c33o). They confirm the existence of a three plate system (Pacific, Marie Byrd Land, Bellingshausen) in the southwest Pacific from before c31o (68.7 Ma) until c28y (62.5 Ma). After c28y, the Bellingshausen and Marie Byrd Land plates moved as a single plate.

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  • Investigations into the Mesophase Behaviour and Equilibrium Dynamics of a Family of Aerosol OT-related Surfactants

    Fairweather, Graham (2017)

    Doctoral thesis
    Victoria University of Wellington

    Surfactants are ubiquitous, with many important applications in physical and biological sciences. Increasingly, they are being utilised for their liquid crystalline properties rather than in their traditional roles as detergents, emulsifiers or wetting agents. This is because these liquid crystalline mesophases offer a large range of different architectures with varying degrees of structural order and functionality. Presently, surfactants used in these applications are those with behaviours that are already understood due to their routine use in industry. However, the concept of producing surfactants by rational design would be a significant step forward and could potentially revolutionise the field. Such a step is not trivial due to the hierarchical and complex nature of self-assembly. In order to realise this goal, we have to understand how the various contributions to the free energy of the system can be manipulated to affect the building blocks of the system on different lengthscales. How do changes made on the sub-nanometre lengthscale alter the physical dynamics of a system as a whole? In order to answer this question, we must gather experimental evidence for different systems using a range of complementary techniques to probe the system dynamics on different lengthscales. Here we report the characterisation of phase behaviour and the evolution of microstructure for a family of surfactants. We investigated this using a combination of small-angle X-ray spectroscopy, polarising optical microscopy and cryogenic scanning electron microscopy, producing seven new phase diagrams. We also report the flow properties of these systems, which were studied using static and dynamic rheology. The starting point of this study is sodium bis(2-ethylhexyl)sulfosuccinate (Na-AOT), which is an industrially important surfactant that forms lyotropic liquid crystalline phases in water. This behaviour has been extensively studied previously and presents over large concentration and temperature ranges. The system has also been a source of debate and confusion in regards to a possible transition within the lamellar phase. We present a review of the literature and, with the addition of our own investigation, show that system does not undergo any of the previously proposed transitions within this region. We show that the anomalies reported in the literature can be explained by a change from a swollen and highly connected lamellar phase to a more classical ordered lamellar phase with increasing surfactant concentration. This change is a result of different intermolecular forces governing the system as the lamellar bilayer repeat distance decreases. The impressive stability of the lamellar phase is mediated through formation of different types of topological defects, which change from positive to negative Gaussian curvature as the bilayer elasticity varies. We then use Na-AOT as a base system for comparison with a family of related surfactants to detail how molecular changes alter the self-assembly of each new system. The first comparison system discussed is the an analogue of Na-AOT with reduced branching, sodium bis(1,3-dimethylbutyl) sulfosuccinate (Na-butylAMA). The use of this system allowed us to investigate the role of the surfactant’s branching ethyl groups in the self-assembly of the Na-AOT system.We show that the less bulky tail region and reduced conformational freedom of Na-butylAMA dramatically reduces the stabilisation of the lamellar phase, with liquid crystalline phases only presenting at surfactant concentrations above 50 wt %. We also report that the phase progression of the Na-AOT system can be closely reproduced by modification of the Na-butylAMA headgroup via ion exchange. Replacing the sodium counterion with potassium restores the balance between headgroup and tail volumes and allows the stabilisation of the lamellar phase at low concentrations. Alterations were made to the steric bulk and electronic properties of the AOT headgroup without changing the chemical functionality by replacing the sodium cation with two other Group 1 alkali metals: lithium and potassium. Cryo-SEM images show how these modifications dramatically alter the elasticity in the system. In the case of Li-AOT, this results in a significant shift of the phase boundaries to lower concentrations, with extended stability of the ordered inverted bicontinuous cubic and inverted hexagonal phases. The K-AOT system shows increased elasticity due to more negative Gaussian curvature. This drives the system to form a more disordered sponge phase, which can reversibly transition to a lamellar phase upon heating. These changes are explained in terms of the different hydration and headgroup affinities of the cations, which both alter the elastic moduli of the surfactant bilayer. The AOT system was also studied with three biologically relevant ammonium-based counterions: ammonium (NH₄⁺), choline (Ch⁺) and acetylcholine (AcCh⁺). With the first being of comparable size to K⁺, the effects of increased polarisability and hydrogen bonding on phase behaviour were investigated. The different microstructures observed in the AOT system when using Na⁺, K⁺ and NH₄⁺ are discussed in terms of the different contributions to the Gaussian curvature, including ion pair formation, hydrogen bonding, hydration and steric constraints on molecular packing. In comparison to the other counterions investigated, choline and acetylcholine are more weakly hydrated, have greater affinity for the sulfonate anion, and provide additional steric bulk to the AOT headgroup. This removes some degrees of freedom in these systems, and hence increases bilayer rigidity, causing significant stabilisation of their lamellar phases. As a result, lamellar phases dominate both systems over nearly their entire concentration ranges. The bilayer elasticity was further investigated by the addition of electrolyte to the lamellar phases of the NH₄-AOT and Ch-AOT systems. Both are shown to exhibit salt-induced phase transitions, forming sponge and cubic phases depending on the starting surfactant concentration. These transitions were achieved with small amounts of NaCl in the case of NH₄-AOT, due to its highly disordered microstructure. By contrast, as a result of the increased bilayer rigidity, the Ch-AOT system is shown to have a highly stable lamellar phase that can accommodate large amounts of salt. Having elucidated the phase behaviour and microstructure of each surfactant system, the viscoelastic properties of a number of the systems were investigated. The flow properties of the lamellar and sponge phases were studied by static and dynamic rheology. The shear behaviour of the sponge phases were observed to be dependent upon concentration, with both Newtonian and shear thinning behaviours displayed. The lamellar phases of several systems demonstrated shear-dependent phase transitions. These transitions were also monitored by cryo-SEM, which showed the formation of multilamellar vesicles (onions) at a critical shear rate. Upon increasing shear, the onions decreased in size and assembled in a close-packed arrangement. In summary, this study addresses the confusion surrounding the widely disputed lamellar phase of the Na-AOT system. The observed changes in physical properties are explained by the evolution of defect behaviours on multiple lengthscales. Through a series of molecular modifications, a range of different systems have been produced and their liquid crystalline phase behaviours and microstructures characterised. The relative stabilisation of lamellar and sponge phases are rationalised in terms of the various competing contributions to the total free energy. These phases are then manipulated through changes in temperature, electrolyte addition and shear, with the induction of several different phase transitions observed. By systematically changing the AOT cation, we have demonstrated the role of the counterion in mesophase stability and system elasticity. We have shown that through careful counterion selection, we can enhance or reduce the stabilisation of curved interfaces. This has allowed us to compile a set of considerations, which can be applied to other surfactant systems in order to better predict the potential outcomes of alterations in these systems, and is a step towards rational design in the field.

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  • Vascular epiphyte assemblage structure and distribution patterns in the south-temperate zone

    Taylor, Amanda (2016)

    Doctoral thesis
    Victoria University of Wellington

    Vascular epiphytes, which are specialised to spend their entire life cycle within trees, are significant contributors to local ecosystem services. However, our current understanding of epiphyte distributions, co-occurrences, and general ecology lags far behind that of terrestrial plants. Furthermore, the majority of epiphyte research is undertaken in tropical forests, with comparatively few studies extending into temperate climates. As such, whether epiphytic plant assemblage structure varies geographically, or is influenced by area and isolation effects needs further scrutiny. In addition, how epiphytes are distributed in relation to host tree ontogeny and microclimates specific to south-temperate forests is poorly understood. Here, I attempt to bridge this gap by researching epiphyte distributions and assemblage structure in New Zealand, southern Chile, and Australia. In the first biogeographic study of epiphyte-host interactions, I determined if epiphyte-host network structure (i.e. nestedness, species co-occurrences, species specialisation) varied among New Zealand and Chilean temperate forests (Chapter 2). At the forest stand level, network structure was consistent with stochastic structuring, which suggests that dispersal and disturbances are important drivers of epiphyte distributions at a biogeographic scale. However, deterministic structure was observed in New Zealand networks with regards to nestedness (i.e. when specialists interact with generalists), which suggests that positive species interactions influence epiphyte distributions at a within-tree scale. Second, I determined whether the composition of plant communities residing in epiphytic birds’ nest ferns (Asplenium goudeyi) on Lord Howe Island, Australia, are influenced by fern size, isolation from a major propagule source and resident plant community richness (Chapter 3). Results suggest that plant communities are structured by dispersal. For one, there was a significant isolation effect on resident plant community richness. Additionally, wind-dispersed taxa were well represented in isolated ferns, while animal-dispersed taxa and taxa with no specific dispersal strategies were absent. This is the first study to test the combined effects of area, isolation and resident plant richness on epiphytic plant assemblage structure. Third, using Darwin’s geological theory of island ontogeny as a theoretical construct, I explored changes in epiphyte species richness throughout tree ontogeny (Chapter 4). Theoretical frameworks have helped bridge the gap between our understanding of vascular epiphytes and terrestrial plants, however, none have been implemented to guide investigations on epiphyte assemblage development. Based on the general features of island ontogeny, I found three stages of epiphyte assemblage development: (i) an initial stage where host trees are devoid of epiphytes, (ii) a second stage where trees acquire epiphytes into maturity, and (iii) a hypothetical stage where epiphyte assemblages follow a period of species decline following host tree mortality. In addition to these results, I found interspecific variation in the ontogenetic stage at which host trees become favourable for epiphyte establishment and the rate at which epiphyte assemblages develop. Lastly, I explored the systematic distribution of epiphytes and mistletoes in relation to microclimate gradients around the trunks of trees (Chapter 5). In addition, I tested the physiological responses of epiphytes and mistletoes to reductions in their most limiting resources to determine if the responses were consistent with their distribution patterns. The radial distributions of epiphytes and mistletoes were highly directional, and paralleled gradients of humidity, light and water. Additionally, the photochemical efficiency of epiphytes and CO₂ assimilation in mistletoe leaves decreased in plants growing in environments with lower water and light availability, respectively. However, mistletoe leaves still assimilated CO₂ in lower light conditions, which suggests a high plasticity of mistletoes to growing in a canopy environment. Despite over 120 years of recognising the importance of vertical microclimates on epiphyte distributions, this is the first systematic study of epiphytic plant distributions in relation to microclimate gradients around the trunks of trees. This thesis has increased our understanding of epiphytic plant assemblage structure, and how it is influenced by host tree species, isolation, area and resident plant species richness. In addition, this thesis has increased our understanding of the effect of host tree ontogeny and microclimate on epiphyte distribution patterns. Together, these studies may be built upon more broadly to further elucidate drivers of epiphyte assembly and distribution patterns.

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  • Magnetic and electronic properties of iron-based superconducting systems

    Sambale, Sebastian (2015)

    Doctoral thesis
    Victoria University of Wellington

    This thesis is motivated by the large variety of high-temperature superconductors that contain iron in the superconducting layer. This number has grown rapidly since the discovery in 2008 of the iron-pnictides (and chalcogenides), where iron and arsenic form the superconducting layer. Also of interest are the iron-cuprate hybrid materials, where one out of three copper atoms is replaced by iron. The aim is to understand the superconducting, magnetic and electronic properties of these materials in respect to their iron content. This thesis describes some of these properties for the iron-pnictide compounds of CeFeAsO₁₋xFx and AFe₂As₂ (A=Ba, Sr), and for the ironcuprate hybrids of FeSr₂YCu₂O₆₊y and FeSr₂Y₂₋xCexCu₂O₁₀₋y. Here it has been found that CeFeAsO₁₋xFx follows a 3D fluctuation conductivity above the superconducting transition and the thermal activation energy is correlated to the critical current density within a two fluid-flux creep model below the superconducting transition. NMR measurements show that there is considerable charge disorder within the superconducting doping region. The AFe₂As₂ show a positive magnetoresistance, which could be interpreted through three-carrier transport. Superconducting samples of SrFe₂As₂ display a large enhancement in the magnetoresistance below the superconducting transition up to 1600 %, which is due to three-carrier transport through metallic and superconducting regions in an inhomogeneous state. The superconducting properties of the iron-cuprate FeSr₂YCu₂O₆₊y in respect to the location of iron was studied under the influence of electron and hole doping and with additional magnetic impurities. FeSr₂Y₂₋xCexCu₂O₁₀₋y shows a disorder induced spin-glass state and strong localization depending on the doping.

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  • A dual-trap optical tweezer approach to study emulsion droplet interactions

    Griffiths, Marjorie (2015)

    Doctoral thesis
    Victoria University of Wellington

    Foods are a highly complex form of soft condensed matter. Their complexity arises from a number of interrelated factors including the natural heterogeneity of raw materials, intricate composition, and the subtle changes in molecular interactions and micro-structural arrangements dictated by food processing and storage. It is highly important to understand the forces dictating the food structure as the assembly and organisation of major structural entities (i.e biopolymers, droplets, bubbles, networks, and particles) are responsible for the foods stability, texture, flow properties and more inclusively their organoleptic properties. The structural entities of foods exhibit numerous forms of self-organization and have significant structure complexity and dynamic behaviour on the mesoscopic length scales from 10 to 1000 nanometres. These dynamic weak interactions between the constituents define the organized state that ranges from simple spatial or temporal ordering to more intricate interactions making up the food microstructure. These interactions are often small in magnitude and are short ranged making them difficult to measure directly. Very few studies have been carried out on direct force measurements in foodstuffs. The focus of this research was to develop a dual-trap optical tweezer method to directly measure interactions between micrometre colloidal particles and ultimately to design an apparatus where interactions between less homogeneous systems, such as emulsion droplets could directly measured as a function of separation. As the name suggests, optical tweezers provide the ability to control the position of particles using a focused laser beam. The general concept of this method is to immobilise two particles in two separate optical traps and step one particle closer to the other stationary particle in a controlled fashion. The droplet’s movement is then recorded using a high-speed camera that provides near-to-real-time images of the particle’s positions. The particle’s positions are determined by a 3-D tracking algorithm developed in-house which determines the position of both particles to a precision of sub-pixel accuracy. The force exerted on each droplet (by the other one) can be extracted as it is proportional to the trap strength (pN/μm) and the displacement of the particle from the centre of the optical trap (μm). To demonstrate the optical tweezer method,the interactions between silica beads of a known size were measured as a function of bead separation. The measured force-distance curves agreed with the electrostatic component of the DLVO theory. Once the method was established it was applied at increasing salt concentrations (decreasing Debye lengths). Interestingly, a salt concentration was found beyond which the experimental data no longer agreed with the predictions of DVLO theory. Above 100 μM sodium chloride the Debye length was reduced to less than the Brownian fluctuations of the particles in the traps, which then dominated the apparent repulsion by restricting their particle trajectories, masking the actual nature of the electrostatic interactions. This resulted in force curves which fitted the exponential function, however, the fitted decay constant bore no resemblance to the actual Debye length. A diffusion experiment was designed to demonstrate the ability to measure interactions in multiple environments using the same pair of beads (at low salt concentrations where Debye lengths are faithfully recovered). The evolution of force-displacement curves was measured as the local salt concentration changed owing to the diffusion of salt from the interface and the results obtained were shown to agree with predictions based on a standard diffusion formalism. Applying the dual-trap optical tweezers method, successfully demonstrated with silica beads, to less homogeneous systems such as emulsion droplets presented challenges which showcased that emulsion design was critical as certain criteria had to be met in order to facilitate undertaking the tweezer experiments. These criteria include particle size (1-3 μm ), low polydispersity, and a reasonable refractive index mismatch between the droplet and continuous phase. In keeping with food systems a protein stabilised oil-in-water emulsion was chosen. Two popular emulsifiers, sodium caseinate and β-lactoglobulin, were investigated at different ionic strength, pH and homogenisation pressures and phase volumes. The emulsion chosen for direct force measurements was a sodium caseinate emulsion when prepared in a 100 mM phosphate buffer at pH 7.0, 60 wt. % soya bean oil and 0.04 wt.% protein which provided an adequate droplet size with minimal polydispersity. Interactions between pairs of sodium caseinate emulsion droplets were measured. Unlike for silica beads, the individual droplet size needed to be measured to deter- mine the surface-to-surface separation of droplet pairs. The droplet’s diameter was determined by measuring the restricted diffusion of the droplet in a weak optical trap and fitting the short time mean squared displacement behaviour to a Brownian motion simulation. It was found that the droplet size can be determined in this fashion to within 50 nm. Moving forward, the interactions between pairs of emulsion droplets were measured in water using the same method gleaned from the silica bead interaction study. The experimental data fitted well to the electrostatic force described by the DLVO theory with reasonable ζ-potentials extracted. To further demonstrate this dual optical tweezer method, interactions between the same pair of droplets were measured at increasing NaCl concentrations by means of diffusion. The expected trend has found to agree from calculations of increased local salt concentration based on a diffusion equation. At salt concentrations above 100 μm significant deviations in the force-curves were observed that may signal salt induced changes of the droplet’s interface or be attributed to the small magnitude of the force being within the noise. This warrants further investigation. In conclusion, the dual-trap optical tweezers have shown incredible potential to become a robust method to measure the interactions between droplets. This method has some clear advantages over current methods including that force, and spatial resolution is superior, sample preparation is straightforward, forces are measured in 3-dimensions, and the droplets are free in solution during measurement, not wetted on surfaces. Accordingly, dual-trap optical tweezer methodology has provided the ability to measure interactions to a precision that has not yet been achieved by any other method for the study of emulsion systems, which in itself is a major achievement. This method is another tool in the toolbox of a colloid chemist, food scientist and physicists to probe interactions in soft materials.

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