Surfactant coated microbubbles are widely used as contrast agents (UCA) in medical ultrasound imaging, due to their high echogenicity and non-linear response to acoustic excitation. Controlling the stability of microbubbles in vivo represents a considerable challenge. Understanding the characteristics of the bubble surface and how they change with production method, composition and environment is key to addressing this problem. The aim of this thesis is to investigate viscosity, bubble dissolution, and acoustic response as functions of their composition, manufacturing method and environment. Bubbles were made using combinations of phospholipid and an emulsifier in different molar ratios. Adding the emulsifier decreased both the size and the surface viscosity of the bubbles and caused changes in the scattered pressure amplitude of bubbles under ultrasound. To increase microbubble stability, solid inorganic nanoparticles were adsorbed on to the microbubble surface. These particles behaved as Pickering stabilisers, and deterred Ostwald ripening. The nanoparticles also enhanced the nonlinear behaviour of bubbles at low acoustic pressures. Three manufacturing methods (sonication, cross-flow and flow focusing) were investigated in order to verify stability differences. Sonication produced bubbles with surface viscosities hundreds of centipoise greater than those produced by microfluidics. Both pressure amplitude and harmonic content for sonicated bubbles were found to be much larger due to a higher liposomal adhesion rate at the surface. Solution temperature and bubble age were also investigated. When the solutions were heated above the phospholipid gelling temperature, microfluidic bubbles showed an increased surface viscosity, due to increased liposome adhesion caused by the increased temperature. Bubble composition, manufacturing method and environment were found to vary the surface characteristics of the microbubbles. Further investigations into the affects of the filling gas, in vitro studies, and low temperature TEM characterisation should be conducted to produce a microbubble with the full range of desired characteristics.

Methods for examination of the crystal structure of crystalline materials include the X-Ray, neutron and synchrotron-radiation diffraction, electron backscattered diffraction in the scanning electron microscope, scanning transmission electron microscopy, transmission electron microscopy and focused ion beam microscopy. The scanning low energy electron microscopy (SLEEM) is less known as yet but already has proven itself very powerful tool for studies of the crystal lattice. By means of very slow electrons reflected from the sample and effectively detected in their full angular and energy distribution the crystalline structure is imaged at high spatial resolution and high contrast is obtained between differently oriented grains in polycrystals. Because of high sensitivity of the image signal to the inner potential distribution in the sample even details like subgrains or twins as well as strain at the microstructural level can be visualized. The aim of this thesis is to demonstrate the scanning low energy electron microscopy as an effective tool for investigation of wide range of materials like steels, non-ferrous alloys and ultra-fine grained materials.

Les travaux présentés portent sur l'élaboration d'une paroi acoustique absorbante de faible épaisseur capable d'absorber des ondes acoustiques de basses fréquences (500 - 1500 Hz). Le bruit est en effet la première source de nuisances environnementales évoquée par le public. Cette gêne nécessite la mise en place de traitements acoustiques dans le but d'améliorer le confort. Cependant, dans certaines conditions, les contraintes portant sur l'encombrement des solutions absorbantes limitent fortement leur utilisation. En effet, de manière générale, plus les fréquences du son à atténuer sont basses plus les éléments à utiliser doivent être épais. La paroi acoustique absorbante conçue dans le cadre de cette thèse est basée sur un réseau de transducteurs électrodynamiques, réalisés en technologie MEMS. Ce type de paroi permet de contrôler l'absorption obtenue à partir de charges électriques adaptées, connectées aux bornes des transducteurs. Afin de dimensionner les différents éléments de cette paroi absorbante, un modèle analytique de l'absorption de la paroi prenant en compte le comportement des transducteurs électrodynamique utilisés ainsi que les couplages acoustiques entre les différentes sources qui sont particulièrement importants dans le domaine des basses fréquences, a été développé. Ce modèle a été validé par 2 moyens : i) des modélisations par éléments finis et ii) la mesure de l'absorption acoustique des prototypes réalisés, mesure obtenue pour deux types de transducteurs. L’une est basée sur des micro-haut-parleurs commerciaux, l'autre sur un transducteur miniature MEMS de dimensions similaires mais dont le rendement de conversion est d’un ordre de grandeur supérieur aux micro-haut-parleurs conventionnels. La modélisation analytique a montrée deux voies d'améliorations qui ont été entreprises, la première sur la suppression des courts-circuits présents au niveau du transducteur, la seconde sur l'optimisation du facteur de force permettant l'amélioration du rendement de conversion électro-mécanique. Les résultats d'absorption acoustique obtenus à partir des transducteurs MEMS montrent que la solution possède un réel intérêt dans le domaine des basses fréquences là où les solutions conventionnelles sont peu efficaces. / The work presented in this thesis focuses on the development of a sound absorbent thin solution able to absorb sound waves of low frequency (500 - 1500 Hz). Noise is, actually, the primary source of environmental pollution raised by the public. This discomfort requires the establishment of acoustic solutions in order to improve the acoustic comfort. However, under certain conditions, the thickness of absorbent solutions strongly limit their use. Indeed, in general, more frequencies are low more the acoustic solutions used must be thick. The sound absorption noise of the solution presented in this work is based on a network of miniature electrodynamic transducers controlled from appropriate electrical loads connected to the terminals of the transducers. An analytical model of the behavior of sound absorbing wall was developed. This model takes into account the behavior of electrodynamic transducers used and the acoustic coupling between the various sources that are particularly important in the area of low frequencies. This model has been validated by two means : i) finite element modeling and ii) measuring the absorption of acoustic prototypes. Two types of absorbent walls were made. One is based on commercial micro-speakers, the other on a miniature MEMS transducer of similar dimensions but the conversion efficiency is an order of magnitude greater than conventional micro-speakers. Analytical modeling has shown two ways of improvements that have been undertaken, the first on the removal of short circuits present at the transducer, the second on optimizing the force factor for improving the conversion efficiency of electro-mechanics. The results sound absorption obtained from the MEMS transducers show that the solution has a real interest in the low frequency range where conventional solutions are not very effective.

Background: Majority of cancer interventions today are performed percutaneously using needle-based procedures, i.e. through the skin and soft tissue. Needle insertion is known as one of the recent needle-based techniques that is used in several diagnostic and therapeutic medical procedures such as brachytherapy, thermal ablations and breast biopsy. The difficulty in most of these procedures is to attain a precise navigation through tissue reaching target locations. Insufficient accuracy using conventional surgical needles motivated researchers to provide actuation forces to the needle’s body for compensating the possible errors of surgeons/physicians. Therefore, active needles were proposed recently where actuation forces provided by shape memory alloys (SMAs) are utilized to assist the maneuverability and accuracy of surgical needles. This work also aims to introduce a novel needle insertion simulation to predict the deflection of a bevel tip needle inside the tissue. Development of a model to predict the behavior of the needle steering in the soft tissue has been always a point of interest as it could improve the performance of many percutaneous needle-based procedures. Methods: In this work first, the actuation capability of a single SMA wire was studied. The complex response of SMAs was investigated via a MATLAB implementation of the Brinson model and verified via experimental tests. The material characteristics of SMAs were simulated by defining multilinear elastic isothermal stress-strain curves. Rigorous experiments with SMA wires were performed to determine the material properties as well as to show the capability of the code to predict a stabilized SMA transformation behavior with sufficient accuracy. The isothermal stress-strain curves of SMAs were simulated and defined as a material model for the Finite Element Analysis of the active needle. In the second part of this work, a three-dimensional finite element (FE) model of the active steerable needle was developed to demonstrate the feasibility of using SMA wires as actuators to bend the surgical needle. In the FE model, birth and death method of defining boundary conditions, available in ANSYS, was used to achieve the pre-strain condition on SMA wire prior to actuation. This numerical model was validated with needle deflection experiments with developed prototypes of the active needle. The third part of this work describes the design optimization of the active using genetic algorithm aiming for its maximum flexibility. Design parameters influencing the steerability include the needle’s diameter, wire diameter, pre-strain, and its offset from the needle. A simplified model was developed to decrease the computation time in iterative analyses of the optimization algorithm. In the fourth part of this work a design of an active needling system was proposed where actuation forces of SMAs as well as shape memory polymers (SMPs) were incorporated. SMP elements provide two major additional advantages to the design: (i) recovery of the SMP’s plastic deformation by heating the element above its glass transition temperature, and (ii) achieving a higher needle deflection by having a softer stage of SMP at higher temperatures with less amount of actuation force. Finally, in the fifth and last part of this study, an Arbitrary-Lagrangian-Eulerian formulation in LS-DYNA software was used to model the solid-fluid interactions between the needle and tissue. A 150mm long needle was considered to bend within the tissue due to the interacting forces on its asymmetric bevel tip. Some additional assumptions were made to maintain a reasonable computational time, with no need of parallel processing, while having practical accuracies. Three experimental tests of needle steering in a soft phantom were performed to validate the simulation. Results: The finite element model of the active needle was first validated experimentally with developed prototypes. Several design parameters affecting the needle’s deflection such as the needle’s Young’s modulus, the SMA’s pre-strain and its offset from the neutral axis of the cannula were studied using the FE model. Then by the integration of the SMA characteristics with the automated optimization schemes an improved design of the active needle was obtained. Real-time experiments with different prototypes showed that the quickest response and the maximum deflection were achieved by the needle with two sections of actuation compared to a single section of actuation. Also the feasibility of providing actuation forces using both SMAs and SMPs for the surgical needle was demonstrated in this study. The needle insertion simulation was validated while observing less than 10% deviation between the estimated amount of needle deflection by the simulation and by the experiments. Using this model the effect of needle diameter and its bevel tip angle on the final shape of the needle was investigated. Conclusion: The numerical and experimental studies of this work showed that a highly maneuverable active needle can be made using the actuation of multiple SMA wires in series. To maneuver around the anatomical obstacles of the human body and reach the target location, thin sharp needles are recommended as they would create a smaller radius of curvature. The insertion model presented in this work is intended to be used as a base structure for path planning and training purposes for future studies. / Mechanical Engineering

The pursuit of high quality, large area graphene has been a major research focus of contemporary materials science research, in the wake of the discovery of the multitude of exceptional properties exhibited by the material. The DPhil project was undertaken with the objective of developing an understanding of the growth of large graphene sheets by chemical vapour deposition (CVD), and also in the subsequent characterisation of their material properties. By conducting atmospheric pressure CVD growth at high methane flow rates, it was found that few-layered graphene (FLG) could be deposited on a copper catalyst. It is demonstrated that the self-limiting property of a copper catalyst is not universal to all deposition conditions, and shown that FLG grows in a terrace-like configuration. In depth transmission electron microscopy (TEM) studies were carried out on FLG. By selective image reconstruction from the inverse power spectrum of the TEM images, it was possible to elucidate the inter-grain connectivity of few-layer graphenes. It was determined that there were two possible inter-grain configurations possible; specifically an overlap of graphene layers or a discrete atomic bonding edge. The perturbation of the few-layer structure when subject to an out of plane distortion was found to incur a shift in the conventional AB-Bernal stacking of FLG. By utilising the aberration corrected TEM (AC-TEM) at Oxford it was possible to resolve atomic detail in CVD synthesised monolayer films, including atomic bond rotations and vacancies. The use of a high current density at low accelerating voltage (80 kV) was demonstrated to allow for the controlled defect creation in graphene sheets. This permitted the creation of monovacancies and iron doped vacancy complexes suitable for further study. The behaviour of these two defect types under electron beam irradiation was subsequently studied.

This thesis describes an investigation into silk spinning, with the objective of producing high performance silk fibres in the laboratory using a novel spinning device based upon observations on natural spinning glands and processes. After an in-depth literature review the work is reported in two sections: natural and artificial spinning. The literature provides fragmented data on different aspects of natural silk production, and artificial spinning has not yet reproduced fibres with the properties of native silk fibres, despite unfounded claims of biomimetic spinning. The first half of the thesis looks at natural silk spinning. The work started with a general study of the morphology of spider and silkworm spinning ducts: First, how the silk fibre develops as the dope flows through the gland; and second the relationship between silk fibre properties and both gland morphology and spinning speed. More detailed studies using histochemical and spectroscopic investigations showed that the silk ducts of the spider Nephila edulis and the silkworm Bombyx mori both contain β-chitin, despite an evolutionarily distant common ancestor. Finally, observations showed that the duct of N. edulis consists of alternating nanoporous discs, and FEA modelling indicated that the duct is optimised for mechanical integrity and permeability. The second half of the thesis describes the development of a spinning device that uses natural silk dope mainly taken from B. mori as feedstock. It begins with a description of the gradual development of the engineering aspects of the spinning device, to meet challenges raised during the spinning investigation. The development of a centrifugal capillary rheometer, for practical quantitative insights into rheological processes is then presented. Finally the spinning investigation is reported: first, the screening of spinning in glass capillaries based upon natural gland dimensions and flow rates, which have been shown to induce fibrillation in silk dope in a rheometer, and also included initiation of instability through heat applied along the capillary; second, the final spinning evaluation, using lessons learned from all the screening trials throughout the project, but also including a key development of a hydrophobic coating on the capillary tip to inhibit droplet formation and massively increase the process stability and ease of fibre production. The main conclusions from this work are that good silk fibre cannot be spun by flow shear stress alone; and, that heat instability induces indiscriminate gelation of the silk, whose disordered molecular structure gives poor silk fibre properties. The body of work behind these conclusions provides fundamental background information and new insights that will contribute to the next stages of development of artificial silk spinning, from obtaining a better understanding of the biology of natural spinning glands to the engineering difficulties of implementing the bioinspired principles.

This thesis is devoted to the development of nano-focusing refractive optics for high energy X-rays using planar microelectronic technology. The availability of such optics is the key for the exploitation of high brilliance third and fourth generation X-ray sources. Advancements in the quality of optics available are commensurate with advancements in the fabrication technology. The fabrication process directly influences the quality and performance, so must be understood and controlled. In the first part of this thesis, the development of high aspect ratio Si kinoform lenses is examined. It is shown that control of the re-entrance angle is critical for successful fabrication; in fact, a large re-entrance angle can destroy the lens during the fabrication process. Through an etch study, it was found that as aspect ratio increases, control of the re-entrance angle becomes harder. To control the re-entrance angle for very high aspect ratios, a novel approach based on sacrificial structures was proposed and initial results presented. The second part is dedicated to an experimental study of refractive lenses made from diamond. Due to its low atomic number, relatively high density and very high thermal conductivity, diamond is one of the most desirable lens materials for refractive X-ray optics. However, due to its extreme hardness, it is very difficult to structure into a form suitable for X-ray lenses. To overcome this difficulty a Si moulding technique was used and focusing down to a 400 nm wide spot was achieved. Several obstacles were encountered and successfully overcome. The hardest obstacle was to obtain selective void-free filling in the Si moulds. Several methods were investigated. A method based on a sacrificial oxide layer and an Electrostatic Self-Assembly process was found to be the most useful. The approach discovered in this thesis is not limited to X-ray lenses, but can be applied to a wide variety of high aspect ratio MEMS requiring void-free diamond filling and smooth sidewalls.

This thesis reports studies done on single-walled carbon nanotubes (SWNTs) using optical spectroscopy as the primary investigative technique. It focuses on advances in sample preparation which have been made possible through improvements to the method of photo-luminescence excitation (PLE) mapping of nanotubes. An introduction to the field and some theoretical models are presented initially to provide a background to the experimental chapters which follow. A description of the standard procedure for sample preparation in aqueous surfactants is then followed by a detailed introduction to PLE mapping, including modeling of SWNT spectra. The next chapter discusses improvements to the sample preparation method by using organic polymer solutions instead of aqueous surfactants for suspending the nanotubes. The results show reductions in the distribution of SWNT species which are solubilised, leading to significant improvements in the resolution of the optical absorbance spectra and an increased photoluminescence yield. Two experiments which were performed on the novel polymer-SWNT systems are then described. The first shows (via PLE mapping) that energy is transfered to the SWNTs when the polymer is photo-excited. The possible mechanisms behind this, as well as the implications for using carbon nanotubes as an additive in polymer photovoltaics, are discussed. The second experiment details a recent magneto-PL study of SWNTs embedded in films produced from the polymer solutions. Here, the improved optical signatures and absence of strain at low temperatures have revealed a previously unseen high field intensity dependence. The behavior has been explained by the magnetic field induced mixing of the excitonic states.

Lithium-rich layered transition metal oxide cathode, represented as the chemical formula of xLi₂MnO₃ · (1 - x)LiMO₂(M = Mn, Ni, Co) , retains immense interest as one of the most promising candidates for energy storage system ranging from mobile devices to electric vehicle applications (EV/HEV/PHEV). This battery type benefits from superior theoretical capacity (>250 mAhg^-1), high chemical potential (>4.6 V vs Li⁰), good thermal stability, high discharge capacity and lower cost compared with conventional cathodes (e.g. LiCoO₂, Li(Ni_1/3Mn_1/3Co_1/3)O₂ cathodes). However, there remain major barriers which still need to be improved in order to achieve a successful commercialization for large-scale devices or electric vehicle applications. The irreversible capacity loss of 40-100 mAhg^-1 during the initial electrochemical cycle and the battery fading phenomena (capacity fading/voltage decay) on further cycles are the major problems which have emerged. The Li⁺ ion extraction accompanied by oxygen release from the active material in the form of oxide known as lithia (Li₂O) along with the transition metal migration has been suggested as the dominant processes underlying the capacity fading mechanism. Those processes, in turn, cause a phase transition from a layered structure into a spinel within the electrode material. The interplay of the local atomic environments between Li₂MnO₃ (monoclinic, C2/m) and LiMO₂ (trigonal/hexagonal, R3m) holds the key to developing better cathodes with enhanced stability. In the present thesis, an in operando XAS study using a specially-designed cell of the graphene- coated Li(Li_0.2Mn_0.54Ni_0.13Co_0.13)O₂ cathode is employed to examine the chemical, electronic, and structural states of the transition metals (Mn, Co, and Ni) during electrochemical cycle(s). Precise oxidation states for the transition metals is evaluated by the combined analyses from the XANES and SQUID measurements. The K-edge XANES spectral shift is quantified to investigate the contribution to the charge compensation mechanism by the oxidation change. Absorption features in K-edge XANES are identified. These features describe the electronic state of the individual atoms in the cathode composite, as well as the local distortion from the octahedral structure of MO₆. The Fourier transform of EXAFS offers a satisfactory description of the local structure changes with the connection to the cation arrangement. The description is generally involved with the peak amplitude, position, shape changes (trend), and coordination numbers in the real space. Hence, similarities or discrepancies in the local atomic environments could be compared at different state of charge. Major structural parameters are deduced from the EXAFS fitting process. These parameters can be used to distinguish different atomic environments upon voltage bias levels or investigate the appearance of the Jahn-Teller effect. A new approach to understand the atomic environment upon charge-discharge is demonstrated, namely, a Continuous Cauchy Wavelet Transform (CCWT) which enables the visualization of the EXAFS spectra in three dimensions by decomposing the k-space and R-space (uncorrected for phase shift) signals. The wavelet transform analysis provides possible evidence of the precursor that leads to the spinel phase transition in this battery system.

X-ray crystallography is a widely used analytical technique for the structure solution of small molecules. Since the determination of the rock salt structure in 1913 by Henry and Lawrence Bragg the technique has developed allowing the solution of larger and more complex structures. The information that can be determined about these structures has increased as X-ray sources, detectors, and computational methods have improved. However, certain properties of molecules cannot always be directly determined from single wavelength X-ray diffraction. These include, inter alia: the site specific oxidation or spin state of an element in compounds where more than one state of the same element exist; discrimination between consecutive heavy elements in the periodic table. As the size of molecules being studied increases, reduced data resolution also becomes a problem. The aim of this research was to determine whether these problems can be addressed by measuring the changing anomalous scattering contribution of heavy atoms within structure through careful selection of the X-ray energy. Firstly, I report an investigation into the problemof discriminating oxidation state, spin state and elements of near identical scattering by exploiting their anomalous signal. I first present DetOx, a program written during the course of the project to deconvolute the fluorescence signal from materials containing more than one state of the same element into their respective spectra. This allows the calculation of anomalous scattering factors for both atomic states of an atom, which can subsequently be used to refine the occupancy of the different states at ambiguous sites within the crystal structure. The approach taken here, to determine differences due to relatively small anomalous signals, is analogous to the refinement of the Flack parameter whereby small changes in many hundreds or thousands of observations can be used to fit a parameter with a high degree of precision and accuracy. I show the application of this technique to the mixed oxidation state compound, GaCl₂, and the two-step spin crossover material, Fe(btr)₃(ClO₄)₂. Refinement of the occupancy of charged ions on multiple sites using data at a single, carefully selected wavelength proved successful for these compounds, although upon extension to materials containing a larger number of anomalous scatterers, the absorption became a major issue in the data along with problems associated with simultaneously refining occupancies at more sites in the structure. We have demonstrated that calculations can be made to select specific experimental data to collect in order to improve the measured signal. However, due to limitation of the current collision model on the diffractometer used we have not yet been able to construct data collection strategies to take advantage of this. I next present a new ratio refinement technique to overcome this absorption problem due to the increased number of scatterers. By using ratios between datasets close in energy, but below the absorption edge, we were able to exploit small changes in f' without encountering absorption problems associated with the increase in f''. These ratio values were then refined against a lab structure using a modified version of CRYSTALS to reveal the site specific occupancies of different atomic species within a given structure. For mixed-valence compounds, e.g. Mn₃ and Mn₆ clusters, the difference in anomalous signal between the different states proved too small for a stable least-squares refinement solution. However, we have shown that using a simulated annealing algorithm (to refine only occupancies), we can consistently obtain the expected structure. For mixed-metal structures e.g. the Mn₅Co₄ cluster, there was enough contrast in the data to refine occupancies with a least-squares approach, and these results were supported using simulated annealing. Lastly, I describe the application of structure solution techniques based on methods used in macromolecular crystallography to 'large' small molecules. Traditionally these have been reserved for non-centrosymmetric protein structures, however with the trend of synthesising larger and larger small molecules, problems encountered in macromolecular crystallography leading to low resolution datasets are becoming increasingly common. I have shown that it is possible to solve the structure of centrosymmetric structures by exploiting the anomalous signal in multiple wavelength diffraction experiments. The technique is applied successfully to two relatively small molecules, however the results are promising for moving to larger structures in the future.