Fracture propagation in brittle materials has been studied using hybrid molecular dynamics simulations, allowing the use of quantum calculations wherever and whenever needed by the problem under investigation. Several timescales of the fracture process have been taken into consideration, going from the lowest possible velocity for a crack growing in subcritical loading conditions, to the highest velocity computed for very cold cracks. The study has been carried out in two prototypical models of brittle fracture: silicon and amorphous silica. Aiming to interpret the lowest crack velocity ever measured by experiments on silicon at room conditions, we used quantum-accurate computer simulations to show that immediate dissociation of oxygen molecules, and consequent oxidation of the highly stressed silicon crack tips, may be the cause of the observed slow crack growth. This theoretical prediction, supported by experimental evidence, claries longstanding discrepancies concerning the role of oxygen as a stress corrosion agent in silicon. Turning the attention to fast crack behaviour, a crossover between activated and catastrophic branches of crack velocities as a function of temperature has been detected in a hybrid classical molecular dynamics model of silicon. Cold cracks travel faster for high loading energies, while this trend is reversed in the region of energies where activated processes become dominant. Finally, the study of catastrophic fracture in amorphous silica has been initiated, testing for the rst time our hybrid approach to non crystalline structures.

In this thesis, we study observational signatures of cosmic (super)strings in the context of D brane inflation and properties of scalar inflaton perturbations on generic homogeneous inflating backgrounds. In the first part, we study the properties of cosmic superstrings produced at the end of inflation in two widely used effective models within a string theory framework, namely the D3/D7 and D3/¯D 3 models. For the D3/D7 model, we show that the symmetry breaking during the waterfall stage is anomalous and cancellation of this anomaly in combination with the moduli stabilisation procedure results in local axionic strings. In addition, we argue that the model is inconsistent with supergravity constraints on constant Fayet Iliopoulos terms. Moreover, we study radiative processes of cosmic superstrings on warped backgrounds. We argue that contrary to previous results in the literature, placing the string formation in a natural context such as D3/¯D 3 inflation, restricts the forms of possible radiation from these objects. Motivated by these string models, which inevitably result in the presence of heavy moduli fields during inflation, in the second part, we construct an effective field theory that captures the effects of massive scalars on the low energy dynamics of inflaton perturbations. We compute the energy scales that define the validity window of the Effective Field Theory (EFT) such as the scale where ultra violet (UV) degrees of freedom become operational and the scale where the EFT becomes strongly coupled. We show that the low energy operators related to heavy fields lift the strong coupling scale of the theory and result in a non linear dispersion relation for the light modes. Assuming that these modes cross the Hubble scale within the dispersive regime, we compute several observables and show how they are directly related with the scale of UV physics.

This thesis is focused on high temperature electronics and how the reliability of solder can be improved for high temperature applications using nanoparticles. Therefore this thesis first investigates a method of using cross-section polishing (CSP) based on argon ion milling to cross section the sample without damaging it. This research will show that this method is very effective in cross-sectioning the delicate samples and allowing embedded nanoparticles to be detected. Once this method of detecting nanoparticles is established, the thesis reports on an investigation into nano-composite solders. This part of the thesis will show that passive Silica (SiO2) nanoparticles can be added to solder to improve room temperature creep resistance of the solder. Compression tests at room temperature and elevated temperature were performed to compare the mechanical characteristics of normal solder and nano-enhanced solder. Compressed samples were cross-sectioned to investigate their microstructure. Results show that while nanoparticles are effective in improving creep resistance of the solder at room temperature, the efficiency of nanoparticles decreases with increasing the temperature. Finally the thesis reports on attempts to combine solders with highly reactive nanoparticles, focussing on the fundamental property of dissolution of thin aluminium layers in solder. As aluminium is highly reactive with oxygen, the main thrust of this chapter is to generate a method to avoid the oxidation of aluminium prior to introducing molten tin to it. After introducing molten tin to aluminium thin films, samples were cross-sectioned by CSP to examine the thickness of the remaining aluminium layer by scanning electron microscopy (SEM). Results did not allow accurate determination of the dissolution rate of aluminium in molten tin and the likely reasons for this are discussed.

Monodisperse emulsions are a special class of emulsion where all droplets are of uniform size. The properties of emulsions (rheology, appearance, stability, reaction kinetics) are determined by the properties of both continuous and dispersed phases but also by the characteristics of the droplets themselves. It is for this reason that monodisperse emulsions are often sought after, as droplet size can have such a large influence upon emulsion behaviour. Having a uniform emulsion results in more predictability and allows for easier design in emulsion properties. Monodisperse emulsions find uses in many academic and industrial fields including pharmaceuticals, food science, paints, and coatings. This work covers two broad approaches to monodisperse emulsion creation; microfluidics and controlled shear. Microfluidics is a rapidly emerging technology where liquid flows are constrained to sub-millimetre channel sizes thus creating highly laminar and controllable flows. The methods are used in various lab on chip, and droplet creating applications. A study is undertaken on the nature of buoyancy-driven formation of drops from microchannels, attempting to further understand the fundamental principles of monodisperse drop generation at nozzles. Droplet producing microfluidic devices often suffer, however, from jetting when the desired emulsions are viscous or have low interfacial tensions, resulting in polydispersity. This work introduces two methods to overcome this, surfactant shielding and core-shell templating. Surfactant shielding is a method by which the nozzle of a droplet producing capillary tip is protected from surfactants by a tertiary, pure continuous phase thus limiting the reduction of interfacial tension at the point of droplet creation. Core-shell templating is a method of introducing water droplets into the stream of a would-be jetting system. These water droplets introduce regular instabilities which have the effect of forcing the system into a quasi-dripping regime and thus create highly monodisperse viscous emulsions. Controlled shear is another method for creating monodisperse emulsions whereby a coarse emulsion is subjected to a uniform shear stress resulting in a smaller more monodisperse emulsion. The work investigates two geometries for doing this, a cylinder-curved plate and a cylinder-flat plate. Both these designs are shown to have higher throughput rates than conventional shear methods. In the final part of this work, microfluidics and controlled shear are combined in an attempt to utilise the contrasting benefits found in both techniques. A study is undertaken in the possibility of shearing monodisperse precursor emulsions created via microfluidic techniques, to obtain uniform emulsions of much smaller size. A microfluidic shear cell is also introduced which aims to combine the benefits of a shear device (increased throughput rates, ability to handle viscous fluids) with the benefits of microfluidics (no moving parts, more control).

The eld of plasmonics studies the interaction of light and free electrons in metals, giving rise to excitation of surface waves, on a metallodielectric interface. One branch of plasmonics is the design of metamaterials in visible and infrared spectral range which are articial structures designed to manipulate the propagation of light in a way not possible with conventional materials. This thesis is categorized in 3 main parts. The rst part examines the effects of waveguided modes in Au nanorod metamaterial waveguides. It shows, both theoretically and experimentally, that these materials can be designed to control the sign and magnitude of modal group velocity depending on the geometry and polarization chosen exhibiting high eective refractive indices (up to 10) and have an unusual cut-o from the high-frequency side, providing deep-subwavelength (0/6 { 0/8 waveguide thickness) single-mode guiding. This allows slow light to exist in such waveguides in a controllable environment which is a critical factor for nonlinear and active nanophotonic devices, quantum information processing, buering and optical data storage components. The second part discusses, analytically and numerically, strategies for biosensing and nonlinearity enhancement with hyperbolic nanorod metamaterials. It shows how the sensitivity of unbound, leaky as well as waveguided modes can be enhanced based on geometrical considerations. Additionally, refractive index variation of the host medium produces 2 orders of magni- tude higher sensitivity compared to nanorod or superstrate refractive index changes. In certain congurations, both TE and TM-modes of the metamaterial transducer have comparable sensitivities opening up opportunities for polarization multiplexing in sensing experiments. The gure of merit of the aforementioned structure is one order of magnitude higher than surface plasmon polariton and localized surface plasmon sensors making it ideal for sensitive-dependant applications such as chemo- and biosensors and nonlinear photonic devices. The third part investigates Strontium Ruthenate thin lms as a new material for near-IR plasmonic applications. It is demonstrated that their plasmonic behavior can be optimized by their deposition conditions leading to a selective and tunable plasma frequency in 324 - 392 nm range and epsilon-near-zero wavelength in 1.11 { 1.47 m range. Applications of these lms range from heat-generating nanostructures in the near-IR spectral range, to metamaterial-based ideal absorbers and epsilon-near-zero components, where the interplay between real and imaginary parts of the permittivity in a given spectral range is needed for optimizing the spectral performance.

We consider three varied soft matter topics from a continuum fluid mechanics perspective, namely: viscoelastic active matter, viscoelastic flows in porous media, and contact line dynamics. Active matter. For the purposes of this thesis, the term active matter describes a collection of active particles which absorb energy from their local environment or from an internal fuel tank and dissipate it to the surrounding fluid. We explore the stability and dynamics of active matter in a biological context in the presence of a polymeric background fluid. Using a novel coarse-grained model, we generalise earlier linear stability analyses (without polymer) and demonstrate that the bulk orientationally ordered phase remains intrinsically unstable to spontaneous flow instabilities. This instability remains even as one takes an ’elastomeric limit’ in which the polymer relaxation time τC → ∞. The 1D nonlinear dynamics in this limit are oscillatory on a timescale set by the rate of active forcing. Then, by considering the rheological response of our model under shear, we explore the mechanism behind the above generic flow instability, which we show exists not only for orientationally ordered phases but also for disordered states deep in the isotropic phase. Our linear stability analysis in 1D for sheared suspensions predicts that initially homogeneous states represented by negatively sloping regions of the constitutive curve are unstable to shear-banding flow instabilities. In some cases, the shear-bands themselves are unstable which leads to a secondary instability that produces rheochaotic flow states. Consistent with recent experiments on active cellular extracts (without applied shear) which show apparently chaotic flow states, we find that the dynamics of active matter are significantly more complex in 2D. Focusing on the turbulent phase that occurs when the activity ζ (or energy input) is large, we show that the characteristic lengthscale of structure in the fluid l∗ scales as l∗ ∝ 1/ √ζ. While this lengthscale decreases with ζ, it also increases with the polymer relaxation time. This can produce a novel ‘drag reduction’ effect in confined geometries where the system forms more coherent flow states, characterised by net material transport. In the elastomeric limit spontaneous flows may still occur, though these appear to be transient in nature. Examples of exotic states that arise when the polymer is strongly coupled to the active particles are also given. Flows in porous media. The second topic treats viscoelastic flows in porous media, which we approximate numerically using geometries consisting of periodic arrays of cylinders. Experimentally, the normalised drag χ (i.e., the ratio of the pressure drop to the flow rate) is observed to undergo a large increase as the Weissenberg number We (which describes the ratio of the polymer relaxation time to the characteristic velocity-gradient timescale) is increased. An analysis of steady flow in the Newtonian limit identifies regions dominated by shear and extension; these are mapped to the rheological behaviour of several popular models for polymer viscoelasticity in simple viscometric protocols, allowing us to study and influence the upturn in the drag. We also attempt to reproduce a recent study in the literature which reported fluctuations for cylinders confined to a channel at high We. At low numerical resolution, we observe fluctuations which increase in magnitude with the same scaling observed in that study. However, these disappear at very high resolutions, suggesting that numerical convergence was not properly obtained by the earlier authors. Contact line dynamics. We finish by investigating the dynamics of the contact line, i.e., the point at which a fluid-fluid interface meets a solid surface, under an externally applied shear flow. The contact line moves relative to the wall, apparently contradicting the conventional no-slip boundary conditions employed in continuum fluid dynamics. A mechanism where material is transported within a ‘slip region’ via diffusive processes resolves this paradox, though the question of how the size of this region (i.e., slip length ξ) scales with fluid properties such as the viscosity η and the width of the interface between phases l, remains disputed within the literature. We reconcile two apparently contradictory scalings, which are shown to describe different limits: (a) a diffuse interface limit where ξ/l is small and (b) a sharp interface limit for large ξ/l. We demonstrate that the physics of the latter (which more closely resembles real fluids in macroscopic experimental geometries) can be captured using simulations in the former regime (which are numerically more accessible).

This thesis explains the development of a quantum cascade laser based gas sensor and the verification of performance of the resulting product. The capability to detect trace levels of multiple species in varying physical conditions, specifically high temperature is shown. Optical designs capable of allowing the measurement technique to be used in a variety of applications have been developed, specifically multi-pass gas absorption cells. The accurate, precise and high speed analysis of the resulting data is made possible by spectral analysis algorithms. These developments and techniques were then applied to high temperature spectroscopic measurements. Spectra recorded at Rutherford Appleton Laboratory were used to calibrate high temperature measurements. The performance of the technology was thoroughly tested at the National Physical Laboratory. The resulting sensors have since been used to study the exhaust gases produced by a variety engine types ranging from cars to ships as well as atmospheric measurements.

The temperature dependence of the electrical double layer (EDL) capacitance of high and low temperature ionic liquids (IL) is under excited debates within experimental, theoretical & computational communities for decades. Using a coarse-grain model of ILs with asymmetric sized ions we studied the EDL behaviour of ILs at an electrified interface to gain understanding in the temperature-effect. We improved the accuracy of the capacitance calculations by increasing the overall simulation time by an order of magnitude compared to former publications and discussed possible error sources. The analysis of different temperatures from 250K to 500K revealed a voltage dependent temperature-effect on the differential capacitance. To our opinion, the temperature dependence can be attributed to the formation of well ordered structures of the EDL at low temperatures, that are ‘melted’ at higher temperatures. The results provide an explanation for the contradicting experimental results published in the past years. By systematically analysing number density profiles, a charge induced structural transition has been revealed. At most surface charge densities under study, the electric double layer forms a multilayered structure. At transition points, a hexatic (also termed as Moiré-like) monolayer structure is formed by the counter-ions. These findings correlate very well with experimental observations on room temperature ionic liquids and metal melts by Freyland and co-workers (Phys. Chem. Chem. Phys., 2008, 10, 923-936). The ions of the hexatic layer screen the electrode charge completely, thereby annihilating the interaction between electrode and subsequent bulk ionic liquid. Moreover we can report the formation of herring-bone structures at higher surface charge densities, that form as the superposition of two hexatic layers.

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