Effects of galaxy--satellite interactions on bar formation

Aims. We aim to show how encounters with low-mass satellite galaxies may alter the bar formation in a Milky Way-like disc galaxy. Methods. We use high-resolution N-body simulations of a disc galaxy prone to mild bar instability. For realistic initial conditions of satellites, we take advantage of cosmological simulations of Milky Way-like dark matter haloes. Results. The satellites may have a significant impact on the time of bar formation. Some runs with satellites demonstrate a delay, while others show an advancement in bar formation compared to the isolated run, with such time differences reaching $\sim$ 1 Gyr. Meanwhile, the final bar configuration, including its very appearance and the bar characteristics such as the pattern speed and the exponential growth rate of its amplitude are independent of the number of encounters and their orbits. The contribution of satellites with masses below $10^9 M_{\odot}$ is insignificant, unless their pericentre distances are small. We suggest that the encounters act indirectly via inducing perturbations across the disc that evolve to delayed waves in the central part and interfere with an emerging seed bar. The predicted effect for the present-day host galaxy is expected to be even more significant at redshifts $z \gtrsim 0.5$.Comment: 16 pages, 14 figures and 4 table

Modification of novel Portland based cement for orthopaedic application

Portland cement (PC) is a ceramic hydraulic cement which has been used in construction for decades and more recently for dental applications. PCs possess high durability and compressive strength and demonstrate good biological responses, that are generating interest as a potential material for orthopaedic applications. The present study investigated the addition of porogens to induce large macropores e.g. > 100 µm to promote potential bone ingrowth, whilst retaining appropriate mechanical and physical properties for vertebroplasty used to stabilise fractured vertebral bodies. The cements containing 20 wt% bismuth oxide (radiopacifying agent) and 5 wt% calcium chloride (setting accelerant) were prepared using a range of powder-to-liquid ratios and porogens (including mannitol, sucrose and sodium bicarbonate or foaming agents) were added from 1-20 wt% to induce macroporosity either after or during setting of the cement. Increasing the concentration of sugars increased the initial setting time 3-fold, whilst causing the cement paste to behave as a liquid. The compressive strengths of modified cements were reduced by up to 90 % after 7 days of storage through increasing flaws and porosity. The macrostructural analysis using scanning electron microscopy (SEM) showed no major difference between the modified cements and controls. 10 wt% foamed gelatine (FG) was found to improve the viscosity of the paste so that it was readily injectable, and demonstrated sufficient cohesion after injection. FG doubled the setting time approximately and generated large interconnected pores ranging from 100-400 µm in diameter according to SEM images. The compressive strengths of foamed cements were sufficiently high after 7 and 30 days of storage to stabilise fractured vertebral bodies. The addition of 10 wt% FG has shown the potential to modify PC by inducing large pores, whilst maintaining high injectability and compressive strength, which warrants further testing for clinical application in verterbroplasty

Impact of cosmological satellite galaxies on the dynamics of the Milky Way disc

I present a high resolution study of the impact of realistic satellite galaxies, extracted from cosmological simulations of Milky Way-like haloes including 6 Aquarius and one Via Lactea II suites, on the dynamics of the Galactic disc. The initial conditions for the multi-component Milky Way galaxy were generated, to ensure an isolated system in a dynamical equilibrium state. After analysing the statistical properties of the subhaloes, candidates likely to impact the disc with the initial mass,tidal mass > 10^8 solar mass, were identified, inserted into our high resolution N-body simulations and evolved for 2 Gyr. I quantified the vertical heating due to such impacts by measuring the disc thickness and squared vertical velocity dispersion across the disc. According to our analysis, the strength of the heating is strongly dependent on the high mass end of the subhalo distribution from cosmological simulations. The mean increase of the vertical dispersion is approximately 20 km^2/s^2/Gyr for R > 4 kpc with a flat radial profile. Excluding Aq-F2 results with a very massive perturber with tidal mass = 6.10^10 solar mass, the mean heating is less than 12 km^^2/s^2/Gyr. These heating values correspond to 28% and 17% of the observed vertical heating rate in the solar neighbourhood, respectively. Taking into account the statistical dispersion around the mean, we miss the observed heating rate by more than 3 sigma. I observed a general flaring of the disc height in the case of all 7 simulations in the outer parts. The dynamics of the disc in the presence of satellites was also studied in the final two chapters where the selected runs were extended to 4 Gyr. The infall of satellites can be responsible for delaying/advancing the bar-formation, depending on the crossing epoch. The passage influences the bar-formation, if the passage occurs between the end of the shot noise regime T1 (0.75 Gyr) and before the exponential growth T2. The Aq-F2 run with the most massive candidate, induces localised vertical heating in the outer disc with R > 15 kpc. I also observed signatures of radial oscillatory behaviour in the outer disc plane, with features similar to that of the Monoceros ring. Disc warping is prominent after 3 Gyr of evolution, reaching median z coordinate of approximately 4 kpc. The asymmetric m = 1 mode is dominant for R > 15 kpc in the form of vertical motion, with the m = 2 mode influencing the rotation of disc particles

Extra-wide deposition in extrusion additive manufacturing: A new convention for improved interlayer mechanical performance

Recent studies have contested long-standing assumptions that mechanical anisotropy is caused by weak interlayer bonding and demonstrated that microscale geometry (the groove between extruded filaments) is the major cause of anisotropy in extrusion additive manufacturing (AM). Inspired by those finding, this study investigates the potential for a new convention for print-path design to improve mechanical properties by setting extrusion width to be at least 250 % of nozzle diameter. The new convention enabled an almost 50 % improvement in mechanical performance, which was supported by finite element analysis data, whilst simultaneously reducing the printing time by 67 %. Whereas a typical extrusion AM part uses several side-by-side extrusions, here, three 0.4-mm-wide extrusions are replaced with a single extra-wide 1.2-mm extrusion; two 0.6-mm-wide extrusions are also studied. The contact area between layers of the extra-wide extrusion was 90 % as opposed to 63 % for the conventional approach. The improved contact area led to a 40–48 % enhancement of strength, strain-at-fracture and toughness. This study presents a compelling case for a methodological shift to extra-wide extruded-filament deposition and explains the underlying cause of anisotropic strength observed in previous studies. Two case studies demonstrate practical applicability for a print run of 1000 nylon visors and lower-limb polylactide prosthetic sockets, for which extra-wide filaments more than doubled load-bearing capabilities. Polylactide material was used for most of the study; potential for translation to other materials is discussed

EXCITATION of COUPLED STELLAR MOTIONS in the GALACTIC DISK by ORBITING SATELLITES

We use a set of high-resolution N-body simulations of the Galactic disk to study its interactions with the population of cosmologically predicted satellites. One simulation illustrates that multiple passages of massive satellites with different velocities through the disk generate a wobble, which has the appearance of rings in face-on projections of the stellar disk. They also produce flares in the outer disk parts and gradually heat the disk through bending waves. A different numerical experiment shows that an individual satellite as massive as the Sagittarius dwarf galaxy passing through the disk will drive coupled horizontal and vertical oscillations of stars in underdense regions with small associated heating. This experiment shows that vertical excursions of stars in these low-density regions can exceed 1 kpc in the Solar neighborhood, resembling the recently locally detected coherent vertical oscillations. They can also induce non-zero vertical streaming motions as large as 10-20 km s-1, which is consistent with recent observations in the Galactic disk. This phenomenon appears as a local ring with modest associated disk heating. © 2016. The American Astronomical Society. All rights reserved

Hydrolytic degradation of polylactide in extrusion additive manufacturing

The combined use of material extrusion additive manufacturing (MEAM) and biodegradable polymers such as polylactide (PLA) is one of the most versatile and valuable manufacturing strategies for biomedical applications. MEAM enables rapid production of personalised PLA medical devices as they degrade by hydrolysis over a period of months or years. Although MEAM presents a range of opportunities, there are a number of limitations, the most critical of which is mechanical anisotropy, specifically low strength in the direction normal to the print platform (Z direction). This limits its use for long-term mechanical application. Numerous studies have attributed the diffusion of the polymer chains across the interface between layers as the main underlying mechanism of mechanical anisotropy. However, attempts to understand mechanical anisotropy of MEAM parts have resulted in considerable inconsistencies, with no consensus on the degree of anisotropy or its dependency on printing parameters. In this thesis, experimental studies describe the development of a new microscale uniaxial tensile specimen, based on the idea of COntinuously Varied EXtrusion (CONVEX) by direct GCode scripting to reduce geometrical complexities of current testing standards and to enable improved manufacturing control. The newly devised PLA specimen comprised of stacked individual extruded filaments enabled an improved fundamental analysis of extruded filaments (F specimens, representing bulk-material properties when extruded filament printed along the print platform) and the interface (Z specimens when extruded filament printed normal to the print platform) between them. Geometrical analysis of specimens by microscopy allowed accurate cross-sectional area measurements to be used in strength calculations and generated new understanding about the effect of testing orientation on mechanical properties of MEAM parts. Mechanical and thermal characterisations of both specimen types were conducted to consider the effects of physiological temperature (PT), hydration and in-aqua testing against the control (non-hydrated specimens tested in air at room temperature). Mechanical studies showed bulk-material bond strength between layers. The filament-scale geometries in Z specimens (i.e. grooves between layers) were responsible for strain concentrations and significantly reducing strain at fracture and toughness. In contrast, for F specimens, the grooves were aligned in the direction of loading and did not impact mechanical properties. Furthermore, the importance of submerged tests at PT for PLA was confirmed by demonstrating a combined plasticisation effect of water and higher temperature, highlighting an important risk of conventional laboratory testing overestimating properties by two-fold. The testing environment has a similar effect on both F and Z specimens. Moreover, during long-term hydrolytic degradation experiments, it was found that the interface degraded in a similar manner to the bulk polymer material. Comparison of thermal and chemical properties revealed that during the early stage of hydrolytic degradation, crystallinity was the dominating factor, whilst at later stages, mechanical properties were mainly defined by the molecular weight. The new understanding developed in this thesis highlights that for MEAM parts, the interface does not affect its long-term properties. This improves confidence in using the MEAM process for high-value applications.</div