L’eau tensile est de l’eau liquide métastable qui persiste dans le champ de stabilité de lavapeur à pression négative, sa durée de vie est finie. Des états de traction de l’eau jusqu’à -1400 baront été mesurés de façon spécifique dans des micro-inclusions intracristallines. La nucléation devapeur (Tn) marque le retour à l’équilibre. Les effets destructeurs liés à la rupture d’états transitoiresd’eau tensile sont observés dans le milieu naturel : explosions phréato-magmatiques, geysers.Modéliser la cinétique de l’eau métastable est fondamental pour gérer les risques qui lui sontassociés. Des inclusions fluides synthétiques (IF) de composition et de densité connues, piégées dansdu quartz, ont été placées dans le champ métastable par refroidissement isochore et leurs gammesde métastabilité ont été mesurées. On montre que la traction maximale de l’eau dans chaque IFdépend de son volume et de sa forme, de la méthode de synthèse de l’IF, de la chimie des solutionsoccluses. Des expériences de durée de vie ont été ensuite réalisées sur des IF placées de 0,5° à10°C au-dessus de leurs Tn. Les 8 IF choisies rende nt compte de la diversité des formes, desvolumes, des densités et gammes de traction observées. Les résultats montrent que la durée de viede l’eau tensile en IF est d’autant plus courte que la traction de l’eau est plus forte. Une loiempirique est proposée qui permet de calculer la durée de vie de la métastabilité pour chaque IF deTn et volume fixés. Par ailleurs, nos données peuvent être rendues compatibles avec la ThéorieClassique de la Nucléation. Nos résultats montrent que l’eau dans les réservoirs poreux naturels peutrester métastable pendant des durées géologiques et ainsi, contrôler les interactions fluides-rochesdans la croûte. / Stretched (tensile) liquid water is a metastable liquid which persists at negative pressures inthe stability field of vapour. The lifetime of metastability is limited. Tensions down to - 1400 bar havebeen specifically measured in aqueous inclusions inside quartz monocrystals. Vapour nucleation (Tn)marks the end of metastability. The destructive effects related to vapour nucleation in transientlytensile fluids are observed in nature: phreato-magmatic explosions, geysers. Modelling the kinetics oftensile water is critical in order to control the risks associated to metastable liquids. Quartz-hostedsynthetic fluid inclusions (FI) with known densities and chemistries have been placed into themetastable tensile field by isochoric cooling and their Tn have been measured. We show that thetensile strength of water in individual FI depends on the FI volume and shape, the method used tosynthetize the FI and the fluid chemistry. Experiments on metastability lifetimes have been performedby placing FI at temperatures 0.5° to 10°C above th eir Tn. Eigth FI were chosen that encompass thediversity of FI volumes, shapes, densities, fluid chemistries and tensile strengths. Our results showthat tensile water lifetimes are all the shorter as the trapped water is more stretched. An empiricalkinetic law is proposed that allows the lifetimes of tensile water in FI to be calculated as a function ofthe FI volume and Tn. Our data can also be reconciled with the Classical Nucleation Theory. Our datafinally show that water in natural porous reservoirs can remain stretched for geologically-relevanttimescales. Tensile water can therefore control fluid-rock interactions in the continental crust.

This thesis describes the synthesis and crystallisation of a selection of known and novel transition-metal – nitrite complexes and their detailed study using non-ambient X-ray diffraction techniques. These coordination compounds are specifically investigated for their ability to undergo nitro – nitrito linkage isomerisation in the single-crystal, with conversion between isomers achieved on either thermal- or photoactivation. The lifetime of the excited-state isomer is strongly dependent on the experimental temperature and, as such, this thesis is also concerned with time-resolved crystallographic methods. This work relies primarily on the technique of photocrystallography, a growing area of crystallographic research that has enjoyed considerable interest in recent decades. Chapter 1 reviews the seminal progress made in both time-resolved and photocrystallographic research, made possible by dramatic technological advances in the field in recent decades. Chapter 2 outlines the goals of this research project, aiming to obtain a more detailed understanding of the factors that influence the solid-state nitro – nitrito isomerisation. Chapter 3 describes the X-ray methods used in these studies, introducing both fundamental crystallographic principles and detailing the experimental procedures that are undertaken. Chapters 4 and 5 record steady-state photocrystallographic studies conducted for a series of complexes containing bulky, chelating ancillary ligands accompanying a monodentate nitrite group expected to be capable of nitro – nitrito isomerism. The response of these systems to both thermal- and photoactivation is investigated via single-crystal diffraction methods. The analysis focusses on the potential steric and electronic influences from the surrounding crystalline environment, which may have an impact on the progress of the solid-state reaction. Chapter 6 introduces photocystallographic kinetic methods to investigate the progress of nitro – nitrito conversion in real time. These studies are the first of their kind to be conducted on nitrite linkage isomer species and some novel methodology is introduced. Finally, Chapter 7 discusses pseudo-steady-state and trial time-resolved experiments conducted using the new time-resolved diffraction set-up on Beamline I19 at the Diamond Light Source. These early results indicate the power of time-resolved methods for elucidating key information on the nitro – nitrito conversion process and show promise for future experiments.

La thématique abordée dans cette thèse relève de la manipulation d’un jet d’atomes d’argon métastables (Ar* ³P2) dans différentes configurations. Premièrement, je présente l’échange de métastabilité entre un atome au fondamental et un atome excité à de faibles énergies de centre de masse (entre 4 et 9 meV). Je propose également l’interprétation théorique par une approche semi-classique (approximation JWKB) qui se révèle validée, dans ce domaine d’énergies, en comparaison avec la résolution exacte de l’équation de Schrödinger radiale mettant en jeu les potentiels concernés par la collision. Les sections efficaces absolues d’échanges, déduites d’une analyse en temps de vol du signal métastable, permettent de réaliser une comparaison sans biais avec les prédictions théoriques. Les caractéristiques d’un jet ralenti par effet Zeeman sont dégradées par le processus de ralentissement et le rende difficilement utilisable en deçà de quelques dizaines de mètres par seconde. C’est pourquoi, dans un deuxième temps, je présente la réalisation d’un jet lent original, issu d’un piège magnéto-optique et présentant des caractéristiques remarquables. La vitesse est accordable entre 10 et 100 m/s, la dispersion de vitesse relative est très faible (6 % à 20 m/s) et le flux est conséquent (10⁹ Ar*/s/sr), pour une ouverture angulaire standard (35 mrad FWHM). Ce nouveau dispositif permet de présenter certaines questions d’interférométrie et d’optique atomique telles que les interactions atome-surface de type van der Waals et l’étude de potentiels comobiles ainsi que leurs applications dans la réalisation de milieux d’indice négatif ou de ralentisseurs. / The topic of this thesis concerns the manipulation of a metastable argon (Ar* ³p2) atomic beam in different configurations. Firstly, I present the metastability exchange between an atom in fundamental state and an excited atom at low center of mass energy (between 4 and 9 meV). I also propose theoretical interpretation by a semi-classical approach (JWKB approximation) which is validated, in this field of energies, compared to the exact solution of the Schrödinger radial equation with potentials involved in collision. The absolute exchange cross-sections, derived from a time of flight analysis of metastable signal, enable an unbiased comparison with theoretical predictions. The characteristics of a Zeeman slowedbeam are degraded by the process of slowing down and makes it difficult to use below a few tens of meters per second. Secondly, I present the realization of an original slow beam from a magneto-optical trap and having outstanding features. The atomic velocity is tunable between 10 and 100 m/s, the relative velocity dispersion is very low (6 % at 20 m/s) and the flow is substantial, (4.7×108 Ar*/s/sr), for a standard angular aperture (35 mrad FWHM). This new device can present some issues in atomic interferometry and atomic optics such as van der Waals atom-surface interactions or study co-moving potentials and their applications in negative-index media for matter wave or slowers.

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No description available.

The current knowledge of the performance characteristics of Centrifugal Tensioned Metastable Fluid Detectors is limited. While a theoretical treatment and experience with bubble chambers may be applied with some degree of success, they are no substitute for experimental and operational knowledge of real CTMFD systems. This research, as with other investigations into CTMFD systems in the past, applies theory and simulations. In addition, however, an experiment was conducted that for the first time attempts to determine the threshold energy for triggering a CTMFD system in a controlled manner. A CTMFD system works in a manner similar to classic bubble chambers. A liquid is brought to an unstable state in which it is favorable to form a volume of vapor; using centrifugal techniques similar to those employed in a Briggs apparatus, the pressure in the sensitive region can be brought to extremely low values, placing the liquid in a tensile state. In such states, the energy necessary to cause the formation of macroscopic bubbles can be vanishingly small, depending on the degree of tension. When such bubbles form in a CTMFD, if they have a size bigger than a critical value, they will grow until a large vapor column forms in the sensitive region of the CTMFD. The experiment developed for this research employed a carefully-controlled laser to fire pulses of known energies into the sensitive region of a CTMFD. By varying the laser power, the threshold values for the triggering energy of a CTMFD can be found. The experiment and simulation demonstrated the ability of the facilities to test CTMFD systems and the potential to extract their operational characteristics. The experiment showed a certain viability for the technique of laser-induced cavitation in a seeded fluid, and demonstrated some of the associated limitations as well. In addition, the CFD framework developed here can be used to cross-compare experimental results with computer simulations as well as with the theoretical models developed for this research.

Repetitive sequences, including transposable elements, represent approximately half of the mammalian genome. Epigenetic mechanisms evolved to repress these potentially deleterious mobile elements. However, such elements can be variably silenced between individuals – so called ‘metastable epialleles’. The best known example is the Avy locus where an endogenous retrovirus (ERV) of the intracisternal A-particle (IAP) class was spontaneously inserted upstream of the agouti coat colour gene, resulting in variable IAP promoter DNA methylation, variable expressivity of coat phenotype, and environmentally modulated transgenerational epigenetic inheritance within genetically identical individuals. It is not known whether the behaviour exhibited by the ERV at Avy represents a common occurrence throughout the genome or is unusual. Taking a genetic approach in purified cell populations, I have conducted a systematic genome-wide screen of murine metastable epialleles. I have identified over 100 murine IAPs with properties of metastable epialleles. Like Avy, each exhibits a stable epigenetic state within an individual but epigenetic variability between individuals. Methylation levels are locus-specific within an individual, suggesting cis-acting control. The same screening strategy was applied for identification of metastable epialleles associated with other types of LTR-retroelements. However, many of identified candidates showed no inter-individual methylation variation upon experimental validation. These results suggest that IAPs are the dominant class of ERVs capable of acquiring epigenetic states that are variable between genetically identical individuals. I have conducted an analysis of IAP induced initiation and termination of transcription events using de novo assembled transcriptomes generated for B and T cells. 143 IAPs have been identified to overlap de novo assembled transcripts. 33 IAPs are metastable epialleles. Several of them show an inverse correlation between LTR promoter methylation and adjacent gene expression. In addition, I have shown that metastable epialleles have a characteristic pattern of histone modification and are flanked by the methylation sensitive binding factor CTCF, providing testable hypotheses concerning the establishment and/or maintenance of the variable methylation state. My findings indicate that metastability is, in general, specific to the IAP class of ERVs, that only around 1% of these elements have this unusual epigenetic property and that the ability to impact transcription, such as at agouti in Avy, is not a ubiquitous feature of these loci.

Predicting events in metastable systems is an important but challenging problem. It can help society forecast, prevent, or prepare for upcoming catastrophes. However, many metastable systems in nature operate near a critical point and are empirically unpredictable. We developed machine learning predictors, applied them to the prediction of nucleation events in the metastable Ising model, near and far from the spinodal critical point. We observed decreasing predictability as the critical point is approached, and found that this unpredictability is due to the vanishing density difference between the nucleating droplet and the background. We also developed a tensor representation of Lennard-Jones con gurations using the symmetry order parameters of the particles and use this representation to predict nucleation in a dense Lennard Jones liquid. Finally, we investigated the noise-induced critical point in two variations of the OFC model - a coupled OFC model and a OFC model with multiplicative noise. In both variations, we found a critical phase boundary that separates the ergodic and non-ergodic phase and the termination point of the phase boundary, which is consistent with a higher-order phase transition.

No description available.

<p>In this work, an in-depth analysis of crystalline characteristics has been performed for a unique set of strictly monodisperse poly-ε-caprolactone (PCL) oligomers. The molecules have different sets of end groups with various degrees of bulkiness and hydrogen bonding potential, affecting their aptitude to pack in ordered crystal structures. The oligomers also have different numbers of repeating units (<em>n </em>= 2-64), affecting the degree to which end groups influence overall molecular characteristics. The presence of bulky end groups leads to an innate confinement effect on crystallisation which in turn makes it possible to utilize the set of PCL oligomers to study confined space crystallisation. Confined space crystallisation is explored as a route to gain further understanding about the early metastable phases in crystal formation.</p><p> </p><p>The monodisperse nature of the samples made it possible to collect very precise small-angle and wide-angle X-ray scattering data (SAXS and WAXS) as well as calorimetric data. Computer modeling studies were performed to support experimental findings. It was shown that end groups strongly affected crystallisation features for the shorter oligomers (<em>n </em>≤ 8) but to a lesser extend for the longer oligomers (<em>n </em>≥ 16). The presence of a bulky end group at one end of an oligomer could inhibit the formation of hydrogen bonds on the other end. Short oligomers (<em>n</em> = 8) with OH-end groups exhibited novel packing characteristics. At one isothermal crystallisation temperature the molecules exhibited not only lamellar ordering but also an additional, likely rectangular or slanted, ordering. The sample was packed in a unique structure with molecular chains lying parallel but not aligned head to head with each other. At a higher crystallisation temperature the molecules packed in a double layered structure and at an even higher temperature in a typical non-folded but tilted single-molecular layer pattern.</p><p> </p><p>Unit cell determination was performed for a short oligomer with two bulky end groups, showing the existence of a tetragonal unit cell with different dimensions than the orthorhombic unit cells previously reported for linear PCL without end groups. To gain greater insight into the earliest stages of molecular packing, in situ WAXS measurements were performed using a synchrotron radiation beam and measuring data each 12 s whilst very slowly going from melt to isothermal crystallisation. It was shown that the crystal unit cell was distorted during the first minutes of slow crystallisation, which might either represent a metastable phase or else a highly distorted orthorhombic phase.</p>