Molecular Mechanisms by which Tetrahydrofuran Affects CO₂ Hydrate Growth: Implications for Carbon Storage

Gas hydrates have attracted siginifcant fundamental and applied interests due to their important role in various technological and enviromental processes. More recently, gas hydrates have shown potential applications for greenhouse gas capture and storage. To facilitate the latter application, introducing chemical additives into clathrate hydrates could help to enhance hydrate formation/growth rates, provided the gas storage capacity is not reduced. Employing equilibrium molecular dynamics, we study the impact of tetrahydrofuran (THF) on the kinetics of carbon dioxide (CO₂) hydrate growth/dissociation and on the CO₂ storage capacity of hydrates. Our simulations reproduce experimental data for CO₂ and CO₂+THF hydrates at selected operating conditions. The simulated results confirm that THF in stoichiometric concentration does reduce CO₂ storage capacity. This is not only due to the shortage of CO₂ trapping in sII hydrate 5^{12} cages, but also because of the favored THF occupancy in hydrate cages due to preferential THF−water hydrogen bonds. An analysis of the dynamical properties for CO₂ and THF at the hydrate-liquid interface reveals that THF can expedite CO₂ diffusion yielding a shift in the conditions conducive to CO₂ hydrate growth and stability to lower pressures and higher temperatures compared to systems without THF. These simulation results augment literature experimental observations, as they provide needed insights into the molecular mechanisms that can be adjusted to achieve optimal CO₂ storage in hydrates

Acoustic characterization of Hofstadter butterfly with resonant scatterers

We are interested in the experimental characterization of the Hofstadter butterfly by means of acoustical waves. The transmission of an acoustic pulse through an array of 60 variable and resonant scatterers periodically distribued along a waveguide is studied. An arbitrary scattering arrangement is realized by using the variable length of each resonator cavity. For a periodic modulation, the structures of forbidden bands of the transmission reproduce the Hofstadter butterfly. We compare experimental, analytical, and computational realizations of the Hofstadter butterfly and we show the influence of the resonances of the scatterers on the structure of the butterfly

Versatile Confocal Raman Imaging Microscope Built from Off-the-Shelf Opto-Mechanical Components

Confocal Raman microscopic (CRM) imaging has evolved to become a key tool for spatially resolved, compositional analysis and imaging, down to the μm-scale, and nowadays one may choose between numerous commercial instruments. That notwithstanding, situations may arise which exclude the use of a commercial instrument, e.g., if the analysis involves toxic or radioactive samples/environments; one may not wish to render an expensive instrument unusable for other uses, due to contamination. Therefore, custom-designed CRM instrumentation—being adaptable to hazardous conditions and providing operational flexibility—may be beneficial. Here, we describe a CRM setup, which is constructed nearly in its entirety from off-the-shelf optomechanical and optical components. The original aim was to develop a CRM suitable for the investigation of samples exposed to tritium. For increased flexibility, the CRM system incorporates optical fiber coupling to both the Raman excitation laser and the spectrometer. Lateral raster scans and axial profiling of samples are facilitated by the use of a motorized xyz-translation assembly. Besides the description of the construction and alignment of the CRM system, we also provide (i) the experimental evaluation of system performance (such as, e.g., spatial resolution) and (ii) examples of Raman raster maps and axial profiles of selected thin-film samples (such as, e.g., graphene sheets)

Das Verhalten von Plasmopara viticola auf resistenten und anfälligen Rebsorten

An Topfreben und Blattscheiben verschiedener resistenter und anfälliger Rebsorten wurde das Verhalten von Plasmopara viticola bei verschiedenen Inokulumdichten, unterschiedlicher Temperatur und Beleuchtung während der Inkubation, sowie veränderter Inkubationsdauer und Blattnässedauer geprüft.Einen starken Einfluß auf die Sporulation des Pilzes auf Blattscheiben hatten die Konzentration des Inokulums sowie die Temperatur und die Belichtung während der Inkubation. Bei diesen Faktoren traten signifikante Interaktionen mit der Sorte auf. Die Dauer der Blattnässe war, soweit sie die für eine Infektion notwendige Dauer nicht unterschritt, ohne Einfluß auf die Sporulation des Pilzes bei verschiedenen Sorten. Ähnliches gilt für die Inkubationsdauer, deren Verlängerung, auch weit über die minimal erforderliche Zeit hinaus, ohne Einfluß auf die Sporulation blieb.The behavior of Plasmopara viticola on resistant and susceptible grapevine varietiesThe behaviour of Plasmopara viticola with respect to inoculum concentration, temperature, level of illumination during incubation, duration of incubation and leaf wetness was investigated with different resistant und susceptible grapevine varieties using leaf discs and potted plants. Inoculum concentration, temperature and illumination during incubation had a great effect on the sporulation of the fungus on leaf discs and proved to be cultivar-dependent. Duration of leaf wetness had no effect on sporulation provided that it was long enough for the infection process to be completed. In a similar way an incubation time longer than that necessary for completion of fungal growth, had no effect on sporulation

Phase transitions in soft-committee machines

Equilibrium statistical physics is applied to layered neural networks with differentiable activation functions. A first analysis of off-line learning in soft-committee machines with a finite number (K) of hidden units learning a perfectly matching rule is performed. Our results are exact in the limit of high training temperatures. For K=2 we find a second order phase transition from unspecialized to specialized student configurations at a critical size P of the training set, whereas for K > 2 the transition is first order. Monte Carlo simulations indicate that our results are also valid for moderately low temperatures qualitatively. The limit K to infinity can be performed analytically, the transition occurs after presenting on the order of N K examples. However, an unspecialized metastable state persists up to P= O (N K^2).Comment: 8 pages, 4 figure

Wirtsbesiedlung durch Plasmopara viticola bei unterschiedlich anfälligen Wirten

Mit Plasmopara viticola infizierte Blattscheiben von verschieden anfalligen Rebsorten und Rebarten wurden nach 6, 24 und 48 h Inkubation fixiert und mit optischen Aufhellern oder Säurefuchsinlösung gefärbt. Die in den Blattscheiben vorgefundenen Entwicklungsstadien des Erregers wurden quantitativ ausgewertet.Die Resistenz einer Rebsorte hat keinen Einfluß auf die Anlagerung der Zoosporen von P. viticola an ihre Spaltöffnungen. Die ersten Behinderungen der Wirtsbesiedlung an resistenten Arten traten auf, wenn der Keimschlauch des Erregers versucht, in die Atemhöhle einzudringen. Bei den verschiedenen Klonen der Arten Vitis riparia und V. rupestris gelang es P. viticola häufig nicht, in der Atemhöhle sichtbare Strukturen zu bilden. Bei V. riparia und der pilzresistenten Sorte Fr 946-60 wurde die Erregerentwicklung in der Atemhöhle häufig schon vor der Bildung des Primärhaustoriums behindert. Bei den pilzresistenten Sorten Fr 946-60, Fr 993-60, Castor und SV 5-276 wurde der Erreger meist nach der Bildung eines Haustoriums in der weiteren Entwicklung behindert . In diesem Entwicklungsstadium kam es häufig zu Degenerationserscheinungen an der befallenen Wirtszelle. Bei der Sorte Pollux traten diese Behinderungen später, meist nach Bildung des zweiten Haustoriums, auf.Host settlement of Plasmopara viticola on different susceptible hostsLeaf discs of grapevine varieties and species of various resistance to downy mildew were infected by Plasmopara viticola, fixed after 6, 24 and 48 h of incubation and coloured with optical brighteners or fuchsin acid solution. The different stages of infection were investigated qualitatively and quantitatively. Zoospores of P. viticola settled at the stomata of leaves of resistant and susceptible cultivars. The first phenomenon of resistance occurs when the germination tube tries to enter the substomatal hole. In clones of Vitis riparia and V. rupestris germination tubes of the pathogen were frequently hindered to enter the substomatal hole. In V. riparia and the resistant variety Fr 946-60 P. viticola was often already stopped before forming a primary haustorium. In the resistant varieties Fr 946-60, Fr 993-60, Castor and SV 5-276 the pathogen was stopped in many cases after the formation of the first haustorium. At this time degeneration of the parasitized host-cells could often be observed. In the resistant variety Pollux cell degenerations mostly occur after the formation of the secondary haustorium

Sheared bioconvection in a horizontal tube

The recent interest in using microorganisms for biofuels is motivation enough to study bioconvection and cell dispersion in tubes subject to imposed flow. To optimize light and nutrient uptake, many microorganisms swim in directions biased by environmental cues (e.g. phototaxis in algae and chemotaxis in bacteria). Such taxes inevitably lead to accumulations of cells, which, as many microorganisms have a density different to the fluid, can induce hydrodynamic instabilites. The large-scale fluid flow and spectacular patterns that arise are termed bioconvection. However, the extent to which bioconvection is affected or suppressed by an imposed fluid flow, and how bioconvection influences the mean flow profile and cell transport are open questions. This experimental study is the first to address these issues by quantifying the patterns due to suspensions of the gravitactic and gyrotactic green biflagellate alga Chlamydomonas in horizontal tubes subject to an imposed flow. With no flow, the dependence of the dominant pattern wavelength at pattern onset on cell concentration is established for three different tube diameters. For small imposed flows, the vertical plumes of cells are observed merely to bow in the direction of flow. For sufficiently high flow rates, the plumes progressively fragment into piecewise linear diagonal plumes, unexpectedly inclined at constant angles and translating at fixed speeds. The pattern wavelength generally grows with flow rate, with transitions at critical rates that depend on concentration. Even at high imposed flow rates, bioconvection is not wholly suppressed and perturbs the flow field.Comment: 19 pages, 9 figures, published version available at http://iopscience.iop.org/1478-3975/7/4/04600

Quantum Hall effect in a p-type heterojunction with a lateral surface quantum dot superlattice

The quantization of Hall conductance in a p-type heterojunction with lateral surface quantum dot superlattice is investigated. The topological properties of the four-component hole wavefunction are studied both in r- and k-spaces. New method of calculation of the Hall conductance in a 2D hole gas described by the Luttinger Hamiltonian and affected by lateral periodic potential is proposed, based on the investigation of four-component wavefunction singularities in k-space. The deviations from the quantization rules for Hofstadter "butterfly" for electrons are found, and the explanation of this effect is proposed. For the case of strong periodic potential the mixing of magnetic subbands is taken into account, and the exchange of the Chern numbers between magnetic subands is discussed.Comment: 12 pages, 5 figures; reported at the 15th Int. Conf. on High Magnetic Fields in Semicond. Phys. (Oxford, UK, 2002