225 research outputs found
Wee1B, Myt1, and Cdc25 function in distinct compartments of the mouse oocyte to control meiotic resumption
Keeping Wee1B in the nucleus is important to maintain meiotic arrest, but its timely export is also required for meiosis to resume
Interactive thermal effects on metal–organic framework Polymer composite membranes
Polymeric membranes are important tools for intensifying separation processes in chemical industries, concerning strategic tasks such as CO2 sequestration, H2 production, and water supply and disposal. Mixed-matrix and supported membranes have been widely developed; recently many of them have been based on metal–organic frameworks (MOFs). However, most of the impacts MOFs have within the polymer matrix have yet to be determined. The effects related to thermal behavior arising from the combination of MOF ZIF-8 and polysulfone have now been quantified. The catalyzed oxidation of the polymer is strongly affected by the MOF crystal size and distribution inside the membrane. A 16 wt¿% 140 nm-sized ZIF-8 loading causes a 40¿% decrease in the observed activation energy of the polysulfone oxidation that takes place at a temperature (545¿°C) 80¿°C lower than in the raw polymer (625¿°C)
Force unfolding kinetics of RNA using optical tweezers. I. Effects of experimental variables on measured results
Experimental variables of optical tweezers instrumentation that affect RNA
folding/unfolding kinetics were investigated. A model RNA hairpin, P5ab, was
attached to two micron-sized beads through hybrid RNA/DNA handles; one bead was
trapped by dual-beam lasers and the other was held by a micropipette. Several
experimental variables were changed while measuring the unfolding/refolding
kinetics, including handle lengths, trap stiffness, and modes of force applied
to the molecule. In constant-force mode where the tension applied to the RNA
was maintained through feedback control, the measured rate coefficients varied
within 40% when the handle lengths were changed by 10 fold (1.1 to 10.2 Kbp);
they increased by two- to three-fold when the trap stiffness was lowered to one
third (from 0.1 to 0.035 pN/nm). In the passive mode, without feedback control
and where the force applied to the RNA varied in response to the end-to-end
distance change of the tether, the RNA hopped between a high-force folded-state
and a low-force unfolded-state. In this mode, the rates increased up to
two-fold with longer handles or softer traps. Overall, the measured rates
remained with the same order-of-magnitude over the wide range of conditions
studied. In the companion paper (1), we analyze how the measured kinetics
parameters differ from the intrinsic molecular rates of the RNA, and thus how
to obtain the molecular rates.Comment: PDF file, 30 pages, 7 figure
Phase field approach with anisotropic interface energy and interface stresses: Large strain formulation
A thermodynamically consistent, large-strain, multi-phase field approach (with consequent interface stresses) is generalized for the case with anisotropic interface (gradient) energy (e.g. an energy density that depends both on the magnitude and direction of the gradients in the phase fields). Such a generalization, if done in the “usual” manner, yields a theory that can be shown to be manifestly unphysical. These theories consider the gradient energy as anisotropic in the deformed configuration, and, due to this supposition, several fundamental contradictions arise. First, the Cauchy stress tensor is non-symmetric and, consequently, violates the moment of momentum principle, in essence the Herring (thermodynamic) torque is imparting an unphysical angular momentum to the system. In addition, this non-symmetric stress implies a violation of the principle of material objectivity. These problems in the formulation can be resolved by insisting that the gradient energy is an isotropic function of the gradient of the order parameters in the deformed configuration, but depends on the direction of the gradient of the order parameters (is anisotropic) in the undeformed configuration. We find that for a propagating nonequilibrium interface, the structural part of the interfacial Cauchy stress is symmetric and reduces to a biaxial tension with the magnitude equal to the temperature- and orientation-dependent interface energy. Ginzburg–Landau equations for the evolution of the order parameters and temperature evolution equation, as well as the boundary conditions for the order parameters are derived. Small strain simplifications are presented. Remarkably, this anisotropy yields a first order correction in the Ginzburg–Landau equation for small strains, which has been neglected in prior works. The next strain-related term is third order. For concreteness, specific orientation dependencies of the gradient energy coefficients are examined, using published molecular dynamics studies of cubic crystals. In order to consider a fully specified system, a typical sixth order polynomial phase field model is considered. Analytical solutions for the propagating interface and critical nucleus are found, accounting for the influence of the anisotropic gradient energy and elucidating the distribution of components of interface stresses. The orientation-dependence of the nonequilibrium interface energy is first suitably defined and explicitly determined analytically, and the associated width is also found. The developed formalism is applicable to melting/solidification and crystal-amorphous transformation and can be generalized for martensitic and diffusive phase transformations, twinning, fracture, and grain growth, for which interface energy depends on interface orientation of crystals from either side
The large area detector onboard the eXTP mission
The Large Area Detector (LAD) is the high-throughput, spectral-timing instrument onboard the eXTP mission, a flagship
mission of the Chinese Academy of Sciences and the China National Space Administration, with a large European
participation coordinated by Italy and Spain. The eXTP mission is currently performing its phase B study, with a target
launch at the end-2027. The eXTP scientific payload includes four instruments (SFA, PFA, LAD and WFM) offering
unprecedented simultaneous wide-band X-ray timing and polarimetry sensitivity. The LAD instrument is based on the
design originally proposed for the LOFT mission. It envisages a deployed 3.2 m2 effective area in the 2-30 keV energy
range, achieved through the technology of the large-area Silicon Drift Detectors - offering a spectral resolution of up to
200 eV FWHM at 6 keV - and of capillary plate collimators - limiting the field of view to about 1 degree. In this paper
we will provide an overview of the LAD instrument design, its current status of development and anticipated
performance
Black hole spin: theory and observation
In the standard paradigm, astrophysical black holes can be described solely
by their mass and angular momentum - commonly referred to as `spin' - resulting
from the process of their birth and subsequent growth via accretion. Whilst the
mass has a standard Newtonian interpretation, the spin does not, with the
effect of non-zero spin leaving an indelible imprint on the space-time closest
to the black hole. As a consequence of relativistic frame-dragging, particle
orbits are affected both in terms of stability and precession, which impacts on
the emission characteristics of accreting black holes both stellar mass in
black hole binaries (BHBs) and supermassive in active galactic nuclei (AGN).
Over the last 30 years, techniques have been developed that take into account
these changes to estimate the spin which can then be used to understand the
birth and growth of black holes and potentially the powering of powerful jets.
In this chapter we provide a broad overview of both the theoretical effects of
spin, the means by which it can be estimated and the results of ongoing
campaigns.Comment: 55 pages, 5 figures. Published in: "Astrophysics of Black Holes -
From fundamental aspects to latest developments", Ed. Cosimo Bambi, Springer:
Astrophysics and Space Science Library. Additional corrections mad
Influência do resfriamento dinâmico na medição da tensão limite de escoamento
Non-Newtonian fluids may present several complex characteristics, such as viscoelasticity, plasticity and thixotropy. Understanding the behaviour of these materials is essential to facilitate its applicability, since many of them are present in daily life, such as gels, chocolate, and mayonnaise. The material studied on the present work was an waxy crude oil. The oil, present in reservoirs located in deep water, when subjected to low temperatures during production shutdowns, precipitates its paraffin, creating a structure which may
be very resistant to flow. Thus, the formed material may represent a great difficulty to production restart. The design of pumps and compressors used in these systems depend on a rheological property called yield stress and therefore, it is essential to obtain reliable values of this rheological parameter. However, the complex behaviour, and the dependence of the crude oil characteristics on the shearing and temperature history make this a challenging task. Besides these, other factors may influence the measurement of yield
stress, as the effect of wall slip and if the cooling is static or dynamic. This work shows the variation of yield stress for tests with dynamic cooling. The imposition of a stress during cooling caused the reduction in yield stress 0. Increasing the imposed stress, 0 decreased until it reached a minimum. Next, it was noticed an increase in yield stress with increasing imposed stress. The experiments carried out in this work were made in a commercial stress control rheometer. The effect of wall slip was tested by comparing smooth and rough geometries and it was verified signs of its presence, with the reduction of yield stress for smooth geometries. The equation of Weissenberg-Rabinowitsch, used for correction of the inhomogeneity of the shear stress along the radius geometry, seemed to be not necessary for the demonstrated results.Fluidos nĂŁo newtonianos podem apresentar diversas caracterĂsticas complexas, como a viscoelasticidade, plasticidade e tixotropia. Entender o comportamento de materiais tĂŁo complexos Ă© fundamental para facilitar sua aplicabilidade, já que muitos estĂŁo presentes no cotidiano das pessoas, como gĂ©is, chocolate e maionese. O material estudado no presente trabalho foi um petrĂłleo parafĂnico. O petrĂłleo, presente em bacias localizadas em águas profundas, quando submetido a baixas temperaturas durante paradas de produção, precipita suas parafinas dando origem a uma estrutura que pode ser muito resistente ao
escoamento. Dessa forma, o material formado pode representar uma grande dificuldade para o reinĂcio da produção. O dimensionamento das bombas e compressores utilizados para retomada da produção depende de uma propriedade reolĂłgica denominada tensĂŁo limite de escoamento e, por isso, Ă© fundamental obter valores confiáveis desse parâmetro reolĂłgico. No entanto, o comportamento complexo, e a dependĂŞncia das caracterĂsticas do petrĂłleo com o histĂłrico de cisalhamento e temperatura tornam a tarefa desafiadora.
Outros fatores podem influenciar na medição da tensĂŁo limite de escoamento, como o efeito do escorregamento na parede do sensor da geometria e se o resfriamento Ă© estático ou dinâmico. Este trabalho mostra a variação da tensĂŁo limite de escoamento para ensaios com resfriamento dinâmico. A imposição de tensĂŁo durante o resfriamento causou a redução da tensĂŁo limite de escoamento 0. Elevando-se a tensĂŁo imposta, 0 reduziu atĂ© atingir um valor mĂnimo. Em seguida, percebeu-se um aumento da tensĂŁo limite de escoamento com o aumento da tensĂŁo imposta. Os experimentos realizados neste trabalho foram feitos em um reĂ´metro comercial do tipo tensĂŁo controlada (“stress controlled”). O efeito de
escorregamento da amostra foi testado comparando geometrias com superfĂcies lisas e ranhuradas e foi possĂvel verificar sinais de sua presença, com a redução da tensĂŁo limite de escoamento para geometrias lisas. A equação de Weissenberg-Rabinowitsch, utilizada para correção da nĂŁo homogeneidade da tensĂŁo de cisalhamento ao longo do raio da geometria, mostrou-se nĂŁo necessária para os resultados demonstrados
Chronostratigraphie et paléoécologie du Paléolithique supérieur morave d'après les fouilles récentes
The Upper Paleolithic evolution in Moravia starts from the end of the first Wurmian Pleniglacial (before 40 000 B.P., earlier industries of the Bohunice- type). During the following Interpleniglacial, two temperate periods were distinguished : at about 38 000 B.P. (younger industries of the Bohunice- type, Szeletian) and between 32-25 000 B.P. (Aurignacian, Pavlovian). There seems to be a settlement hiatus during the second Pleniglacial maximum, followed by a resettlement in the Late Glacial. Within the Late Glacial period, three phases of wood extension were recognized. They are related subsequently to an undetermined Upper Palaeolithic, the Magdalenian and the Epimagdalenian.L'évolution du Paléolithique supérieur morave commence à la fin du premier Pléniglaciaire würmien (avant 40 000 ans, industnes anciennes du type Bohunice). Dans I ' Interpléniglaciaire suivant, on peut distinguer deux périodes de réchauffement, celle autour de 38 000 ans (industries récentes du type Bohunice, Szélétien) et celle entre 32-25 000 ans (Aurignacien, Pavlovien). On observe un hiatus dans l'occupation pendant le maximum du second Pléniglaciaire et une réoccupation dans le Tardiglaciaire suivant. Pendant le Tardiglaciaire, on a distingué trois périodes de boisement, liées successivement à un Paléolithique supérieur indéterminable, le Magdalénien et l'Epimagdalénien.Svobodova Helena, Svoboda Jin. Chronostratigraphie et paléoécologie du Paléolithique supérieur morave d'après les fouilles récentes. In: Revue archéologique de Picardie, n°1-2, 1988. Cultures et industries lithiques en milieu lœssique. Actes du colloque international, Amiens 9-11 décembre 1986. pp. 11-15
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