567 research outputs found
Microscopic energy flows in disordered Ising spin systems
An efficient microcanonical dynamics has been recently introduced for Ising
spin models embedded in a generic connected graph even in the presence of
disorder i.e. with the spin couplings chosen from a random distribution. Such a
dynamics allows a coherent definition of local temperatures also when open
boundaries are coupled to thermostats, imposing an energy flow. Within this
framework, here we introduce a consistent definition for local energy currents
and we study their dependence on the disorder. In the linear response regime,
when the global gradient between thermostats is small, we also define local
conductivities following a Fourier dicretized picture. Then, we work out a
linearized "mean-field approximation", where local conductivities are supposed
to depend on local couplings and temperatures only. We compare the approximated
currents with the exact results of the nonlinear system, showing the
reliability range of the mean-field approach, which proves very good at high
temperatures and not so efficient in the critical region. In the numerical
studies we focus on the disordered cylinder but our results could be extended
to an arbitrary, disordered spin model on a generic discrete structures.Comment: 12 pages, 6 figure
Continuum approach to wide shear zones in quasi-static granular matter
Slow and dense granular flows often exhibit narrow shear bands, making them
ill-suited for a continuum description. However, smooth granular flows have
been shown to occur in specific geometries such as linear shear in the absence
of gravity, slow inclined plane flows and, recently, flows in split-bottom
Couette geometries. The wide shear regions in these systems should be amenable
to a continuum description, and the theoretical challenge lies in finding
constitutive relations between the internal stresses and the flow field. We
propose a set of testable constitutive assumptions, including
rate-independence, and investigate the additional restrictions on the
constitutive relations imposed by the flow geometries. The wide shear layers in
the highly symmetric linear shear and inclined plane flows are consistent with
the simple constitutive assumption that, in analogy with solid friction, the
effective-friction coefficient (ratio between shear and normal stresses) is a
constant. However, this standard picture of granular flows is shown to be
inconsistent with flows in the less symmetric split-bottom geometry - here the
effective friction coefficient must vary throughout the shear zone, or else the
shear zone localizes. We suggest that a subtle dependence of the
effective-friction coefficient on the orientation of the sliding layers with
respect to the bulk force is crucial for the understanding of slow granular
flows.Comment: 11 pages, 7 figure
Laser Marking of Titanium Coating for Aerospace Applications
Abstract In the aerospace industry, in order to ensure the identification and the traceability of the products, high repeatability, non-invasive and durable marking processes are required. Laser marking is one of the most advanced marking technologies. Compared to traditional marking processes, like punches, microdot, scribing or electric discharge pencil etcher, laser marking offers several advantages, such us: non-contact working, high repeatability, high scanning speed, mark width comparable to the laser spot dimension, high flexibility and high automation of the process itself. In order to assure the mark visibility for the component lifetime, an appropriate depth of the mark is required. In this way, a stable behaviour is ensured also when the component operates in aggressive environments (i.e. in presence of oxidation, corrosion and wear phenomena). The mark depth is strongly affected by the laser source kind and by the process parameters, such us average power, pulse frequency and scanning speed. Moreover, an excessive mark penetration could cause stress concentrations and reduce the fatigue life of the component. Consequently, an appropriate selection of the process parameters is required in order to assure visibility and to avoid excessive damage. Cold Spray Deposition (CSD) is a relative new technology that allows to produce surface coatings without significant substrate temperature increasing. In aeronautics fields this technology is useful to coat materials sensible to temperature, such as solution tempered aluminum alloy, with a titanium layer. Aim of the work is to characterize the laser marking process on CSD Ti coating, in order to study the influence of the laser marking process parameters (pulse power and scanning speed), on the groove geometry of the marking. The experimental marking tests were carried out through a 30 W MOPA Q-Switched Yb:YAG fibre laser; under different process conditions. The groove geometry was measured through a HIROX HK9700 optical microscope. The results showed the effectiveness of the laser process to produce high quality marks on the titanium layer. Moreover, a correlation between the process parameters and the mark's geometry was clearly observed
Chlamydophila pecorum in fetuses of mediterranean buffalo (bubalus bubalis) bred in Italy
In order to study the role played by the different species of Chlamydophila in causing abortions in Mediterranean buffalo, the Authors examined 164 fetuses from 80 different buffalo herds in Southern Italy. Three fetuses, came from two different herds, were positive. Our study confirms the pathogenic role of C. pecorum in buffalo, not only as a cause of neuropathology in calves but as an infectious abortive agent
Reactions at polymer interfaces: A Monte Carlo Simulation
Reactions at a strongly segregated interface of a symmetric binary polymer
blend are investigated via Monte Carlo simulations. End functionalized
homopolymers of different species interact at the interface instantaneously and
irreversibly to form diblock copolymers. The simulations, in the framework of
the bond fluctuation model, determine the time dependence of the copolymer
production in the initial and intermediate time regime for small reactant
concentration . The results are compared to
recent theories and simulation data of a simple reaction diffusion model. For
the reactant concentration accessible in the simulation, no linear growth of
the copolymer density is found in the initial regime, and a -law is
observed in the intermediate stage.Comment: to appear in Macromolecule
Deficient liver biosynthesis of docosahexaenoic acid correlates with cognitive impairment in Alzheimer\u27s disease
Reduced brain levels of docosahexaenoic acid (C22:6n-3), a neurotrophic and neuroprotective fatty acid, may contribute to cognitive decline in Alzheimer\u27s disease. Here, we investigated whether the liver enzyme system that provides docosahexaenoic acid to the brain is dysfunctional in this disease. Docosahexaenoic acid levels were reduced in temporal cortex, mid-frontal cortex and cerebellum of subjects with Alzheimer\u27s disease, compared to control subjects (P = 0.007). Mini Mental State Examination (MMSE) scores positively correlated with docosahexaenoic/α-linolenic ratios in temporal cortex (P = 0.005) and mid-frontal cortex (P = 0.018), but not cerebellum. Similarly, liver docosahexaenoic acid content was lower in Alzheimer\u27s disease patients than control subjects (P = 0.011). Liver docosahexaenoic/α-linolenic ratios correlated positively with MMSE scores (r = 0.78; P\u3c0.0001), and negatively with global deterioration scale grades (P = 0.013). Docosahexaenoic acid precursors, including tetracosahexaenoic acid (C24:6n-3), were elevated in liver of Alzheimer\u27s disease patients (P = 0.041), whereas expression of peroxisomal d-bifunctional protein, which catalyzes the conversion of tetracosahexaenoic acid into docosahexaenoic acid, was reduced (P = 0.048). Other genes involved in docosahexaenoic acid metabolism were not affected. The results indicate that a deficit in d-bifunctional protein activity impairs docosahexaenoic acid biosynthesis in liver of Alzheimer\u27s disease patients, lessening the flux of this neuroprotective fatty acid to the brain
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