1,347 research outputs found
Thermal denaturation of fluctuating finite DNA chains: the role of bending rigidity in bubble nucleation
Statistical DNA models available in the literature are often effective models
where the base-pair state only (unbroken or broken) is considered. Because of a
decrease by a factor of 30 of the effective bending rigidity of a sequence of
broken bonds, or bubble, compared to the double stranded state, the inclusion
of the molecular conformational degrees of freedom in a more general mesoscopic
model is needed. In this paper we do so by presenting a 1D Ising model, which
describes the internal base pair states, coupled to a discrete worm like chain
model describing the chain configurations [J. Palmeri, M. Manghi, and N.
Destainville, Phys. Rev. Lett. 99, 088103 (2007)]. This coupled model is
exactly solved using a transfer matrix technique that presents an analogy with
the path integral treatment of a quantum two-state diatomic molecule. When the
chain fluctuations are integrated out, the denaturation transition temperature
and width emerge naturally as an explicit function of the model parameters of a
well defined Hamiltonian, revealing that the transition is driven by the
difference in bending (entropy dominated) free energy between bubble and
double-stranded segments. The calculated melting curve (fraction of open base
pairs) is in good agreement with the experimental melting profile of
polydA-polydT. The predicted variation of the mean-square-radius as a function
of temperature leads to a coherent novel explanation for the experimentally
observed thermal viscosity transition. Finally, the influence of the DNA strand
length is studied in detail, underlining the importance of finite size effects,
even for DNA made of several thousand base pairs.Comment: Latex, 28 pages pdf, 9 figure
Ionic Capillary Evaporation in Weakly Charged Nanopores
Using a variational field theory, we show that an electrolyte confined to a
neutral cylindrical nanopore traversing a low dielectric membrane exhibits a
first-order ionic liquid-vapor pseudo-phase-transition from an
ionic-penetration "liquid" phase to an ionic-exclusion "vapor" phase,
controlled by nanopore-modified ionic correlations and dielectric repulsion.
For weakly charged nanopores, this pseudotransition survives and may shed light
on the mechanism behind the rapid switching of nanopore conductivity observed
in experiments.Comment: This version is accepted for publication in PR
Copper fine-structure K-shell electron impact ionization cross sections for fast-electron diagnostic in laser-solid experiments
AN INTEGRATED APPROACH TO PREVENT THE EROSION OF SALT MARSHES IN THE LAGOON OF VENICE
The loss of coastal habitats is a widespread problem in Europe. To
protect the intertidal salt marshes of the lagoon of Venice from the
erosion due to natural and human causes which is diffusely and intensely
impacting them, the European Commission has funded the demonstrative
project LIFE VIMINE. LIFE VIMINE aims to protect the most interior,
hard-to-access salt marshes in the northern lagoon of Venice through an
integrated approach, whose core is the prevention of erosion through
numerous, small but spatially-diffuse soil-bioengineering protections
works, mainly placed through semi-manual labour and with low impact on
the environment and the landscape. The effectiveness of protection works
in the long term is ensured through routine, temporally-continuous and
spatially-diffuse actions of monitoring and maintenance. This method
contrasts the common approach to managing hydraulic risk and erosion in
Italy which is based on large, one-off and irreversible protection
actions. The sustainability of the LIFE VIMINE approach is ensured by
the participatory involvement of stakeholders and the recognition that
protecting salt marshes means defending the benefits they provide to
society through their ecological functions, as well as protecting the
jobs linked to the existence or conservation of this habitat
Inhibitory control in mind and brain 2.0: blocked-input models of saccadic countermanding.
The interactive race model of saccadic countermanding assumes that response inhibition results from an interaction between a go unit, identified with gaze-shifting neurons, and a stop unit, identified with gaze-holding neurons, in which activation of the stop unit inhibits the growth of activation in the go unit to prevent it from reaching threshold. The interactive race model accounts for behavioral data and predicts physiological data in monkeys performing the stop-signal task. We propose an alternative model that assumes that response inhibition results from blocking the input to the go unit. We show that the blocked-input model accounts for behavioral data as accurately as the original interactive race model and predicts aspects of the physiological data more accurately. We extend the models to address the steady-state fixation period before the go stimulus is presented and find that the blocked-input model fits better than the interactive race model. We consider a model in which fixation activity is boosted when a stop signal occurs and find that it fits as well as the blocked input model but predicts very high steady-state fixation activity after the response is inhibited. We discuss the alternative linking propositions that connect computational models to neural mechanisms, the lessons to be learned from model mimicry, and generalization from countermanding saccades to countermanding other kinds of responses
Selective Laser Melting of Ti6Al4V: Effects of Heat Accumulation Phenomena Due to Building Orientation
Titanium alloy Ti6Al4V is one of the most utilized alloys in the field of additive manufacturing due to the excellent combination of mechanical properties, density and good corrosion behavior. These characteristics make the use of this material particularly attractive for additively manufacturing components with complex geometry in sectors such as aeronautics and biomedical. Selective Laser Melting (SLM), by which a component is fabricated by selectively melting of stacked layers of powder using a laser beam, is the one of most promising additive manufacturing technologies for Ti6Al4V alloy. Although this technique offers numerous advantages, it has some critical issues related to the high thermal gradients, associated with the process, promoting the formation of a metastable martensitic microstructure resulting in high tensile strength but poor ductility of the produced parts. The formation of microstructural defects such as balling and porosity can occur together with the presence of residual stresses that may significantly affect the mechanical characteristics of the component. Specific process parameters and geometries can determine heat accumulation phenomena that result in a progressive decrease in thermal gradients between layers. These heat accumulation phenomena are influenced by the number of layers deposited, but also by the building orientation that, for a given geometry, determines a variation of the deposited surface for each layer. © 2022 The Author(s). Published by Trans Tech Publications Ltd, Switzerland
Variational approach for electrolyte solutions: from dielectric interfaces to charged nanopores
A variational theory is developed to study electrolyte solutions, composed of
interacting point-like ions in a solvent, in the presence of dielectric
discontinuities and charges at the boundaries. Three important and non-linear
electrostatic effects induced by these interfaces are taken into account:
surface charge induced electrostatic field, solvation energies due to the ionic
cloud, and image charge repulsion. Our variational equations thus go beyond the
mean-field theory. The influence of salt concentration, ion valency, dielectric
jumps, and surface charge is studied in two geometries. i) A single neutral
air-water interface with an asymmetric electrolyte. A charge separation and
thus an electrostatic field gets established due to the different image charge
repulsions for coions and counterions. Both charge distributions and surface
tension are computed and compared to previous approximate calculations. For
symmetric electrolyte solutions close to a charged surface, two zones are
characterized. In the first one, with size proportional to the logarithm of the
coupling parameter, strong image forces impose a total ion exclusion, while in
the second zone the mean-field approach applies. ii) A symmetric electrolyte
confined between two dielectric interfaces as a simple model of ion rejection
from nanopores. The competition between image charge repulsion and attraction
of counterions by the membrane charge is studied. For small surface charge, the
counterion partition coefficient decreases with increasing pore size up to a
critical pore size, contrary to neutral membranes. For larger pore sizes, the
whole system behaves like a neutral pore. The prediction of the variational
method is also compared with MC simulations and a good agreement is observed.Comment: This version is accepted for publication in Phys. Rev. E
Simplified primary energy models for the selection of Electron Beam Melting over turning in the production of titanium alloys components
Over the last years two factors have deeply affected research in Manufacturing: the growing interest around Additive Manufacturing (AM) processes and the need to reduce the anthropogenic environmental impact. As result, a large papers concerning the environmental impact performance of AM compared to conventional processes have been published. Specifically, very complex models accounting for the impact of each life cycle stage of AMed components have been released. Results revealed that AM, at present, guarantees energy savings only within some domains, and the potential saving depends on several factors: product complexity, eco-properties of the material, energy intensity of the material deposition, light-weighting enabled by AM approaches and extent of the use phase. Above all, the result of the comparative analysis depends both on the considered factors and on the selected system boundaries. The already proposed models are very complex and many inventory data are needed, this could make them unapplicable at industrial level. In the presented paper, simplified models are proposed and the performances of these are quantified with varying the analyzed scenario (considered factors and selected system boundary). Results revealed that, for given scenarios, simplified models characterized by low computational effort, can provide reliable results. Guidelines for the implementation of different models with varying the system boundary are provided for the cumulative energy comparison of Electron Beam Melting and conventional turning for the production of titanium alloys components
Ductility and linear energy density of Ti6Al4V parts produced with additive powder bed fusion technology
Hybrid metal forming processes involve the integration of commonly used sheet metal
forming processes, as bending, deep drawing and incremental forming, with additive
manufacturing processes as Powder Bed Fusion. In recent ybears, these integrations have been more developed for manufacturing sectors characterized by components with complex geometries in low numbers, as the aerospace sector. Hybrid additive manufacturing overcomes the typical limitations of additive manufacturing related to low productivity, metallurgical defects and low dimensional accuracy. In this perspective, a key aspect of hybrid processes is the production of parts characterized by high strength and ductility. In the present work, a study was carried out on
the influence of process parameters, such as laser power and scanning speed, on material ductility for Ti6Al4V alloy samples produced by Selective Laser Melting. In particular, the material strength and ductility were related to the process linear energy density (LED)
Hybrid prediction-optimization approaches for maximizing parts density in SLM of Ti6Al4V titanium alloy
It is well known that the processing parameters of selective laser melting (SLM) highly influence mechanical and physical properties of the manufactured parts. Also, the energy density is insufficient to detect the process window for producing full dense components. In fact, parts produced with the same energy density but different combinations of parameters may present different properties even under the microstructural viewpoint. In this context, the need to assess the influence of the process parameters and to select the best parameters set able to optimize the final properties of SLM parts has been capturing the attention of both academics and practitioners. In this paper different hybrid prediction-optimization approaches for maximizing the relative density of Ti6Al4V SLM manufactured parts are proposed. An extended design of experiments involving six process parameters has been configured for constructing two surrogate models based on response surface methodology (RSM) and artificial neural network (ANN), respectively. The optimization phase has been performed by means of evolutionary computations. To this end, three nature-inspired metaheuristic algorithms have been integrated with the prediction modelling structures. A series of experimental tests has been carried out to validate the results from the proposed hybrid optimization procedures. Also, a sensitivity analysis based on the results from the analysis of variance was executed to evaluate the influence of the processing parameter and their reciprocal interactions on the part porosity
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