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Pisani L.,Center for Advanced Studies
Transport in Porous Media | Year: 2011

In this article, we derive a simple expression for the tortuosity of porous media as a function of porosity and of a single parameter characterizing the shape of the porous medium components. Following its value, a very large range of porous materials is described, from non-tortuous to high tortuosity ones with percolation limits. The proposed relation is compared with a widely used expression derived from percolation theory, and its predictive power is demonstrated through comparison with numerical simulations of diffusion phenomena. Application to the tortuosity of hydrated polymeric membranes is shown. © 2011 Springer Science+Business Media B.V.

Pisani L.,Center for Advanced Studies
Transport in Porous Media | Year: 2016

In this article, the dependence of tortuosity on the geometrical structure of a porous medium is studied. In particular, the considered porous media have anisotropic structures, being composed by a collection of object of similar shape, with a well-defined orientation. Geometrical expressions for the tortuosity as a function of the porosity, of shape factors characterizing the geometry of the solid objects and of the orientation of the flow with respect to the object axes are derived. Besides the general case, two simpler expressions are derived for 2D porous media and for media composed by axisymmetric objects. The expressions for the two particular cases are validated through a series of numerical simulations of diffusion phenomena, finding a good agreement. The model is also compared with experimental data from literature, showing its possible use in the prediction of the transport properties of a porous medium made by assembling similar solid particles, by a simple geometrical characterization of its components. Finally, a parametric analysis is performed, showing a strong dependence of the tortuosity on the objects shape and on their orientation with respect to the flow. The capability of the presented model to predict such effects can be used to design materials with particular non-isotropic transport characteristics. © 2016 Springer Science+Business Media Dordrecht

Palomo A.M.,Center for Advanced Studies
Journal of Public Health Policy | Year: 2015

The Ebola epidemic exemplifies the importance of social determinants of health: poverty and illiteracy, among others. © 2015 Macmillan Publishers Ltd.

Palomo A.M.,Center for Advanced Studies
Journal of Public Health Policy | Year: 2016

This Viewpoint discusses the World Health Organization's Declaration on 1 February 2016 that the epidemic infection caused by the Zika virus is a public health emergency of international concern - the basis of the decision and controversy surrounding it. © 2016 Macmillan Publishers Ltd.

Leonardi E.,Center for Advanced Studies | Angeli C.,University of Ferrara
Journal of Physical Chemistry B | Year: 2010

The diffusion process in a multicomponent system can be formulated in a general form by the generalized Maxwell - Stefan equations. This formulation is able to describe the diffusion process in different systems, such as, for instance, bulk diffusion (in the gas, liquid, and solid phase) and diffusion in microporous materials (membranes, zeolites, nanotubes, etc.). The Maxwell - Stefan equations can be solved analytically (only in special cases) or by numerical approaches. Different numerical strategies have been previously presented, but the number of diffusing species is normally restricted, with only few exceptions, to three in bulk diffusion and to two in microporous systems, unless simplifications of the Maxwell - Stefan equations are considered. In the literature, a large effort has been devoted to the derivation of the analytic expression of the elements of the Fick-like diffusion matrix and therefore to the symbolic inversion of a square matrix with dimensions n × n (n being the number of independent components). This step, which can be easily performed for n = 2 and remains reasonable for n = 3, becomes rapidly very complex in problems with a large number of components. This paper addresses the problem of the numerical resolution of the Maxwell - Stefan equations in the transient regime for a one-dimensional system with a generic number of components, avoiding the definition of the analytic expression of the elements of the Fick-like diffusion matrix. To this aim, two approaches have been implemented in a computational code; the first is the simple finite difference second- order accurate in time Crank - Nicolson scheme for which the full mathematical derivation and the relevant final equations are reported. The second is based on the more accurate backward differentiation formulas, BDF, or Gear's method (Shampine, L. F.; Gear, C. W. SIAM Rev. 1979, 21, 1.), as implemented in the Livermore solver for ordinary differential equations, LSODE (Hindmarsh, A. C. Serial Fortran Solvers for ODE Initial Value Problems, Technical Report; https://computation.llnl.gov/casc/odepack/odepack-home.html (2006).). Both methods have been applied to a series of specific problems, such as bulk diffusion of acetone and methanol through stagnant air, uptake of two components on a microporous material in a model system, and permeation across a microporous membrane in model systems, both with the aim to validate the method and to add new information to the comprehension of the peculiar behavior of these systems. The approach is validated by comparison with different published results and with analytic expressions for the steady-state concentration profiles or fluxes in particular systems. The possibility to treat a generic number of components (the limitation being essentially the computational power) is also tested, and results are reported on the permeation of a five component mixture through a membrane in a model system. It is worth noticing that the algorithm here reported can be applied also to the Fick formulation of the diffusion problem with concentration-dependent diffusion coefficients. © 2010 American Chemical Society.

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