Biomedical subjects

A D Araújo

Publications and source records attributed to A D Araújo.

9 recordsLinked to original sources

PubMed2006

Critical role of gravity in filters.

The efficiency of filters depends crucially on the mass of the particles one wants to capture. Using analytical and numerical calculations we reveal a very rich scenario of scaling laws relating this efficiency to particle size and density and the velocity and viscosity of the carrying fluid. These are combined in the dimensionless, so-called Stokes number St. In the case of horizontal flow or neutrally buoyant particles, we find a critical number St{c} below which no particles are trapped; i.e., the filter does not work. Above St{c} the capture efficiency increases like the square root of (St-St{c}). Under the action of gravity, the threshold abruptly vanishes and capture occurs at any Stokes number increasing linearly in St. We discovered further scaling laws in the penetration profile and as function of the porosity of the filter.

Biophysics↗

PubMed2006

Periodic neural activity induced by network complexity.

We study a model for neural activity on the small-world topology of Watts and Strogatz and on the scale-free topology of Barabási and Albert. We find that the topology of the network connections may spontaneously induce periodic neural activity, contrasting with nonperiodic neural activities exhibited by regular topologies. Periodic activity exists only for relatively small networks and occurs with higher probability when the rewiring probability is larger. The average length of the periods increases with the square root of the network size.

Journal Article↗

PubMed2005

Invasion percolation between two sites.

We investigate the process of invasion percolation between two sites (injection and extraction sites) separated by a distance r in two-dimensional lattices of size L. Our results for the nontrapping invasion percolation model indicate that the statistics of the mass of invaded clusters is significantly dependent on the local occupation probability (pressure) Pe at the extraction site. For Pe = 0, we show that the mass distribution of invaded clusters P(M) follows a power-law P(M) approximately M(-alpha) for intermediate values of the mass M, with an exponent alpha = 1.39+/-0.03. When the local pressure is set to Pe = Pc, where Pc corresponds to the site percolation threshold of the lattice topology, the distribution P(M) still displays a scaling region, but with an exponent alpha = 1.02+/-0.03. This last behavior is consistent with previous results for the cluster statistics in standard percolation. In spite of these differences, the results of our simulations indicate that the fractal dimension of the invaded cluster does not depend significantly on the local pressure Pe and it is consistent with the fractal dimension values reported for standard invasion percolation. Finally, we perform extensive numerical simulations to determine the effect of the lattice borders on the statistics of the invaded clusters and also to characterize the self-organized critical behavior of the invasion percolation process.

Journal Article↗

PubMed2004

Multiple invaded consolidating materials.

We study a multiple invasion model to simulate corrosion or intrusion processes. Estimated values for the fractal dimension of the invaded region reveal that the critical exponents vary as a function of the generation number G , i.e., with the number of times the invasion process takes place. The averaged mass M of the invaded region decreases with a power law as a function of G , M approximately Gbeta , where the exponent beta approximately 0.6 . We also find that the fractal dimension of the invaded cluster changes from d(1) =1.887+/-0.002 to d(s) =1.217+/-0.005 . This result confirms that the multiple invasion process (for the case in which uninvaded regions are forbidden) follows a continuous transition from one universality class (nontrapping invasion percolation) to another (optimal path). In addition, we report extensive numerical simulations that indicate that the mass distribution of avalanches P (S,L) has a power-law behavior and we find that the exponent tau governing the power-law P (S,L) approximately S-tau changes continuously as a function of the parameter G . We propose a scaling law for the mass distribution of avalanches for different number of generations G .

Journal Article↗

PubMed2003

Scaling behavior of diffusion and reaction processes in percolating porous media.

We investigate the diffusion-reaction behavior of two-dimensional pore networks at the critical percolation point. Our results indicate the existence of three distinct regimes of reactivity, determined by parameter xi[triple bond]D/(Kl2), where D is the molecular diffusivity of the reagent, K is its chemical reaction coefficient, and l is the length scale of the pore. First, when the diffusion transport is strongly limited by chemical reaction (i.e., D< >K), the flux of reagent reaches a saturation limit Phi(sat) that scales with the system size as Phi(sat) approximately L(alpha), with an exponent alpha approximately 1.89, corresponding to the fractal dimension of the sample-spanning cluster. We then show that the variation of flux Phi calculated for different network sizes at the second and third regimes can be adequately described in terms of the scaling relation, Phi approximately L(alpha)f(xi/L(z)), where the crossover exponent z approximately 2.69 is consistent with the predicted scaling law alpha=2betaz.

Journal Article↗

PubMed2003

Statistics of the critical percolation backbone with spatial long-range correlations.

We study the statistics of the backbone cluster between two sites separated by distance r in two-dimensional percolation networks subjected to spatial long-range correlations. We find that the distribution of backbone mass follows the scaling ansatz, P(M(B)) approximately M(-(alpha+1))(B)f(M(B)/M(0)), where f(x)=(alpha+etax(eta))exp(-x(eta)) is a cutoff function and M0 and eta are cutoff parameters. Our results from extensive computational simulations indicate that this scaling form is applicable to both correlated and uncorrelated cases. We show that the exponent alpha can be directly related to the fractal dimension of the backbone d(B), and should therefore depend on the imposed degree of long-range correlations.

Journal Article↗

PubMed2002

Traveling length and minimal traveling time for flow through percolation networks with long-range spatial correlations.

We study the distributions of traveling length l and minimal traveling time t(min) through two-dimensional percolation porous media characterized by long-range spatial correlations. We model the dynamics of fluid displacement by the convective movement of tracer particles driven by a pressure difference between two fixed sites ("wells") separated by Euclidean distance r. For strongly correlated pore networks at criticality, we find that the probability distribution functions P(l) and P(t(min)) follow the same scaling ansatz originally proposed for the uncorrelated case, but with quite different scaling exponents. We relate these changes in dynamical behavior to the main morphological difference between correlated and uncorrelated clusters, namely, the compactness of their backbones. Our simulations reveal that the dynamical scaling exponents d(l) and d(t) for correlated geometries take values intermediate between the uncorrelated and homogeneous limiting cases, where l(*) approximately r(d(l)) and t(*)(min) approximately r(d(t)), and l(*) and t(*)(min) are the most probable values of l and t(min), respectively.

Journal Article↗

PubMed2001

Dynamics of viscous penetration in percolation porous media.

We investigate the dynamics of viscous penetration in two-dimensional percolation networks at criticality for the case in which the ratio between the viscosities of displaced and injected fluids is very large. We report extensive numerical simulations that indicate that the scaling exponents for the breakthrough time distribution are the same as the previously reported values computed for the case of unit viscosity ratio. Our results are consistent with the possibility that viscous displacement through critical percolation networks constitutes a single universality class, independent of the viscosity ratio. We also find that the distributions of mass and breakthrough time of the invaded clusters have the same scaling form, but with different critical exponents.

Journal Article↗

PubMed1996

[Neuropathogenesis of HIV infection].

The spreading of human immunodeficiency virus (HIV) infection and its increasing scientific knowledge keep the medical staff involved with these patients in permanent need of updating themselves. The different neurologic manifestations caused by HIV are related to a variety of pathogenic mechanisms, as follows: immunodeficiency, autoimmunity, direct effects of the virus on the nervous system, and toxic and metabolic effects. The opportunistic infections are caused by the immunodeficiency due to the action of the virus on CD4+ T cells and on cells of the monocytic-macrophage lineage. Demyelinating polyradiculoneuropathy and polymyositis-like syndromes are related to autoimmune mechanisms involving, probably, the non-specific stimulation of T cells by viral proteins. The primary action of the virus on the nervous system brings out aseptic meningitis, cognitive dysfunction, dementia, vacuolar myelopathy and sensory polyneuropathy probably through liberation of neurotoxic products by the infected macrophages. Antiretroviral drugs and others used to treat patients with AIDS may also have neurotoxic effects. The better understanding of the neuropathogenesis of HIV infection will permit the use of new, and more specific, therapeutical options in the future as well as a more precocious control of its neurologic complications.

AIDS Dementia Complex↗