Biomedical subjects

R N Costa Filho

Publications and source records attributed to R N Costa Filho.

6 recordsLinked to original sources

PubMed2005

Structure, normal mode spectra, and mixing of a binary system of charged particles confined in a parabolic trap.

We study the mixing of two different kinds of particles, having different charge and/or mass, interacting through a pure Coulomb potential, and confined in a parabolic trap. The structure of the cluster and its normal mode spectrum are analyzed as a function of the ratio of the charges (mass ratio) of the two types of particles. We show that particles are not always arranged in a shell structure. Mixing of the particles goes hand in hand with a large number of metastable states. The normal modes of the system are obtained, and we find that some of the special modes can be tuned by varying the ratio between the charges (masses) of the two species. The degree of mixing of the two type of particles is summarized in a phase diagram, and an order parameter that describes quantitatively the mixing between particles is defined.

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PubMed2005

Memory effects on the statistics of fragmentation.

We investigate through extensive molecular dynamics simulations the fragmentation process of two-dimensional Lennard-Jones systems. After thermalization, the fragmentation is initiated by a sudden increment to the radial component of the particles' velocities. We study the effect of temperature of the thermalized system as well as the influence of the impact energy of the "explosion" event on the statistics of mass fragments. Our results indicate that the cumulative distribution of fragments follows the scaling ansatz F(m) proportional to m(1-alpha)exp-(m/m(0))(gamma), where m is the mass, m(0) and gamma are cutoff parameters, and alpha is a scaling exponent that is dependent on the temperature. More precisely, we show clear evidence that there is a characteristic scaling exponent alpha for each macroscopic phase of the thermalized system, i.e., that the nonuniversal behavior of the fragmentation process is dictated by the state of the system before it breaks down.

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PubMed2005

Majority-vote model on a random lattice.

The stationary critical properties of the isotropic majority vote model on random lattices with quenched connectivity disorder are calculated by using Monte Carlo simulations and finite size analysis. The critical exponents gamma and beta are found to be different from those of the Ising and majority vote on the square lattice model and the critical noise parameter is found to be q(c) =0.117+/-0.005 .

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PubMed2005

Nanopercolation.

We investigate through direct molecular mechanics calculations the geometrical properties of hydrocarbon mantles subjected to percolation disorder. We show that the structures of mantles generated at the critical percolation point have a fractal dimension df approximately 2.5. In addition, the solvent access surface As and volume Vs of these molecules follow power-law behavior, As approximately L(alphaA) and Vs approximately L(alphaV), where L is the system size, and with both exponents alphaA and alphaV being significantly dependent on the radius of the accessing probing molecule, r(p). Our results from extensive simulations with two distinct microscopic topologies (i.e., square and honeycomb) indicate the consistency of the statistical analysis and confirm the self-similar characteristic of the percolating hydrocarbons. Due to their highly branched topology, this new class of disordered molecules can be of potential use in a variety of practical applications.

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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.

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PubMed1999

Scaling behavior in a proportional voting process.

We perform a statistical analysis on the proportional elections held in Brazil in October 1998. We show that the distribution of votes among candidates for the whole country follows a power law N(v) is proportional to v(-alpha), with alpha=1.00+/-0.02, extending over two orders of magnitude. The voting distributions for several states of the federation also display scale-invariant behavior with alpha approximately equal to 1. We argue that this particular voting system can be modeled as a typical multiplicative process in which the choice of the candidate is governed by a product of probabilities.

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