Biomedical subjects

R E de Souza

Publications and source records attributed to R E de Souza.

4 recordsLinked to original sources

PubMed2006

Water ingress in Y-type zeolite: anomalous moisture-dependent transport diffusivity.

Nuclear magnetic resonance imaging measurements of liquid water ingress in a large number of nonactivated Y-type (Na) zeolite samples prepared under different conditions are reported on. Using an experimental arrangement that permits the application of Boltzmann's transformation of the 1D (one-dimensional) diffusion equation, the spatiotemporal scaling variables required for a collapse of the measured profiles into universal curves revealed subdiffusive behavior in all cases. It is shown that the one-dimensional fractal time diffusion equation constitutes a powerful tool to analyze the data and provides a connection between the moisture dependence of the effective transport diffusivities and the shapes of the universal curves. Thus, even for anomalous diffusion, the relationship between the universal curves and structural characteristics of the system; such as porosity, tortuosity of the pore space and, in some cases, the interplay between mesopores and nanopores can be addressed.

Journal Article↗

PubMed2003

Scaling properties in the packing of crumpled wires.

Statistical properties of configurations of a metallic wire injected into a transparent planar two-dimensional cavity for three different injection geometries are investigated with the aid of high-resolution digital imaging techniques. The observed patterns of folds are studied as a function of the packing fraction of the wire within the cavity. In particular, we have examined the dependence of the mass of wire within a circle of radius R, as well as the dependence of the number of contacts wire-wire with the packing fraction. The distribution function n(s) of connected loops with internal area s formed as a consequence of the folded structure of the wire, and the average coordination number for these loops are also examined. Several scaling laws connecting variables of physical interest are obtained and discussed and a relation of this problem with disordered two-dimensional foam and random packing of disks is examined.

Journal Article↗

PubMed2002

Crumpled wires in two dimensions.

Geometric and statistical properties of wires injected into a two-dimensional cavity with three different injection geometries are investigated. Complex patterns of folds are observed and studied as a function of the length of the wire. The mass-size relation and the distribution function n(s) of loops with internal area s formed as a consequence of the folded structure of the wire are examined. Several scaling laws are found and a hierarchical model is introduced to explain the experimental behavior observed in this two-dimensional crumpling process.

Journal Article↗

PubMed1998

Mobility and free radical concentration effects in proton-electron double-resonance imaging.

The dependence of the enhancement of proton-electron double-resonance images upon the mobility of the proton bearing molecules, of the concentration of free radicals, and of the pulsed saturating RF power is studied in a magnetic field of 16 mT. The data exhibit a behavior which, in the potentially interesting region of small free radical concentration, may differ substantially from the high-concentration regime depending upon experimental conditions. The results permit a clearer understanding of the factors determining enhancement and contrast in images obtained by dynamic nuclear polarization.

Electron Spin Resonance Spectroscopy↗