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Biomedical subjects

M Sahimi

Publications and source records attributed to M Sahimi.

At least 19 recordsLinked to original sources
PubMed2000

Invasion percolation with long-range correlations: first-order phase transition and nonuniversal scaling properties

We present the results of extensive Monte Carlo simulations of the invasion percolation model with trapping (TIP) with long-range correlations, a problem which is relevant to multiphase flow in field-scale porous media, such as oil reservoirs and groundwater aquifers, as well as flow in rock fractures. The correlations are generated by a fractional Brownian motion characterized by a Hurst exponent H. We employ a highly efficient algorithm for simulating TIP, and a novel method for identifying the backbone of TIP clusters. Both site and bond TIP are studied. Our study indicates that the backbone of bond TIP is loopless and completely different from that of site TIP. We obtain precise estimates for the fractal dimensions of the sample-spanning cluster (SSC), the minimal path, and the backbone of site and bond TIP, and analyze the size distribution of the trapped clusters, in order to identify all the possible universality classes of TIP with long-range correlations. For site TIP with H > 1/2 the SSC and its backbone are compact, indicating a first-order phase transition at the percolation threshold, while the minimal paths are essentially straigth lines. For H < 1/2 the SSC, its backbone, and the minimal paths are all fractal with fractal dimensions that depend on the Hurst exponent H. The fractal dimension of the loopless backbone for bond TIP is much less than that of site TIP for any H.

Journal Article↗
PubMed2000

Nonequilibrium molecular dynamics simulations of transport and separation of gas mixtures in nanoporous materials

The nonequilibrium molecular dynamics simulations of transport and separation of a binary gas mixture through a porous membrane with interconnected pores of distributed sizes are reported. The membrane is modeled by a three-dimensional disordered molecular network of interconnected pores consisting of tens of thousands of atoms, based on a Voronoi tessellation of space. Results are presented for transport and adsorption of the gases, including the existence of an optimal pore structure for maximum separation of the gases.

Journal Article↗
PubMed1998

Fractal dimension of the bone marrow in metastatic lesions.

We previously reported that the distribution of the cells in normal bone marrow is fractal and self-similar. The purpose of this study was to determine whether the same is true in metastatic tumors. Thirty-two bone marrow biopsy sections (3 to 5 microm thick) of 28 patients were used to measure the fractal dimensions of the metastatic tumor cells' distribution. Microscopic images were obtained and were used for the fractal measurements. In the two-dimensional images, the fractal dimensions were 1.98 +/- 0.02 (95% +/- 5% cellularity), suggesting a compact nonfractal structure. The dimensions, however, were 1.72 +/- 0.1 (56% +/- 11% cellularity) for the normal components, with a P-value of <.0001 that is in agreement with our previous study. These results suggest that loss of the fractal structure in the metastatic lesions may be attributable to loss or suppression of the regulatory mechanisms maintaining the fractal morphogenesis of the bone marrow. This report provides a novel objective approach in the study of pathophysiology of the bone marrow disorders.

Adolescent↗
PubMed1996

Morphogenesis of the bone marrow: fractal structures and diffusion-limited growth.

Bone marrow (BM) provides a particular spatial organization that allows interaction between its various components. Characterization of the spatial patterns in the BM and understanding the mechanisms that give rise to them may play a role in better understanding of the BM pathologic processes. Morphometric analyses were performed in BM biopsy samples from 30 patients (16 men and 14 women) with an average age of 46 years, ranging from 17 to 77 years. The biopsies were obtained during the course of patient care to rule out BM involvement in a variety of hematologic disorders before or after therapy. Three different, but structurally interrelated, parameters were measured: (A) cellular area, (B) nuclear area, and (C) cell numbers. All three methods, in all cases, showed that the spatial structure of the BM is fractal. The average values of the fractal dimensions (Df) were 1.7 +/- 0.08, 1.64 +/- 0.1, and 1.69 +/- 0.04 for categories A, B, and C, respectively. The overall value of Df for the cellularity in the range of 40% to 60% was about 1.67 +/- 0.09. Fractal dimensions of 1.6 to 1.7 represent configurations that correspond to two-dimensional diffusion limited aggregation structures, suggesting that the structural configuration of hematopoietic cells is dependent on the diffusion of regulatory cytokines in the BM.

Adolescent↗
PubMed1994

High-dimensional simulation of simple immunological models.

We propose a simple model for idiotypic-antiidiotypic immunological networks which represents a simplification of the model originally suggested by Stewart & Varela, and de Boer, van der Laan & Hogeweg. Window cellular automata enlarge the antibody concentration if the influence of the immediate neighborhood lies between 70 and 99% of its maximum value; in addition random recruitment is possible. We simulate the model on large square lattices as well as in higher dimensions. The results are in qualitative agreement with those of the earlier model. Moreover, in five to ten dimensions we find phase separation for not too large recruitment, which then leads to domains growing to "infinity" for infinite times.

Animals↗