Francis Moiny

On T2-shortening by strongly magnetized spheres: A partial refocusing model

Computer simulations of water transverse relaxation induced by superparamagnetic particles are shown to disagree with the available theories, covering the slow diffusion domain. Understanding these new simulations, not in the slow diffusion domain, thus requires a new theoretical approach. A “partial refocusing model” is introduced for this purpose; it is based on a spatial division between an inner region where the gradients are too strong for the refocusing pulses to be efficient and an outer region where they are efficient. This model agrees with published simulations of relaxation induced by magnetic dipoles approximated as points. The validity domains of the various models are also compared. Magn Reson Med 47:257–263, 2002.

On T2-Shortening by weakly magnetized particles : The chemical exchange model

Chemical exchange (CE) theory is compared with two theories of T2‐shortening caused by microscopic magnetic centers: inner‐ and outer‐sphere relaxation theory (long‐echo limit) and mean gradient diffusion theory (short‐echo limit). The CE equation is shown to be identical to these theories in the respective limits and appropriate parameter relationships are derived for spherical particles. The theories are then compared with computer simulations of spherical particles and with a recent general theory, with good agreement in the asymptotic regions. The CE model also reproduces the essential relaxation characteristics in the intermediate range. Finally, good agreement of a CE model with simulations for magnetized cylinders is also demonstrated. The discussion is limited to weakly magnetized particles such that the maximum phase shift during an echo interval is less than one radian, permitting the use of the Luz‐Meiboom CE equation. Magn Reson Med 45:1014–1020, 2001. Published 2001 Wiley‐Liss, Inc.

ABOUT SURFACE REACTION KINETICS ON PROBABILISTIC AND FRACTAL SUBSTRATES

Two models of bimolecular surface reactions A+(1/n)Bn⟶ lim surfacecatalystAB (n=1,2) occurring on irregular substrates (probabilistic supports and Sierpinski carpets) are investigated by means of Monte Carlo Simulations. Steady-state regimes controlled by the reaction exhibit fractal kinetics features which are compared to those observed when these reactions are simulated on a regular lattice. Noninteger rate orders and modifications of the phase diagram (case n=2) are presented as a function of the concentration of the inactive sites of the surface. These fractal kinetics properties are tentatively interpreted in terms of structural specifics that characterize the ramification of both the substrate and the reactive adsorbate.

Fractal kinetics and surface reactions

Simple models of bimolecular surface reactions occurring on a regular lattice are investigated by means of Monte-Carlo simulations. Fractal kinetics, observed in various steady-state regimes, appear to be due to the fractal nature of the adsorbate clusters generated by the microscopic mechanisms of the kinetic scheme. Two-site mechanism rates involve anomalous orders which are tentatively assigned to cluster properties defined in terms of 2d-percolation specifics.

WATER MAGNETIC RELAXATION IN SUPERPARAMAGNETIC COLLOID SUSPENSIONS: THE EFFECT OF AGGLOMERATION

Superparamagnetic (SPM) nanoparticles in aqueous suspensions are known to shorten the nuclear magnetic relaxation of water protons. For transverse relaxation, this effect is enhanced when agglomeration of elementary SPM cores occurs. This feature is explained by considering the agglomerate as a large magnetized sphere, which at high field contributes strongly to the secular part of the transverse relaxivity. Longitudinal relaxation is modeled by analyzing diffusion through the permeable coating of the agglomerate as a virtual exchange between normal decaying modes and the bulk water. This model is used to interpret the flattening of the Nuclear Magn~tic Relaxation Dispersion profiles during a chemically induced agglomeration of ferrite particles.

Substrate effects on the fractal kinetics of a simple surface reaction

The reversible monomer–monomer model occurring without diffusion on irregular substrates (probabilistic supports and Sierpinski carpets) is investigated by means of Monte Carlo simulations. The strong influence of the catalyst surface structure on the kinetics of the reaction and its fractal like features such as noninteger reaction rate orders is investigated for steady-state regimes controlled by the reaction. We succeed in separating the dependence of the reaction rate orders on adsorbate cluster parameters from that on bare surface geometry specifics. The inactivity ratio ρ appears to be the relevant parameter to characterize the bare surface structure effects on the Langmuir–Hinshelwood reaction kinetics.

The monomer-monomer model revisited

Abstract The monomer-monomer model for surface reactions has been investigated by means of Monte Carlo simulations performed for different kinetic regimes. The poisoning time and the width of the “chaos window” has been specified in terms of the control rate constant and the lattice size. Indicative lateral interactions acting on the adsorption process have been shown to be opposed to the chaotic behaviour of the model.

Impact of Wind Power on the Gaseous Pollutants Emissions in Electrical Systems Operating under Constraints

An original nonsequential Monte Carlo simulation tool is developed. It permits to compute the optimal dispatch of classical thermal generation in order to minimize pollutants emissions in presence of wind power and under operating constraints.

Magnetic resonance susceptibility contrast induced by capillaries: a numerical comparison of two models

Monte-Carlo computer simulations have proven to be very powerful tools for the analysis of the magnetization decay induced by susceptibility gradients, as well for contrast agent characterization, as for the BOLD effect allowing fMRI. A recent vasculature model containing capillaries and venules uses homogeneous magnetized cylinders as models for the vessels. This modeling is questioned by comparing results obtained from simulation results based on two different models, one using homogeneous cylinders and another taking into account the existence of red blood cells, treated as homogeneous magnetized spheres. The results show the nonequivalence of both models, with the modeling by cylinders systematically overestimating the transverse relaxation rates, and the difference increasing with the adopted value of the diffusion coefficient. The discrepancy is attributed to the dominating role, regarding relaxation, of the local magnetic field in the immediate vicinity of the capillaries, which results in the suggestion of elaborating a “mixed modeling”: the analytical expressions corresponding to the homogeneous cylinder model could be used except when the spin packets are wandering in the immediate vicinity of the capillaries, where accounting for the existence of individual red blood cells (whose motion may be neglected) seems unavoidable.

A new estimator for the long-term reduction of gaseous pollutants emissions in electrical systems with wind generation

In this paper, the long-term reduction of gaseous pollutant emissions in electrical systems with wind generation is realistically evaluated thanks to a Monte Carlo simulation tool. In that way, a new estimator is proposed in order to quantify the long term impact of wind power on the gaseous pollutants emissions of a given system under cost, reliability and unexpected outages constraints. This estimator will allow producers, or even providers, to better dispatch their generation park from the pollutant emissions point of view.