Characteristic Exchange Time Research Articles

Representability and Dynamical Consistency in Coarse-Grained Models.

We address the challenge of representativity and dynamical consistency when unbonded fine-grained particles are collected together into coarse-grained particles. We implement a hybrid procedure for identifying and tracking the underlying fine-grained particles─e.g., atoms or molecules─by exchanging them between the coarse-grained particles periodically at a characteristic time. The exchange involves a back-mapping of the coarse-grained particles into fine-grained particles and a subsequent reassignment to coarse-grained particles conserving total mass and momentum. We find that an appropriate choice of the characteristic exchange time can lead to the correct effective diffusion rate of the fine-grained particles when simulated in hybrid coarse-grained dynamics. In the compressed (supercritical) fluid regime, without the exchange term, fine-grained particles remain associated with a given coarse-grained particle, leading to substantially lower diffusion rates than seen in all-atom molecular dynamics of the fine-grained particles. Thus, this work confirms the need for addressing the representativity of fine-grained particles within coarse-grained particles and offers a simple exchange mechanism so as to retain dynamical consistency between the fine- and coarse-grained scales.

Describing NMR chemical exchange by effective phase diffusion approach

This paper proposes an effective phase diffusion method to analyze chemical exchange in nuclear magnetic resonance (NMR). The chemical exchange involves spin jumps around different sites where the spin angular frequencies vary, which is a random phase walk viewed from the rotating frame reference. Such a random walk in phase space can be treated by the effective phase diffusion method. Both the coupled and uncoupled normal and fractional phase diffusions are considered. Based on the phase diffusion results, the line shape of NMR exchange spectrum can be obtained. By comparing these theoretical results with the conventional theory, this phase diffusion approach works for fast exchange, ranging from slightly faster than intermediate exchange to very fast exchange. For normal diffusion models, the theoretically predicted curves agree with those predicted from traditional models in the literature, and the characteristic exchange time obtained from phase diffusion with a fixed jump time is the same as that obtained from the conventional model. However, the phase diffusion with a monoexponential time distribution gives a relatively shorter characteristic exchange time constant, which is half of that obtained from the traditional model. Additionally, the fractional diffusion obtains a significantly different line shape than that predicted based on normal diffusion.

Modeling the Nd isotopic composition in the North Atlantic basin using an eddy-permitting model

Abstract. Boundary Exchange (BE – exchange of elements between continental margins and the open ocean) has been emphasized as a key process in the oceanic cycle of neodymium (Nd) (Lacan and Jeandel, 2005a). Here, we use a regional eddy-permitting resolution Ocean General Circulation Model (1/4°) of the North Atlantic basin to simulate the distribution of the Nd isotopic composition, considering BE as the only source. Results show good agreement with the data, confirming previous results obtained using the same parameterization of the source in a coarse resolution global model (Arsouze et al., 2007), and therefore the major control played by the BE processes in the Nd cycle on the regional scale. We quantified the exchange rate of the BE, and found that the time needed for the continental margins to significantly imprint the chemical composition of the surrounding seawater (further referred as characteristic exchange time) is of the order of 0.2 years. However, the timescale of the BE may be subject to large variations as a very short exchange time (a few days) is needed to reproduce the highly negative values of surface waters in the Labrador Sea, whereas a longer one (up to 0.5 years) is required to simulate the radiogenic influence of basaltic margins and distinguish the negative isotopic signatures of North Atlantic Deep Water from the more radiogenic southern origin water masses. This likely represents geographical variations in erosion fluxes and the subsequent particle load onto the continental margins. Although the parameterization of the BE is the same in both configurations of the model, the characteristic exchange time in the eddy-permitting configuration is significantly lower than the previous evaluations using a low resolution configuration (6 months to 10 years), but however in agreement with the available seawater Nd isotope data. This results highlights the importance of the model dynamics in simulating the BE process.

Modeling the neodymium isotopic composition with a global ocean circulation model

For the first time, modeling of the neodymium (Nd) isotopic composition (expressed as ε Nd) is performed in an Ocean Global Circulation Model (OPA-ORCA2.0 model). In the interest of simplicity, Nd concentration is not explicitly included and ε Nd is modeled as a passive tracer. Recent studies suggest that dissolved/particulate exchanges between continental margin sediments and seawater (termed boundary exchange, BE), could be the dominant source-sink terms that determine the distribution of neodymium isotopes in the global ocean. To test this hypothesis, we only included this BE source-sink term in the model. BE was parameterized as a relaxing term that forces ε Nd of seawater to the isotopic composition of the continental margin over a characteristic exchange time. Sensitivity tests were made using different exchange times and parameterizations. The model reproduces most of the general trends of ε Nd composition in seawater at global scale (inter-basin gradient, composition of some of the main water masses). These results suggest that BE process may account for the major sources and sinks of oceanic Nd, while indicating that the characteristic time of interaction between seawater and margins varies from about half a year in surface waters to several years at depth. The disparities between the model and observations might reflect the typical shortcomings of utilizing a coarse resolution ocean model circulation fields or neglected sources of neodymium such as dissolution of aeolian inputs or dissolved river loads.

Tracer dispersion in packings of porous activated carbon grains

A new method based on radioactive dispersion is reported which permits the accurate determination of the parameters which describe the transport and exchange mechanisms for packed beds of porous particles: characteristic exchange time between the internal and external pores (including adsorption effects), mean tracer velocity, mobile fluid fraction and longitudinal hydrodynamic dispersion coefficient. The method relies upon the fitting of the Coats–Smith model to radioactive tracer transit time distributions at regularly spaced intervals along a packed column for low Peclet numbers. Relationships between the transport and exchange mechanism parameters and relatively easily measured properties of the porous particles (size, porosity, intraparticle diffusion coefficient, adsorption isotherm) and packed bed (packing density) have been determined. These relationships may be used to estimate the hydrodynamic parameters. It is shown that the transport and exchange mechanism parameters determined at low Pe numbers using the new method can be used to accurately predict behavior of the packed bed at high Pe numbers provided transport within the porous particles is explicitly included within the model.

Tracer dispersion in rough channels: A two-dimensional numerical study

Numerical results are presented on the hydrodynamic dispersion of passive tracer in flow through a two-dimensional channel bounded by parallel plates, one having substantial rugosities perpendicular to the flow. The simulation is based on the numerical solution of the microscopic Stokes and convention-diffusion equations, over a range of Péclet numbers and of degrees of rugosity of the channel. One observes a tenfold increase of the effective dispersion between the smooth plate case (classical Taylor dispersion) and the maximum rugosity case, attributable to the trapping of tracer inside deep rugosities. The results are analyzed in terms of tracer exchange models, involving a characteristic exchange time between the low- and high-velocity zones and their relative volume fractions, as a function of the Péclet number and the relative depth of the rugosities. The simulations are compared to ‘‘echo’’ dispersion experiments in which the tracer is first injected into the model and then, upon reversal of the fluid flow, drawn back through the inlet.

Correlation between the rotational mobility and the kinetics of the intramolecular exchange of the unpaired electron and bond in a tin-containing free radical with two pyrocatechol ligands

EPR spectroscopy was used to study the effect of the mobility of the medium on the dynamics of the interligand exchange of the unpaired electron and bond in the free radical formed upon the reaction of stannous bis-3,6-di-tert-butyl-o-semiquinolate with 3,6-di-tert-butyl-2-hydroxyphenoxyl in the presence of triphenylphosphine. A dependence exists between the characteristic exchange time τex and the correlation time for the rotational mobility of the radicalτc;τex=ατc. A temperature dependence was found for coefficient α.

Experimental study of tracer dispersion in sintered glass porous materials of variable compaction

Experimental measurements of tracer dispersion in sintered glass samples of variable compaction are reported. The results are compared to measurements in nonconsolidated packings of the same glass beads used to prepare the sintered material. While normal ‘‘Gaussian’’ dispersion is obtained in nonconsolidated samples (37% porosity), clear-cut anomalies (such as dispersion ‘‘tails’’ at long times) are observed in sintered materials even with a very low degree of compaction (34% porosity). Experimental curves are analyzed with a ‘‘capacitance’’-type model. The effective dispersion coefficient increases by a factor of 60 when the porosity decreases from 37% to 14% at higher degrees of compaction. The characteristic exchange time between the low-velocity regions and the main flow varies linearly with the transit time through the sample. These results are interpreted by the presence of inhomogeneities with a size of several grain diameters.