Inhomogeneous diffusion-limited aggregation
NASA Technical Reports Server (NTRS)
Selinger, Robin Blumberg; Nittmann, Johann; Stanley, H. E.
1989-01-01
It is demonstrated here that inhomogeneous diffusion-limited aggregation (DLA) model can be used to simulate viscous fingering in a medium with inhomogeneous permeability and homogeneous porosity. The medium consists of a pipe-pore square-lattice network in which all pores have equal volume and the pipes have negligible volume. It is shown that fluctuations in a DLA-based growth process may be tuned by noise reduction, and that fluctuations in the velocity of the moving interface are multiplicative in form.
Attracted diffusion-limited aggregation.
Rahbari, S H Ebrahimnazhad; Saberi, A A
2012-07-01
In this paper we present results of extensive Monte Carlo simulations of diffusion-limited aggregation (DLA) with a seed placed on an attractive plane as a simple model in connection with the electrical double layers. We compute the fractal dimension of the aggregated patterns as a function of the attraction strength α. For the patterns grown in both two and three dimensions, the fractal dimension shows a significant dependence on the attraction strength for small values of α and approaches that of the ordinary two-dimensional (2D) DLA in the limit of large α. For the nonattracting case with α = 1, our results in three dimensions reproduce the patterns of 3D ordinary DLA, while in two dimensions our model leads to the formation of a compact cluster with dimension 2. For intermediate α, the 3D clusters have a quasi-2D structure with a fractal dimension very close to that of the ordinary 2D DLA. This allows one to control the morphology of a growing cluster by tuning a single external parameter α. PMID:23005417
Attracted diffusion-limited aggregation
NASA Astrophysics Data System (ADS)
Rahbari, S. H. Ebrahimnazhad; Saberi, A. A.
2012-07-01
In this paper we present results of extensive Monte Carlo simulations of diffusion-limited aggregation (DLA) with a seed placed on an attractive plane as a simple model in connection with the electrical double layers. We compute the fractal dimension of the aggregated patterns as a function of the attraction strength α. For the patterns grown in both two and three dimensions, the fractal dimension shows a significant dependence on the attraction strength for small values of α and approaches that of the ordinary two-dimensional (2D) DLA in the limit of large α. For the nonattracting case with α=1, our results in three dimensions reproduce the patterns of 3D ordinary DLA, while in two dimensions our model leads to the formation of a compact cluster with dimension 2. For intermediate α, the 3D clusters have a quasi-2D structure with a fractal dimension very close to that of the ordinary 2D DLA. This allows one to control the morphology of a growing cluster by tuning a single external parameter α.
Anisotropic diffusion-limited aggregation.
Popescu, M N; Hentschel, H G E; Family, F
2004-06-01
Using stochastic conformal mappings, we study the effects of anisotropic perturbations on diffusion-limited aggregation (DLA) in two dimensions. The harmonic measure of the growth probability for DLA can be conformally mapped onto a constant measure on a unit circle. Here we map m preferred directions for growth to a distribution on the unit circle, which is a periodic function with m peaks in [-pi,pi) such that the angular width sigma of the peak defines the "strength" of anisotropy kappa= sigma(-1) along any of the m chosen directions. The two parameters (m,kappa) map out a parameter space of perturbations that allows a continuous transition from DLA (for small enough kappa ) to m needlelike fingers as kappa--> infinity. We show that at fixed m the effective fractal dimension of the clusters D(m,kappa) obtained from mass-radius scaling decreases with increasing kappa from D(DLA) approximately 1.71 to a value bounded from below by D(min) = 3 / 2. Scaling arguments suggest a specific form for the dependence of the fractal dimension D(m,kappa) on kappa for large kappa which compares favorably with numerical results. PMID:15244564
Diffusion Limited Aggregation: Algorithm optimization revisited
NASA Astrophysics Data System (ADS)
Braga, F. L.; Ribeiro, M. S.
2011-08-01
The Diffusion Limited Aggregation (DLA) model developed by Witten and Sander in 1978 is useful in modeling a large class of growth phenomena with local dependence. Besides its simplicity this aggregation model has a complex behavior that can be observed at the patterns generated. We propose on this work a brief review of some important proprieties of this model and present an algorithm to simulate a DLA aggregates that simpler and efficient compared to others found in the literature.
Diffusion-limited aggregation on curved surfaces
NASA Astrophysics Data System (ADS)
Choi, J.; Crowdy, D.; Bazant, M. Z.
2010-08-01
We develop a general theory of transport-limited aggregation phenomena occurring on curved surfaces, based on stochastic iterated conformal maps and conformal projections to the complex plane. To illustrate the theory, we use stereographic projections to simulate diffusion-limited aggregation (DLA) on surfaces of constant Gaussian curvature, including the sphere (K>0) and the pseudo-sphere (K<0), which approximate "bumps" and "saddles" in smooth surfaces, respectively. Although the curvature affects the global morphology of the aggregates, the fractal dimension (in the curved metric) is remarkably insensitive to curvature, as long as the particle size is much smaller than the radius of curvature. We conjecture that all aggregates grown by conformally invariant transport on curved surfaces have the same fractal dimension as DLA in the plane. Our simulations suggest, however, that the multifractal dimensions increase from hyperbolic (K<0) to elliptic (K>0) geometry, which we attribute to curvature-dependent screening of tip branching.
Thermodynamically reversible generalization of diffusion limited aggregation.
D'Souza, R M; Margolus, N H
1999-07-01
We introduce a lattice gas model of cluster growth via the diffusive aggregation of particles in a closed system obeying a local, deterministic, microscopically reversible dynamics. This model roughly corresponds to placing the irreversible diffusion limited aggregation model (DLA) in contact with a heat bath. Particles release latent heat when aggregating, while singly connected cluster members can absorb heat and evaporate. The heat bath is initially empty, hence we observe the flow of entropy from the aggregating gas of particles into the heat bath, which is being populated by diffusing heat tokens. Before the population of the heat bath stabilizes, the cluster morphology (quantified by the fractal dimension) is similar to a standard DLA cluster. The cluster then gradually anneals, becoming more tenuous, until reaching configurational equilibrium when the cluster morphology resembles a quenched branched random polymer. As the microscopic dynamics is invertible, we can reverse the evolution, observe the inverse flow of heat and entropy, and recover the initial condition. This simple system provides an explicit example of how macroscopic dissipation and self-organization can result from an underlying microscopically reversible dynamics. We present a detailed description of the dynamics for the model, discuss the macroscopic limit, and give predictions for the equilibrium particle densities obtained in the mean field limit. Empirical results for the growth are then presented, including the observed equilibrium particle densities, the temperature of the system, the fractal dimension of the growth clusters, scaling behavior, finite size effects, and the approach to equilibrium. We pay particular attention to the temporal behavior of the growth process and show that the relaxation to the maximum entropy state is initially a rapid nonequilibrium process, then subsequently it is a quasistatic process with a well defined temperature. PMID:11969759
Diffusion-Limited Aggregation with Polygon Particles
NASA Astrophysics Data System (ADS)
Deng, Li; Wang, Yan-Ting; Ou-Yang, Zhong-Can
2012-12-01
Diffusion-limited aggregation (DLA) assumes that particles perform pure random walk at a finite temperature and aggregate when they come close enough and stick together. Although it is well known that DLA in two dimensions results in a ramified fractal structure, how the particle shape influences the formed morphology is still unclear. In this work, we perform the off-lattice two-dimensional DLA simulations with different particle shapes of triangle, quadrangle, pentagon, hexagon, and octagon, respectively, and compare with the results for circular particles. Our results indicate that different particle shapes only change the local structure, but have no effects on the global structure of the formed fractal cluster. The local compactness decreases as the number of polygon edges increases.
Scaling in the Diffusion Limited Aggregation Model
NASA Astrophysics Data System (ADS)
Menshutin, Anton
2012-01-01
We present a self-consistent picture of diffusion limited aggregation (DLA) growth based on the assumption that the probability density P(r,N) for the next particle to be attached within the distance r to the center of the cluster is expressible in the scale-invariant form P[r/Rdep(N)]. It follows from this assumption that there is no multiscaling issue in DLA and there is only a single fractal dimension D for all length scales. We check our assumption self-consistently by calculating the particle-density distribution with a measured P(r/Rdep) function on an ensemble with 1000 clusters of 5×107 particles each. We also show that a nontrivial multiscaling function D(x) can be obtained only when small clusters (N<10000) are used to calculate D(x). Hence, multiscaling is a finite-size effect and is not intrinsic to DLA.
Linear relationship statistics in diffusion limited aggregation
NASA Astrophysics Data System (ADS)
Saberi, Abbas Ali
2009-11-01
We show that various surface parameters in two-dimensional diffusion limited aggregation (DLA) grow linearly with the number of particles. We find the ratio of the average length of the perimeter and the accessible perimeter of a DLA cluster together with its external perimeters to the cluster size, and define a microscopic schematic procedure for attachment of an incident new particle to the cluster. We measure the fractal dimension of the red sites (i.e., the sites such that cutting each of them splits the cluster) as equal to that of the DLA cluster. It is also shown that the average number of dead sites and the average number of red sites have linear relationships with the cluster size.
Scaling in the diffusion limited aggregation model.
Menshutin, Anton
2012-01-01
We present a self-consistent picture of diffusion limited aggregation (DLA) growth based on the assumption that the probability density P(r,N) for the next particle to be attached within the distance r to the center of the cluster is expressible in the scale-invariant form P[r/R{dep}(N)]. It follows from this assumption that there is no multiscaling issue in DLA and there is only a single fractal dimension D for all length scales. We check our assumption self-consistently by calculating the particle-density distribution with a measured P(r/R{dep}) function on an ensemble with 1000 clusters of 5×10{7} particles each. We also show that a nontrivial multiscaling function D(x) can be obtained only when small clusters (N<10 000) are used to calculate D(x). Hence, multiscaling is a finite-size effect and is not intrinsic to DLA. PMID:22304265
Beyond diffusion-limited aggregation kinetics in microparticle suspensions.
Erb, Randall M; Krebs, Melissa D; Alsberg, Eben; Samanta, Bappaditya; Rotello, Vincent M; Yellen, Benjamin B
2009-11-01
Aggregation in nondiffusion limited colloidal particle suspensions follows a temporal power-law dependence that is consistent with classical diffusion limited cluster aggregation models; however, the dynamic scaling exponents observed in these systems are not adequately described by diffusion limited cluster aggregation models, which expect these scaling exponents to be constant over all experimental conditions. We show here that the dynamic scaling exponents for 10 microm particles increase with the particle concentration and the particle-particle free energy of interaction. We provide a semiquantitative explanation for the scaling behavior in terms of the long-ranged particle-particle interaction potential. PMID:20364980
Diffusion-limited aggregates grown on nonuniform substrates
NASA Astrophysics Data System (ADS)
Cornette, V.; Centres, P. M.; Ramirez-Pastor, A. J.; Nieto, F.
2013-12-01
In the present paper, patterns of diffusion-limited aggregation (DLA) grown on nonuniform substrates are investigated by means of Monte Carlo simulations. We consider a nonuniform substrate as the largest percolation cluster of dropped particles with different structures and forms that occupy more than a single site on the lattice. The aggregates are grown on such clusters, in the range the concentration, p, from the percolation threshold, pc up to the jamming coverage, pj. At the percolation threshold, the aggregates are asymmetrical and the branches are relatively few. However, for larger values of p, the patterns change gradually to a pure DLA. Tiny qualitative differences in this behavior are observed for different k sizes. Correspondingly, the fractal dimension of the aggregates increases as p raises in the same range pc≤p≤pj. This behavior is analyzed and discussed in the framework of the existing theoretical approaches.
Optimizing off-lattice Diffusion-Limited Aggregation
NASA Astrophysics Data System (ADS)
Kuijpers, Kasper R.; de Martín, Lilian; van Ommen, J. Ruud
2014-03-01
We present a technique to improve the time scaling of Diffusion-Limited Aggregation simulations. The proposed method reduces the number of calculations by making an extensive use of the RAM memory to store information about the particles’ positions and distances. We have simulated clusters up to 5ṡ106 particles in 2D and up to 1ṡ106 particles in 3D and compared the calculation times with previous algorithms proposed in the literature. Our method scales t∝Np1.08, outperforming the current optimization techniques.
Abnormal Stability in Growth of Diffusion-Limited Aggregation
NASA Astrophysics Data System (ADS)
Ohta, Shonosuke
2009-01-01
An abnormal and unsteady growth of an isotropic cluster in diffusion-limited aggregation (DLA) is observed in stability analyses. Macroscopic fluctuation due to the delay of transition from a dendritic tip to a tip-splitting growth induces the anisotropy of DLA. An asymptotic deformation factor \\varepsilon∞ = 0.0888 is obtained from large DLA clusters. A symmetric oval model proposed from the dual-stability growth of DLA gives an asymptotic fractal dimension of 1.7112 using \\varepsilon∞. The correspondence of this model to the box dimension is excellent.
Growth of Silicon Nanosheets Under Diffusion-Limited Aggregation Environments.
Lee, Jaejun; Kim, Sung Wook; Kim, Ilsoo; Seo, Dongjea; Choi, Heon-Jin
2015-12-01
The two-dimensional (2D) growth of cubic-structured (silicon) Si nanosheets (SiNSs) was investigated. Freestanding, single-crystalline SiNSs with a thickness of 5-20 nm were grown on various Si substrates under an atmospheric chemical vapor deposition process. Systematic investigation indicated that a diffusion-limited aggregation (DLA) environment that leads to dendritic growth in <110> directions at the initial stage is essential for 2D growth. The kinetic aspects under DLA environments that ascribe to the dendritic and 2D growth were discussed. Under the more dilute conditions made by addition of Ar to the flow of H2, the SiNSs grew epitaxially on the substrates with periodic arrangement at a specific angle depending on the orientation of the substrate. It reveals that SiNSs always grew two dimensionally with exposing (111) surfaces. That is thermodynamically favorable. PMID:26518028
Growth of Silicon Nanosheets Under Diffusion-Limited Aggregation Environments
NASA Astrophysics Data System (ADS)
Lee, Jaejun; Kim, Sung Wook; Kim, Ilsoo; Seo, Dongjea; Choi, Heon-Jin
2015-10-01
The two-dimensional (2D) growth of cubic-structured (silicon) Si nanosheets (SiNSs) was investigated. Freestanding, single-crystalline SiNSs with a thickness of 5-20 nm were grown on various Si substrates under an atmospheric chemical vapor deposition process. Systematic investigation indicated that a diffusion-limited aggregation (DLA) environment that leads to dendritic growth in <110> directions at the initial stage is essential for 2D growth. The kinetic aspects under DLA environments that ascribe to the dendritic and 2D growth were discussed. Under the more dilute conditions made by addition of Ar to the flow of H2, the SiNSs grew epitaxially on the substrates with periodic arrangement at a specific angle depending on the orientation of the substrate. It reveals that SiNSs always grew two dimensionally with exposing (111) surfaces. That is thermodynamically favorable.
Influence of particle size on diffusion-limited aggregation.
Tan, Z J; Zou, X W; Zhang, W B; Jin, Z Z
1999-11-01
The influence of particle size on diffusion-limited aggregation (DLA) has been investigated by computer simulations. For DLA clusters consisting of two kinds of particles with different sizes, when large particles are in the minority, the patterns of clusters appear asymmetrical and nonuniform, and their fractal dimensions D(f) increase compared with one-component DLA. With increasing size of large particles, D(f) increases. This increase can be attributed to two reasons: one is that large particles become new growth centers; the other is the big masses of large particles. As the concentration ratio x(n) of large particles increases, D(f) will reach a maximum value D(f(m)) and then decrease. When x(n) exceeds a certain value, the morphology and D(f) of the two-component DLA clusters are similar to those of one-component DLA clusters. PMID:11970534
Anisotropic diffusion limited aggregation in three dimensions: universality and nonuniversality.
Goold, Nicholas R; Somfai, Ellák; Ball, Robin C
2005-09-01
We explore the macroscopic consequences of lattice anisotropy for diffusion limited aggregation (DLA) in three dimensions. Simple cubic and bcc lattice growths are shown to approach universal asymptotic states in a coherent fashion, and the approach is accelerated by the use of noise reduction. These states are strikingly anisotropic dendrites with a rich hierarchy of structure. For growth on an fcc lattice, our data suggest at least two stable fixed points of anisotropy, one matching the bcc case. Hexagonal growths, favoring six planar and two polar directions, appear to approach a line of asymptotic states with continuously tunable polar anisotropy. The more planar of these growths visually resembles real snowflake morphologies. Our simulations use a new and dimension-independent implementation of the DLA model. The algorithm maintains a hierarchy of sphere coverings of the growth, supporting efficient random walks onto the growth by spherical moves. Anisotropy was introduced by restricting growth to certain preferred directions. PMID:16241431
Anisotropic diffusion limited aggregation in three dimensions: Universality and nonuniversality
NASA Astrophysics Data System (ADS)
Goold, Nicholas R.; Somfai, Ellák; Ball, Robin C.
2005-09-01
We explore the macroscopic consequences of lattice anisotropy for diffusion limited aggregation (DLA) in three dimensions. Simple cubic and bcc lattice growths are shown to approach universal asymptotic states in a coherent fashion, and the approach is accelerated by the use of noise reduction. These states are strikingly anisotropic dendrites with a rich hierarchy of structure. For growth on an fcc lattice, our data suggest at least two stable fixed points of anisotropy, one matching the bcc case. Hexagonal growths, favoring six planar and two polar directions, appear to approach a line of asymptotic states with continuously tunable polar anisotropy. The more planar of these growths visually resembles real snowflake morphologies. Our simulations use a new and dimension-independent implementation of the DLA model. The algorithm maintains a hierarchy of sphere coverings of the growth, supporting efficient random walks onto the growth by spherical moves. Anisotropy was introduced by restricting growth to certain preferred directions.
Modeling realistic breast lesions using diffusion limited aggregation
NASA Astrophysics Data System (ADS)
Rashidnasab, Alaleh; Elangovan, Premkumar; Dance, David R.; Young, Kenneth C.; Diaz, Oliver; Wells, Kevin
2012-03-01
Synthesizing the appearance of malignant masses and inserting these into digital mammograms can be used as part of a wider framework for investigating the radiological detection task in X-ray mammography. However, the randomness associated with cell division within cancerous masses and the associated complex morphology challenges the realism of the modeling process. In this paper, Diffusion Limited Aggregation (DLA), a type of fractal growth process is proposed and utilized for modeling breast lesions. Masses of different sizes, shapes and densities were grown by controlling DLA growth parameters either prior to growth, or dynamically updating these during growth. A validation study was conducted by presenting 30 real and 30 simulated masses in a random order to a team of radiologists. The results from the validation study suggest that the observers found it difficult to differentiate between the real and simulated lesions.
Scaling laws in the diffusion limited aggregation of persistent random walkers
NASA Astrophysics Data System (ADS)
Nogueira, Isadora R.; Alves, Sidiney G.; Ferreira, Silvio C.
2011-11-01
We investigate the diffusion limited aggregation of particles executing persistent random walks. The scaling properties of both random walks and large aggregates are presented. The aggregates exhibit a crossover between ballistic and diffusion limited aggregation models. A non-trivial scaling relation ξ∼ℓ1.25 between the characteristic size ξ, in which the cluster undergoes a morphological transition, and the persistence length ℓ, between ballistic and diffusive regimes of the random walk, is observed.
Diffusion-limited aggregation with power-law pinning.
Hentschel, H G E; Popescu, M N; Family, F
2004-01-01
Using stochastic conformal mapping techniques we study the patterns emerging from Laplacian growth with a power-law decaying threshold for growth R(-gamma)(N) (where R(N) is the radius of the N-particle cluster). For gamma>1 the growth pattern is in the same universality class as diffusion limited aggregation (DLA), while for gamma<1 the resulting patterns have a lower fractal dimension D(gamma) than a DLA cluster due to the enhancement of growth at the hot tips of the developing pattern. Our results indicate that a pinning transition occurs at gamma=1/2, significantly smaller than might be expected from the lower bound alpha(min) approximately 0.67 of multifractal spectrum of DLA. This limiting case shows that the most singular tips in the pruned cluster now correspond to those expected for a purely one-dimensional line. Using multifractal analysis, analytic expressions are established for D(gamma) both close to the breakdown of DLA universality class, i.e., gamma less, similar 1, and close to the pinning transition, i.e., gamma greater, similar 1/2. PMID:14995617
NASA Astrophysics Data System (ADS)
Lu, Jing; Liu, Dongmei; Yang, Xiaonan; Zhao, Ying; Liu, Haixing; Tang, Huan; Cui, Fuyi
2015-12-01
Due to the limitations of experimental methods at the atomic level, research on the aggregation of small nanoparticles (D < 5 nm) in aqueous solutions is quite rare. The aggregation of small nanoparticles in aqueous solutions is very different than that of normal sized nanoparticles. The interfacial interactions play a dominant role in the aggregation of small nanoparticles. In this paper, molecular dynamics simulations, which can explore the microscopic behavior of nanoparticles during the diffusion-limited aggregation at an atomic level, were employed to reveal the aggregation mechanism of small nanoparticles in aqueous solutions. First, the aggregation processes and aggregate structure were depicted. Second, the particle-particle interaction and surface diffusion of nanoparticles during aggregation were investigated. Third, the water-mediated interactions during aggregation were ascertained. The results indicate that the aggregation of nanoparticle in aqueous solutions is affected by particle size. The strong particle-particle interaction and high surface diffusion result in the formation of particle-particle bonds of 2 nm TiO2 nanoparticles, and the water-mediated interaction plays an important role in the aggregation process of 3 and 4 nm TiO2 nanoparticles.
An extended fractal growth regime in the diffusion limited aggregation including edge diffusion
NASA Astrophysics Data System (ADS)
Ghosh, Aritra; Batabyal, R.; Das, G. P.; Dev, B. N.
2016-01-01
We have investigated on-lattice diffusion limited aggregation (DLA) involving edge diffusion and compared the results with the standard DLA model. For both cases, we observe the existence of a crossover from the fractal to the compact regime as a function of sticking coefficient. However, our modified DLA model including edge diffusion shows an extended fractal growth regime like an earlier theoretical result using realistic growth models and physical parameters [Zhang et al., Phys. Rev. Lett. 73 (1994) 1829]. While the results of Zhang et al. showed the existence of the extended fractal growth regime only on triangular but not on square lattices, we find its existence on the square lattice. There is experimental evidence of this growth regime on a square lattice. The standard DLA model cannot characterize fractal morphology as the fractal dimension (Hausdorff dimension, DH) is insensitive to morphology. It also predicts DH = DP (the perimeter dimension). For the usual fractal structures, observed in growth experiments on surfaces, the perimeter dimension can differ significantly (DH ≠ DP) depending on the morphology. Our modified DLA model shows minor sensitivity to this difference.
Multifractal analysis of the branch structure of diffusion-limited aggregates
NASA Astrophysics Data System (ADS)
Hanan, W. G.; Heffernan, D. M.
2012-02-01
We examine the branch structure of radial diffusion-limited aggregation (DLA) clusters for evidence of multifractality. The lacunarity of DLA clusters is measured and the generalized dimensions D(q) of their mass distribution is estimated using the sandbox method. We find that the global n-fold symmetry of the aggregates can induce anomalous scaling behavior into these measurements. However, negating the effects of this symmetry, standard scaling is recovered.
Multifractal analysis of the branch structure of diffusion-limited aggregates.
Hanan, W G; Heffernan, D M
2012-02-01
We examine the branch structure of radial diffusion-limited aggregation (DLA) clusters for evidence of multifractality. The lacunarity of DLA clusters is measured and the generalized dimensions D(q) of their mass distribution is estimated using the sandbox method. We find that the global n-fold symmetry of the aggregates can induce anomalous scaling behavior into these measurements. However, negating the effects of this symmetry, standard scaling is recovered. PMID:22463212
Multifractal analysis of the branch structure of diffusion-limited aggregates.
Hanan, W G; Heffernan, D M
2012-02-01
We examine the branch structure of radial diffusion-limited aggregation (DLA) clusters for evidence of multifractality. The lacunarity of DLA clusters is measured and the generalized dimensions D(q) of their mass distribution is estimated using the sandbox method. We find that the global n-fold symmetry of the aggregates can induce anomalous scaling behavior into these measurements. However, negating the effects of this symmetry, standard scaling is recovered.
Electrochemical Growth of Ag Junctions and Diffusion Limited Aggregate (DLA) Fractal Simulation
NASA Astrophysics Data System (ADS)
Olson, Zak; Tuppan, Sam; Kim, Woo-Joong; Seattle University Team
2015-03-01
We attempt construction of a single atom connection between two copper wires. By applying a DC voltage across the wires when immersed in a silver nitrate solution, we deposit silver until a junction is formed. The deposited silver forms a fractal structure that can be simulated with a diffusion limited aggregation model.
Computer simulation of diffusion-limited cluster-cluster aggregation with an Epstein drag force.
Pierce, F; Sorensen, C M; Chakrabarti, A
2006-08-01
The motion of particles, dispersed in a medium, between collisions with each other can, in limiting situations, be either ballistic (straight line) or diffusive (random walker). The diffusive regime can be divided into two distinct subregimes. The "continuum regime" exhibits Stokes-Einstein-type diffusion (no-slip surface boundary condition) with a frictional coefficient proportional to the particle size (linear dimension). The "Epstein regime," as we shall refer to it, is characterized by a frictional coefficient proportional to the particle cross-sectional area, hence an Epstein-type diffusion (slip surface). The purpose of the current study is to illuminate the dynamics of dilute-limit aggregation in the Epstein regime. We present results from low volume fraction Monte Carlo simulations of cluster-cluster aggregation in the Epstein regime with the particle motion based on each particle's cross-sectional area. Our findings indicate that aggregates grown under Epstein conditions have a fractal dimension of approximately 1.8, similar to that of diffusion-limited cluster-cluster aggregates (DLCA) in the continuum regime. The kinetic exponent z in the Epstein regime is found to be z approximately 0.8, lower than its value for both the continuum regime DLCA (z = 1) and for the ballistic cluster aggregation regime (z approximately 2). Cluster size distribution data for Epstein systems are found to scale at large cluster sizes with exponents consistent with the kinetic data. A scaling argument for predicting the kinetic exponent and kernel homogeneity based on the mass or size dependence of the particle velocity and collision cross section is presented and is seen to give accurate results for dilute and intermediate values of particle volume fractions not only for the current study, but also for work done by other researchers with various choices for the aggregation kernel. PMID:17025429
Lattuada, Marco
2012-01-12
Smoluchowski's equation for the rate of aggregation of colloidal particles under diffusion-limited conditions has set the basis for the interpretation of kinetics of aggregation phenomena. Nevertheless, its use is limited to sufficiently dilute conditions. In this work we propose a correction to Smoluchowski's equation by using a result derived by Richards ( J. Phys. Chem. 1986 , 85 , 3520 ) within the framework of trapping theory. This corrected aggregation kernel, which accounts for concentration dependence effects, has been implemented in a population-balance equations scheme and used to model the aggregation kinetics of colloidal particles undergoing diffusion-limited aggregation under concentrated conditions (up to a particle volume fraction of 30%). The predictions of population balance calculations have been validated by means of Brownian dynamic simulations. It was found that the corrected kernel can very well reproduce the results from Brownian dynamic simulations for all concentration values investigated, and is also able to accurately predict the time required by a suspension to reach the gel point. On the other hand, classical Smoluchowski's theory substantially underpredicts the rate of aggregation as well as the onset of gelation, with deviations becoming progressively more severe as the particle volume fraction increases. PMID:22148884
Kinetics of cold-set diffusion-limited aggregations of denatured whey protein isolate colloids.
Wu, Hua; Xie, Jianjun; Morbidelli, Massimo
2005-01-01
The CaCl2-induced cold-set aggregation kinetics of the denatured whey protein isolate (WPI) colloids has been investigated under dilute diffusion-limited cluster aggregation (DLCA) conditions, using small-angle light scattering. In particular, the structure factor, the scattered intensity at zero angle and the average radius of gyration have been measured for the aggregating system as a function of time. It is found that the fractal dimension of the clusters is df= 1.85, in the range typical of clusters aggregated under DLCA conditions. The aggregation kinetics in this transition region can be described by a power law relation in the initial stage of the aggregation, but the exponent of the power law is equal to 0.7, i.e., significantly larger than 1/df= 0.54, which is the typical value of the DLCA kinetics. Since it is found that the average gyration radius of the clusters has reached a value of 80 microm, leading to a cumulative volume fraction of clusters equal to 0.25, it is legitimate to expect that the process is in the region of transition from aggregation to gelation. This confirmed by the fact that, at the later stage of the aggregation, the growth of the average cluster size further accelerates with time and eventually becomes explosive, leading to gelation. The observed aggregation kinetics has been compared with that reported in the literature from DLCA Monte Carlo simulations, and a good agreement has been found with the data corresponding to the transition region from aggregation to gelation. Numerical simulations using the Smoluchowski kinetic model have also been carried out in order to support the experimental findings.
Marchetti, Riccardo; Taloni, Alessandro; Caglioti, Emanuele; Loreto, Vittorio; Pietronero, Luciano
2012-08-10
We prove that the harmonic measure is stationary, unique, and invariant on the interface of diffusion limited aggregation (DLA) growing on a cylinder surface. We provide a detailed theoretical analysis puzzling together multiscaling, multifractality, and conformal invariance, supported by extensive numerical simulations of clusters built using conformal mappings and on a lattice. The growth properties of the active and frozen zones are clearly elucidated. We show that the unique scaling exponent characterizing the stationary growth is the DLA fractal dimension. PMID:23006279
Stationary Growth and Unique Invariant Harmonic Measure of Cylindrical Diffusion Limited Aggregation
NASA Astrophysics Data System (ADS)
Marchetti, Riccardo; Taloni, Alessandro; Caglioti, Emanuele; Loreto, Vittorio; Pietronero, Luciano
2012-08-01
We prove that the harmonic measure is stationary, unique, and invariant on the interface of diffusion limited aggregation (DLA) growing on a cylinder surface. We provide a detailed theoretical analysis puzzling together multiscaling, multifractality, and conformal invariance, supported by extensive numerical simulations of clusters built using conformal mappings and on a lattice. The growth properties of the active and frozen zones are clearly elucidated. We show that the unique scaling exponent characterizing the stationary growth is the DLA fractal dimension.
Interplay of model ingredients affecting aggregate shape plasticity in diffusion-limited aggregation
NASA Astrophysics Data System (ADS)
Duarte-Neto, P.; Stošić, T.; Stošić, B.; Lessa, R.; Milošević, M. V.
2014-07-01
We analyze the combined effect of three ingredients of an aggregation model—surface tension, particle flow and particle source—representing typical characteristics of many aggregation growth processes in nature. Through extensive numerical experiments and for different underlying lattice structures we demonstrate that the location of incoming particles and their preferential direction of flow can significantly affect the resulting general shape of the aggregate, while the surface tension controls the surface roughness. Combining all three ingredients increases the aggregate shape plasticity, yielding a wider spectrum of shapes as compared to earlier works that analyzed these ingredients separately. Our results indicate that the considered combination of effects is fundamental for modeling the polymorphic growth of a wide variety of structures in confined geometries and/or in the presence of external fields, such as rocks, crystals, corals, and biominerals.
Duarte-Neto, P; Stošić, T; Stošić, B; Lessa, R; Milošević, M V
2014-07-01
We analyze the combined effect of three ingredients of an aggregation model--surface tension, particle flow and particle source--representing typical characteristics of many aggregation growth processes in nature. Through extensive numerical experiments and for different underlying lattice structures we demonstrate that the location of incoming particles and their preferential direction of flow can significantly affect the resulting general shape of the aggregate, while the surface tension controls the surface roughness. Combining all three ingredients increases the aggregate shape plasticity, yielding a wider spectrum of shapes as compared to earlier works that analyzed these ingredients separately. Our results indicate that the considered combination of effects is fundamental for modeling the polymorphic growth of a wide variety of structures in confined geometries and/or in the presence of external fields, such as rocks, crystals, corals, and biominerals. PMID:25122308
NASA Astrophysics Data System (ADS)
Menshutin, A. Yu.; Shchur, L. N.; Vinokur, V. M.
2007-01-01
We develop a technique for probing the harmonic measure of a diffusion-limited-aggregation (DLA) cluster surface with variable-size particles and generate 1000 clusters with 50×106 particles using an original off-lattice killing-free algorithm. Taking, in sequence, the limit of the vanishing size of the probing particles and then sending the growing cluster size to infinity, we achieve unprecedented accuracy in determining the fractal dimension D=1.7100(2) crucial to the characterization of the geometric properties of DLA clusters.
Meier, Christoph; Wu, Yuzhou; Pramanik, Goutam; Weil, Tanja
2014-01-13
The assembly of high molecular weight polypeptides into complex architectures exhibiting structural complexity ranging from the nano- to the mesoscale is of fundamental importance for various protein-related diseases but also hold great promise for various nano- and biotechnological applications. Here, the aggregation of partially unfolded high molecular weight polypeptides into multiscale fractal structures is investigated by means of diffusion limited aggregation and atomic force microscopy. The zeta potential, the hydrodynamic radius, and the obtained fractal morphologies were correlated with the conformation of the polypeptide backbones as obtained from circular dichroism measurements. The polypeptides are modified with polyethylene oxide side chains to stabilize the polypeptides and to normalize intermolecular interactions. The modification with the hydrophobic thioctic acid alters the folding of the polypeptide backbone, resulting in a change in solution aggregation and fractal morphology. We found that a more compact folding results in dense and highly branched structures, whereas a less compact folded polypeptide chain yields a more directional assembly. Our results provide first evidence for the role of compactness of polypeptide folding on aggregation. Furthermore, the mesoscale-structured biofilms were used to achieve a hierarchical protein assembly, which is demonstrated by deposition of Rhodamine-labeled HSA with the preassembled fractal structures. These results contribute important insights to the fundamental understanding of the aggregation of high molecular weight polypeptides in general and provide opportunities to study nanostructure-related effects on biological systems such as adhesion, proliferation, and the development of, for example, neuronal cells. PMID:24354281
Meier, Christoph; Wu, Yuzhou; Pramanik, Goutam; Weil, Tanja
2014-01-13
The assembly of high molecular weight polypeptides into complex architectures exhibiting structural complexity ranging from the nano- to the mesoscale is of fundamental importance for various protein-related diseases but also hold great promise for various nano- and biotechnological applications. Here, the aggregation of partially unfolded high molecular weight polypeptides into multiscale fractal structures is investigated by means of diffusion limited aggregation and atomic force microscopy. The zeta potential, the hydrodynamic radius, and the obtained fractal morphologies were correlated with the conformation of the polypeptide backbones as obtained from circular dichroism measurements. The polypeptides are modified with polyethylene oxide side chains to stabilize the polypeptides and to normalize intermolecular interactions. The modification with the hydrophobic thioctic acid alters the folding of the polypeptide backbone, resulting in a change in solution aggregation and fractal morphology. We found that a more compact folding results in dense and highly branched structures, whereas a less compact folded polypeptide chain yields a more directional assembly. Our results provide first evidence for the role of compactness of polypeptide folding on aggregation. Furthermore, the mesoscale-structured biofilms were used to achieve a hierarchical protein assembly, which is demonstrated by deposition of Rhodamine-labeled HSA with the preassembled fractal structures. These results contribute important insights to the fundamental understanding of the aggregation of high molecular weight polypeptides in general and provide opportunities to study nanostructure-related effects on biological systems such as adhesion, proliferation, and the development of, for example, neuronal cells.
Random walks, diffusion limited aggregation in a wedge, and average conformal maps.
Sander, Leonard M; Somfai, Ellák
2005-06-01
We investigate diffusion-limited aggregation (DLA) in a wedge geometry. Arneodo and collaborators have suggested that the ensemble average of DLA cluster density should be close to the noise-free selected Saffman-Taylor finger. We show that a different, but related, ensemble average, that of the conformal maps associated with random clusters, yields a nontrivial shape which is also not far from the Saffman-Taylor finger. However, we have previously demonstrated that the same average of DLA in a channel geometry is not the Saffman-Taylor finger. This casts doubt on the idea that the average of noisy diffusion-limited growth is governed by a simple transcription of noise-free results. PMID:16035911
Electrochemical deposition of layered copper thin films based on the diffusion limited aggregation
Wei, Chenhuinan; Wu, Guoxing; Yang, Sanjun; Liu, Qiming
2016-01-01
In this work layered copper films with smooth surface were successfully fabricated onto ITO substrate by electrochemical deposition (ECD) and the thickness of the films was nearly 60 nm. The resulting films were characterized by SEM, TEM, AFM, XPS, and XRD. We have investigated the effects of potential and the concentration of additives and found that 2D dendritic-like growth process leaded the formation of films. A suitable growth mechanism based on diffusion limited aggregation (DLA) mechanism for the copper films formation is presented, which are meaningful for further designing homogeneous and functional films. PMID:27734900
Analysis of patterns formed by two-component diffusion limited aggregation.
Postnikov, E B; Ryabov, A B; Loskutov, A
2010-11-01
We consider diffusion limited aggregation of particles of two different kinds. It is assumed that a particle of one kind may adhere only to another particle of the same kind. The particles aggregate on a linear substrate which consists of periodically or randomly placed particles of different kinds. We analyze the influence of initial patterns on the structure of growing clusters. It is shown that at small distances from the substrate, the cluster structures repeat initial patterns. However, starting from a critical distance the initial periodicity is abruptly lost, and the particle distribution tends to a random one. An approach describing the evolution of the number of branches is proposed. Our calculations show that the initial pattern can be detected only at the distance which is not larger than approximately one and a half of the characteristic pattern size. PMID:21230475
Bias-free simulation of diffusion-limited aggregation on a square lattice
NASA Astrophysics Data System (ADS)
Loh, Yen Lee
We identify sources of systematic error in traditional simulations of the Witten-Sander model of diffusion-limited aggregation (DLA) on a square lattice. Based on semi-analytic solutions of the walk-to-line and walk-to-square first-passage problems, we develop an algorithm that reduces the simulation bias to below 10-12. We grow clusters of 108 particles on 65536 × 65536 lattices. We verify that lattice DLA clusters inevitably grow into anisotropic shapes, dictated by the anisotropy of the aggregation process. We verify that the fractal dimension evolves from the continuum DLA value, D = 1 . 71 , for small disk-shaped clusters, towards Kesten's bound of D = 3 / 2 for highly anisotropic clusters with long protruding arms.
Diffusion-limited aggregation as a markovian process: bond-sticking conditions
Kol; Aharony
2000-08-01
Cylindrical lattice diffusion limited aggregation (DLA), with a narrow width N, is solved using a Markovian matrix method. This matrix contains the probabilities that the front moves from one configuration to another at each growth step, calculated exactly by solving the Laplace equation and using the proper normalization. The method is applied for a series of approximations, which include only a finite number of rows near the front. The matrix is then used to find the weights of the steady-state growing configurations and the rate of approaching this steady-state stage. The former are then used to find the average upward growth probability, the average steady-state density and the fractal dimensionality of the aggregate, which is extrapolated to a value near 1.64. PMID:11088734
Is it really possible to grow isotropic on-lattice diffusion-limited aggregates?
NASA Astrophysics Data System (ADS)
Alves, S. G.; Ferreira, S. C., Jr.
2006-03-01
In a recent paper (Bogoyavlenskiy V A 2002 J. Phys. A: Math. Gen. 35 2533), an algorithm aiming to generate isotropic clusters of the on-lattice diffusion-limited aggregation (DLA) model was proposed. The procedure consists of aggregation probabilities proportional to the squared number of occupied sites (k2). In the present work, we analysed this algorithm using the noise reduced version of the DLA model and large-scale simulations. In the noiseless limit, instead of isotropic patterns, a 45° (30°) rotation in the anisotropy directions of the clusters grown on square (triangular) lattices was observed. A generalized algorithm, in which the aggregation probability is proportional to kν, was proposed. The exponent ν has a nonuniversal critical value νc, for which the patterns generated in the noiseless limit exhibit the original (axial) anisotropy for ν < νc and the rotated one (diagonal) for ν > νc. The values νc = 1.395 ± 0.005 and νc = 0.82 ± 0.01 were found for square and triangular lattices, respectively. Moreover, large-scale simulations show that there is a nontrivial relation between the noise reduction and anisotropy direction. The case ν = 2 (Bogoyavlenskiy's rule) is an example where the patterns exhibit the axial anisotropy for small and the diagonal one for large noise reduction.
Morphological transition between diffusion-limited and ballistic aggregation growth patterns.
Ferreira, S C; Alves, S G; Brito, A Faissal; Moreira, J G
2005-05-01
In this work, the transition between diffusion-limited (DLA) and ballistic aggregation (BA) models was reconsidered using a model in which biased random walks simulate the particle trajectories. The bias is controlled by a parameter lambda, which assumes the value lambda=0 (1) for the ballistic (diffusion-limited) aggregation model. Patterns growing from a single seed were considered. In order to simulate large clusters, an efficient algorithm was developed. For lambda (not equal to) 0 , the patterns are fractal on small length scales, but homogeneous on large ones. We evaluated the mean density of particles (-)rho in the region defined by a circle of radius r centered at the initial seed. As a function of r, (-)rho reaches the asymptotic value rho(0)(lambda) following a power law (-)rho = rho(0) +Ar(-gamma) with a universal exponent gamma=0.46 (2) , independent of lambda . The asymptotic value has the behavior rho(0) approximately |1-lambda|(beta) , where beta=0.26 (1) . The characteristic crossover length that determines the transition from DLA- to BA-like scaling regimes is given by xi approximately |1-lambda|(-nu) , where nu=0.61 (1) , while the cluster mass at the crossover follows a power law M(xi) approximately |1-lambda(-alpha) , where alpha=0.97 (2) . We deduce the scaling relations beta=nugamma and beta=2nu-alpha between these exponents. PMID:16089530
A new example of the diffusion-limited aggregation: Ni-Cu film patterns
NASA Astrophysics Data System (ADS)
Kockar, Hakan; Bayirli, Mehmet; Alper, Mursel
2010-02-01
The mechanism of the growth of the dendrites in the Ni-Cu films is studied by comparing them with the aggregates obtained by Monte Carlo (MC) simulations according to the diffusion-limited aggregation (DLA) model. The films were grown by electrodeposition. The structural analysis of the films carried out using the x-ray diffraction showed that the films have a face-centered cubic structure. Scanning electron microscope (SEM) was used for morphological observations and the film compositions were determined by energy dispersive x-ray spectroscopy. The observed SEM images are compared with the patterns obtained by MC simulations according to DLA model in which the sticking probability, P between the particles is used as a parameter. For all samples between the least and the densest aggregates in the films, the critical exponents of the density-density correlation functions, α were within the interval 0.160 ± 0.005-0.124 ± 0.006, and the fractal dimensions, Df, varies from 1.825 ± 0.006 to 1.809 ± 0.008 according to the method of two-point correlation function. These values are also verified by the mass-radius method. The pattern with α and Df within these intervals was obtained by MC simulations to DLA model while the sticking probability, P was within the interval from 0.35 to 0.40 obtained by varying P (1-0.001). The results showed that the DLA model in this binary system is a possible mechanism for the formation of the ramified pattern of Ni-Cu within the Ni-rich base part of the Ni-Cu films due to the diffusive characteristics of Cu.
Liu, Zhenyu; Kong, Xiaohui
2010-08-28
A regular array of fractal patterns with macroscopic dendritic carbon architecture was prepared by catalytic chemical vapor deposition (CVD). The dendritic carbon architectures have micrometre-sized stems and hyperbranches evolved by lateral growth, and they are formed by diffusion-limited aggregation of carbon-encapsulated iron nanoparticle building blocks generated from catalytic pyrolysis of toluene, which is directed by carbon nanotube cores, and followed by subsequent restructuring from surface to bulk. Incorporation of such proposed processes in Monte Carlo simulations generates dendritic architectures similar to the morphologies observed from the experiments. The findings provide direct information to the time resolved evolution of the morphology and microstructure of the dendritic carbon architecture, which mimic the nature behavior as snowflake attaching on the tree branches. Those will be important to understand the growth of vapor grown carbon fibers and carbon filamentous structures, and further possibility to control branching out of vapor grown carbon fibers. PMID:20607160
Scaling exponent of the maximum growth probability in diffusion-limited aggregation.
Jensen, Mogens H; Mathiesen, Joachim; Procaccia, Itamar
2003-04-01
An early (and influential) scaling relation in the multifractal theory of diffusion limited aggregation (DLA) is the Turkevich-Scher conjecture that relates the exponent alpha(min) that characterizes the "hottest" region of the harmonic measure and the fractal dimension D of the cluster, i.e., D=1+alpha(min). Due to lack of accurate direct measurements of both D and alpha(min), this conjecture could never be put to a serious test. Using the method of iterated conformal maps, D was recently determined as D=1.713+/-0.003. In this paper, we determine alpha(min) accurately with the result alpha(min)=0.665+/-0.004. We thus conclude that the Turkevich-Scher conjecture is incorrect for DLA. PMID:12786408
Phase transition in diffusion limited aggregation with patchy particles in two dimensions
NASA Astrophysics Data System (ADS)
Kartha, Moses J.; Sayeed, Ahmed
2016-08-01
The influence of patchy interactions on diffusion-limited aggregation (DLA) has been investigated by computer simulations. In this model, the adsorption of the particle is irreversible, but the adsorption occurs only when the 'sticky patch' makes contact with the sticky patch of a previously adsorbed particle. As we vary the patch size, growth rate of the cluster decreases, and below a well-defined critical patch size, pc the steady state growth rate goes to zero. The system reaches an absorbing phase producing a non-equilibrium continuous phase transition. The order parameter close to the critical value of the patch size shows a power law behavior ρ (∞) ∼(p -pc) β, where β = 0.2840. We have found that the value of the critical exponent convincingly shows that this transition in patchy DLA belongs to the directed percolation universality class.
A diffusion-limited aggregation model for the evolution of drainage networks
NASA Technical Reports Server (NTRS)
Masek, Jeffrey G.; Turcotte, Donald L.
1993-01-01
We propose a modified diffusion-limited aggregation (DLA) model for the evolution of fluvial drainage networks. Random walkers are introduced randomly on a grid, and each two-dimensional random walk proceeds until the walker finds a drainage network on which to accrete. This model for headward growth of drainage networks generates drainage patterns remarkably similar to actual drainages. The model also predicts statistical features which agree with actual networks, including the frequency-order (bifurcation) ratio (R(sub b) = 3.98) and the stream length-order (R(sub r) = 2.09). Using the definition of network fractal dimension D = log R(sub b)/log R(sub r), we find that our DLA model gives D = 1.87, near the observed range of D approximately equal to 1.80 - 1.85.
NASA Astrophysics Data System (ADS)
Srivastava, Rohit; Srivastava, P. K.
2014-03-01
Nanostructured diffusion-limited-aggregation (DLA) crystal pattern formation in microemulsion consisting of water, styrene, cetyltrimethylammonium chloride (CTACl), potassium persulphate and an oscillating Belousov-Zhabotinsky (BZ) reactant is reported. A variety of spatiotemporal patterns like concentric wave, spatial (stripe) and chaotic patterns appear. A colloidal phase composed of numerous nano-sized particles has been observed. The solid phase nucleation has been found to occur in the colloidal phase and has been found to grow in a symmetric crystal pattern with the progress of the reaction finally exhibiting DLA structures. We show that the formation of a nanostructured DLA crystal pattern is governed by spatial structures emerging in the BZ microemulsion system. Without any spatial structure in the microemulsion system only hydrogel of high viscosity is formed. A nano-sized branched crystal pattern was formed with a particle diameter in the range of 60-100 nm, as evident by transmission electron microscope, powder x-ray diffraction and particle size analyser studies.
Mean-field diffusion-limited aggregation: a "density" model for viscous fingering phenomena.
Bogoyavlenskiy, V A
2001-12-01
We explore a universal "density" formalism to describe nonequilibrium growth processes, specifically, the immiscible viscous fingering in Hele-Shaw cells (usually referred to as the Saffman-Taylor problem). For that we develop an alternative approach to the viscous fingering phenomena, whose basic concepts have been recently published in a Rapid Communication [Phys. Rev. E 63, 045305(R) (2001)]. This approach uses the diffusion-limited aggregation (DLA) paradigm as a core: we introduce a mean-field DLA generalization in stochastic and deterministic formulations. The stochastic model, a quasicontinuum DLA, simulates Monte Carlo patterns, which demonstrate a striking resemblance to natural Hele-Shaw fingers and, for steady-state growth regimes, follow precisely the Saffman-Taylor analytical solutions in channel and sector configurations. The relevant deterministic theory, a complete set of differential equations for a time development of density fields, is derived from that stochastic model. As a principal conclusion, we prove an asymptotic equivalency of both the stochastic and deterministic mean-field DLA formulations to the classic Saffman-Taylor hydrodynamics in terms of an interface evolution. PMID:11736272
Hentschel, H George E; Levermann, Anders; Procaccia, Itamar
2002-07-01
We study the fractal and multifractal properties (i.e., the generalized dimensions of the harmonic measure) of a two-parameter family of growth patterns that result from a growth model that interpolates between diffusion-limited aggregation (DLA) and Laplacian growth patterns in two dimensions. The two parameters are beta that determines the size of particles accreted to the interface, and C that measures the degree of coverage of the interface by each layer accreted to the growth pattern at every growth step. DLA and Laplacian growth are obtained at beta=0, C=0 and beta=2, C=1, respectively. The main purpose of this paper is to show that there exists a line in the beta-C phase diagram that separates fractal (D<2) from nonfractal (D=2) growth patterns. Moreover, Laplacian growth is argued to lie in the nonfractal part of the phase diagram. Some of our arguments are not rigorous, but together with the numerics they indicate this result rather strongly. We first consider the family of models obtained for beta=0, C>0, and derive for them a scaling relation D=2D(3). We then propose that this family has growth patterns for which D=2 for some C>C(cr), where C(cr) may be zero. Next we consider the whole beta-C phase diagram and define a line that separates two-dimensional growth patterns from fractal patterns with D<2. We explain that Laplacian growth lies in the region belonging to two-dimensional growth patterns, motivating the main conjecture of this paper, i.e., that Laplacian growth patterns are two dimensional. The meaning of this result is that the branches of Laplacian growth patterns have finite (and growing) area on scales much larger than any ultraviolet cutoff length. PMID:12241482
Ferretti, R.; Zhang, J.; Buffle, J.
1998-12-15
The structure of hematite aggregates in the presence of fairly monodisperse polyacrylic acid (PAA) with two different molecular weights (M{sub w} = 1.36 {times} 10{sup 6}, M{sub w}/M{sub n} = 1.53; M{sub w} = 3.69 {times} 10{sup 4}, M{sub w}/M{sub n} = 1.60) was studied using static light scattering (SLS). The fractal dimensions were calculated from the scattering exponents, after taking into account the finite size of aggregates, using exponential and Gaussian cutoff functions. Three flocculation regimes, namely, pre-DLA, DLA (diffusion-limited aggregation), and post-DLA, were defined based on the polymer concentration. In the DLA regime, fractal dimension values, D{sub f} = 1.84 {+-} 0.02 and 1.73 {+-} 0.02, were obtained using exponential and Gaussian cutoff functions, respectively. A fractal dimension of approximately 2.0 was found, as expected, in the pre-DLA regime (at PAA concentrations lower than the optimal dosage for a DLA regime) where the flocculation rate was reaction limited. In contrast, in the post-DLA regime, the flocculation was slow but the structure of aggregates was as tenuous as in the DLA regime with a fractal dimension D{sub f} {approx} 1.8. Moreover, for all three regimes, the D{sub f} values were independent of the molecular weights of PAA. The lower fractal dimension in post-DLA was probably due to the increased concentration of polymer chains between adjacent particles in aggregates. The steric hindrance favored tip-to-tip aggregation, leading to a more tenuous structure.
NASA Astrophysics Data System (ADS)
Braga, F. L.; Mattos, O. A.; Amorin, V. S.; Souza, A. B.
2015-07-01
Clusters formation models have been extensively studied in literature, and one of the main task of this research area is the analysis of the particle aggregation processes. Some work support that the main characteristics of this processes are strictly correlated to the cluster morphology, for example in DLA. It is expected that in the DLA clusters formation with particles containing different sizes the modification of the aggregation processes can be responsible for changes in the DLA morphology. The present article is going to analyze the formation of DLA clusters of particles with different sizes and show that the aggregates obtained by this approach generate an angle selection mechanism on dendritic growth that influences the shielding effect of the DLA edge and affect the fractal dimension of the clusters.
NASA Astrophysics Data System (ADS)
Majumdar, Satya N.
2003-08-01
We use the traveling front approach to derive exact asymptotic results for the statistics of the number of particles in a class of directed diffusion-limited aggregation models on a Cayley tree. We point out that some aspects of these models are closely connected to two different problems in computer science, namely, the digital search tree problem in data structures and the Lempel-Ziv algorithm for data compression. The statistics of the number of particles studied here is related to the statistics of height in digital search trees which, in turn, is related to the statistics of the length of the longest word formed by the Lempel-Ziv algorithm. Implications of our results to these computer science problems are pointed out.
NASA Astrophysics Data System (ADS)
Mohammadi, F.; Saberi, A. A.; Rouhani, S.
2009-09-01
In this paper, we analyze the scaling behavior of a diffusion-limited aggregation (DLA) simulated by the Hastings-Levitov method. We obtain the fractal dimension of the clusters by direct analysis of the geometrical patterns, in good agreement with one obtained from an analytical approach. We compute the two-point density correlation function and we show that, in the large-size limit, it agrees with the obtained fractal dimension. These support the statistical agreement between the patterns and DLA clusters. We also investigate the scaling properties of various length scales and their fluctuations, related to the boundary of the cluster. We find that all of the length scales do not have a simple scaling with the same correction to scaling exponent. The fractal dimension of the perimeter is obtained equal to that of the cluster. The growth exponent is computed from the evolution of the interface width equal to β = 0.557(2). We also show that the perimeter of the DLA cluster has an asymptotic multiscaling behavior.
Mohammadi, F; Saberi, A A; Rouhani, S
2009-09-16
In this paper, we analyze the scaling behavior of a diffusion-limited aggregation (DLA) simulated by the Hastings-Levitov method. We obtain the fractal dimension of the clusters by direct analysis of the geometrical patterns, in good agreement with one obtained from an analytical approach. We compute the two-point density correlation function and we show that, in the large-size limit, it agrees with the obtained fractal dimension. These support the statistical agreement between the patterns and DLA clusters. We also investigate the scaling properties of various length scales and their fluctuations, related to the boundary of the cluster. We find that all of the length scales do not have a simple scaling with the same correction to scaling exponent. The fractal dimension of the perimeter is obtained equal to that of the cluster. The growth exponent is computed from the evolution of the interface width equal to β = 0.557(2). We also show that the perimeter of the DLA cluster has an asymptotic multiscaling behavior. PMID:21832341
NASA Astrophysics Data System (ADS)
Menshutin, Anton Yu.; Shchur, Lev N.
2006-01-01
We test the multiscaling issue of diffusion-limited-aggregation (DLA) clusters using a modified algorithm. This algorithm eliminates killing the particles at the death circle. Instead, we return them to the birth circle at a random relative angle taken from the evaluated distribution. In addition, we use a two-level hierarchical memory model that allows using large steps in conjunction with an off-lattice realization of the model. Our algorithm still seems to stay in the framework of the original DLA model. We present an accurate estimate of the fractal dimensions based on the data for a hundred clusters with 50 million particles each. We find that multiscaling cannot be ruled out. We also find that the fractal dimension is a weak self-averaging quantity. In addition, the fractal dimension, if calculated using the harmonic measure, is a nonmonotonic function of the cluster radius. We argue that the controversies in the data interpretation can be due to the weak self-averaging and the influence of intrinsic noise.
Average shape of transport-limited aggregates.
Davidovitch, Benny; Choi, Jaehyuk; Bazant, Martin Z
2005-08-12
We study the relation between stochastic and continuous transport-limited growth models. We derive a nonlinear integro-differential equation for the average shape of stochastic aggregates, whose mean-field approximation is the corresponding continuous equation. Focusing on the advection-diffusion-limited aggregation (ADLA) model, we show that the average shape of the stochastic growth is similar, but not identical, to the corresponding continuous dynamics. Similar results should apply to DLA, thus explaining the known discrepancies between average DLA shapes and viscous fingers in a channel geometry. PMID:16196793
Average Shape of Transport-Limited Aggregates
NASA Astrophysics Data System (ADS)
Davidovitch, Benny; Choi, Jaehyuk; Bazant, Martin Z.
2005-08-01
We study the relation between stochastic and continuous transport-limited growth models. We derive a nonlinear integro-differential equation for the average shape of stochastic aggregates, whose mean-field approximation is the corresponding continuous equation. Focusing on the advection-diffusion-limited aggregation (ADLA) model, we show that the average shape of the stochastic growth is similar, but not identical, to the corresponding continuous dynamics. Similar results should apply to DLA, thus explaining the known discrepancies between average DLA shapes and viscous fingers in a channel geometry.
Characterization and modeling of thermal diffusion and aggregation in nanofluids.
Gharagozloo, Patricia E.; Goodson, Kenneth E.
2010-05-01
Fluids with higher thermal conductivities are sought for fluidic cooling systems in applications including microprocessors and high-power lasers. By adding high thermal conductivity nanoscale metal and metal oxide particles to a fluid the thermal conductivity of the fluid is enhanced. While particle aggregates play a central role in recent models for the thermal conductivity of nanofluids, the effect of particle diffusion in a temperature field on the aggregation and transport has yet to be studied in depth. The present work separates the effects of particle aggregation and diffusion using parallel plate experiments, infrared microscopy, light scattering, Monte Carlo simulations, and rate equations for particle and heat transport in a well dispersed nanofluid. Experimental data show non-uniform temporal increases in thermal conductivity above effective medium theory and can be well described through simulation of the combination of particle aggregation and diffusion. The simulation shows large concentration distributions due to thermal diffusion causing variations in aggregation, thermal conductivity and viscosity. Static light scattering shows aggregates form more quickly at higher concentrations and temperatures, which explains the increased enhancement with temperature reported by other research groups. The permanent aggregates in the nanofluid are found to have a fractal dimension of 2.4 and the aggregate formations that grow over time are found to have a fractal dimension of 1.8, which is consistent with diffusion limited aggregation. Calculations show as aggregates grow the viscosity increases at a faster rate than thermal conductivity making the highly aggregated nanofluids unfavorable, especially at the low fractal dimension of 1.8. An optimum nanoparticle diameter for these particular fluid properties is calculated to be 130 nm to optimize the fluid stability by reducing settling, thermal diffusion and aggregation.
Fractal aggregation growth and the surrounding diffusion field
NASA Astrophysics Data System (ADS)
Miyashita, Satoru; Saito, Yukio; Uwaha, Makio
2005-10-01
Silver metal trees grow and form a forest at the edge of a Cu plate in the AgNO3 water solution in a two-dimensional ( d=2) cell. The local structure of the forest is similar to that of the diffusion-limited aggregation (DLA), but the whole pattern approaches a uniform structure. Its growth dynamics is characterized by the fractal dimension Df of DLA. Time-dependence of the tip height is found to satisfy the scaling relation with the solute concentration c, and the asymptotic growth velocity V is consistent with the power law V˜c expected from the theory. The thickness ξc of the diffusion boundary layer is measured by the Michelson interferometry, and the scaling relation is also confirmed.
Aggregate influence on chloride ion diffusion into concrete
Hobbs, D.W.
1999-12-01
An attempt is made to predict the probable effect of the aggregate on chloride ion diffusion into saturated concrete. It is shown that if the chloride ion diffusion coefficient of an aggregate ranges from 0.2 to 10 times that of the cement past matrix, then this could result in variations in the concrete chloride ion diffusion coefficient of up to 10:1. Such a variation is equivalent to a change in free water-cement ration from 0.77 to 0.45.
Diffusion Limited Aggregation on a Cylinder
NASA Astrophysics Data System (ADS)
Benjamini, Itai; Yadin, Ariel
2008-04-01
We consider the DLA process on a cylinder G × {mathbb{N}} . It is shown that this process “grows arms”, provided that the base graph G has small enough mixing time. Specifically, if the mixing time of G is at most log^{(2-ɛ)}left\\vert G right\\vert , the time it takes the cluster to reach the m th layer of the cylinder is at most of order m \\cdot left\\vert G right\\vert/loglogleft\\vert G right\\vert . In particular we get examples of infinite Cayley graphs of degree 5, for which the DLA cluster on these graphs has arbitrarily small density. In addition, we provide an upper bound on the rate at which the “arms” grow. This bound is valid for a large class of base graphs G, including discrete tori of dimension at least 3. It is also shown that for any base graph G, the density of the DLA process on a G-cylinder is related to the rate at which the arms of the cluster grow. This implies that for any vertex transitive G, the density of DLA on a G-cylinder is bounded by 2/3.
5 CFR 9901.313 - Aggregate compensation limitations.
Code of Federal Regulations, 2011 CFR
2011-01-01
... 5 Administrative Personnel 3 2011-01-01 2011-01-01 false Aggregate compensation limitations. 9901... Aggregate compensation limitations. (a) General. (1) Except as provided in paragraphs (a)(2) and (a)(3) of... in another Federal agency, the payment would cause the total aggregate compensation to exceed...
Fluctuations and discreteness in diffusion limited growth
NASA Astrophysics Data System (ADS)
Devita, Jason P.
This thesis explores the effects of fluctuations and discreteness on the growth of physical systems where diffusion plays an important role. It focuses on three related problems, all dependent on diffusion in a fundamental way, but each with its own unique challenges. With diffusion-limited aggregation (DLA), the relationship between noisy and noise-free Laplacian growth is probed by averaging the results of noisy growth. By doing so in a channel geometry, we are able to compare to known solutions of the noise-free problem. We see that while the two are comparable, there are discrepancies which are not well understood. In molecular beam epitaxy (MBE), we create efficient computational algorithms, by replacing random walkers (diffusing atoms) with approximately equivalent processes. In one case, the atoms are replaced by a continuum field. Solving for the dynamics of the field yields---in an average sense---the dynamics of the atoms. In the other case, the atoms are treated as individual random-walking particles, but the details of the dynamics are changed to an (approximately) equivalent set of dynamics. This approach involves allowing adatoms to take long hops. We see approximately an order of magnitude speed up for simulating island dynamics, mound growth, and Ostwald ripening. Some ideas from the study of MBE are carried over to the study of front propagation in reaction-diffusion systems. Many of the analytic results about front propagation are derived from continuum models. It is unclear, however, that these results accurately describe the properties of a discrete system. It is reasonable to think that discrete systems will converge to the continuum results when sufficiently many particles are included. However, computational evidence of this is difficult to obtain, since the interesting properties tend to depend on a power law of the logarithm of the number of particles. Thus, the number of particles included in simulations must be exceedingly large. By
47 CFR 20.6 - CMRS spectrum aggregation limit.
Code of Federal Regulations, 2012 CFR
2012-10-01
... CMRS (see 47 CFR 20.9) shall have an attributable interest in a total of more than 55 MHz of licensed... 47 Telecommunication 2 2012-10-01 2012-10-01 false CMRS spectrum aggregation limit. 20.6 Section... COMMERCIAL MOBILE SERVICES § 20.6 CMRS spectrum aggregation limit. (a) Spectrum limitation. No licensee...
Field theory and diffusion creep predictions in polycrystalline aggregates
NASA Astrophysics Data System (ADS)
Villani, A.; Busso, E. P.; Forest, S.
2015-07-01
In polycrystals, stress-driven vacancy diffusion at high homologous temperatures leads to inelastic deformation. In this work, a novel continuum mechanics framework is proposed to describe the strain fields resulting from such a diffusion-driven process in a polycrystalline aggregate where grains and grain boundaries are explicitly considered. The choice of an anisotropic eigenstrain in the grain boundary region provides the driving force for the diffusive creep processes. The corresponding inelastic strain rate is shown to be related to the gradient of the vacancy flux. Dislocation driven deformation is then introduced as an additional mechanism, through standard crystal plasticity constitutive equations. The fully coupled diffusion-mechanical model is implemented into the finite element method and then used to describe the biaxial creep behaviour of FCC polycrystalline aggregates. The corresponding results revealed for the first time that such a coupled diffusion-stress approach, involving the gradient of the vacancy flux, can accurately predict the well-known macroscopic strain rate dependency on stress and grain size in the diffusion creep regime. They also predict strongly heterogeneous viscoplastic strain fields, especially close to grain boundaries triple junctions. Finally, a smooth transition from Herring and Coble to dislocation creep behaviour is predicted and compared to experimental results for copper.
NASA Astrophysics Data System (ADS)
Ebrahimi, Ali; Or, Dani
2015-12-01
The constantly changing soil hydration status affects gas and nutrient diffusion through soil pores and thus the functioning of soil microbial communities. The conditions within soil aggregates are of particular interest due to limitations to oxygen diffusion into their core, and the presence of organic carbon often acting as binding agent. We developed a model for microbial life in simulated soil aggregates comprising of 3-D angular pore network model (APNM) that mimics soil hydraulic and transport properties. Within these APNM, we introduced individual motile (flagellated) microbial cells with different physiological traits that grow, disperse, and respond to local nutrients and oxygen concentrations. The model quantifies the dynamics and spatial extent of anoxic regions that vary with hydration conditions, and their role in shaping microbial community size and activity and the spatial (self) segregation of anaerobes and aerobes. Internal carbon source and opposing diffusion directions of oxygen and carbon within an aggregate were essential to emergence of stable coexistence of aerobic and anaerobic communities (anaerobes become extinct when carbon sources are external). The model illustrates a range of hydration conditions that promote or suppress denitrification or decomposition of organic matter and thus affect soil GHG emissions. Model predictions of CO2 and N2O production rates were in good agreement with limited experimental data. These limited tests support the dynamic modeling approach whereby microbial community size, composition, and spatial arrangement emerge from internal interactions within soil aggregates. The upscaling of the results to a population of aggregates of different sizes embedded in a soil profile is underway.
Tumbek, L; Winkler, A
2012-08-01
The nucleation and growth of organic molecules is usually discussed in the framework of diffusion limited aggregation (DLA). In this letter we demonstrate for the rod-like organic molecules hexaphenyl (6P) on sputter-modified mica, that under specific experimental conditions the nucleation has to be described by attachment limited aggregation (ALA). The crucial parameter for the growth mode is the roughness of the substrate surface, as induced by ion sputtering. With decreasing surface roughness the diffusion probability of the molecules increases and the growth mode changes from DLA to ALA. This was derived from the deposition rate dependence of the island density. A critical size of i = 7 molecules was determined for the nucleation of 6P on a moderately sputtered mica surface. PMID:23470898
NASA Astrophysics Data System (ADS)
Tumbek, L.; Winkler, A.
2012-08-01
The nucleation and growth of organic molecules is usually discussed in the framework of diffusion limited aggregation (DLA). In this letter we demonstrate for the rod-like organic molecules hexaphenyl (6P) on sputter-modified mica, that under specific experimental conditions the nucleation has to be described by attachment limited aggregation (ALA). The crucial parameter for the growth mode is the roughness of the substrate surface, as induced by ion sputtering. With decreasing surface roughness the diffusion probability of the molecules increases and the growth mode changes from DLA to ALA. This was derived from the deposition rate dependence of the island density. A critical size of i = 7 molecules was determined for the nucleation of 6P on a moderately sputtered mica surface.
Kinetic theory of diffusion-limited nucleation.
Philippe, T; Bonvalet, M; Blavette, D
2016-05-28
We examine binary nucleation in the size and composition space {R,c} using the formalism of the multivariable theory [N. V. Alekseechkin, J. Chem. Phys. 124, 124512 (2006)]. We show that the variable c drops out of consideration for very large curvature of the new phase Gibbs energy with composition. Consequently nuclei around the critical size have the critical composition, which is derived from the condition of criticality for the canonical variables and is found not to depend on surface tension. In this case, nucleation kinetics can be investigated in the size space only. Using macroscopic kinetics, we determine the general expression for the condensation rate when growth is limited by bulk diffusion, which accounts for both diffusion and capillarity and exhibits a different dependence with the critical size, as compared with the interface-limited regime. This new expression of the condensation rate for bulk diffusion-limited nucleation is the counterpart of the classical interface-limited result. We then extend our analysis to multicomponent solutions.
Kinetic theory of diffusion-limited nucleation
NASA Astrophysics Data System (ADS)
Philippe, T.; Bonvalet, M.; Blavette, D.
2016-05-01
We examine binary nucleation in the size and composition space {R,c} using the formalism of the multivariable theory [N. V. Alekseechkin, J. Chem. Phys. 124, 124512 (2006)]. We show that the variable c drops out of consideration for very large curvature of the new phase Gibbs energy with composition. Consequently nuclei around the critical size have the critical composition, which is derived from the condition of criticality for the canonical variables and is found not to depend on surface tension. In this case, nucleation kinetics can be investigated in the size space only. Using macroscopic kinetics, we determine the general expression for the condensation rate when growth is limited by bulk diffusion, which accounts for both diffusion and capillarity and exhibits a different dependence with the critical size, as compared with the interface-limited regime. This new expression of the condensation rate for bulk diffusion-limited nucleation is the counterpart of the classical interface-limited result. We then extend our analysis to multicomponent solutions.
Kinetic theory of diffusion-limited nucleation.
Philippe, T; Bonvalet, M; Blavette, D
2016-05-28
We examine binary nucleation in the size and composition space {R,c} using the formalism of the multivariable theory [N. V. Alekseechkin, J. Chem. Phys. 124, 124512 (2006)]. We show that the variable c drops out of consideration for very large curvature of the new phase Gibbs energy with composition. Consequently nuclei around the critical size have the critical composition, which is derived from the condition of criticality for the canonical variables and is found not to depend on surface tension. In this case, nucleation kinetics can be investigated in the size space only. Using macroscopic kinetics, we determine the general expression for the condensation rate when growth is limited by bulk diffusion, which accounts for both diffusion and capillarity and exhibits a different dependence with the critical size, as compared with the interface-limited regime. This new expression of the condensation rate for bulk diffusion-limited nucleation is the counterpart of the classical interface-limited result. We then extend our analysis to multicomponent solutions. PMID:27250310
Coquel, Anne-Sophie; Jacob, Jean-Pascal; Primet, Mael; Demarez, Alice; Dimiccoli, Mariella; Julou, Thomas; Moisan, Lionel; Lindner, Ariel B; Berry, Hugues
2013-04-01
Aggregates of misfolded proteins are a hallmark of many age-related diseases. Recently, they have been linked to aging of Escherichia coli (E. coli) where protein aggregates accumulate at the old pole region of the aging bacterium. Because of the potential of E. coli as a model organism, elucidating aging and protein aggregation in this bacterium may pave the way to significant advances in our global understanding of aging. A first obstacle along this path is to decipher the mechanisms by which protein aggregates are targeted to specific intercellular locations. Here, using an integrated approach based on individual-based modeling, time-lapse fluorescence microscopy and automated image analysis, we show that the movement of aging-related protein aggregates in E. coli is purely diffusive (Brownian). Using single-particle tracking of protein aggregates in live E. coli cells, we estimated the average size and diffusion constant of the aggregates. Our results provide evidence that the aggregates passively diffuse within the cell, with diffusion constants that depend on their size in agreement with the Stokes-Einstein law. However, the aggregate displacements along the cell long axis are confined to a region that roughly corresponds to the nucleoid-free space in the cell pole, thus confirming the importance of increased macromolecular crowding in the nucleoids. We thus used 3D individual-based modeling to show that these three ingredients (diffusion, aggregation and diffusion hindrance in the nucleoids) are sufficient and necessary to reproduce the available experimental data on aggregate localization in the cells. Taken together, our results strongly support the hypothesis that the localization of aging-related protein aggregates in the poles of E. coli results from the coupling of passive diffusion-aggregation with spatially non-homogeneous macromolecular crowding. They further support the importance of "soft" intracellular structuring (based on macromolecular
Calculating effective diffusivities in the limit of vanishing molecular diffusion
Pavliotis, G.A. Stuart, A.M. Zygalakis, K.C.
2009-03-01
In this paper we study the problem of the numerical calculation (by Monte Carlo methods) of the effective diffusivity for a particle moving in a periodic divergent-free velocity field, in the limit of vanishing molecular diffusion. In this limit traditional numerical methods typically fail, since they do not represent accurately the geometry of the underlying deterministic dynamics. We propose a stochastic splitting method that takes into account the volume-preserving property of the equations of motion in the absence of noise, and when inertial effects can be neglected. An extension of the method is then proposed for the cases where the noise has a non-trivial time-correlation structure and when inertial effects cannot be neglected. The method of modified equations is used to explain failings of Euler-based methods. The new stochastic geometric integrators are shown to outperform standard Euler-based integrators. Various asymptotic limits of physical interest are investigated by means of numerical experiments, using the new integrators.
Diffusion-limited hyperbranched polymers with substitution effect.
Wang, Long; He, Xuehao; Chen, Yu
2011-03-14
Highly branched structure has the essential influence on macromolecular property and functionality in physics and chemistry. In this work, we proposed a diffusion-limited reaction model with the consideration of macromolecular unit relaxations and substitution effect of monomers to study the structure of hyperbranched polymers prepared by slow monomer addition to a core molecule. The exponential relationship (R(g) ∼ N(λ)) between the radius of gyration R(g) and the degree of polymerization N, was systematically analyzed at various branching degrees. It is shown that the effective exponent λ(eff) decreases at lower N and but increases toward that of diffusion-limited aggregation (DLA) clusters (λ(DLA) = 0.4) with the degree of polymerization increasing. The substitution effect of monomers in reaction strongly influences the evolution pathway of λ(eff). With the static light scattering technique, the fractal property of internal chains was further calculated. A general law about the radial distribution of the units of diffusion-limited hyperbranched polymers was found that, at smaller reactivity ratio k(12), the radial density of all monomer units D(A) declines from the center region to the peripheral layer revealing the dense core structure; however, at larger k(12), the density distribution shows a loose-dense-loose structure. These structural characteristics are helpful to deeply understand the property of hyperbranched polymers. PMID:21405187
Shape and scale dependent diffusivity of colloidal nanoclusters and aggregates
NASA Astrophysics Data System (ADS)
Alcanzare, M. M. T.; Ollila, S. T. T.; Thakore, V.; Laganapan, A. M.; Videcoq, A.; Cerbelaud, M.; Ferrando, R.; Ala-Nissila, T.
2016-07-01
The diffusion of colloidal nanoparticles and nanomolecular aggregates, which plays an important role in various biophysical and physicochemical phenomena, is currently under intense study. Here, we examine the shape and size dependent diffusion of colloidal nano- particles, fused nanoclusters and nanoaggregates using a hybrid fluctuating lattice Boltzmann-Molecular Dynamics method. We use physically realistic parameters characteristic of an aqueous solution, with explicitly implemented microscopic no-slip and full-slip boundary conditions. Results from nanocolloids below 10 nm in radii demonstrate how the volume fraction of the hydrodynamic boundary layer influences diffusivities. Full-slip colloids are found to diffuse faster than no-slip particles. We also characterize the shape dependent anisotropy of the diffusion coefficients of nanoclusters through the Green-Kubo relation. Finally, we study the size dependence of the diffusion of nanoaggregates comprising N ≤ 108 monomers and demonstrate that the diffusion coefficient approaches the continuum scaling limit of N-1/3.
Xu, Zhijie; Meakin, Paul
2011-01-28
Two-dimensional dendritic growth due to solute precipitation was simulated using a phase-field model reported earlier [Z. Xu and P. Meakin, J. Chem. Phys. 129, 014705 (2008)]. It was shown that diffusion-limited precipitation due to the chemical reaction at the solid-liquid interface posses similarities with diffusion-limited aggregation (DLA). The diffusion-limited precipitation is attained by setting the chemical reaction rate much larger compared to the solute diffusion to eliminate the effect of the interface growth kinetics. The phase-field simulation results were in reasonable agreement with the analytical solutions. The fractal solid fingers can be formed in the diffusion-limited precipitation and have a fractal dimension measured , close to 1.64, the fractal dimensionality of large square lattice diffusion-limited aggregation (DLA) clusters.
A qualitative model for aggregation and diffusion of β-amyloid in Alzheimer's disease.
Achdou, Yves; Franchi, Bruno; Marcello, Norina; Tesi, Maria Carla
2013-12-01
In this paper we present a mathematical model for the aggregation and diffusion of Aβ amyloid in the brain affected by Alzheimer's disease, at the early stage of the disease. The model is based on a classical discrete Smoluchowski aggregation equation modified to take diffusion into account. We also describe a numerical scheme and discuss the results of the simulations in the light of the recent biomedical literature.
Fireproof impact limiter aggregate packaging inside shipping containers
Byington, Gerald A.; Oakes, Jr., Raymon Edgar; Feldman, Matthew Rookes
2001-01-01
The invention is a product and a process for making a fireproof, impact limiter, homogeneous aggregate material for casting inside a hazardous material shipping container, or a double-contained Type-B nuclear shipping container. The homogeneous aggregate material is prepared by mixing inorganic compounds with water, pouring the mixture into the void spaces between an inner storage containment vessel and an outer shipping container, vibrating the mixture inside the shipping container, with subsequent curing, baking, and cooling of the mixture to form a solidified material which encapsulates an inner storage containment vessel inside an outer shipping container. The solidified material forms a protective enclosure around an inner storage containment vessel which may store hazardous, toxic, or radioactive material. The solidified material forms a homogeneous fire-resistant material that does not readily transfer heat, and provides general shock and specific point-impact protection, providing protection to the interior storage containment vessel. The material is low cost, may contain neutron absorbing compounds, and is easily formed into a variety of shapes to fill the interior void spaces of shipping containers.
Tuning the thermal diffusivity of silver based nanofluids by controlling nanoparticle aggregation
NASA Astrophysics Data System (ADS)
Agresti, Filippo; Barison, Simona; Battiston, Simone; Pagura, Cesare; Colla, Laura; Fedele, Laura; Fabrizio, Monica
2013-09-01
With the aim of preparing stable nanofluids for heat exchange applications and to study the effect of surfactant on the aggregation of nanoparticles and thermal diffusivity, stable silver colloids were synthesized in water by a green method, reducing AgNO3 with fructose in the presence of poly-vinylpyrollidone (PVP) of various molecular weights. A silver nanopowder was precipitated from the colloids and re-dispersed at 4 vol% in deionized water. The Ag colloids were characterized by UV-visible spectroscopy, combined dynamic light scattering and ζ-potential measurements, and laser flash thermal diffusivity. The Ag nanopowders were characterized by scanning electron microscopy and thermal gravimetric analysis. It was found that the molecular weight of PVP strongly affects the ζ-potential and the aggregation of nanoparticles, thereby affecting the thermal diffusivity of the obtained colloids. In particular, it was observed that on increasing the molecular weight of PVP the absolute value of the ζ-potential is reduced, leading to increased aggregation of nanoparticles. A clear relation was identified between thermal diffusivity and aggregation, showing higher thermal diffusivity for nanofluids having higher aggregation. A maximum improvement of thermal diffusivity by about 12% was found for nanofluids prepared with PVP having higher molecular weight.
Tuning the thermal diffusivity of silver based nanofluids by controlling nanoparticle aggregation.
Agresti, Filippo; Barison, Simona; Battiston, Simone; Pagura, Cesare; Colla, Laura; Fedele, Laura; Fabrizio, Monica
2013-09-13
With the aim of preparing stable nanofluids for heat exchange applications and to study the effect of surfactant on the aggregation of nanoparticles and thermal diffusivity, stable silver colloids were synthesized in water by a green method, reducing AgNO₃ with fructose in the presence of poly-vinylpyrollidone (PVP) of various molecular weights. A silver nanopowder was precipitated from the colloids and re-dispersed at 4 vol% in deionized water. The Ag colloids were characterized by UV-visible spectroscopy, combined dynamic light scattering and ζ-potential measurements, and laser flash thermal diffusivity. The Ag nanopowders were characterized by scanning electron microscopy and thermal gravimetric analysis. It was found that the molecular weight of PVP strongly affects the ζ-potential and the aggregation of nanoparticles, thereby affecting the thermal diffusivity of the obtained colloids. In particular, it was observed that on increasing the molecular weight of PVP the absolute value of the ζ-potential is reduced, leading to increased aggregation of nanoparticles. A clear relation was identified between thermal diffusivity and aggregation, showing higher thermal diffusivity for nanofluids having higher aggregation. A maximum improvement of thermal diffusivity by about 12% was found for nanofluids prepared with PVP having higher molecular weight.
Some free boundary problems involving non-local diffusion and aggregation
Carrillo, José Antonio; Vázquez, Juan Luis
2015-01-01
We report on recent progress in the study of evolution processes involving degenerate parabolic equations which may exhibit free boundaries. The equations we have selected follow two recent trends in diffusion theory: considering anomalous diffusion with long-range effects, which leads to fractional operators or other operators involving kernels with large tails; and the combination of diffusion and aggregation effects, leading to delicate long-term equilibria whose description is still incipient. PMID:26261360
A numerical study of soot aggregate formation in a laminar coflow diffusion flame
Zhang, Q.; Thomson, M.J.; Guo, H.; Liu, F.; Smallwood, G.J.
2009-03-15
Soot aggregate formation in a two-dimensional laminar coflow ethylene/air diffusion flame is studied with a pyrene-based soot model, a detailed sectional aerosol dynamics model, and a detailed radiation model. The chemical kinetic mechanism describes polycyclic aromatic hydrocarbon formation up to pyrene, the dimerization of which is assumed to lead to soot nucleation. The growth and oxidation of soot particles are characterized by the HACA surface mechanism and pyrene-soot surface condensation. The mass range of the solid soot phase is divided into thirty-five discrete sections and two equations are solved in each section to model the formation of the fractal-like soot aggregates. The coagulation model is improved by implementing the aggregate coagulation efficiency. Several physical processes that may cause sub-unitary aggregate coagulation efficiency are discussed. Their effects on aggregate structure are numerically investigated. The average number of primary soot particles per soot aggregate n{sub p} is found to be a strong function of the aggregate coagulation efficiency. Compared to the available experimental data, n{sub p} is well reproduced with a constant 20% aggregate coagulation efficiency. The predicted axial velocity, OH mole fraction, and C{sub 2}H{sub 2} mole fraction are validated against experimental data in the literature. Reasonable agreements are obtained. Finally, a sensitivity study of the effects of particle coalescence on soot volume fraction and soot aggregate nanostructure is conducted using a coalescence cutoff diameter method. (author)
Asymptotic Diffusion-Limit Accuracy of Sn Angular Differencing Schemes
Bailey, T S; Morel, J E; Chang, J H
2009-11-05
In a previous paper, Morel and Montry used a Galerkin-based diffusion analysis to define a particular weighted diamond angular discretization for S{sub n}n calculations in curvilinear geometries. The weighting factors were chosen to ensure that the Galerkin diffusion approximation was preserved, which eliminated the discrete-ordinates flux dip. It was also shown that the step and diamond angular differencing schemes, which both suffer from the flux dip, do not preserve the diffusion approximation in the Galerkin sense. In this paper we re-derive the Morel and Montry weighted diamond scheme using a formal asymptotic diffusion-limit analysis. The asymptotic analysis yields more information than the Galerkin analysis and demonstrates that the step and diamond schemes do in fact formally preserve the diffusion limit to leading order, while the Morel and Montry weighted diamond scheme preserves it to first order, which is required for full consistency in this limit. Nonetheless, the fact that the step and diamond differencing schemes preserve the diffusion limit to leading order suggests that the flux dip should disappear as the diffusion limit is approached for these schemes. Computational results are presented that confirm this conjecture. We further conjecture that preserving the Galerkin diffusion approximation is equivalent to preserving the asymptotic diffusion limit to first order.
Endocannabinoids Control Platelet Activation and Limit Aggregate Formation under Flow
De Angelis, Valentina; Koekman, Arnold C.; Weeterings, Cees; Roest, Mark; de Groot, Philip G.; Herczenik, Eszter; Maas, Coen
2014-01-01
Background The endocannabinoid system has previously been implicated in the regulation of neurons and inflammatory cells. Additionally, it has been reported that endocannabinoid receptors are present on circulating platelets, but there has been conflicting evidence on their contribution to platelet function. Objectives Our aim was to examine the role of endocannabinoids in platelet function in vitro and in vivo. Methods and Results We studied the effects of the well-characterized endogenous endocannabinoid anandamide on platelet aggregation in suspension, α-granule release, calcium mobilization, Syk phosphorylation, as well as platelet spreading and aggregate formation under flow. Anandamide inhibits platelet aggregation and α-granule release by collagen, collagen-derived peptide CRP-XL, ADP, arachidonic acid and thromboxane A2 analogue U46619. However, activation via thrombin receptor PAR-1 stays largely unaffected. Calcium mobilization is significantly impaired when platelets are stimulated with collagen or CRP-XL, but remains normal in the presence of the other agonists. In line with this finding, we found that anandamide prevents collagen-induced Syk phosphorylation. Furthermore, anandamide-treated platelets exhibit reduced spreading on immobilized fibrinogen, have a decreased capacity for binding fibrinogen in solution and show perturbed platelet aggregate formation under flow over collagen. Finally, we investigated the influence of Cannabis sativa consumption by human volunteers on platelet activation. Similar to our in vitro findings with anandamide, ex vivo collagen-induced platelet aggregation and aggregate formation on immobilized collagen under flow were impaired in whole blood of donors that had consumed Cannabis sativa. Conclusions Endocannabinoid receptor agonists reduce platelet activation and aggregate formation both in vitro and ex vivo after Cannabis sativa consumption. Further elucidation of this novel regulatory mechanism for platelet function
Limits on ion radial diffusion coefficients in Saturn's inner magnetosphere
NASA Technical Reports Server (NTRS)
Paonessa, M.; Cheng, A. F.
1986-01-01
The development of upper and lower limits for the rate of radial diffusion of energetic ions in Saturn's inner magnetosphere is discussed. Improved calculations of the satellite-sweeping rate and phase space density profiles for a wide range of ion invariants are utilized to determine the limits. The lower limit for the radial diffusion coefficient is established by requiring the rate of inward diffusion to be large enough to balance satellite sweeping losses; the upper limit is obtained by requiring the rate of inward diffusion to be less than the observable ultraviolet aurora on plasma torus L shell. It is concluded that the radial diffusion coefficient for ions in Saturn's inner magnetosphere is calculated to about two orders of magnitude.
NASA Astrophysics Data System (ADS)
Meienberg, Kyle; Papaioannou, John; Park, Cheol; Glaser, Matt; Maclennan, Joe; Clark, Noel; Kuriabova, Tatiana; Powers, Thomas
2015-03-01
We observe directly the diffusion and aggregation of nanoparticles (buckyballs) embedded in thin, freely suspended smectic A liquid crystal films of 8CB using reflected light microscopy Individual buckyballs, initially homogeneously dispersed in the film, are too small to see but after some hours form nanoscale clusters. These, in turn, aggregate to form extended, micron-scale objects which diffuse in the film, enabling the measurement of 2D rotational and translational mobilities of inclusions with a wide variety of different shapes. The experimental mobilities are compared with predictions of the extended Saffman-Delbrück (SD) model used successfully to describe the diffusion of micron-sized objects in thin fluid membranes in a variety of experimental systems. This work was supported by NASA Grant No. NNX-13AQ81G, NSF MRSEC Grant No. DMR-0820579, and by NSF Grant No. CBET-0854108.
Invisibility cloaking in the diffusive-light limit (presentation video)
NASA Astrophysics Data System (ADS)
Schittny, Robert; Kadic, Muamer; Wegener, Martin
2014-09-01
Albert Einstein's theory of relativity imposes stringent limitations to making macroscopic objects invisible with respect to electromagnetic light waves propagating in vacuum. These limitations are not relevant though for propagation of light in diffusive media like fog or milk because the effective energy speed is significantly lower than in vacuum due to multiple scattering events. Here, by exploiting the close mathematical analogy between the electrostatic or near-field limit of optics on the one hand and light diffusion on the other hand, we design, fabricate, and characterize simple core-shell cloaking structures for diffusive light propagation in cylindrical and spherical geometry.
42 CFR 441.354 - Aggregate projected expenditure limit (APEL).
Code of Federal Regulations, 2014 CFR
2014-10-01
... individuals age 65 or older furnished as an alternative to care in an SNF or ICF (NF effective October 1, 1990... × (1+Z), where P=The aggregate amount of the State's medical assistance under title XIX for SNF and ICF... adjusted) for SNF services, ICF-other services, and mental health facility services for the base...
42 CFR 441.354 - Aggregate projected expenditure limit (APEL).
Code of Federal Regulations, 2011 CFR
2011-10-01
... individuals age 65 or older furnished as an alternative to care in an SNF or ICF (NF effective October 1, 1990... × (1+Z), where P=The aggregate amount of the State's medical assistance under title XIX for SNF and ICF... adjusted) for SNF services, ICF-other services, and mental health facility services for the base...
42 CFR 441.354 - Aggregate projected expenditure limit (APEL).
Code of Federal Regulations, 2010 CFR
2010-10-01
... individuals age 65 or older furnished as an alternative to care in an SNF or ICF (NF effective October 1, 1990... × (1+Z), where P=The aggregate amount of the State's medical assistance under title XIX for SNF and ICF... adjusted) for SNF services, ICF-other services, and mental health facility services for the base...
42 CFR 441.354 - Aggregate projected expenditure limit (APEL).
Code of Federal Regulations, 2012 CFR
2012-10-01
... individuals age 65 or older furnished as an alternative to care in an SNF or ICF (NF effective October 1, 1990... × (1+Z), where P=The aggregate amount of the State's medical assistance under title XIX for SNF and ICF... adjusted) for SNF services, ICF-other services, and mental health facility services for the base...
42 CFR 441.354 - Aggregate projected expenditure limit (APEL).
Code of Federal Regulations, 2013 CFR
2013-10-01
... individuals age 65 or older furnished as an alternative to care in an SNF or ICF (NF effective October 1, 1990... × (1+Z), where P=The aggregate amount of the State's medical assistance under title XIX for SNF and ICF... adjusted) for SNF services, ICF-other services, and mental health facility services for the base...
42 CFR 447.512 - Drugs: Aggregate upper limits of payment.
Code of Federal Regulations, 2010 CFR
2010-10-01
... 42 Public Health 4 2010-10-01 2010-10-01 false Drugs: Aggregate upper limits of payment. 447.512 Section 447.512 Public Health CENTERS FOR MEDICARE & MEDICAID SERVICES, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL ASSISTANCE PROGRAMS PAYMENTS FOR SERVICES Payment for Drugs § 447.512 Drugs: Aggregate upper limits of payment....
Limits on ion radial diffusion coefficients in Saturn's inner magnetosphere
NASA Astrophysics Data System (ADS)
Paonessa, M.; Cheng, A. F.
1986-02-01
Voyager low energy charged particle (LECP) ion phase space densities at constant first and second adiabatic invariants have been used to place limits on the rate of radial diffusion of energetic ions (30 keV to 1 MeV) in Saturn's inner magnetosphere. Upper and lower limits to the radial diffusion coefficient, DLL, are deduced from physical requirements on the rates of diffusion and loss. If DLL is near the lower limit found in this work, then satellite sweeping accounts for a large fraction of the total ion losses. If DLL is near the upper limit, then ion losses can approach 10% of the strong diffusion rate. In this case, ion losses are dominated by wave-particle interactions, and sweeping losses are relatively unimportant.
NASA Astrophysics Data System (ADS)
Yochelis, Arik; Bar-On, Tomer; Gov, Nir S.
2016-04-01
Unconventional myosins belong to a class of molecular motors that walk processively inside cellular protrusions towards the tips, on top of actin filament. Surprisingly, in addition, they also form retrograde moving self-organized aggregates. The qualitative properties of these aggregates are recapitulated by a mass conserving reaction-diffusion-advection model and admit two distinct families of modes: traveling waves and pulse trains. Unlike the traveling waves that are generated by a linear instability, pulses are nonlinear structures that propagate on top of linearly stable uniform backgrounds. Asymptotic analysis of isolated pulses via a simplified reaction-diffusion-advection variant on large periodic domains, allows to draw qualitative trends for pulse properties, such as the amplitude, width, and propagation speed. The results agree well with numerical integrations and are related to available empirical observations.
Xu, Zhijie; Meakin, Paul
2011-01-28
Two-dimensional dendritic growth due to solute precipitation was simulated using a phase-field model reported earlier [Z. Xu and P. Meakin, J. Chem. Phys. 129, 014705 (2008)]. It was shown that diffusion-limited precipitation due to the chemical reaction at the solid-liquid interface has similarities with diffusion-limited aggregation (DLA). The diffusion-limited precipitation is attained by setting the chemical reaction rate much larger compared to the solute diffusion to eliminate the effect of the interface growth kinetics. The phase-field simulation results were in reasonable agreement with the analytical solutions. The fractal solid fingers can be formed in the diffusion-limited precipitation and have a fractal dimension measured d(f)=1.68, close to 1.64, the fractal dimensionality of large square lattice DLA clusters. PMID:21280717
NASA Astrophysics Data System (ADS)
Xu, Zhijie; Meakin, Paul
2011-01-01
Two-dimensional dendritic growth due to solute precipitation was simulated using a phase-field model reported earlier [Z. Xu and P. Meakin, J. Chem. Phys. 129, 014705 (2008)]. It was shown that diffusion-limited precipitation due to the chemical reaction at the solid-liquid interface has similarities with diffusion-limited aggregation (DLA). The diffusion-limited precipitation is attained by setting the chemical reaction rate much larger compared to the solute diffusion to eliminate the effect of the interface growth kinetics. The phase-field simulation results were in reasonable agreement with the analytical solutions. The fractal solid fingers can be formed in the diffusion-limited precipitation and have a fractal dimension measured d_f = 1.68, close to 1.64, the fractal dimensionality of large square lattice DLA clusters.
Zhijie Xu; Paul Meakin
2011-01-01
Two-dimensional dendritic growth due to solute precipitation was simulated using a phase-field model reported earlier [Z. Xu and P. Meakin, J. Chem. Phys. 129, 014705 (2008)]. It was shown that diffusion-limited precipitation due to the chemical reaction at the solid–liquid interface has similarities with diffusion-limited aggregation (DLA). The diffusion-limited precipitation is attained by setting the chemical reaction rate much larger compared to the solute diffusion to eliminate the effect of the interface growth kinetics. The phase-field simulation results were in reasonable agreement with the analytical solutions. The fractal solid fingers can be formed in the diffusion-limited precipitation and have a fractal dimension measured df = 1.68, close to 1.64, the fractal dimensionality of large square lattice DLA clusters.
5 CFR 530.203 - Administration of aggregate limitation on pay.
Code of Federal Regulations, 2011 CFR
2011-01-01
.... (2) An employee covered by a performance appraisal system that has been certified under 5 CFR part... system under 5 CFR part 430, subpart D. (c) The aggregate limitations described in paragraphs (a) and (b...), would cause the employee's aggregate compensation to exceed the rate for level I of the...
Latent heat induced rotation limited aggregation in 2D ice nanocrystals.
Bampoulis, Pantelis; Siekman, Martin H; Kooij, E Stefan; Lohse, Detlef; Zandvliet, Harold J W; Poelsema, Bene
2015-07-21
The basic science responsible for the fascinating shapes of ice crystals and snowflakes is still not understood. Insufficient knowledge of the interaction potentials and the lack of relevant experimental access to the growth process are to blame for this failure. Here, we study the growth of fractal nanostructures in a two-dimensional (2D) system, intercalated between mica and graphene. Based on our scanning tunneling spectroscopy data, we provide compelling evidence that these fractals are 2D ice. They grow while they are in material contact with the atmosphere at 20 °C and without significant thermal contact to the ambient. The growth is studied in situ, in real time and space at the nanoscale. We find that the growing 2D ice nanocrystals assume a fractal shape, which is conventionally attributed to Diffusion Limited Aggregation (DLA). However, DLA requires a low mass density mother phase, in contrast to the actual currently present high mass density mother phase. Latent heat effects and consequent transport of heat and molecules are found to be key ingredients for understanding the evolution of the snow (ice) flakes. We conclude that not the local availability of water molecules (DLA), but rather them having the locally required orientation is the key factor for incorporation into the 2D ice nanocrystal. In combination with the transport of latent heat, we attribute the evolution of fractal 2D ice nanocrystals to local temperature dependent rotation limited aggregation. The ice growth occurs under extreme supersaturation, i.e., the conditions closely resemble the natural ones for the growth of complex 2D snow (ice) flakes and we consider our findings crucial for solving the "perennial" snow (ice) flake enigma. PMID:26203037
Latent heat induced rotation limited aggregation in 2D ice nanocrystals
NASA Astrophysics Data System (ADS)
Bampoulis, Pantelis; Siekman, Martin H.; Kooij, E. Stefan; Lohse, Detlef; Zandvliet, Harold J. W.; Poelsema, Bene
2015-07-01
The basic science responsible for the fascinating shapes of ice crystals and snowflakes is still not understood. Insufficient knowledge of the interaction potentials and the lack of relevant experimental access to the growth process are to blame for this failure. Here, we study the growth of fractal nanostructures in a two-dimensional (2D) system, intercalated between mica and graphene. Based on our scanning tunneling spectroscopy data, we provide compelling evidence that these fractals are 2D ice. They grow while they are in material contact with the atmosphere at 20 °C and without significant thermal contact to the ambient. The growth is studied in situ, in real time and space at the nanoscale. We find that the growing 2D ice nanocrystals assume a fractal shape, which is conventionally attributed to Diffusion Limited Aggregation (DLA). However, DLA requires a low mass density mother phase, in contrast to the actual currently present high mass density mother phase. Latent heat effects and consequent transport of heat and molecules are found to be key ingredients for understanding the evolution of the snow (ice) flakes. We conclude that not the local availability of water molecules (DLA), but rather them having the locally required orientation is the key factor for incorporation into the 2D ice nanocrystal. In combination with the transport of latent heat, we attribute the evolution of fractal 2D ice nanocrystals to local temperature dependent rotation limited aggregation. The ice growth occurs under extreme supersaturation, i.e., the conditions closely resemble the natural ones for the growth of complex 2D snow (ice) flakes and we consider our findings crucial for solving the "perennial" snow (ice) flake enigma.
von Deuster, Constantin; Sammut, Eva; Asner, Liya; Nordsletten, David; Lamata, Pablo; Stoeck, Christian T.; Razavi, Reza
2016-01-01
Background— The objective of this study is to assess the dynamic alterations of myocardial microstructure and strain between diastole and systole in patients with dilated cardiomyopathy relative to healthy controls using the magnetic resonance diffusion tensor imaging, myocardial tagging, and biomechanical modeling. Methods and Results— Dual heart-phase diffusion tensor imaging was successfully performed in 9 patients and 9 controls. Tagging data were acquired for the diffusion tensor strain correction and cardiac motion analysis. Mean diffusivity, fractional anisotropy, and myocyte aggregate orientations were compared between both cohorts. Cardiac function was assessed by left ventricular ejection fraction, torsion, and strain. Computational modeling was used to study the impact of cardiac shape on fiber reorientation and how fiber orientations affect strain. In patients with dilated cardiomyopathy, a more longitudinal orientation of diastolic myofiber aggregates was measured compared with controls. Although a significant steepening of helix angles (HAs) during contraction was found in the controls, consistent change in HAs during contraction was absent in patients. Left ventricular ejection fraction, cardiac torsion, and strain were significantly lower in the patients compared with controls. Computational modeling revealed that the dilated heart results in reduced HA changes compared with a normal heart. Reduced torsion was found to be exacerbated by steeper HAs. Conclusions— Diffusion tensor imaging revealed reduced reorientation of myofiber aggregates during cardiac contraction in patients with dilated cardiomyopathy relative to controls. Left ventricular remodeling seems to be an important factor in the changes to myocyte orientation. Steeper HAs are coupled with a worsening in strain and torsion. Overall, the findings provide new insights into the structural alterations in patients with dilated cardiomyopathy. PMID:27729361
77 FR 31767 - Aggregation, Position Limits for Futures and Swaps
Federal Register 2010, 2011, 2012, 2013, 2014
2012-05-30
... Contracts'').\\4\\ \\4\\ See Position Limits for Futures and Swaps, 76 FR 71626, Nov. 18, 2011. The regulations... numerical level of the non-spot month limits based upon a formula provided in part 151.\\12\\ \\9\\ See 76 FR at... ``Security Based Swap Agreement''; Mixed Swaps; Security-Based Swap Agreement Recordkeeping, 76 FR 29818,...
Effect of diffusive transport limitations on UO2 dissolution.
Giammar, Daniel E; Cerrato, José M; Mehta, Vrajesh; Wang, Zimeng; Wang, Yin; Pepping, Troy J; Ulrich, Kai-Uwe; Lezama-Pacheco, Juan S; Bargar, John R
2012-11-15
The effects of diffusive transport limitations on the dissolution of UO(2) were investigated using an artificial groundwater prepared to simulate the conditions at the Old Rifle aquifer site in Colorado, USA. Controlled batch, continuously-stirred tank (CSTR), and plug flow reactors were used to study UO(2) dissolution in the absence and presence of diffusive limitations exerted by permeable sample cells. The net rate of uranium release following oxidative UO(2) dissolution obtained from diffusion-limited batch experiments was ten times lower than that obtained for UO(2) dissolution with no permeable sample cells. The release rate of uranium to bulk solution from UO(2) contained in permeable sample cells under advective flow conditions was more than 100 times lower than that obtained from CSTR experiments without diffusive limitations. A 1-dimensional transport model was developed that could successfully simulate diffusion-limited release of U following oxidative UO(2) dissolution with the dominant rate-limiting process being the transport of U(VI) out of the cells. Scanning electron microscopy, X-ray diffraction, and extended X-ray absorption fine structure spectroscopy (EXAFS) characterization of the UO(2) solids recovered from batch experiments suggest that oxidative dissolution was more evident in the absence of diffusive limitations. Ca-EXAFS spectra indicate the presence of Ca in the reacted UO(2) solids with a coordination environment similar to that of a Ca-O-Si mineral. The findings from this study advance our overall understanding of the coupling of geochemical and transport processes that can lead to differences in dissolution rates measured in the field and in laboratory experiments.
Estimation of the diffusion-limited rate of microtubule assembly.
Odde, D J
1997-01-01
Microtubule assembly is a complex process with individual microtubules alternating stochastically between extended periods of assembly and disassembly, a phenomenon known as dynamic instability. Since the discovery of dynamic instability, molecular models of assembly have generally assumed that tubulin incorporation into the microtubule lattice is primarily reaction-limited. Recently this assumption has been challenged and the importance of diffusion in microtubule assembly dynamics asserted on the basis of scaling arguments, with tubulin gradients predicted to extend over length scales exceeding a cell diameter, approximately 50 microns. To assess whether individual microtubules in vivo assemble at diffusion-limited rates and to predict the theoretical upper limit on the assembly rate, a steady-state mean-field model for the concentration of tubulin about a growing microtubule tip was developed. Using published parameter values for microtubule assembly in vivo (growth rate = 7 microns/min, diffusivity = 6 x 10(-12) m2/s, tubulin concentration = 10 microM), the model predicted that the tubulin concentration at the microtubule tip was approximately 89% of the concentration far from the tip, indicating that microtubule self-assembly is not diffusion-limited. Furthermore, the gradients extended less than approximately 50 nm (the equivalent of about two microtubule diameters) from the microtubule tip, a distance much less than a cell diameter. In addition, a general relation was developed to predict the diffusion-limited assembly rate from the diffusivity and bulk tubulin concentration. Using this relation, it was estimated that the maximum theoretical assembly rate is approximately 65 microns/min, above which tubulin can no longer diffuse rapidly enough to support faster growth. Images FIGURE 1 PMID:9199774
A Computational Investigation of Sooting Limits of Spherical Diffusion Flames
NASA Technical Reports Server (NTRS)
Lecoustre, V. R.; Chao, B. H.; Sunderland, P. B.; Urban, D. L.; Stocker, D. P.; Axelbaum, R. L.
2007-01-01
Limiting conditions for soot particle inception in spherical diffusion flames were investigated numerically. The flames were modeled using a one-dimensional, time accurate diffusion flame code with detailed chemistry and transport and an optically thick radiation model. Seventeen normal and inverse flames were considered, covering a wide range of stoichiometric mixture fraction, adiabatic flame temperature, and residence time. These flames were previously observed to reach their sooting limits after 2 s of microgravity. Sooting-limit diffusion flames with residence times longer than 200 ms were found to have temperatures near 1190 K where C/O = 0.6, whereas flames with shorter residence times required increased temperatures. Acetylene was found to be a reasonable surrogate for soot precursor species in these flames, having peak mole fractions of about 0.01.
Fractal Aggregates in Tennis Ball Systems
ERIC Educational Resources Information Center
Sabin, J.; Bandin, M.; Prieto, G.; Sarmiento, F.
2009-01-01
We present a new practical exercise to explain the mechanisms of aggregation of some colloids which are otherwise not easy to understand. We have used tennis balls to simulate, in a visual way, the aggregation of colloids under reaction-limited colloid aggregation (RLCA) and diffusion-limited colloid aggregation (DLCA) regimes. We have used the…
Shear-limited test particle diffusion in 2-dimensional plasmas
NASA Astrophysics Data System (ADS)
Anderegg, Francois; Driscoll, C. Fred; Dubin, Daniel H. E.
2002-01-01
Measurements of test-particle diffusion in pure ion plasmas show 2D enhancements over the 3D rates, limited by shear in the plasma rotation ωE(r). The diffusion is due to "long-range" ion-ion collisions in the quiescent, steady-state Mg+ plasma. For short plasma length Lp and low shear S≡r∂ωE/∂r, thermal ions bounce axially many times before shear separates them in θ, so the ions move in (r,θ) as bounce averaged "rods" of charge (i.e. 2D point vortices). Experimentally, we vary the number of bounces over the range 0.2⩽Nb⩽10,000. For long plasmas with Nb⩽1, we observe diffusion in quantitative agreement with the 3D theory of long-range E×B drift collisions. For shorter plasmas or lower shear, with Nb>1, we measure diffusion rates enhanced by up to 100×. For exceedingly small she0ar, i.e. Nb⩾1000, we observe diffusion rates consistent with the Taylor-McNamara estimates for a shear-free thermal plasma. Overall, the data shows fair agreement with Dubin's new theory of 2D diffusion in shear, which predicts an enhancement of D2D/D3D≈Nb up to the Taylor-McNamara limit.
Kee, Kerk F; Sparks, Lisa; Struppa, Daniele C; Mannucci, Mirco A; Damiano, Alberto
2016-01-01
By integrating the simplicial model of social aggregation with existing research on opinion leadership and diffusion networks, this article introduces the constructs of simplicial diffusers (mathematically defined as nodes embedded in simplexes; a simplex is a socially bonded cluster) and simplicial diffusing sets (mathematically defined as minimal covers of a simplicial complex; a simplicial complex is a social aggregation in which socially bonded clusters are embedded) to propose a strategic approach for information diffusion of cancer screenings as a health intervention on Facebook for community cancer prevention and control. This approach is novel in its incorporation of interpersonally bonded clusters, culturally distinct subgroups, and different united social entities that coexist within a larger community into a computational simulation to select sets of simplicial diffusers with the highest degree of information diffusion for health intervention dissemination. The unique contributions of the article also include seven propositions and five algorithmic steps for computationally modeling the simplicial model with Facebook data.
NASA Astrophysics Data System (ADS)
Menshutin, A. Yu.; Shchur, L. N.
2011-09-01
Two-dimensional structures grown with Witten and Sander algorithm are investigated. We analyze clusters grown off-lattice and clusters grown with antenna method with N=3,4,5,6,7 and 8 allowed growth directions. With the help of variable probe particles technique we measure fractal dimension of such clusters D(N) as a function of their size N. We propose that in the thermodynamic limit of infinite cluster size the aggregates grown with high degree of anisotropy ( N=3,4,5) tend to have fractal dimension D equal to 3/2, while off-lattice aggregates and aggregates with lower anisotropy ( N>6) have D≈1.710. Noise-reduction procedure results in the change of universality class for DLA. For high enough noise-reduction value clusters with N⩾6 have fractal dimension going to 3/2 when N→∞.
Intercellular Diffusion Limits to CO2 Uptake in Leaves 1
Parkhurst, David F.; Mott, Keith A.
1990-01-01
We studied plants of five species with hypostomatous leaves, and six with amphistomatous leaves, to determine the extent to which gaseous diffusion of CO2 among the mesophyll cells limits photosynthetic carbon assimilation. In helox (air with nitrogen replaced by helium), the diffusivities of CO2 and water vapor are 2.3 times higher than in air. For fixed estimated CO2 pressure at the evaporating surfaces of the leaf (pi), assimilation rates in helox ranged up to 27% higher than in air for the hypostomatous leaves, and up to 7% higher in the amphistomatous ones. Thus, intercellular diffusion must be considered as one of the processes limiting photosynthesis, especially for hypostomatous leaves. A corollary is that CO2 pressure should not be treated as uniform through the mesophyll in many leaves. To analyze our helox data, we had to reformulate the usual gas-exchange equation used to estimate CO2 pressure at the evaporating surfaces of the leaf; the new equation is applicable to any gas mixture for which the diffusivities of CO2 and H2O are known. Finally, we describe a diffusion-biochemistry model for CO2 assimilation that demonstrates the plausibility of our experimental results. PMID:16667792
42 CFR 447.512 - Drugs: Aggregate upper limits of payment.
Code of Federal Regulations, 2011 CFR
2011-10-01
...: Aggregate upper limits of payment. (a) (b) Other drugs. The agency payments for brand name drugs certified... of brand name drugs. (1) The upper limit for payment for multiple source drugs for which a specific... an electronic alternative means approved by the Secretary) that a specific brand is...
NASA Astrophysics Data System (ADS)
Wang, Sibo; Xu, Junbo; Wen, Hao
2014-12-01
The heavy crude oil consists of thousands of compounds and much of them have large molecular weights and complex structures. Studying the aggregation and diffusion behavior of asphaltenes can facilitate the understanding of the heavy crude oil. In previous studies, the fused aromatic rings were treated as rigid bodies so that dissipative particle dynamics (DPD) integrated with the quaternion method can be used to study asphaltene systems. In this work, DPD integrated with the quaternion method is implemented on graphics processing units (GPUs). Compared with the serial program, tens of times speedup can be achieved when simulations performed on a single GPU. Using multiple GPUs can provide faster computation speed and more storage space for simulations of significant large systems. By using large systems, simulations of the asphaltene-toluene system at extremely dilute concentrations can be performed. The determined diffusion coefficients of asphaltenes are similar to that in experimental studies. At last, the aggregation behavior of asphaltenes in heptane was investigated, and the simulation results agreed with the modified Yen model. Monomers, nanoaggregates and clusters were observed from the simulations at different concentrations.
Why the Phosphotransferase System of Escherichia coli Escapes Diffusion Limitation
Francke, Christof; Postma, Pieter W.; Westerhoff, Hans V.; Blom, Joke G.; Peletier, Mark A.
2003-01-01
We calculated the implications of diffusion for the phosphoenolpyruvate:glucose phosphotransferase system (glucose-PTS) of Escherichia coli in silicon cells of various magnitudes. For a cell of bacterial size, diffusion limitation of glucose influx was negligible. Nevertheless, a significant concentration gradient for one of the enzyme species, nonphosphorylated IIAGlc, was found. This should have consequences because the phosphorylation state of IIAGlc is an important intracellular signal. For mammalian cell sizes we found significant diffusion limitation, as well as strong concentration gradients in many PTS components, and strong effects on glucose and energy signaling. We calculated that the PTS may sense both extracellular glucose and the intracellular free-energy state. We discuss i), that the effects of diffusion on cell function should prevent this highly effective bacterial system from functioning in eukaryotic cells, ii), that in the larger eukaryotic cell any similar chain of mobile group-transfer proteins can neither sustain the same volumetric flux as in bacteria nor transmit a signal far into the cell, and iii), that systems such as these may exhibit spatial differentiation in their sensitivity to different signals. PMID:12829515
Reaction-diffusion master equation in the microscopic limit
NASA Astrophysics Data System (ADS)
Hellander, Stefan; Hellander, Andreas; Petzold, Linda
2012-04-01
Stochastic modeling of reaction-diffusion kinetics has emerged as a powerful theoretical tool in the study of biochemical reaction networks. Two frequently employed models are the particle-tracking Smoluchowski framework and the on-lattice reaction-diffusion master equation (RDME) framework. As the mesh size goes from coarse to fine, the RDME initially becomes more accurate. However, recent developments have shown that it will become increasingly inaccurate compared to the Smoluchowski model as the lattice spacing becomes very fine. Here we give a general and simple argument for why the RDME breaks down. Our analysis reveals a hard limit on the voxel size for which no local RDME can agree with the Smoluchowski model and lets us quantify this limit in two and three dimensions. In this light we review and discuss recent work in which the RDME has been modified in different ways in order to better agree with the microscale model for very small voxel sizes.
Diffusive limits of nonlinear hyperbolic systems with variable coefficients
NASA Astrophysics Data System (ADS)
Miyoshi, Hironari; Tsutsumi, Masayoshi
2016-09-01
We consider the initial-boundary value problem for a 2-speed system of first-order nonhomogeneous semilinear hyperbolic equations whose leading terms have a small positive parameter. Using energy estimates and a compactness lemma, we show that the diffusion limit of the sum of the solutions of the hyperbolic system, as the parameter tends to zero, verifies the nonlinear parabolic equation of the p-Laplacian type.
Shear-Limited Diffusion and Viscosity: Experiments and Theory
NASA Astrophysics Data System (ADS)
Driscoll, C. Fred
2001-10-01
Experiments and theory on collisional diffusion and viscosity demonstrate enhanced transport in the 2D bounce-averaged regime, limited by shear in the plasma rotation. The experiments are performed on relatively quiescent pure-ion or pure electron plasma columns, where the shear in the drift rotation ωE (r) can be controlled accurately. For long plasma columns, we measure test particle diffusion(F. Anderegg, et al.), Phys. Rev. Lett. 78, 2128 (1997). and bulk viscosity(J.M. Kriesel and C.F. Driscoll, submitted to Phys. Rev. Lett. (2001).) coefficients which quantitatively agree with recent 3D theories(D.H.E. Dubin, Phys. Plasmas 5), 1688 (1998). of E × B drift collisions with impact parameters in the range rc < ρ < λ_D. In general, this transport is substantially greater than would be expected for velocity-scattering collisions with ρ < r_c. For finite plasma length L_p, thermal particles may bounce axially many times before rotational shear separates them in θ and this number of bounces Nb ≡ ( barv / 2L_p) / (r ; partial ωE / partial r) characterizes the approach to the 2D bounce-averaged regime. Experiments measuring electron viscosity coefficients and separate experiments measuring tagged ion diffusion coefficients each show transport enhancements up to 100×, scaling quantitatively as Nb over the range 1 < Nb < 10^2. In the zero-shear limit of Nb arrow ∞ , theory treats the particles as z-averaged rods of charge undergoing 2D E × B drift dynamics. For this case, Taylor and McNamara showed that Bohm-like diffusion results from large-scale thermally-excited ``Dawson-Okuda'' vortices. More recently, Dubin(D.H.E. Dubin and D.Z. Jin, Phys. Lett. A 284), 112 (2001). analyzed the 2D test-particle diffusion with applied background shear, showing that the particle diffusion decreases with increasing shear. Overall, this new theory gives fair quantitative agreement with the diffusion experiments from the 3D (or high shear) regime with Nb <= 1 to the 2D (or
Chen, Jun; Zhang, Tao; Wang, Shuangqing; Hu, Rui; Li, Shayu; Ma, Jin Shi; Yang, Guoqiang
2015-10-01
A series of triazine-linked mono-, bis- and tris-phthalocyanines are synthesized, intramolecular aggregation is found in bis- and tris-phthalocyanines via π-π stacking interaction. Theoretical and experimental studies reveal the formation of the intramolecular aggregation. The spectrographic, photophysical and nonlinear optical properties of these compounds are adjusted for the formation of the intramolecular aggregation. The bis-phthalocyanine dimer presents smaller fluorescence quantum yield, lower triplet formation yield and the triplet-minus-ground state extinction coefficient, which causes poorer optical limiting performance. It is interesting that the tris-phthalocyanine is composed of a mono-phthalocyanine part and a bis-phthalocyanine part, the optical limiting property of the tris-phthalocyanine is similar to that of mono-phthalocyanine.
Fractal aggregates in tennis ball systems
NASA Astrophysics Data System (ADS)
Sabin, J.; Bandín, M.; Prieto, G.; Sarmiento, F.
2009-09-01
We present a new practical exercise to explain the mechanisms of aggregation of some colloids which are otherwise not easy to understand. We have used tennis balls to simulate, in a visual way, the aggregation of colloids under reaction-limited colloid aggregation (RLCA) and diffusion-limited colloid aggregation (DLCA) regimes. We have used the images of the cluster of balls, following Forrest and Witten's pioneering studies on the aggregation of smoke particles, to estimate their fractal dimension.
Diffusion-limited gas exchange across amphibian skin.
Gatz, R N
1982-04-01
Cutaneous gas exchange function of amphibians is conveniently analyzed in lungless salamanders in which all gas exchange is through the skin. Measurements in Desmognathus fuscus (Plethodontidae) have yielded the following results: 1) Cutaneous transfer of O2 and CO2 is limited predominantly by diffusion. 2) Accordingly, in environmental hypoxia, O2 uptake decreases. 3) Anaerobiosis contributes to energy balance during the first few hours of hypoxia only. Thereafter unknown biochemical adjustments take place that allow the animal to restore metabolite levels characteristic of normoxia regardless of the continued reduced oxygen uptake.
SVE design: Mass transfer limitation due to molecular diffusion
Yang, Y.J.; Gates, T.M.; Edwards, S.
1999-09-01
Vaporization and soil adsorption are the two mass transfer mechanisms that control contaminant recovery rates for soil vapor extraction (SVE) systems. At most soil remediation sites, contaminants are distributed among three phases, namely, soil particles, pore water, and soil vapor. Contaminant mass transfer from adsorption sites into a convective vapor stream involves desorption, diffusion through pore water, and vaporization into soil vapor. An SVE design model is proposed to describe this three-phase mass transfer process and assist the design and evaluation of SVE systems. The model contains analytical solutions developed to estimate contaminant concentrations in the vapor phase and predict contaminant removal rates. Monitoring data from two full-scale SVE systems were used for model development and calibration. The results suggest that contaminant diffusion through the pore water is the rate-limiting step and leads to remediation inefficiency of an SVE system. Mass Transfer retardation from molecular diffusion in water is likely the major contributing component to the venting efficiency coefficient of Staudinger et al.
On the continuum time limit of reaction-diffusion systems
NASA Astrophysics Data System (ADS)
Grassberger, Peter
2013-09-01
The parity-conserving branching-annihilating random walk (pc-BARW) model is a reaction-diffusion system on a lattice where particles can branch into m offsprings with even m and hop to neighboring sites. If two or more particles land on the same site, they immediately annihilate pairwise. In this way the number of particles is preserved modulo two. It is well known that the pc-BARW with m = 2 in 1 spatial dimension has no phase transition (it is always subcritical), if the hopping is described by a continuous time random walk. In contrast, the m = 2 1-d pc-BARW has a phase transition when formulated in discrete time, but we show that the continuous time limit is non-trivial: When the time step \\delta t\\to 0 , the branching and hopping probabilities at the critical point scale with different powers of \\delta t . These powers are different for different microscopic realizations. Although this phenomenon is not observed in some other reaction-diffusion systems like, e.g., the contact process, we argue that it should be generic and not restricted to the 1-d pc-BARW model.
Diffusion Limited Supercritical Water Oxidation (SCWO) in Microgravity Environments
NASA Technical Reports Server (NTRS)
Hicks, M. C.; Lauver, R. W.; Hegde, U. G.; Sikora, T. J.
2006-01-01
Tests designed to quantify the gravitational effects on thermal mixing and reactant injection in a Supercritical Water Oxidation (SCWO) reactor have recently been performed in the Zero Gravity Facility (ZGF) at NASA s Glenn Research Center. An artificial waste stream, comprising aqueous mixtures of methanol, was pressurized to approximately 250 atm and then heated to 450 C. After uniform temperatures in the reactor were verified, a controlled injection of air was initiated through a specially designed injector to simulate diffusion limited reactions typical in most continuous flow reactors. Results from a thermal mapping of the reaction zone in both 1-g and 0-g environments are compared. Additionally, results of a numerical model of the test configuration are presented to illustrate first order effects on reactant mixing and thermal transport in the absence of gravity.
Diffusive shock acceleration - Acceleration rate, magnetic-field direction and the diffusion limit
NASA Technical Reports Server (NTRS)
Jokipii, J. R.
1992-01-01
This paper reviews the concept of diffusive shock acceleration, showing that the acceleration of charged particles at a collisionless shock is a straightforward consequence of the standard cosmic-ray transport equation, provided that one treats the discontinuity at the shock correctly. This is true for arbitrary direction of the upstream magnetic field. Within this framework, it is shown that acceleration at perpendicular or quasi-perpendicular shocks is generally much faster than for parallel shocks. Paradoxically, it follows also that, for a simple scattering law, the acceleration is faster for less scattering or larger mean free path. Obviously, the mean free path can not become too large or the diffusion limit becomes inapplicable. Gradient and curvature drifts caused by the magnetic-field change at the shock play a major role in the acceleration process in most cases. Recent observations of the charge state of the anomalous component are shown to require the faster acceleration at the quasi-perpendicular solar-wind termination shock.
Cooper, Keith M
2012-08-01
In the UK, Government policy requires marine aggregate extraction companies to leave the seabed in a similar physical condition after the cessation of dredging. This measure is intended to promote recovery, and the return of a similar faunal community to that which existed before dredging. Whilst the policy is sensible, and in line with the principles of sustainable development, the use of the word 'similar' is open to interpretation. There is, therefore, a need to set quantifiable limits for acceptable change in sediment composition. Using a case study site, it is shown how such limits could be defined by the range of sediment particle size composition naturally found in association with the faunal assemblages in the wider region. Whilst the approach offers a number of advantages over the present system, further testing would be required before it could be recommended for use in the regulatory context. PMID:22721693
NASA Astrophysics Data System (ADS)
Lv, Neng; Zhang, Chu-Hang; Yang, Bo; Pan, Qi-Fa; Ye, Gao-Xiang
2012-09-01
Ramified iron aggregates are fabricated on silicone oil surface by thermal evaporation method at room temperature and the crossover from ramified aggregates to continuous films is studied by atomic force microscopy. It is shown that the aggregates are composed of numerous granules with the average diameter around 34 nm. The dynamic scaling analysis shows that the growth exponent β = 0.23 ± 0.02 and the roughness exponent α evolves from 0.65 to 0.42 with the nominal film thickness, indicating a roughening behavior transfer may exist during the growth process. The physical interpretation for the scaling behavior is presented.
Limitations in Determining Multifractal Spectra from Pore-Solid Soil Aggregate Images
Kravchenko, A N; Martin, M A; Smucker, A J.M.; Rivers, M L
2011-11-16
Multifractal methods have the potential to be useful tools for characterizing spatial distributions of soil pores from microtomographic images of undisturbed soil cores and soil aggregates. The objective of this study was to examine the limitations of multifractal analyses in binary (void and solid) soil images and to explore conditions under which multifractal spectra can be obtained. Multifractal characteristics of binary soil images are bounded within certain limiting values corresponding to nonfractal scaling. In this study, we first addressed the theoretical limitations of multifractal analysis of binary images and examined the nonfractal scaling boundaries in multifractal calculations by the method of moments. Then we developed boundary conditions for multifractal calculations by the direct method. Results revealed that fractal scaling is potentially possible only across a relatively narrow range of cell sizes restricted by the nonfractal scaling boundaries. Moreover, the range of cell sizes where fractal scaling is potentially possible varies with pore size. That is, in multifractal calculations it changes continuously with changes in the q value. For the soil aggregates examined in this study, this range varied from two to eight pixels for low q values to 128 pixels for high q values. The varying range makes calculations of true multifractal spectra for binary soil image data impossible. These results are consistent with a general theoretical notion that binary soil images are not multifractal in a strict mathematical sense. We suggest, however, that application of multifractal formalism can generate 'pseudo-multifractal spectra' that might still be useful for summarizing pore distribution information and for comparing pore data among different agricultural management regimes and soil type.
Note: The effect of viscosity on the rate of diffusion-limited association of nanoparticles.
Zhdanov, Vladimir P
2015-10-28
In the treatments of diffusion-limited association of suspended nanoparticles, their diffusion coefficients are usually considered to be constant and equal to those given by conventional hydrodynamics for diffusion of single nanoparticles. In reality, according to hydrodynamics, these coefficients depend, however, on the distance between nanoparticles. I show how this dependence can influence the association rate.
NASA Technical Reports Server (NTRS)
Srinivasan, R. Srini; Gerth, Wayne A.; Powell, Michael R.; Paloski, William H. (Technical Monitor)
2000-01-01
A three-region mathematical model of gas bubble dynamics has been shown suitable for describing diffusion-limited dynamics of more than one bubble in a given volume of extravascular tissue. The model is based on the dynamics of gas exchange between a bubble and a well-stirred tissue region through an intervening unperfused diffusion region previously assumed to have constant thickness and uniform gas diffusivity. As a result, the gas content of the diffusion region remains constant as the volume of the region increases with bubble growth, causing dissolved gas in the region to violate Henry's law. Earlier work also neglected the relationship between the varying diffusion region volume and the fixed total tissue volume, because only cases in which the diffusion region volume is a small fraction of the overall tissue volume were considered. We herein extend the three-region model to correct these theoretical inconsistencies by allowing both the thickness and gas content of the diffusion region to vary during bubble evolution. A postulated difference in gas diffusivity between an infinitesimally thin layer at the bubble surface and the remainder of the diffusion region leads to variation in diffusion region gas content and thickness during bubble growth and resolution. This variable thickness, differential diffusivity (VTDD) model can yield bubble lifetimes considerably longer than those yielded by earlier three-region models for given model and decompression parameters, and meets a need for theoretically consistent but relatively simple bubble dynamics models for use in studies of decompression sickness (DCS) in human subjects, Keywords: decompression sickness, gas diffusion in tissue, diffusivity
Nonlinear diffusion equations as asymptotic limits of Cahn-Hilliard systems
NASA Astrophysics Data System (ADS)
Colli, Pierluigi; Fukao, Takeshi
2016-05-01
An asymptotic limit of a class of Cahn-Hilliard systems is investigated to obtain a general nonlinear diffusion equation. The target diffusion equation may reproduce a number of well-known model equations: Stefan problem, porous media equation, Hele-Shaw profile, nonlinear diffusion of singular logarithmic type, nonlinear diffusion of Penrose-Fife type, fast diffusion equation and so on. Namely, by setting the suitable potential of the Cahn-Hilliard systems, all these problems can be obtained as limits of the Cahn-Hilliard related problems. Convergence results and error estimates are proved.
Uncertainty limits for the macroscopic elastic moduli of random polycrystalline aggregates
NASA Astrophysics Data System (ADS)
Chinh, Pham Duc
2000-08-01
Practical polycrystalline aggregates are expected to have macroscopic properties that depend upon the properties of constituent crystals and the aggregate geometry. Since that microgeometry is usually random, there will be some uncertainty in the observed macroscopic behavior of the aggregates. The general shape-independent upper and lower estimates for those uncertainty intervals for the elastic moduli of completely random polycrystals are constructed from the minimum energy and complementary energy principles. Applications to aggregates of cubic crystals are presented.
Murray, Brian; Rosenthal, Joseph; Zheng, Zhongli; Isaacson, David; Zhu, Yingxi; Belfort, Georges
2015-04-14
The effects of cosolutes on amyloid aggregation kinetics in vivo are critical and not fully understood. To explore the effects of cosolute additives, the in vitro behavior of destabilizing and stabilizing osmolytes with polymer cosolutes on the aggregation of a model amyloid, human insulin, is probed using experiments coupled with an amyloid aggregation reaction model. The destabilizing osmolyte, guanidine hydrochloride (GuHCl), induces biphasic behavior on the amyloid aggregation rate exhibited by an enhancement of the aggregation kinetics at low concentrations of GuHCl (<0.6 M) and a reduction in kinetics at higher GuHCl concentrations. Stabilizing osmolytes, glycerol, sorbitol and trimethylamine N-oxide, slow the rate of aggregation by reducing the rate of monomer unfolding. Polymer cosolutes, polyvinylpyrrolidone 3.5 kDa and 40 kDa, delay amyloid aggregation mainly through a decrease in the nucleation reaction. These results are in good agreement with the volume exclusion principle for polymer crowding and supports the need to include conformational rearrangement of monomers prior to nucleation. Using fluorescence correlation spectroscopy, we demonstrate that amyloid aggregation is nondiffusion limited, except during fibril accumulation in the presence of high concentrations of long chain polymers. Lastly, the neutral surface area of osmolytes correlates well with the time to initiate fibril formation, tlag, which implicates an intrinsic osmolyte property underlying preferential interactions. PMID:25803421
Effective reaction rates for diffusion-limited reaction cycles.
Nałęcz-Jawecki, Paweł; Szymańska, Paulina; Kochańczyk, Marek; Miękisz, Jacek; Lipniacki, Tomasz
2015-12-01
Biological signals in cells are transmitted with the use of reaction cycles, such as the phosphorylation-dephosphorylation cycle, in which substrate is modified by antagonistic enzymes. An appreciable share of such reactions takes place in crowded environments of two-dimensional structures, such as plasma membrane or intracellular membranes, and is expected to be diffusion-controlled. In this work, starting from the microscopic bimolecular reaction rate constants and using estimates of the mean first-passage time for an enzyme-substrate encounter, we derive diffusion-dependent effective macroscopic reaction rate coefficients (EMRRC) for a generic reaction cycle. Each EMRRC was found to be half of the harmonic average of the microscopic rate constant (phosphorylation c or dephosphorylation d), and the effective (crowding-dependent) motility divided by a slowly decreasing logarithmic function of the sum of the enzyme concentrations. This implies that when c and d differ, the two EMRRCs scale differently with the motility, rendering the steady-state fraction of phosphorylated substrate molecules diffusion-dependent. Analytical predictions are verified using kinetic Monte Carlo simulations on the two-dimensional triangular lattice at the single-molecule resolution. It is demonstrated that the proposed formulas estimate the steady-state concentrations and effective reaction rates for different sets of microscopic reaction rates and concentrations of reactants, including a non-trivial example where with increasing diffusivity the fraction of phosphorylated substrate molecules changes from 10% to 90%.
Effective reaction rates for diffusion-limited reaction cycles
NASA Astrophysics Data System (ADS)
Nałecz-Jawecki, Paweł; Szymańska, Paulina; Kochańczyk, Marek; Miekisz, Jacek; Lipniacki, Tomasz
2015-12-01
Biological signals in cells are transmitted with the use of reaction cycles, such as the phosphorylation-dephosphorylation cycle, in which substrate is modified by antagonistic enzymes. An appreciable share of such reactions takes place in crowded environments of two-dimensional structures, such as plasma membrane or intracellular membranes, and is expected to be diffusion-controlled. In this work, starting from the microscopic bimolecular reaction rate constants and using estimates of the mean first-passage time for an enzyme-substrate encounter, we derive diffusion-dependent effective macroscopic reaction rate coefficients (EMRRC) for a generic reaction cycle. Each EMRRC was found to be half of the harmonic average of the microscopic rate constant (phosphorylation c or dephosphorylation d), and the effective (crowding-dependent) motility divided by a slowly decreasing logarithmic function of the sum of the enzyme concentrations. This implies that when c and d differ, the two EMRRCs scale differently with the motility, rendering the steady-state fraction of phosphorylated substrate molecules diffusion-dependent. Analytical predictions are verified using kinetic Monte Carlo simulations on the two-dimensional triangular lattice at the single-molecule resolution. It is demonstrated that the proposed formulas estimate the steady-state concentrations and effective reaction rates for different sets of microscopic reaction rates and concentrations of reactants, including a non-trivial example where with increasing diffusivity the fraction of phosphorylated substrate molecules changes from 10% to 90%.
Multiple-to-dominant path collapse of linked-flux model for diffusion-limited nucleation
NASA Astrophysics Data System (ADS)
Lau, Y. H.; Wu, D. T.
2013-01-01
While capable of estimating diffusion-limited nucleation rates, Kelton's linked-flux model has no simple solution. To increase the model's usability, we simplify the model by retaining only the dominant nucleation path to obtain a series solution. The solution agrees well with the Kelton's model's predictions of the nucleation rate, and thus provides a simple estimate of diffusion-limited nucleation rates.
Diffusion limited soil vapor extraction: Geologic and bed thickness controls
Beckett, G.D. ); Benson, D.A. )
1996-01-01
Soil vapor extraction (SVE) can remove volatile contaminants from the subsurface environment. In a heterogeneous geologic setting, SVE cleanup will progress rapidly through advective mass transfer in permeable sediments and primarily through slow diffusion in lower permeability soil. The contrast in rates of cleanup between high and low permeability soils is further increased by the associated soil moisture retention contrasts (i.e., capillarity) in the same soils. Low permeability soil generally has a higher soil suction capacity and moisture content than high permeability soil. This results in further diminishment of cleanup rate in fine-grained sediments in a heterogeneous environment. This paper investigates how contrasts in soil type and bed thickness affect the rate of SVE diffusive cleanup. The numerical model VENT3D is used to simulate three heterogeneous geologic settings with differing soil contrasts. Within each geologic setting, four simulations are performed with varying bed thicknesses in each, effectively changing the diffusive half-length of the fine-grained soils while maintaining the total bulk percentages of fine-to coarse-grained material. Under these conditions, the bulk flow parameters measured during SVE field testing would be constant for each of the four simulations within a single geologic domain while the cleanup times would not.
Diffusion limited soil vapor extraction: Geologic and bed thickness controls
Beckett, G.D.; Benson, D.A.
1996-12-31
Soil vapor extraction (SVE) can remove volatile contaminants from the subsurface environment. In a heterogeneous geologic setting, SVE cleanup will progress rapidly through advective mass transfer in permeable sediments and primarily through slow diffusion in lower permeability soil. The contrast in rates of cleanup between high and low permeability soils is further increased by the associated soil moisture retention contrasts (i.e., capillarity) in the same soils. Low permeability soil generally has a higher soil suction capacity and moisture content than high permeability soil. This results in further diminishment of cleanup rate in fine-grained sediments in a heterogeneous environment. This paper investigates how contrasts in soil type and bed thickness affect the rate of SVE diffusive cleanup. The numerical model VENT3D is used to simulate three heterogeneous geologic settings with differing soil contrasts. Within each geologic setting, four simulations are performed with varying bed thicknesses in each, effectively changing the diffusive half-length of the fine-grained soils while maintaining the total bulk percentages of fine-to coarse-grained material. Under these conditions, the bulk flow parameters measured during SVE field testing would be constant for each of the four simulations within a single geologic domain while the cleanup times would not.
Conditions for diffusion-limited and reaction-limited recombination in nanostructured solar cells
Ansari-Rad, Mehdi; Anta, Juan A.; Arzi, Ezatollah
2014-04-07
The performance of Dye-sensitized solar cells (DSC) and related devices made of nanostructured semiconductors relies on a good charge separation, which in turn is achieved by favoring charge transport against recombination. Although both processes occur at very different time scales, hence ensuring good charge separation, in certain cases the kinetics of transport and recombination can be connected, either in a direct or an indirect way. In this work, the connection between electron transport and recombination in nanostructured solar cells is studied both theoretically and by Monte Carlo simulation. Calculations using the Multiple-Trapping model and a realistic trap distribution for nanostructured TiO{sub 2} show that for attempt-to-jump frequencies higher than 10{sup 11}–10{sup 13} Hz, the system adopts a reaction limited (RL) regime, with a lifetime which is effectively independent from the speed of the electrons in the transport level. For frequencies lower than those, and depending on the concentration of recombination centers in the material, the system enters a diffusion-limited regime (DL), where the lifetime increases if the speed of free electrons decreases. In general, the conditions for RL or DL recombination depend critically on the time scale difference between recombination kinetics and free-electron transport. Hence, if the former is too rapid with respect to the latter, the system is in the DL regime and total thermalization of carriers is not possible. In the opposite situation, a RL regime arises. Numerical data available in the literature, and the behavior of the lifetime with respect to (1) density of recombination centers and (2) probability of recombination at a given center, suggest that a typical DSC in operation stays in the RL regime with complete thermalization, although a transition to the DL regime may occur for electrolytes or hole conductors where recombination is especially rapid or where there is a larger dispersion of energies of
Diffusion-Limited Cargo Loading of an Engineered Protein Container.
Zschoche, Reinhard; Hilvert, Donald
2015-12-30
The engineered bacterial nanocompartment AaLS-13 is a promising artificial encapsulation system that exploits electrostatic interactions for cargo loading. In order to study its ability to take up and retain guests, a pair of fluorescent proteins was developed which allows spectroscopic determination of the extent of encapsulation by Förster resonance energy transfer (FRET). The encapsulation process is generally complete within a second, suggesting low energetic barriers for proteins to cross the capsid shell. Formation of intermediate aggregates upon mixing host and guest in vitro complicates capsid loading at low ionic strength, but can be sidestepped by increasing salt concentrations or diluting the components. Encapsulation of guests is completely reversible, and the position of the equilibrium is easily tuned by varying the ionic strength. These results, which challenge the notion that AaLS-13 is a continuous rigid shell, provide valuable information about cargo loading that will guide ongoing efforts to engineer functional host-guest complexes. Moreover, it should be possible to adapt the protein FRET pair described in this report to characterize functional capsid-cargo complexes generated by other encapsulation systems.
Xiao, Gaoyu; Madabhushi, Anant
2011-01-01
The focus of image classification through supervised distance metric learning is to find an appropriate measure of similarity between images. Although this approach is effective in the presence of large amounts of training data, classification accuracy will deteriorate when the number of training samples is small, which, unfortunately, is often the situation in several medical applications. We present a novel image classification method called aggregated distance metric (ADM) learning for situations where the training image data are limited. Our approach is novel in that it combines the merits of boosted distance metric learning (BDM, a recently published learning scheme) and bagging theory. This approach involves selecting several sub-sets of the original training data to form a number of new training sets and then performing BDM on each of these training sub-sets. The distance metrics learned from each of the training sets are then combined for image classification. We present a theoretical proof of the superiority of classification by ADM over BDM. Using both clinical (X-ray) and non-clinical (toy car) images in our experiments (with altogether 10 sets of different parameters) and image classification accuracy as the measure, our method is shown to be more accurate than BDM and the traditional bagging strategy. PMID:22003681
Xiao, Gaoyu; Madabhushi, Anant
2011-01-01
The focus of image classification through supervised distance metric learning is to find an appropriate measure of similarity between images. Although this approach is effective in the presence of large amounts of training data, classification accuracy will deteriorate when the number of training samples is small, which, unfortunately, is often the situation in several medical applications. We present a novel image classification method called aggregated distance metric (ADM) learning for situations where the training image data are limited. Our approach is novel in that it combines the merits of boosted distance metric learning (BDM, a recently published learning scheme) and bagging theory. This approach involves selecting several sub-sets of the original training data to form a number of new training sets and then performing BDM on each of these training sub-sets. The distance metrics learned from each of the training sets are then combined for image classification. We present a theoretical proof of the superiority of classification by ADM over BDM. Using both clinical (X-ray) and non-clinical (toy car) images in our experiments (with altogether 10 sets of different parameters) and image classification accuracy as the measure, our method is shown to be more accurate than BDM and the traditional bagging strategy.
A multigrid Newton-Krylov method for flux-limited radiation diffusion
Rider, W.J.; Knoll, D.A.; Olson, G.L.
1998-09-01
The authors focus on the integration of radiation diffusion including flux-limited diffusion coefficients. The nonlinear integration is accomplished with a Newton-Krylov method preconditioned with a multigrid Picard linearization of the governing equations. They investigate the efficiency of the linear and nonlinear iterative techniques.
Fluctuation Limit for Interacting Diffusions with Partial Annihilations Through Membranes
NASA Astrophysics Data System (ADS)
Chen, Zhen-Qing; Fan, Wai-Tong Louis
2016-08-01
We study fluctuations of the empirical processes of a non-equilibrium interacting particle system consisting of two species over a domain that is recently introduced in Chen and Fan (Ann Probab, to appear) and establish its functional central limit theorem. This fluctuation limit is a distribution-valued Gaussian Markov process which can be represented as a mild solution of a stochastic partial differential equation. The drift of our fluctuation limit involves a new partial differential equation with nonlinear coupled term on the interface that characterized the hydrodynamic limit of the system. The covariance structure of the Gaussian part consists two parts, one involving the spatial motion of the particles inside the domain and other involving a boundary integral term that captures the boundary interactions between two species. The key is to show that the Boltzmann-Gibbs principle holds for our non-equilibrium system. Our proof relies on generalizing the usual correlation functions to the join correlations at two different times.
Code of Federal Regulations, 2010 CFR
2010-10-01
... Disproportionate Number of Low-Income Patients § 447.296 Limitations on aggregate payments for disproportionate... disproprotionate number of low-income patients with special needs only if the payments are made in accordance with... disproportionate share hospitals to those hospitals with Medicaid inpatient utilization rates or...
Code of Federal Regulations, 2014 CFR
2014-10-01
... 42 Public Health 4 2014-10-01 2014-10-01 false Limitations on aggregate payments for disproportionate share hospitals for the period January 1, 1992 through September 30, 1992. 447.296 Section 447.296... share hospitals for the period January 1, 1992 through September 30, 1992. (a) The provisions of...
11 CFR 110.5 - Aggregate biennial contribution limitation for individuals (2 U.S.C. 441a(a)(3)).
Code of Federal Regulations, 2010 CFR
2010-01-01
... CONTRIBUTION AND EXPENDITURE LIMITATIONS AND PROHIBITIONS § 110.5 Aggregate biennial contribution limitation... political committees, making independent expenditures under 11 CFR part 109. (e) Contributions to delegates... 11 Federal Elections 1 2010-01-01 2010-01-01 false Aggregate biennial contribution limitation...
Experimental limit on the cosmic diffuse ultrahigh energy neutrino flux.
Gorham, P W; Hebert, C L; Liewer, K M; Naudet, C J; Saltzberg, D; Williams, D
2004-07-23
We report results from 120 h of live time with the Goldstone lunar ultrahigh energy neutrino experiment (GLUE). The experiment searches for < or = 10 ns microwave pulses from the lunar regolith, appearing in coincidence at two large radio telescopes separated by 22 km and linked by optical fiber. Such pulses would arise from subsurface electromagnetic cascades induced by interactions of > or = 100 EeV (1 EeV = 10(18) eV neutrinos in the lunar regolith. No candidates are yet seen, and the implied limits constrain several current models for ultrahigh energy neutrino fluxes. PMID:15323748
Effective reaction rates in diffusion-limited phosphorylation-dephosphorylation cycles.
Szymańska, Paulina; Kochańczyk, Marek; Miękisz, Jacek; Lipniacki, Tomasz
2015-02-01
We investigate the kinetics of the ubiquitous phosphorylation-dephosphorylation cycle on biological membranes by means of kinetic Monte Carlo simulations on the triangular lattice. We establish the dependence of effective macroscopic reaction rate coefficients as well as the steady-state phosphorylated substrate fraction on the diffusion coefficient and concentrations of opposing enzymes: kinases and phosphatases. In the limits of zero and infinite diffusion, the numerical results agree with analytical predictions; these two limits give the lower and the upper bound for the macroscopic rate coefficients, respectively. In the zero-diffusion limit, which is important in the analysis of dense systems, phosphorylation and dephosphorylation reactions can convert only these substrates which remain in contact with opposing enzymes. In the most studied regime of nonzero but small diffusion, a contribution linearly proportional to the diffusion coefficient appears in the reaction rate. In this regime, the presence of opposing enzymes creates inhomogeneities in the (de)phosphorylated substrate distributions: The spatial correlation function shows that enzymes are surrounded by clouds of converted substrates. This effect becomes important at low enzyme concentrations, substantially lowering effective reaction rates. Effective reaction rates decrease with decreasing diffusion and this dependence is more pronounced for the less-abundant enzyme. Consequently, the steady-state fraction of phosphorylated substrates can increase or decrease with diffusion, depending on relative concentrations of both enzymes. Additionally, steady states are controlled by molecular crowders which, mostly by lowering the effective diffusion of reactants, favor the more abundant enzyme.
Effective reaction rates in diffusion-limited phosphorylation-dephosphorylation cycles
NASA Astrophysics Data System (ADS)
Szymańska, Paulina; Kochańczyk, Marek; Miekisz, Jacek; Lipniacki, Tomasz
2015-02-01
We investigate the kinetics of the ubiquitous phosphorylation-dephosphorylation cycle on biological membranes by means of kinetic Monte Carlo simulations on the triangular lattice. We establish the dependence of effective macroscopic reaction rate coefficients as well as the steady-state phosphorylated substrate fraction on the diffusion coefficient and concentrations of opposing enzymes: kinases and phosphatases. In the limits of zero and infinite diffusion, the numerical results agree with analytical predictions; these two limits give the lower and the upper bound for the macroscopic rate coefficients, respectively. In the zero-diffusion limit, which is important in the analysis of dense systems, phosphorylation and dephosphorylation reactions can convert only these substrates which remain in contact with opposing enzymes. In the most studied regime of nonzero but small diffusion, a contribution linearly proportional to the diffusion coefficient appears in the reaction rate. In this regime, the presence of opposing enzymes creates inhomogeneities in the (de)phosphorylated substrate distributions: The spatial correlation function shows that enzymes are surrounded by clouds of converted substrates. This effect becomes important at low enzyme concentrations, substantially lowering effective reaction rates. Effective reaction rates decrease with decreasing diffusion and this dependence is more pronounced for the less-abundant enzyme. Consequently, the steady-state fraction of phosphorylated substrates can increase or decrease with diffusion, depending on relative concentrations of both enzymes. Additionally, steady states are controlled by molecular crowders which, mostly by lowering the effective diffusion of reactants, favor the more abundant enzyme.
Detailed numerical investigation of the Bohm limit in cosmic ray diffusion theory
Hussein, M.; Shalchi, A. E-mail: andreasm4@yahoo.com
2014-04-10
A standard model in cosmic ray diffusion theory is the so-called Bohm limit in which the particle mean free path is assumed to be equal to the Larmor radius. This type of diffusion is often employed to model the propagation and acceleration of energetic particles. However, recent analytical and numerical work has shown that standard Bohm diffusion is not realistic. In the present paper, we perform test-particle simulations to explore particle diffusion in the strong turbulence limit in which the wave field is much stronger than the mean magnetic field. We show that there is indeed a lower limit of the particle mean free path along the mean field. In this limit, the mean free path is directly proportional to the unperturbed Larmor radius like in the traditional Bohm limit, but it is reduced by the factor δB/B {sub 0} where B {sub 0} is the mean field and δB the turbulent field. Although we focus on parallel diffusion, we also explore diffusion across the mean field in the strong turbulence limit.
Moll, Remington J; Killion, Alexander K; Montgomery, Robert A; Tambling, Craig J; Hayward, Matt W
2016-05-01
The "landscape of fear" model, recently advanced in research on the non-lethal effects of carnivores on ungulates, predicts that prey will exhibit detectable antipredator behavior not only during risky times (i.e., predators in close proximity) but also in risky places (i.e., habitat where predators kill prey or tend to occur). Aggregation is an important antipredator response in numerous ungulate species, making it a useful metric to evaluate the strength and scope of the landscape of fear in a multi-carnivore, multi-ungulate system. We conducted ungulate surveys over a 2-year period in South Africa to test the influence of three broad-scale sources of variation in the landscape on spatial patterns in aggregation: (1) habitat structure, (2) where carnivores tended to occur (i.e., population-level utilization distributions), and (3) where carnivores tended to kill ungulate prey (i.e., probabilistic kill site maps). We analyzed spatial variation in aggregation for six ungulate species exposed to predation from recently reintroduced lion (Panthera leo) and spotted hyena (Crocuta crocuta). Although we did detect larger aggregations of ungulates in "risky places," these effects existed primarily for smaller-bodied (<150 kg) ungulates and were relatively moderate (change of 4 individuals across all habitats). In comparison, ungulate aggregations tended to increase at a slightly lower rate in habitat that was more open. The lion, an ambush (stalking) carnivore, had stronger influence on ungulate aggregation than the hyena, an active (coursing) carnivore. In addition, places where lions tended to kill prey had a greater effect on ungulate aggregation than places where lions tended to occur, but an opposing pattern existed for hyena. Our study reveals heterogeneity in the landscape of fear and suggests broad-scale risk effects following carnivore reintroduction only moderately influence ungulate aggregation size and vary considerably by predator hunting mode, type of
Moll, Remington J; Killion, Alexander K; Montgomery, Robert A; Tambling, Craig J; Hayward, Matt W
2016-05-01
The "landscape of fear" model, recently advanced in research on the non-lethal effects of carnivores on ungulates, predicts that prey will exhibit detectable antipredator behavior not only during risky times (i.e., predators in close proximity) but also in risky places (i.e., habitat where predators kill prey or tend to occur). Aggregation is an important antipredator response in numerous ungulate species, making it a useful metric to evaluate the strength and scope of the landscape of fear in a multi-carnivore, multi-ungulate system. We conducted ungulate surveys over a 2-year period in South Africa to test the influence of three broad-scale sources of variation in the landscape on spatial patterns in aggregation: (1) habitat structure, (2) where carnivores tended to occur (i.e., population-level utilization distributions), and (3) where carnivores tended to kill ungulate prey (i.e., probabilistic kill site maps). We analyzed spatial variation in aggregation for six ungulate species exposed to predation from recently reintroduced lion (Panthera leo) and spotted hyena (Crocuta crocuta). Although we did detect larger aggregations of ungulates in "risky places," these effects existed primarily for smaller-bodied (<150 kg) ungulates and were relatively moderate (change of 4 individuals across all habitats). In comparison, ungulate aggregations tended to increase at a slightly lower rate in habitat that was more open. The lion, an ambush (stalking) carnivore, had stronger influence on ungulate aggregation than the hyena, an active (coursing) carnivore. In addition, places where lions tended to kill prey had a greater effect on ungulate aggregation than places where lions tended to occur, but an opposing pattern existed for hyena. Our study reveals heterogeneity in the landscape of fear and suggests broad-scale risk effects following carnivore reintroduction only moderately influence ungulate aggregation size and vary considerably by predator hunting mode, type of
Diffusion into human islets is limited to molecules below 10 kDa.
Williams, S J; Schwasinger-Schmidt, T; Zamierowski, D; Stehno-Bittel, L
2012-10-01
Isolated islets are important tools in diabetes research and are used for islet transplantation as a treatment for type 1 diabetes. Yet these cell clusters have a dramatic diffusion barrier that leads to core cell death. Computer modeling has provided theoretical size limitations, but little has been done to measure the actual rate of diffusion in islets. The purpose of this study was to directly measure the diffusion barrier in intact human islets and determine its role in restricting insulin secretion. Impeded diffusion into islets was monitored with fluorescent dextran beads. Dextran beads of 10-70 kDa failed to diffuse into the core of the intact islets, while 0.9 kDa probe was observed within the core of smaller islets. Diffusion of the fluorescent form of glucose, 2-NBDG, had similar diffusion limitations as the beads, with an average intra-islet diffusion rate of 1.5 ± 0.2 μm/min. The poor diffusion properties were associated with core cell death from necrosis, not apoptosis. Short-term exposure to a mild papain/0 Ca(2+) cocktail, dramatically reduced the diffusion barrier so that all cells within islets were exposed to media components. Lowering the diffusion barrier increased the immediate and long-term viability of islet cells, and tended to increase the amount of insulin released, especially in low glucose conditions. However, it failed to improve the large islet's glucose-stimulated insulin secretion. Thus, the islet diffusion barrier leads to low viability and poor survival of large islets, but is not solely responsible for the reduced insulin secretion of large isolated islets.
Turnover of subsoil organic carbon controlled by substrate limitation and aggregation?
NASA Astrophysics Data System (ADS)
Dietrich, Patrick; Don, Axel; Helfrich, Mirjam
2014-05-01
Subsoils (>30 cm depth) store more than 50% of the total soil organic carbon (SOC) and subsoil SOC is characterised by high mean residence times compared to topsoil SOC. However, little is known about the mechanisms controlling the turnover of SOC in the subsoil. The purpose of this study was to evaluate the effect of temperature, substrate limitation and aggregation disturbance on subsoil SOC turnover. We assumed that temperature limits SOC turnover in subsoil, but the temperature response of SOC is obscured by an increasing stabilization of organic material with soil depth. In a laboratory incubation experiment the production of CO2 from undisturbed and disturbed soil samples and disturbed soil samples with added 13C labelled roots were investigated at two different temperatures (10 and 20° C). Soil samples were taken from 2-12 cm (depth 1), 30-60 cm (depth2) and 130-160 cm (depth 3) in a deciduous forest from a podzolic Cambisol and were placed in microcosms with an inner diameter of 14.2 cm and a height of 20 cm for depth 1 and 40 cm for depth 2 and 3. The microcosms were incubated for 30 days at 60% of water holding capacity. The incubation experiment showed an average increase of 80-150% in CO2 production for disturbed and undisturbed samples in depth 1 and depth 2 with increasing temperature. However, this was not observed in depth 3. This temperature influence was not found in the disturbed samples with added substrate. Instead, the increase in CO2 production of the labelled samples from depth 2 and 3 had a lag time of 5 to 8 days compared to samples from depth 1. Reasons for this delayed reaction on substrate might be dormant microorganisms in the subsoil at the beginning of the incubation experiment or spatial separation of microorganisms and the labelled substrate. Disturbance of the samples from depth 1 and 2 initially increase the CO2 production, but this effect was minor after day 15. Contrary to expectation, the CO2 production in depth 3 was greater
Zhdanov, Vladimir P; Höök, Fredrik
2015-05-01
Relatively large (~100 nm) spherical particles, e.g., virions, vesicles, or metal nanoparticles, often interact with short (<10 nm) flexible receptors immobilized in a lipid membrane or on other biologically relevant surfaces. The attachment kinetics of such particles may be limited globally by their diffusion toward a membrane or locally by diffusion around receptors. The detachment kinetics, also, can be limited by diffusion. Focusing on local diffusion limitations and using suitable approximations, we present expressions for the corresponding rate constants and identify their dependence on particle size and receptor length. We also illustrate features likely to be observed in such kinetics for particles (e.g., vesicles) with a substantial size distribution. The results obtained are generic and can be used to interpret a variety of situations. For example, we estimate upper values of virion attachment rate constants and clarify the likely effect of vesicle size distribution on previously observed non-exponential kinetics of vesicle detachment.
Endeward, Volker
2012-05-01
A mathematical model describing facilitation of O(2) diffusion by the diffusion of myoglobin and hemoglobin is presented. The equations are solved numerically by a finite-difference method for the conditions as they prevail in cardiac and skeletal muscle and in red cells without major simplifications. It is demonstrated that, in the range of intracellular diffusion distances, the degree of facilitation is limited by the rate of the chemical reaction between myglobin or hemoglobin and O(2). The results are presented in the form of relationships between the degree of facilitation and the length of the diffusion path on the basis of the known kinetics of the oxygenation-deoxygenation reactions. It is concluded that the limitation by reaction kinetics reduces the maximally possible facilitated oxygen diffusion in cardiomyoctes by ∼50% and in skeletal muscle fibers by ∼ 20%. For human red blood cells, a reduction of facilitated O(2) diffusion by 36% is obtained in agreement with previous reports. This indicates that, especially in cardiomyocytes and red cells, chemical equilibrium between myoglobin or hemoglobin and O(2) is far from being established, an assumption that previously has often been made. Although the "O(2) transport function" of myoglobin in cardiac muscle cells thus is severely limited by the chemical reaction kinetics, and to a lesser extent also in skeletal muscle, it is noteworthy that the speed of release of O(2) from MbO(2), the "storage function," is not limited by the reaction kinetics under physiological conditions.
Central limit theorems and suppression of anomalous diffusion for systems with symmetry
NASA Astrophysics Data System (ADS)
Gottwald, Georg A.; Melbourne, Ian
2016-10-01
We give general conditions for the central limit theorem and weak convergence to Brownian motion (the weak invariance principle/functional central limit theorem) to hold for observables of compact group extensions of nonuniformly expanding maps. In particular, our results include situations where the central limit theorem would fail, and anomalous behaviour would prevail, if the compact group were not present. This has important consequences for systems with noncompact Euclidean symmetry and provides the rigorous proof for a conjecture made in our paper: a Huygens principle for diffusion and anomalous diffusion in spatially extended systems. Gottwald and Melbourne (2013 Proc. Natl Acad. Sci. USA 110 8411-6).
Effects of C/O Ratio and Temperature on Sooting Limits of Spherical Diffusion Flames
NASA Technical Reports Server (NTRS)
Lecoustre, V. R.; Sunderland, P. B.; Chao, B. H.; Urban, D. L.; Stocker, D. P.; Axelbaum, R. L.
2008-01-01
Limiting conditions for soot particle inception in spherical diffusion flames were investigated numerically. The flames were modeled using a one-dimensional, time accurate diffusion flame code with detailed chemistry and transport and an optically thick radiation model. Seventeen normal and inverse flames were considered, covering a wide range of stoichiometric mixture fraction, adiabatic flame temperature, residence time and scalar dissipation rate. These flames were previously observed to reach their sooting limits after 2 s of microgravity. Sooting-limit diffusion flames with scalar dissipation rate lower than 2/s were found to have temperatures near 1400 K where C/O = 0.51, whereas flames with greater scalar dissipation rate required increased temperatures. This finding was valid across a broad range of fuel and oxidizer compositions and convection directions.
An asymptotic-preserving scheme for linear kinetic equation with fractional diffusion limit
NASA Astrophysics Data System (ADS)
Wang, Li; Yan, Bokai
2016-05-01
We present a new asymptotic-preserving scheme for the linear Boltzmann equation which, under appropriate scaling, leads to a fractional diffusion limit. Our scheme rests on novel micro-macro decomposition to the distribution function, which splits the original kinetic equation following a reshuffled Hilbert expansion. As opposed to classical diffusion limit, a major difficulty comes from the fat tail in the equilibrium which makes the truncation in velocity space depending on the small parameter. Our idea is, while solving the macro-micro part in a truncated velocity domain (truncation only depends on numerical accuracy), to incorporate an integrated tail over the velocity space that is beyond the truncation, and its major component can be precomputed once with any accuracy. Such an addition is essential to drive the solution to the correct asymptotic limit. Numerical experiments validate its efficiency in both kinetic and fractional diffusive regimes.
NASA Astrophysics Data System (ADS)
Pink, David A.; Peyronel, Fernanda; Quinn, Bonnie; Singh, Pratham; Marangoni, Alejandro G.
2015-09-01
Understanding how solid fats structures come about in edible oils and quantifying their structures is of fundamental importance in developing edible oils with pre-selected characteristics. We considered the great range of fractal dimensions, from 1.91 to 2.90, reported from rheological measurements. We point out that, if the structures arise via DLA/RLA or DLCA/RLCA, as has been established using ultra small angle x-ray scattering (USAXS), we would expect fractal dimensions in the range ~1.7 to 2.1, and ~2.5 or ~3.0. We present new data for commercial fats and show that the fractal dimensions deduced lie outside these values. We have developed a model in which competition between two processes can lead to the range of fractal dimensions observed. The two processes are (i) the rate at which the solid fat particles are created as the temperature is decreased, and (ii) the rate at which these particles diffuse, thereby meeting and forming aggregates. We assumed that aggregation can take place essentially isotropically and we identified two characteristic times: a time characterizing the rate of creation of solid fats, {τ\\text{create}}(T)\\equiv 1/{{R}S}(T) , where {{R}S}(T) is the rate of solid condensation (cm3 s-1), and the diffusion time of solid fats, {τ\\text{diff}}≤ft(T,{{c}S}\\right)=< {{r}2}> /6{D}≤ft(T,{{c}S}\\right) , where {D}≤ft(T,{{c}S}\\right) is their diffusion coefficient and < {{r}2}> is the typical average distance that fats must move in order to aggregate. The intent of this model is to show that a simple process can lead to a wide range of fractal dimensions. We showed that in the limit of very fast solid creation, {τ\\text{create}}\\ll {τ\\text{diff}} the fractal dimension is predicted to be that of DLCA, ~1.7, relaxing to that of RLCA, 2.0-2.1, and that in the limit of very slow solid creation, {τ\\text{create}}\\gg {τ\\text{diff}} , the fractal dimension is predicted to be that obtained via DLA, ~2.5, relaxing to that of RLA, 3
Models of diffusion-limited uptake of trace elements in fossils and rates of fossilization
NASA Astrophysics Data System (ADS)
Kohn, Matthew J.
2008-08-01
Many fossils are assumed to take up trace elements by a process of combined diffusion plus adsorption (DA), yet in principle composition profiles can be explained by several different diffusion-limited processes, including diffusion plus reaction or recrystallization (DR) and double-medium diffusion (DMD). The DA and DMD models are supported by REE and U composition profiles across fossil teeth, measured by laser-ablation ICP-MS, that show error-function - like diffusion profiles into enamel from the dentine-enamel interface and concentrations in the interior of enamel that are at original biogenic levels or higher. Published composition and age profiles in some Pleistocene bones may be better explained by a DR model. All three diffusion models imply linear behavior between age and distance squared, vastly simplifying U-series dating methods for Pleistocene fossils. Modeled uptake rates for fossil teeth yield a strict minimum bound on durations of about one decade to one century. The similarity of diffusion profiles in teeth, irrespective of depositional ages ranging from ˜30 ka to >30 Ma, implies that uptake occurred quickly, with a maximum duration of a few tens of kyr for typical fossil enamel; faster uptake is implied for typical fossil bone and dentine. Disparities in these uptake estimates compared to some archeological bone may reflect sampling and preservation bias for paleontological vs. archeological materials.
Underdamped scaled Brownian motion: (non-)existence of the overdamped limit in anomalous diffusion
NASA Astrophysics Data System (ADS)
Bodrova, Anna S.; Chechkin, Aleksei V.; Cherstvy, Andrey G.; Safdari, Hadiseh; Sokolov, Igor M.; Metzler, Ralf
2016-07-01
It is quite generally assumed that the overdamped Langevin equation provides a quantitative description of the dynamics of a classical Brownian particle in the long time limit. We establish and investigate a paradigm anomalous diffusion process governed by an underdamped Langevin equation with an explicit time dependence of the system temperature and thus the diffusion and damping coefficients. We show that for this underdamped scaled Brownian motion (UDSBM) the overdamped limit fails to describe the long time behaviour of the system and may practically even not exist at all for a certain range of the parameter values. Thus persistent inertial effects play a non-negligible role even at significantly long times. From this study a general questions on the applicability of the overdamped limit to describe the long time motion of an anomalously diffusing particle arises, with profound consequences for the relevance of overdamped anomalous diffusion models. We elucidate our results in view of analytical and simulations results for the anomalous diffusion of particles in free cooling granular gases.
Underdamped scaled Brownian motion: (non-)existence of the overdamped limit in anomalous diffusion.
Bodrova, Anna S; Chechkin, Aleksei V; Cherstvy, Andrey G; Safdari, Hadiseh; Sokolov, Igor M; Metzler, Ralf
2016-01-01
It is quite generally assumed that the overdamped Langevin equation provides a quantitative description of the dynamics of a classical Brownian particle in the long time limit. We establish and investigate a paradigm anomalous diffusion process governed by an underdamped Langevin equation with an explicit time dependence of the system temperature and thus the diffusion and damping coefficients. We show that for this underdamped scaled Brownian motion (UDSBM) the overdamped limit fails to describe the long time behaviour of the system and may practically even not exist at all for a certain range of the parameter values. Thus persistent inertial effects play a non-negligible role even at significantly long times. From this study a general questions on the applicability of the overdamped limit to describe the long time motion of an anomalously diffusing particle arises, with profound consequences for the relevance of overdamped anomalous diffusion models. We elucidate our results in view of analytical and simulations results for the anomalous diffusion of particles in free cooling granular gases. PMID:27462008
Underdamped scaled Brownian motion: (non-)existence of the overdamped limit in anomalous diffusion
Bodrova, Anna S.; Chechkin, Aleksei V.; Cherstvy, Andrey G.; Safdari, Hadiseh; Sokolov, Igor M.; Metzler, Ralf
2016-01-01
It is quite generally assumed that the overdamped Langevin equation provides a quantitative description of the dynamics of a classical Brownian particle in the long time limit. We establish and investigate a paradigm anomalous diffusion process governed by an underdamped Langevin equation with an explicit time dependence of the system temperature and thus the diffusion and damping coefficients. We show that for this underdamped scaled Brownian motion (UDSBM) the overdamped limit fails to describe the long time behaviour of the system and may practically even not exist at all for a certain range of the parameter values. Thus persistent inertial effects play a non-negligible role even at significantly long times. From this study a general questions on the applicability of the overdamped limit to describe the long time motion of an anomalously diffusing particle arises, with profound consequences for the relevance of overdamped anomalous diffusion models. We elucidate our results in view of analytical and simulations results for the anomalous diffusion of particles in free cooling granular gases. PMID:27462008
Aggregation Limits Surface Expression of Homomeric GluA3 Receptors.
Coleman, Sarah K; Hou, Ying; Willibald, Marina; Semenov, Artur; Möykkynen, Tommi; Keinänen, Kari
2016-04-15
AMPA receptors are glutamate-gated cation channels assembled from GluA1-4 subunits and have properties that are strongly dependent on the subunit composition. The subunits have different propensities to form homomeric or various heteromeric receptors expressed on cell surface, but the underlying mechanisms are still poorly understood. Here, we examined the biochemical basis for the poor ability of GluA3 subunits to form homomeric receptors, linked previously to two amino acid residues, Tyr-454 and Arg-461, in its ligand binding domain (LBD). Surface expression of GluA3 was improved by co-assembly with GluA2 but not with stargazin, a trafficking chaperone and modulator of AMPA receptors. The secretion efficiency of GluA2 and GluA3 LBDs paralleled the transport difference between the respective full-length receptors and was similarly dependent on Tyr-454/Arg-461 but not on LBD stability. In comparison to GluA2, GluA3 homomeric receptors showed a strong and Tyr-454/Arg-461-dependent tendency to aggregate both in the macroscopic scale measured as lower solubility in nonionic detergent and in the microscopic scale evident as the preponderance of hydrodynamically large structures in density gradient centrifugation and native gel electrophoresis. We conclude that the impaired surface expression of homomeric GluA3 receptors is caused by nonproductive assembly and aggregation to which LBD residues Tyr-454 and Arg-461 strongly contribute. This aggregation inhibits the entry of newly synthesized GluA3 receptors to the secretory pathway. PMID:26912664
Yang, Qing-Song; Shen, Guo-Chun; Liu, He-Ming; Wang, Zhang-Hua; Ma, Zun-Ping; Fang, Xiao-Feng; Zhang, Jian; Wang, Xi-Hua
2016-01-01
The pervasive pattern of aggregated tree distributions in natural communities is commonly explained by the joint effect of two clustering processes: environmental filtering and dispersal limitation, yet little consensus remains on the relative importance of the two clustering processes on tree aggregations. Different life stages of examined species were thought to be one possible explanation of this disagreement, because the effect of environmental filtering and dispersal limitation are expected to increase and decrease with tree life stages, respectively. However, few studies have explicitly tested these expectations. In this study, we evaluated these expectations by three different methods (species-habitat association test based on Poisson Clustering model and spatial point pattern analyses based on Heterogeneous Poisson model and the jointly modeling approach) using 36 species in a 20-ha subtropical forest plot. Our results showed that the percentage of species with significant habitat association increased with life stages, and there were fewer species affected by dispersal limitation in later life stages compared with those in earlier stages. Percentage of variance explained by the environmental filtering and dispersal limitation also increases and decreases with life stages. These results provided a promising alternative explanation on the existing mixed results about the relative importance of the two clustering processes. These findings also highlighted the importance of plant life stages for fully understanding species distributions and species coexistence. PMID:27227538
Liu, He-Ming; Wang, Zhang-Hua; Ma, Zun-Ping; Fang, Xiao-Feng; Zhang, Jian; Wang, Xi-Hua
2016-01-01
The pervasive pattern of aggregated tree distributions in natural communities is commonly explained by the joint effect of two clustering processes: environmental filtering and dispersal limitation, yet little consensus remains on the relative importance of the two clustering processes on tree aggregations. Different life stages of examined species were thought to be one possible explanation of this disagreement, because the effect of environmental filtering and dispersal limitation are expected to increase and decrease with tree life stages, respectively. However, few studies have explicitly tested these expectations. In this study, we evaluated these expectations by three different methods (species-habitat association test based on Poisson Clustering model and spatial point pattern analyses based on Heterogeneous Poisson model and the jointly modeling approach) using 36 species in a 20-ha subtropical forest plot. Our results showed that the percentage of species with significant habitat association increased with life stages, and there were fewer species affected by dispersal limitation in later life stages compared with those in earlier stages. Percentage of variance explained by the environmental filtering and dispersal limitation also increases and decreases with life stages. These results provided a promising alternative explanation on the existing mixed results about the relative importance of the two clustering processes. These findings also highlighted the importance of plant life stages for fully understanding species distributions and species coexistence. PMID:27227538
Yang, Qing-Song; Shen, Guo-Chun; Liu, He-Ming; Wang, Zhang-Hua; Ma, Zun-Ping; Fang, Xiao-Feng; Zhang, Jian; Wang, Xi-Hua
2016-01-01
The pervasive pattern of aggregated tree distributions in natural communities is commonly explained by the joint effect of two clustering processes: environmental filtering and dispersal limitation, yet little consensus remains on the relative importance of the two clustering processes on tree aggregations. Different life stages of examined species were thought to be one possible explanation of this disagreement, because the effect of environmental filtering and dispersal limitation are expected to increase and decrease with tree life stages, respectively. However, few studies have explicitly tested these expectations. In this study, we evaluated these expectations by three different methods (species-habitat association test based on Poisson Clustering model and spatial point pattern analyses based on Heterogeneous Poisson model and the jointly modeling approach) using 36 species in a 20-ha subtropical forest plot. Our results showed that the percentage of species with significant habitat association increased with life stages, and there were fewer species affected by dispersal limitation in later life stages compared with those in earlier stages. Percentage of variance explained by the environmental filtering and dispersal limitation also increases and decreases with life stages. These results provided a promising alternative explanation on the existing mixed results about the relative importance of the two clustering processes. These findings also highlighted the importance of plant life stages for fully understanding species distributions and species coexistence.
The effect of receptor clustering on diffusion-limited forward rate constants.
Goldstein, B; Wiegel, F W
1983-01-01
The effect of receptor clustering on the diffusion-limited forward rate constant (k+) is studied theoretically by modeling cell surface receptors by hemispheres distributed on a plane. We give both exact results and bounds. The exact results are obtained using an electrostatic analogue and applying the method of the images. Accurate upper bounds on k+ are found from a variational principle. PMID:6309261
Asymptotic current-voltage relations for currents exceeding the diffusion limit
NASA Astrophysics Data System (ADS)
Yariv, Ehud
2009-11-01
We consider the one-dimensional transport of ions into a perm-selective solid. Direct attempts to evaluate the current-voltage characteristics for currents exceeding the diffusion limit are frustrated by the appearance of nonconverging integrals. We describe how to overcome this obstacle using a regularization scheme.
Mathematical model of diffusion-limited evolution of multiple gas bubbles in tissue.
Srinivasan, R Srini; Gerth, Wayne A; Powell, Michael R
2003-04-01
Models of gas bubble dynamics employed in probabilistic analyses of decompression sickness incidence in man must be theoretically consistent and simple, if they are to yield useful results without requiring excessive computations. They are generally formulated in terms of ordinary differential equations that describe diffusion-limited gas exchange between a gas bubble and the extravascular tissue surrounding it. In our previous model (Ann. Biomed. Eng. 30: 232-246, 2002), we showed that with appropriate representation of sink pressures to account for gas loss or gain due to heterogeneous blood perfusion in the unstirred diffusion region around the bubble, diffusion-limited bubble growth in a tissue of finite volume can be simulated without postulating a boundary layer across which gas flux is discontinuous. However, interactions between two or more bubbles caused by competition for available gas cannot be considered in this model, because the diffusion region has a fixed volume with zero gas flux at its outer boundary. The present work extends the previous model to accommodate interactions among multiple bubbles by allowing the diffusion region volume of each bubble to vary during bubble evolution. For given decompression and tissue volume, bubble growth is sustained only if the bubble number density is below a certain maximum.
Mathematical model of diffusion-limited evolution of multiple gas bubbles in tissue
NASA Technical Reports Server (NTRS)
Srinivasan, R. Srini; Gerth, Wayne A.; Powell, Michael R.
2003-01-01
Models of gas bubble dynamics employed in probabilistic analyses of decompression sickness incidence in man must be theoretically consistent and simple, if they are to yield useful results without requiring excessive computations. They are generally formulated in terms of ordinary differential equations that describe diffusion-limited gas exchange between a gas bubble and the extravascular tissue surrounding it. In our previous model (Ann. Biomed. Eng. 30: 232-246, 2002), we showed that with appropriate representation of sink pressures to account for gas loss or gain due to heterogeneous blood perfusion in the unstirred diffusion region around the bubble, diffusion-limited bubble growth in a tissue of finite volume can be simulated without postulating a boundary layer across which gas flux is discontinuous. However, interactions between two or more bubbles caused by competition for available gas cannot be considered in this model, because the diffusion region has a fixed volume with zero gas flux at its outer boundary. The present work extends the previous model to accommodate interactions among multiple bubbles by allowing the diffusion region volume of each bubble to vary during bubble evolution. For given decompression and tissue volume, bubble growth is sustained only if the bubble number density is below a certain maximum.
Shear-Limited Diffusion of Test Particles in Pure Ion Plasmas.
NASA Astrophysics Data System (ADS)
Anderegg, F.; Driscoll, C. F.; Dubin, D. H. E.
2001-10-01
Measurements of test-particle diffusion in pure ion plasmas show 2D enhancements over the 3D rates, limited by shear in the plasma rotation ωE (r). The diffusion is due to ``long-range'' ion-ion collisions in the quiescent, steady-state Mg^+ plasma. For short plasma length Lp and low shear ω_E^' ≡ partial ωE / partial r, thermal ions bounce axially many times before shear separates them in θ, so the ions may move in (r, θ ) as bounce averaged ``rods'' of charge (i.e. 2D point vortices). Experimentally, we vary the number of bounces over the range 0.2 <= Nb ≡ ( barv / 2 Lp ) / r ω_E^' <= 10,000. For long plasmas with Nb <= 1, we observe diffusion in quantitative agreement with the 3D theory of long-range E × B drift collisions.(F. Anderegg et al.), Phys. Rev. Lett. 78, 2128 (1997). For shorter plasmas or lower shear, with Nb > 1, we measure diffusion rates enhanced by approximately N_b. For exceedingly small shear, i.e. Nb >= 1000, we observe diffusion rates consistent with the Taylor-McNamara estimates for a shear-free plasma. Overall, the data shows fair agreement with Dubin's new theory of 2D diffusion in shear.(D.H.E. Dubin and D.Z. Jin, Phys. Lett. A 284), 112-117 (2001).
Limiting diffusion current at rotating disk electrode with dense particle layer.
Weroński, P; Nosek, M; Batys, P
2013-09-28
Exploiting the concept of diffusion permeability of multilayer gel membrane and porous multilayer we have derived a simple analytical equation for the limiting diffusion current at rotating disk electrode (RDE) covered by a thin layer with variable tortuosity and porosity, under the assumption of negligible convection in the porous film. The variation of limiting diffusion current with the porosity and tortuosity of the film can be described in terms of the equivalent thickness of stagnant solution layer, i.e., the average ratio of squared tortuosity to porosity. In case of monolayer of monodisperse spherical particles, the equivalent layer thickness is an algebraic function of the surface coverage. Thus, by means of cyclic voltammetry of RDE with a deposited particle monolayer we can determine the monolayer surface coverage. The effect of particle layer adsorbed on the surface of RDE increases non-linearly with surface coverage. We have tested our theoretical results experimentally by means of cyclic voltammetry measurements of limiting diffusion current at the glassy carbon RDE covered with a monolayer of 3 μm silica particles. The theoretical and experimental results are in a good agreement at the surface coverage higher than 0.7. This result suggests that convection in a monolayer of 3 μm monodisperse spherical particles is negligibly small, in the context of the coverage determination, in the range of very dense particle layers. PMID:24089793
Microwave extinction characteristics of nanoparticle aggregates
NASA Astrophysics Data System (ADS)
Wu, Y. P.; Cheng, J. X.; Liu, X. X.; Wang, H. X.; Zhao, F. T.; Wen, W. W.
2016-07-01
Structure of nanoparticle aggregates plays an important role in microwave extinction capacity. The diffusion-limited aggregation model (DLA) for fractal growth is utilized to explore the possible structures of nanoparticle aggregates by computer simulation. Based on the discrete dipole approximation (DDA) method, the microwave extinction performance by different nano-carborundum aggregates is numerically analyzed. The effects of the particle quantity, original diameter, fractal structure, as well as orientation on microwave extinction are investigated, and also the extinction characteristics of aggregates are compared with the spherical nanoparticle in the same volume. Numerical results give out that proper aggregation of nanoparticle is beneficial to microwave extinction capacity, and the microwave extinction cross section by aggregated granules is better than that of the spherical solid one in the same volume.
Limitations and Prospects for Diffusion-Weighted MRI of the Prostate
Bourne, Roger; Panagiotaki, Eleftheria
2016-01-01
Diffusion-weighted imaging (DWI) is the most effective component of the modern multi-parametric magnetic resonance imaging (mpMRI) scan for prostate pathology. DWI provides the strongest prediction of cancer volume, and the apparent diffusion coefficient (ADC) correlates moderately with Gleason grade. Notwithstanding the demonstrated cancer assessment value of DWI, the standard measurement and signal analysis methods are based on a model of water diffusion dynamics that is well known to be invalid in human tissue. This review describes the biophysical limitations of the DWI component of the current standard mpMRI protocol and the potential for significantly improved cancer assessment performance based on more sophisticated measurement and signal modeling techniques. PMID:27240408
Boron-enhanced-diffusion of boron: The limiting factor for ultra-shallow junctions
Agarwal, A. |; Eaglesham, D.J.; Gossmann, H.J.; Pelaz, L.; Herner, S.B.; Jacobson, D.C.; Haynes, T.E.; Erokhin, Y.; Simonton, R.
1997-12-01
Reducing implant energy is an effective way to eliminate transient enhanced diffusion (TED) due to excess interstitials from the implant. It is shown that TED from a fixed Si dose implanted at energies from 0.5 to 20 keV into boron doping-superlattices decreases linearly with decreasing Si ion range, virtually disappearing at sub-keV energies. However, for sub-keV B implants diffusion remains enhanced and x{sub j} is limited to {ge} 100 nm at 1,050 C. The authors term this enhancement, which arises in the presence of B atomic concentrations at the surface of {approx} 6%, Boron-Enhanced-Diffusion (BED).
Ye, Chuyang; Murano, Emi; Stone, Maureen; Prince, Jerry L
2015-10-01
The tongue is a critical organ for a variety of functions, including swallowing, respiration, and speech. It contains intrinsic and extrinsic muscles that play an important role in changing its shape and position. Diffusion tensor imaging (DTI) has been used to reconstruct tongue muscle fiber tracts. However, previous studies have been unable to reconstruct the crossing fibers that occur where the tongue muscles interdigitate, which is a large percentage of the tongue volume. To resolve crossing fibers, multi-tensor models on DTI and more advanced imaging modalities, such as high angular resolution diffusion imaging (HARDI) and diffusion spectrum imaging (DSI), have been proposed. However, because of the involuntary nature of swallowing, there is insufficient time to acquire a sufficient number of diffusion gradient directions to resolve crossing fibers while the in vivo tongue is in a fixed position. In this work, we address the challenge of distinguishing interdigitated tongue muscles from limited diffusion magnetic resonance imaging by using a multi-tensor model with a fixed tensor basis and incorporating prior directional knowledge. The prior directional knowledge provides information on likely fiber directions at each voxel, and is computed with anatomical knowledge of tongue muscles. The fiber directions are estimated within a maximum a posteriori (MAP) framework, and the resulting objective function is solved using a noise-aware weighted ℓ1-norm minimization algorithm. Experiments were performed on a digital crossing phantom and in vivo tongue diffusion data including three control subjects and four patients with glossectomies. On the digital phantom, effects of parameters, noise, and prior direction accuracy were studied, and parameter settings for real data were determined. The results on the in vivo data demonstrate that the proposed method is able to resolve interdigitated tongue muscles with limited gradient directions. The distributions of the
Mathematical model of diffusion-limited gas bubble dynamics in unstirred tissue with finite volume.
Srinivasan, R Srini; Gerth, Wayne A; Powell, Michael R
2002-02-01
Models of gas bubble dynamics for studying decompression sickness have been developed by considering the bubble to be immersed in an extravascular tissue with diffusion-limited gas exchange between the bubble and the surrounding unstirred tissue. In previous versions of this two-region model, the tissue volume must be theoretically infinite, which renders the model inapplicable to analysis of bubble growth in a finite-sized tissue. We herein present a new two-region model that is applicable to problems involving finite tissue volumes. By introducing radial deviations to gas tension in the diffusion region surrounding the bubble, the concentration gradient can be zero at a finite distance from the bubble, thus limiting the tissue volume that participates in bubble-tissue gas exchange. It is shown that these deviations account for the effects of heterogeneous perfusion on gas bubble dynamics, and are required for the tissue volume to be finite. The bubble growth results from a difference between the bubble gas pressure and an average gas tension in the surrounding diffusion region that explicitly depends on gas uptake and release by the bubble. For any given decompression, the diffusion region volume must stay above a certain minimum in order to sustain bubble growth.
Mathematical model of diffusion-limited gas bubble dynamics in unstirred tissue with finite volume
NASA Technical Reports Server (NTRS)
Srinivasan, R. Srini; Gerth, Wayne A.; Powell, Michael R.
2002-01-01
Models of gas bubble dynamics for studying decompression sickness have been developed by considering the bubble to be immersed in an extravascular tissue with diffusion-limited gas exchange between the bubble and the surrounding unstirred tissue. In previous versions of this two-region model, the tissue volume must be theoretically infinite, which renders the model inapplicable to analysis of bubble growth in a finite-sized tissue. We herein present a new two-region model that is applicable to problems involving finite tissue volumes. By introducing radial deviations to gas tension in the diffusion region surrounding the bubble, the concentration gradient can be zero at a finite distance from the bubble, thus limiting the tissue volume that participates in bubble-tissue gas exchange. It is shown that these deviations account for the effects of heterogeneous perfusion on gas bubble dynamics, and are required for the tissue volume to be finite. The bubble growth results from a difference between the bubble gas pressure and an average gas tension in the surrounding diffusion region that explicitly depends on gas uptake and release by the bubble. For any given decompression, the diffusion region volume must stay above a certain minimum in order to sustain bubble growth.
NASA Astrophysics Data System (ADS)
Smedley-Stevenson, Richard P.; McClarren, Ryan G.
2015-04-01
This paper attempts to unify the asymptotic diffusion limit analysis of thermal radiation transport schemes, for a linear-discontinuous representation of the material temperature reconstructed from cell centred temperature unknowns, in a process known as 'source tilting'. The asymptotic limits of both Monte Carlo (continuous in space) and deterministic approaches (based on linear-discontinuous finite elements) for solving the transport equation are investigated in slab geometry. The resulting discrete diffusion equations are found to have nonphysical terms that are proportional to any cell-edge discontinuity in the temperature representation. Based on this analysis it is possible to design accurate schemes for representing the material temperature, for coupling thermal radiation transport codes to a cell centred representation of internal energy favoured by ALE (arbitrary Lagrange-Eulerian) hydrodynamics schemes.
Smedley-Stevenson, Richard P.; McClarren, Ryan G.
2015-04-01
This paper attempts to unify the asymptotic diffusion limit analysis of thermal radiation transport schemes, for a linear-discontinuous representation of the material temperature reconstructed from cell centred temperature unknowns, in a process known as ‘source tilting’. The asymptotic limits of both Monte Carlo (continuous in space) and deterministic approaches (based on linear-discontinuous finite elements) for solving the transport equation are investigated in slab geometry. The resulting discrete diffusion equations are found to have nonphysical terms that are proportional to any cell-edge discontinuity in the temperature representation. Based on this analysis it is possible to design accurate schemes for representing the material temperature, for coupling thermal radiation transport codes to a cell centred representation of internal energy favoured by ALE (arbitrary Lagrange–Eulerian) hydrodynamics schemes.
Self-similar fast-reaction limits for reaction-diffusion systems on unbounded domains
NASA Astrophysics Data System (ADS)
Crooks, E. C. M.; Hilhorst, D.
2016-08-01
We present a unified approach to characterising fast-reaction limits of systems of either two reaction-diffusion equations, or one reaction-diffusion equation and one ordinary differential equation, on unbounded domains, motivated by models of fast chemical reactions where either one or both reactant(s) is/are mobile. For appropriate initial data, solutions of four classes of problems each converge in the fast-reaction limit k → ∞ to a self-similar limit profile that has one of four forms, depending on how many components diffuse and whether the spatial domain is a half or whole line. For fixed k, long-time convergence to these same self-similar profiles is also established, thanks to a scaling argument of Kamin. Our results generalise earlier work of Hilhorst, van der Hout and Peletier to a much wider class of problems, and provide a quantitative description of the penetration of one substance into another in both the fast-reaction and long-time regimes.
Hertel, Tobias; Himmelein, Sabine; Ackermann, Thomas; Stich, Dominik; Crochet, Jared
2010-12-28
Photoluminescence quantum yields and nonradiative decay of the excitonic S(1) state in length fractionated (6,5) single-wall carbon nanotubes (SWNTs) are studied by continuous wave and time-resolved fluorescence spectroscopy. The experimental data are modeled by diffusion limited contact quenching of excitons at stationary quenching sites including tube ends. A combined analysis of the time-resolved photoluminescence decay and the length dependence of photoluminescence quantum yields (PL QYs) from SWNTs in sodium cholate suspensions allows to determine the exciton diffusion coefficient D = 10.7 ± 0.4 cm(2)s(-1) and lifetime τ(PL) for long tubes of 20 ± 1 ps. PL quantum yields Φ(PL) are found to scale with the inverse diffusion coefficient and the square of the mean quenching site distance, here l(d) = 120 ± 25 nm. The results suggest that low PL QYs of SWNTs are due to the combination of high-diffusive exciton mobility with the presence of only a few quenching sites.
Nano Vacancy Clusters and Trap Limited Diffusion of Si Interstitials in Silicon
Prof. Wei-Kan Chu
2010-05-05
consecutive steps. (a) First, high energy self ion irradiation is used to create a wide vacancy-rich region, and to form voids by post implantation annealing. (b) In an additional annealing step in oxygen ambient, Si interstitials are injected in by surface oxidation. (c) Analyzing trap-limited diffusion of Si interstitials, which is experimentally detectable by studying the diffusion of multiple boron superlattices grown in Si, and enables us to characterize the nano voids, e.g. their sizes and densities.
Pushing the limits of in vivo diffusion MRI for the Human Connectome Project.
Setsompop, K; Kimmlingen, R; Eberlein, E; Witzel, T; Cohen-Adad, J; McNab, J A; Keil, B; Tisdall, M D; Hoecht, P; Dietz, P; Cauley, S F; Tountcheva, V; Matschl, V; Lenz, V H; Heberlein, K; Potthast, A; Thein, H; Van Horn, J; Toga, A; Schmitt, F; Lehne, D; Rosen, B R; Wedeen, V; Wald, L L
2013-10-15
Perhaps more than any other "-omics" endeavor, the accuracy and level of detail obtained from mapping the major connection pathways in the living human brain with diffusion MRI depend on the capabilities of the imaging technology used. The current tools are remarkable; allowing the formation of an "image" of the water diffusion probability distribution in regions of complex crossing fibers at each of half a million voxels in the brain. Nonetheless our ability to map the connection pathways is limited by the image sensitivity and resolution, and also the contrast and resolution in encoding of the diffusion probability distribution. The goal of our Human Connectome Project (HCP) is to address these limiting factors by re-engineering the scanner from the ground up to optimize the high b-value, high angular resolution diffusion imaging needed for sensitive and accurate mapping of the brain's structural connections. Our efforts were directed based on the relative contributions of each scanner component. The gradient subsection was a major focus since gradient amplitude is central to determining the diffusion contrast, the amount of T2 signal loss, and the blurring of the water PDF over the course of the diffusion time. By implementing a novel 4-port drive geometry and optimizing size and linearity for the brain, we demonstrate a whole-body sized scanner with G(max) = 300 mT/m on each axis capable of the sustained duty cycle needed for diffusion imaging. The system is capable of slewing the gradient at a rate of 200 T/m/s as needed for the EPI image encoding. In order to enhance the efficiency of the diffusion sequence we implemented a FOV shifting approach to Simultaneous MultiSlice (SMS) EPI capable of unaliasing 3 slices excited simultaneously with a modest g-factor penalty allowing us to diffusion encode whole brain volumes with low TR and TE. Finally we combine the multi-slice approach with a compressive sampling reconstruction to sufficiently undersample q-space to
Diffuse reflectance optical topography: location of inclusions in 3D and detectability limits
Carbone, N. A.; Baez, G. R.; García, H. A.; Waks Serra, M. V.; Di Rocco, H. O.; Iriarte, D. I.; Pomarico, J. A.; Grosenick, D.; Macdonald, R.
2014-01-01
In the present contribution we investigate the images of CW diffusely reflected light for a point-like source, registered by a CCD camera imaging a turbid medium containing an absorbing lesion. We show that detection of μa variations (absorption anomalies) is achieved if images are normalized to background intensity. A theoretical analysis based on the diffusion approximation is presented to investigate the sensitivity and the limitations of our proposal and a novel procedure to find the location of the inclusions in 3D is given and tested. An analysis of the noise and its influence on the detection capabilities of our proposal is provided. Experimental results on phantoms are also given, supporting the proposed approach. PMID:24876999
Theoretical limit of spatial resolution in diffuse optical tomography using a perturbation model
Konovalov, A B; Vlasov, V V
2014-03-28
We have assessed the limit of spatial resolution of timedomain diffuse optical tomography (DOT) based on a perturbation reconstruction model. From the viewpoint of the structure reconstruction accuracy, three different approaches to solving the inverse DOT problem are compared. The first approach involves reconstruction of diffuse tomograms from straight lines, the second – from average curvilinear trajectories of photons and the third – from total banana-shaped distributions of photon trajectories. In order to obtain estimates of resolution, we have derived analytical expressions for the point spread function and modulation transfer function, as well as have performed a numerical experiment on reconstruction of rectangular scattering objects with circular absorbing inhomogeneities. It is shown that in passing from reconstruction from straight lines to reconstruction using distributions of photon trajectories we can improve resolution by almost an order of magnitude and exceed the accuracy of reconstruction of multi-step algorithms used in DOT. (optical tomography)
The Significance of Diffusion Limitation for Oxygen Isotope Fractionation in Soil Respiration
NASA Astrophysics Data System (ADS)
Angert, A.; Luz, B.
2001-05-01
Oxygen concentration and its isotopic composition have been monitored in light and heavy soils. Steep oxygen gradients were present at the heavy soil site (minimal O2 concentration was 1% at 150cm depth) and delta O18 values typically ranged from 0 to -1.6 permil relative to atmospheric oxygen. In the light-soil site, the O2 concentration was 20.38% to 20.53% and delta O18 values ranged from -0.06 to 0.06 permil relative to atmospheric O2. The fractionation in soil respiration was estimated from the observed [O2] and delta O18 profiles and their change with time by a one-box analytical model and a five-box numerical model. Diffusion due to concentration and temperature gradients was taken into account. Good agreement was found between the two models, and between the model results and the measured values. The Discrimination against O18 during O2 uptake ranged from 11% to 14%, with an averaged value of 12 permil. Low discrimination (8.4-16.9 permil) was also determined in closed-system soil incubation experiments. The current understanding of the composition of air O2 attributes the magnitude of the fractionation in soil respiration to biochemical mechanisms alone. Thus the discrimination against O18 is assumed to be 18 permil in cyanide-sensitive dark respiration and 25 to 30 permil in cyanide-resistant respiration. The discrimination we report is significantly less than in dark respiration. This overall low discrimination is explained by slow diffusion in soil aggregates, and in root tissues that result in low O2 concentration in the consumption site. Since soil respiration is a major component of the global oxygen uptake this new discrimination factor should be taken into account in models of the isotopic composition of atmospheric O2.
Wagner, K; Hempel, D C
1988-04-20
Naphthalene-2-sulfonate was degraded by submerse growing Pseudomonads in a chemostat culture. The kinetic parameters for the Monod equation, including Pirts maintenance energy, were calculated from these experiments regarding naphthalene-2-sulfonate as substrate and oxygene as cosubstrate. By immobilizing the bacteria on sand particles, the degradation of naphthalene-2-sulfonate was carried out in a specialy designed three-phase airlift-loop reactor in a completely fluidized state. From these experiments, the influence of biofilm diffusion limitation on reaction kinetics and criteria for stable biofilm formation on sand particles were obtained.
Development of the new approach to the diffusion-limited reaction rate theory
Veshchunov, M. S.
2012-04-15
The new approach to the diffusion-limited reaction rate theory, recently proposed by the author, is further developed on the base of a similar approach to Brownian coagulation. The traditional diffusion approach to calculation of the reaction rate is critically analyzed. In particular, it is shown that the traditional approach is applicable only in the special case of reactions with a large reaction radius and the mean inter-particle distances, and become inappropriate in calculating the reaction rate in the case of a relatively small reaction radius. In the latter case, most important for chemical reactions, particle collisions occur not in the diffusion regime but mainly in the kinetic regime characterized by homogeneous (random) spatial distribution of particles on the length scale of the mean inter-particle distance. The calculated reaction rate for a small reaction radius in three dimensions formally (and fortuitously) coincides with the expression derived in the traditional approach for reactions with a large reaction radius, but notably deviates at large times from the traditional result in the planar two-dimensional geometry. In application to reactions on discrete lattice sites, new relations for the reaction rate constants are derived for both three-dimensional and two-dimensional lattices.
Lattice Boltzmann method for diffusion-limited partial dissolution of fluids
NASA Astrophysics Data System (ADS)
Aursjø, Olav; Pride, Steven R.
2015-07-01
A lattice Boltzmann model for two partially miscible fluids is developed. By partially miscible we mean that, although there is a definite interfacial region separating the two fluids with a surface tension force acting at all points of the transition region, each fluid can nonetheless accept molecules from the other fluid up to a set solubility limit. We allow each fluid to diffuse into the other with the solubility and diffusivity in each fluid being input parameters. The approach is to define two regions within the fluid: one interfacial region having finite width, across which most of the concentration change occurs, and in which a surface tension force and color separation step are allowed for and one miscible fluid region where the concentration of the binary fluids follows an advection-diffusion equation and the mixture as a whole obeys the Navier-Stokes incompressible flow equations. Numerical examples are presented in which the algorithm produces results that are quantitatively compared to exact analytical results as well as qualitatively examined for their reasonableness. The model has the ability to simulate how bubbles of one fluid flow through another while dissolving their contents as well as to simulate a range of practical invasion problems such as injecting supercritical CO2 into a porous material saturated with water for sequestration purposes.
Srinivasan, R S; Gerth, W A; Powell, M R
2000-10-01
The three-region model of gas bubble dynamics consists of a bubble and a well-stirred tissue region with an intervening unperfused diffusion region previously assumed to have constant thickness and uniform gas diffusivity. As a result, the diffusion region gas content remains unchanged as its volume increases with bubble growth, causing dissolved gas in the region to violate Henry's law. Earlier work also neglected the relationship between the varying diffusion region volume and the fixed total tissue volume. The present work corrects these theoretical inconsistencies by postulating a difference in gas diffusivity between an infinitesimally thin layer at the bubble surface and the remainder of the diffusion region, thus allowing both thickness and gas content of the diffusion region to vary during bubble evolution. The corrected model can yield bubble lifetimes considerably longer than those yielded by earlier three-region models, and meets a need for theoretically consistent but relatively simple bubble dynamics models for use in studies of decompression sickness (DCS) in human subjects.
Upper limit on the diffuse flux of ultrahigh energy tau neutrinos from the Pierre Auger Observatory.
Abraham, J; Abreu, P; Aglietta, M; Aguirre, C; Allard, D; Allekotte, I; Allen, J; Allison, P; Alvarez-Muñiz, J; Ambrosio, M; Anchordoqui, L; Andringa, S; Anzalone, A; Aramo, C; Argirò, S; Arisaka, K; Armengaud, E; Arneodo, F; Arqueros, F; Asch, T; Asorey, H; Assis, P; Atulugama, B S; Aublin, J; Ave, M; Avila, G; Bäcker, T; Badagnani, D; Barbosa, A F; Barnhill, D; Barroso, S L C; Bauleo, P; Beatty, J J; Beau, T; Becker, B R; Becker, K H; Bellido, J A; BenZvi, S; Berat, C; Bergmann, T; Bernardini, P; Bertou, X; Biermann, P L; Billoir, P; Blanch-Bigas, O; Blanco, F; Blasi, P; Bleve, C; Blümer, H; Bohácová, M; Bonifazi, C; Bonino, R; Boratav, M; Brack, J; Brogueira, P; Brown, W C; Buchholz, P; Bueno, A; Burton, R E; Busca, N G; Caballero-Mora, K S; Cai, B; Camin, D V; Caramete, L; Caruso, R; Carvalho, W; Castellina, A; Catalano, O; Cataldi, G; Cazon, L; Cester, R; Chauvin, J; Chiavassa, A; Chinellato, J A; Chou, A; Chye, J; Clark, P D J; Clay, R W; Colombo, E; Conceição, R; Connolly, B; Contreras, F; Coppens, J; Cordier, A; Cotti, U; Coutu, S; Covault, C E; Creusot, A; Criss, A; Cronin, J; Curutiu, A; Dagoret-Campagne, S; Daumiller, K; Dawson, B R; de Almeida, R M; De Donato, C; de Jong, S J; De La Vega, G; de Mello Junior, W J M; de Mello Neto, J R T; DeMitri, I; de Souza, V; del Peral, L; Deligny, O; Della Selva, A; Delle Fratte, C; Dembinski, H; Di Giulio, C; Diaz, J C; Dobrigkeit, C; D'Olivo, J C; Dornic, D; Dorofeev, A; dos Anjos, J C; Dova, M T; D'Urso, D; Dutan, I; DuVernois, M A; Engel, R; Epele, L; Erdmann, M; Escobar, C O; Etchegoyen, A; Facal San Luis, P; Falcke, H; Farrar, G; Fauth, A C; Fazzini, N; Ferrer, F; Ferry, S; Fick, B; Filevich, A; Filipcic, A; Fleck, I; Fonte, R; Fracchiolla, C E; Fulgione, W; García, B; García Gámez, D; Garcia-Pinto, D; Garrido, X; Geenen, H; Gelmini, G; Gemmeke, H; Ghia, P L; Giller, M; Glass, H; Gold, M S; Golup, G; Gomez Albarracin, F; Gómez Berisso, M; Gómez Herrero, R; Gonçalves, P; Gonçalves do Amaral, M; Gonzalez, D; Gonzalez, J G; González, M; Góra, D; Gorgi, A; Gouffon, P; Grassi, V; Grillo, A F; Grunfeld, C; Guardincerri, Y; Guarino, F; Guedes, G P; Gutiérrez, J; Hague, J D; Hamilton, J C; Hansen, P; Harari, D; Harmsma, S; Harton, J L; Haungs, A; Hauschildt, T; Healy, M D; Hebbeker, T; Hebrero, G; Heck, D; Hojvat, C; Holmes, V C; Homola, P; Hörandel, J; Horneffer, A; Horvat, M; Hrabovský, M; Huege, T; Hussain, M; Iarlori, M; Insolia, A; Ionita, F; Italiano, A; Kaducak, M; Kampert, K H; Karova, T; Kégl, B; Keilhauer, B; Kemp, E; Kieckhafer, R M; Klages, H O; Kleifges, M; Kleinfeller, J; Knapik, R; Knapp, J; Koang, D-H; Krieger, A; Krömer, O; Kuempel, D; Kunka, N; Kusenko, A; La Rosa, G; Lachaud, C; Lago, B L; Lebrun, D; Lebrun, P; Lee, J; Leigui de Oliveira, M A; Letessier-Selvon, A; Leuthold, M; Lhenry-Yvon, I; López, R; Lopez Agüera, A; Lozano Bahilo, J; Luna García, R; Maccarone, M C; Macolino, C; Maldera, S; Mancarella, G; Manceñido, M E; Mandat, D; Mantsch, P; Mariazzi, A G; Maris, I C; Marquez Falcon, H R; Martello, D; Martínez, J; Martínez Bravo, O; Mathes, H J; Matthews, J; Matthews, J A J; Matthiae, G; Maurizio, D; Mazur, P O; McCauley, T; McEwen, M; McNeil, R R; Medina, M C; Medina-Tanco, G; Meli, A; Melo, D; Menichetti, E; Menschikov, A; Meurer, Chr; Meyhandan, R; Micheletti, M I; Miele, G; Miller, W; Mollerach, S; Monasor, M; Monnier Ragaigne, D; Montanet, F; Morales, B; Morello, C; Moreno, J C; Morris, C; Mostafá, M; Muller, M A; Mussa, R; Navarra, G; Navarro, J L; Navas, S; Necesal, P; Nellen, L; Newman-Holmes, C; Newton, D; Nguyen Thi, T; Nierstenhoefer, N; Nitz, D; Nosek, D; Nozka, L; Oehlschläger, J; Ohnuki, T; Olinto, A; Olmos-Gilbaja, V M; Ortiz, M; Ortolani, F; Ostapchenko, S; Otero, L; Pacheco, N; Pakk Selmi-Dei, D; Palatka, M; Pallotta, J; Parente, G; Parizot, E; Parlati, S; Pastor, S; Patel, M; Paul, T; Pavlidou, V; Payet, K; Pech, M; Pekala, J; Pelayo, R; Pepe, I M; Perrone, L; Petrera, S; Petrinca, P; Petrov, Y; Pham Ngoc, Diep; Pham Ngoc, Dong; Pham Thi, T N; Pichel, A; Piegaia, R; Pierog, T; Pimenta, M; Pinto, T; Pirronello, V; Pisanti, O; Platino, M; Pochon, J; Privitera, P; Prouza, M; Quel, E J; Rautenberg, J; Redondo, A; Reucroft, S; Revenu, B; Rezende, F A S; Ridky, J; Riggi, S; Risse, M; Rivière, C; Rizi, V; Roberts, M; Robledo, C; Rodriguez, G; Rodríguez Frías, D; Rodriguez Martino, J; Rodriguez Rojo, J; Rodriguez-Cabo, I; Ros, G; Rosado, J; Roth, M; Rouillé-d'Orfeuil, B; Roulet, E; Rovero, A C; Salamida, F; Salazar, H; Salina, G; Sánchez, F; Santander, M; Santo, C E; Santos, E M; Sarazin, F; Sarkar, S; Sato, R; Scherini, V; Schieler, H; Schmidt, A; Schmidt, F; Schmidt, T; Scholten, O; Schovánek, P; Schüssler, F; Sciutto, S J; Scuderi, M; Segreto, A; Semikoz, D; Settimo, M; Shellard, R C; Sidelnik, I; Siffert, B B; Sigl, G; Smetniansky De Grande, N; Smiałkowski, A; Smída, R; Smith, A G K; Smith, B E; Snow, G R; Sokolsky, P; Sommers, P; Sorokin, J; Spinka, H; Squartini, R; Strazzeri, E; Stutz, A; Suarez, F; Suomijärvi, T; Supanitsky, A D; Sutherland, M S; Swain, J; Szadkowski, Z; Takahashi, J; Tamashiro, A; Tamburro, A; Taşcău, O; Tcaciuc, R; Thomas, D; Ticona, R; Tiffenberg, J; Timmermans, C; Tkaczyk, W; Todero Peixoto, C J; Tomé, B; Tonachini, A; Torres, I; Torresi, D; Travnicek, P; Tripathi, A; Tristram, G; Tscherniakhovski, D; Tueros, M; Tunnicliffe, V; Ulrich, R; Unger, M; Urban, M; Valdés Galicia, J F; Valiño, I; Valore, L; van den Berg, A M; van Elewyck, V; Vázquez, R A; Veberic, D; Veiga, A; Velarde, A; Venters, T; Verzi, V; Videla, M; Villaseñor, L; Vorobiov, S; Voyvodic, L; Wahlberg, H; Wainberg, O; Walker, P; Warner, D; Watson, A A; Westerhoff, S; Wieczorek, G; Wiencke, L; Wilczyńska, B; Wilczyński, H; Wileman, C; Winnick, M G; Wu, H; Wundheiler, B; Yamamoto, T; Younk, P; Zas, E; Zavrtanik, D; Zavrtanik, M; Zech, A; Zepeda, A; Ziolkowski, M
2008-05-30
The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from nu(tau) charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of nu(tau) at EeV energies. Assuming an E(nu)(-2) differential energy spectrum the limit set at 90% C.L. is E(nu)(2)dN(nu)(tau)/dE(nu)<1.3 x 10(-7) GeV cm(-2) s(-1) sr(-1) in the energy range 2 x 10(17) eV< E(nu)< 2 x 10(19) eV.
Operating limit study for the proposed solid waste landfill at Paducah Gaseous Diffusion Plant
Lee, D.W.; Wang, J.C.; Kocher, D.C.
1995-06-01
A proposed solid waste landfill at Paducah Gaseous Diffusion Plant (PGDP) would accept wastes generated during normal operations that are identified as non-radioactive. These wastes may include small amounts of radioactive material from incidental contamination during plant operations. A site-specific analysis of the new solid waste landfill is presented to determine a proposed operating limit that will allow for waste disposal operations to occur such that protection of public health and the environment from the presence of incidentally contaminated waste materials can be assured. Performance objectives for disposal were defined from existing regulatory guidance to establish reasonable dose limits for protection of public health and the environment. Waste concentration limits were determined consistent with these performance objectives for the protection of off-site individuals and inadvertent intruders who might be directly exposed to disposed wastes. Exposures of off-site individuals were estimated using a conservative, site-specific model of the groundwater transport of contamination from the wastes. Direct intrusion was analyzed using an agricultural homesteader scenario. The most limiting concentrations from direct intrusion or groundwater transport were used to establish the concentration limits for radionuclides likely to be present in PGDP wastes.
Alveolar-Membrane Diffusing Capacity Limits Performance in Boston Marathon Qualifiers
Lavin, Kaleen M.; Straub, Allison M.; Uhranowsky, Kathleen A.; Smoliga, James M.; Zavorsky, Gerald S.
2012-01-01
Purpose (1) to examine the relation between pulmonary diffusing capacity and marathon finishing time, and (2), to evaluate the accuracy of pulmonary diffusing capacity for nitric oxide (DLNO) in predicting marathon finishing time relative to that of pulmonary diffusing capacity for carbon monoxide (DLCO). Methods 28 runners [18 males, age = 37 (SD 9) years, body mass = 70 (13) kg, height = 173 (9) cm, percent body fat = 17 (7) %] completed a test battery consisting of measurement of DLNO and DLCO at rest, and a graded exercise test to determine running economy and aerobic capacity prior to the 2011 Steamtown Marathon (Scranton, PA). One to three weeks later, all runners completed the marathon (range: 2∶22:38 to 4∶48:55). Linear regressions determined the relation between finishing time and a variety of anthropometric characteristics, resting lung function variables, and exercise parameters. Results In runners meeting Boston Marathon qualification standards, 74% of the variance in marathon finishing time was accounted for by differences in DLNO relative to body surface area (BSA) (SEE = 11.8 min, p<0.01); however, the relation between DLNO or DLCO to finishing time was non-significant in the non-qualifiers (p = 0.14 to 0.46). Whereas both DLCO and DLNO were predictive of finishing time for all finishers, DLNO showed a stronger relation (r2 = 0.30, SEE = 33.4 min, p<0.01) compared to DLCO when considering BSA. Conclusion DLNO is a performance-limiting factor in only Boston qualifiers. This suggests that alveolar-capillary membrane conductance is a limitation to performance in faster marathoners. Additionally, DLNO/BSA predicts marathon finishing time and aerobic capacity more accurately than DLCO. PMID:22984520
Ma, Yun; Hoepker, Alexander C.; Gupta, Lekha; Faggin, Marc F.; Collum, David B.
2010-01-01
Lithium diisopropylamide (LDA) in tetrahydrofuran at −78 °C undergoes 1,4-addition to an unsaturated ester via a rate-limiting deaggregation of LDA dimer followed by a post-rate-limiting reaction with the substrate. Muted autocatalysis is traced to a lithium enolate-mediated deaggregation of the LDA dimer and the intervention of LDA-lithium enolate mixed aggregates displaying higher reactivities than LDA. Striking accelerations are elicited by <1.0 mol % LiCl. Rate and mechanistic studies reveal that the uncatalyzed and catalyzed pathways funnel through a common monosolvated-monomer-based intermediate. Four distinct classes of mixed aggregation effects are discussed. PMID:20961095
Mechanism of Suppression of Protein Aggregation by α-Crystallin
Markossian, Kira A.; Yudin, Igor K.; Kurganov, Boris I.
2009-01-01
This review summarizes experimental data illuminating the mechanism of suppression of heat-induced protein aggregation by α-crystallin, one of the small heat shock proteins. The dynamic light scattering data show that the initial stage of thermal aggregation of proteins is the formation of the initial aggregates involving hundreds of molecules of the denatured protein. Further sticking of the starting aggregates proceeds in a regime of diffusion-limited cluster-cluster aggregation. The protective effect of α-crystallin is due to transition of the aggregation process to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower than unity. PMID:19399251
Universal diffusion-limited injection and the hook effect in organic thin-film transistors
NASA Astrophysics Data System (ADS)
Liu, Chuan; Huseynova, Gunel; Xu, Yong; Long, Dang Xuan; Park, Won-Tae; Liu, Xuying; Minari, Takeo; Noh, Yong-Young
2016-07-01
The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials.
Gillani, Nabeel; Yasseri, Taha; Eynon, Rebecca; Hjorth, Isis
2014-01-01
Massive Open Online Courses (MOOCs) bring together a global crowd of thousands of learners for several weeks or months. In theory, the openness and scale of MOOCs can promote iterative dialogue that facilitates group cognition and knowledge construction. Using data from two successive instances of a popular business strategy MOOC, we filter observed communication patterns to arrive at the "significant" interaction networks between learners and use complex network analysis to explore the vulnerability and information diffusion potential of the discussion forums. We find that different discussion topics and pedagogical practices promote varying levels of 1) "significant" peer-to-peer engagement, 2) participant inclusiveness in dialogue, and ultimately, 3) modularity, which impacts information diffusion to prevent a truly "global" exchange of knowledge and learning. These results indicate the structural limitations of large-scale crowd-based learning and highlight the different ways that learners in MOOCs leverage, and learn within, social contexts. We conclude by exploring how these insights may inspire new developments in online education. PMID:25244925
Gillani, Nabeel; Yasseri, Taha; Eynon, Rebecca; Hjorth, Isis
2014-09-23
Massive Open Online Courses (MOOCs) bring together a global crowd of thousands of learners for several weeks or months. In theory, the openness and scale of MOOCs can promote iterative dialogue that facilitates group cognition and knowledge construction. Using data from two successive instances of a popular business strategy MOOC, we filter observed communication patterns to arrive at the "significant" interaction networks between learners and use complex network analysis to explore the vulnerability and information diffusion potential of the discussion forums. We find that different discussion topics and pedagogical practices promote varying levels of 1) "significant" peer-to-peer engagement, 2) participant inclusiveness in dialogue, and ultimately, 3) modularity, which impacts information diffusion to prevent a truly "global" exchange of knowledge and learning. These results indicate the structural limitations of large-scale crowd-based learning and highlight the different ways that learners in MOOCs leverage, and learn within, social contexts. We conclude by exploring how these insights may inspire new developments in online education.
NASA Astrophysics Data System (ADS)
Gillani, Nabeel; Yasseri, Taha; Eynon, Rebecca; Hjorth, Isis
2014-09-01
Massive Open Online Courses (MOOCs) bring together a global crowd of thousands of learners for several weeks or months. In theory, the openness and scale of MOOCs can promote iterative dialogue that facilitates group cognition and knowledge construction. Using data from two successive instances of a popular business strategy MOOC, we filter observed communication patterns to arrive at the ``significant'' interaction networks between learners and use complex network analysis to explore the vulnerability and information diffusion potential of the discussion forums. We find that different discussion topics and pedagogical practices promote varying levels of 1) ``significant'' peer-to-peer engagement, 2) participant inclusiveness in dialogue, and ultimately, 3) modularity, which impacts information diffusion to prevent a truly ``global'' exchange of knowledge and learning. These results indicate the structural limitations of large-scale crowd-based learning and highlight the different ways that learners in MOOCs leverage, and learn within, social contexts. We conclude by exploring how these insights may inspire new developments in online education.
Universal diffusion-limited injection and the hook effect in organic thin-film transistors
Liu, Chuan; Huseynova, Gunel; Xu, Yong; Long, Dang Xuan; Park, Won-Tae; Liu, Xuying; Minari, Takeo; Noh, Yong-Young
2016-01-01
The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials. PMID:27440253
Forecasting sales of new vehicle with limited data using Bass diffusion model and Grey theory
NASA Astrophysics Data System (ADS)
Abu, Noratikah; Ismail, Zuhaimy
2015-02-01
New product forecasting is a process that determines a reasonable estimate of sales attainable under a given set of conditions. There are several new products forecasting method in practices and Bass Diffusion Model (BDM) is one of the most common new product diffusion model used in many industries to forecast new product and technology. Hence, this paper proposed a combining BDM with Grey theory to forecast sales of new vehicle in Malaysia that certainly have limited data to build a model on. The aims of this paper is to examine the accuracy of different new product forecasting models and thus identify which is the best among the basic BDM and combining BDM with Grey theory. The results show that combining BDM with Grey theory performs better than the basic BDM based on in-sample and out-sample mean absolute percentage error (MAPE). Results also reveals combining model forecast more effectively and accurately even with insufficient previous data on the new vehicle in Malaysia.
Universal diffusion-limited injection and the hook effect in organic thin-film transistors.
Liu, Chuan; Huseynova, Gunel; Xu, Yong; Long, Dang Xuan; Park, Won-Tae; Liu, Xuying; Minari, Takeo; Noh, Yong-Young
2016-01-01
The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials. PMID:27440253
Kinetic effects of toluene blending on the extinction limit of n-decane diffusion flames
Won, Sang Hee; Sun, Wenting; Ju, Yiguang
2010-03-15
The impact of toluene addition in n-decane on OH concentrations, maximum heat release rates, and extinction limits were studied experimentally and computationally by using counterflow diffusion flames with laser induced fluorescence imaging. Sensitivity analyses of kinetic path ways and species transport on flame extinction were also conducted. The results showed that the extinction strain rate of n-decane/toluene/nitrogen flames decreased significantly with an increase of toluene addition and depended linearly on the maximum OH concentration. It was revealed that the maximum OH concentration, which depends on the fuel H/C ratio, can be used as an index of the radical pool and chemical heat release rate, since it plays a significant role on the heat production via the reaction with other species, such as CO, H{sub 2}, and HCO. Experimental results further demonstrated that toluene addition in n-decane dramatically reduced the peak OH concentration via H abstraction reactions and accelerated flame extinction via kinetic coupling between toluene and n-decane mechanisms. Comparisons between experiments and simulations revealed that the current toluene mechanism significantly over-predicts the radical destruction rate, leading to under-prediction of extinction limits and OH concentrations, especially caused by the uncertainty of the H abstraction reaction from toluene, which rate coefficient has a difference by a factor of 5 in the tested toluene models. In addition, sensitivity analysis of diffusive transport showed that in addition to n-decane and toluene, the transport of OH and H also considerably affects the extinction limit. A reduced linear correlation between the extinction limits of n-decane/toluene blended fuels and the H/C ratio as well as the mean fuel molecular weight was obtained. The results suggest that an explicit prediction of the extinction limits of aromatic and alkane blended fuels can be established by using H/C ratio (or radical index) and the
The nonlinear diffusion limit for generalized Carleman models: the initial-boundary value problem
NASA Astrophysics Data System (ADS)
Golse, François; Salvarani, Francesco
2007-04-01
Consider the initial-boundary value problem for the 2-speed Carleman model of the Boltzmann equation of the kinetic theory of gases, (see Carleman 1957 Problèmes Mathématiques Dans la Théorie Cinétique des Gaz (Uppsala: Almqvist-Wiksells)), set in some bounded interval with boundary conditions prescribing the density of particles entering the interval. Under the usual parabolic scaling, a nonlinear diffusion limit is established for this problem. In fact, the techniques presented here allow treatment generalizations of the Carleman system where the collision frequency is proportional to the αth power of the macroscopic density, with α ∈ [-1, 1].
Mass fractionation of noble gases in diffusion-limited hydrodynamic hydrogen escape
NASA Technical Reports Server (NTRS)
Zahnle, Kevin; Pollack, James B.; Kasting, James F.
1990-01-01
The theory of mass fractionation by hydrogen is presently extended to atmospheres in which hydrogen is not the major constituent. This theoretical framework is applied to three different cases. In the first, it is shown that the fractionation of terrestrial atmospheric neon with respect to mantle neon is explainable as a consequence of diffusion-limited hydrogen escape from a steam atmosphere toward the end of the accretion process. In the second, the anomalously high Ar-38/Ar-36 ratio of Mars is shown to be due to hydrodynamic fractionation by a vigorously escaping and very pure hydrogen wind. In the last case, it is speculated that the currently high Martian D/H ratio emerged during the hydrodynamic escape phase which fractionated Ar.
Gambini, Andrés; De Stefano, Adrian; Bevacqua, Romina Jimena; Karlanian, Florencia; Salamone, Daniel Felipe
2014-01-01
Embryo aggregation has been demonstrated to improve cloning efficiency in mammals. However, since no more than three embryos have been used for aggregation, the effect of using a larger number of cloned zygotes is unknown. Therefore, the goal of the present study was to determine whether increased numbers of cloned aggregated zygotes results in improved in vitro and in vivo embryo development in the equine. Zona-free reconstructed embryos (ZFRE's) were cultured in the well of the well system in four different experimental groups: I. 1x, only one ZFRE per microwell; II. 3x, three per microwell; III. 4x, four per microwell; and IV. 5x, five ZFRE's per microwell. Embryo size was measured on day 7, after which blastocysts from each experimental group were either a) maintained in culture from day 8 until day 16 to follow their growth rates, b) fixed to measure DNA fragmentation using the TUNEL assay, or c) transferred to synchronized mares. A higher blastocyst rate was observed on day 7 in the 4x group than in the 5x group. Non-aggregated embryos were smaller on day 8 compared to those aggregated, but from then on the in vitro growth was not different among experimental groups. Apoptotic cells averaged 10% of total cells of day 8 blastocysts, independently of embryo aggregation. Only pregnancies resulting from the aggregation of up to four embryos per microwell went beyond the fifth month of gestation, and two of these pregnancies, derived from experimental groups 3x and 4x, resulted in live cloned foals. In summary, we showed that the in vitro and in vivo development of cloned zona-free embryos improved until the aggregation of four zygotes and declined when five reconstructed zygotes were aggregated. PMID:25396418
Salgado-García, R; Maldonado, Cesar
2013-12-01
We study the diffusion of an ensemble of overdamped particles sliding over a tilted random potential (produced by the interaction of a particle with a random polymer) with long-range correlations. We found that the diffusion properties of such a system are closely related to the correlation function of the corresponding potential. We model the substrate as a symbolic trajectory of a shift space which enables us to obtain a general formula for the diffusion coefficient when normal diffusion occurs. The total time that the particle takes to travel through n monomers can be seen as an ergodic sum to which we can apply the central limit theorem. The latter can be implemented if the correlations decay fast enough in order for the central limit theorem to be valid. On the other hand, we presume that when the central limit theorem breaks down the system give rise to anomalous diffusion. We give two examples exhibiting a transition from normal to anomalous diffusion due to this mechanism. We also give analytical expressions for the diffusion exponents in both cases by assuming convergence to a stable law. Finally we test our predictions by means of numerical simulations.
Aggregates and Superaggregates of Soot with Four Distinct Fractal Morphologies
NASA Technical Reports Server (NTRS)
Sorensen, C. M.; Kim, W.; Fry, D.; Chakrabarti, A.
2004-01-01
Soot formed in laminar diffusion flames of heavily sooting fuels evolves through four distinct growth stages which give rise to four distinct aggregate fractal morphologies. These results were inferred from large and small angle static light scattering from the flames, microphotography of the flames, and analysis of soot sampled from the flames. The growth stages occur approximately over four successive orders of magnitude in aggregate size. Comparison to computer simulations suggests that these four growth stages involve either diffusion limited cluster aggregation or percolation in either three or two dimensions.
Near-Limit Flamelet Phenomena in Buoyant Low Stretch Diffusion Flames Beneath a Solid Fuel
NASA Technical Reports Server (NTRS)
Olson, S. L.; Tien, J. S.
2000-01-01
A unique near-limit low stretch multidimensional stable flamelet phenomena has been observed for the first time which extends the material flammability limit beyond the one-dimensional low stretch flammability limit to lower burning rates and higher relative heat losses than is possible with uniform flame coverage. During low stretch experiments burning the underside of very large radii (greater than or = 75 cm stretch rate less than or = 3/s) cylindrical cast PMMA samples, multidimensional flamelets were observed, in contrast with a one-dimensional flame that was found to blanket the surface for smaller radii samples ( higher stretch rate). Flamelets were observed by decreasing the stretch rate or by increasing the conductive heat loss from the flame. Flamelets are defined as flames that cover only part of the burning sample at any given time, but persist for many minutes. Flamelet phenomena is viewed as the flame's method of enhancing oxygen flow to the flame, through oxygen transport into the edges of the flamelet. Flamelets form as heat losses (surface radiation and solid-phase conduction) become large relative to the weakened heat release of the low stretch flame. While heat loss rates remain fairly constant, the limiting factor in the heat release of the flame is hypothesized to be the oxygen transport to the flame in this low stretch (low convective) environment. Flamelet extinction is frequently caused by encroachment of an adjacent flamelet. Large-scale whole-body flamelet oscillations at 1.2 - 1.95 Hz are noted prior to extinction of a flamelet. This oscillation is believed to be due a repeated process of excess fuel leakage through the dark channels between the flamelets, fuel premixing with slow incoming oxidizer, and subsequent rapid flame spread and retreat of the flamelet through the premixed layer. The oscillation frequency is driven by gas-phase diffusive time scales.
Brahma, Neil; Talbot, Jan B
2014-04-01
The aggregation rate and mechanism of 150 nm alumina particles in 1mM KNO3 with various additives used in chemical mechanical planarization of copper were investigated. The pH of each suspension was ∼8 such that the aggregation rate was slow enough to be measured and analyzed over ∼120 min. In general, an initial exponential growth was observed for most suspensions indicating reaction-limited aggregation. After aggregate sizes increase to >500 nm, the rate followed a power law suggesting diffusion-limited aggregation. Stability ratios and fractal dimension numbers were also calculated to further elucidate the aggregation mechanism. PMID:24491325
Upper Limit on the Diffuse Flux of Ultrahigh Energy Tau Neutrinos from the Pierre Auger Observatory
Abraham, J.; Garcia, B.; Otero, L.; Abreu, P.; Andringa, S.; Assis, P.; Brogueira, P.; Conceicao, R.; Goncalves, P.; Pimenta, M.; Santo, C. E.; Tome, B.; Aglietta, M.; Bonino, R.; Castellina, A.; Chiavassa, A.; Fulgione, W.; Gorgi, A.; Hauschildt, T.; Maldera, S.
2008-05-30
The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from {nu}{sub {tau}} charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of {nu}{sub {tau}} at EeV energies. Assuming an E{sub {nu}}{sup -2} differential energy spectrum the limit set at 90% C.L. is E{sub {nu}}{sup 2}dN{sub {nu}{sub {tau}}}/dE{sub {nu}}<1.3x10{sup -7} GeV cm{sup -2} s{sup -1} sr{sup -1} in the energy range 2x10{sup 17} eV
Effects of Radiative and Diffusive Transport Processes on Premixed Flames near Flammability Limits
NASA Technical Reports Server (NTRS)
Abbud-Madrid, Angel; Ronney, Paul D.
1991-01-01
A study of the mechanisms of flammability limits and the dynamics of flame extinguishment in premixed gas flames is described, a novel feature of which is the use of diluent gases having a wide range of radiative and diffusive transport properties. This feature enables an assessment of the importance of volumetric heat losses and Lewis number effects on these mechanisms. Additionally, effects of flame dynamics and flame front curvature are studied by employing spherically expanding flames obtained in a microgravity environment whereby natural convection is eliminated. New diagnostics include chamber pressure measurements and the first reported species concentration measurements in a microgravity combustion experiment. The limit mechanisms and extinguishment phenomena are found to be strongly influenced by the combined effects of radiant heat loss, Lewis number and flame curvature. Two new and as yet not well understood phenomena are reported: 'double flames' in rich H2-O2-CO2 mixtures and an 'inverse flammability region' in rich C3H8-O2-CO2 mixtures.
Upper limit on the diffuse flux of UHE tau neutrinos from the Pierre Auger Observatory
Collaboration, The Pierre Auger
2007-12-01
The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau-neutrinos {nu}{sub {tau}} that interact in the Earth's crust. Tau leptons from {tau}{sub {tau}} charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 is used to place an upper limit on the diffuse flux of {nu}{sub {tau}} at EeV energies. Assuming an E{sub {nu}}{sup -2} differential energy spectrum the limit set at 90 % C.L. is E{sub {nu}}{sup 2} dN{sub {nu}{sub {tau}}}/dE{sub {nu}} < 1.3 x 10{sup -7} GeV cm{sup -2} s{sup -1} sr{sup -1} in the energy range 2 x 10{sup 17} eV < E{sub {nu}} < 2 x 10{sup 19} eV.
Self-Similarity in Game-Locked Aggregation
NASA Astrophysics Data System (ADS)
Wang, Chao; Xiong, Wan-Ting; Wang, You-Gui
2012-12-01
A collective game is studied via agent-based modeling approach, where a group of adaptive learning players seek for their best positions on a vertical line. The movements of players are driven by benefits obtained from interactions. The game falls into an evolutionary stable state, at which aggregations of players on the line emerge. The pattern of these aggregates exhibits self-similarity at different scales with a fractal dimension of 0.58. The underlying mechanism of this aggregation is unique in that aggregates are resulted from mutual lock-in of players. This game-locked aggregation, in contrast with the diffusion limited aggregation, is applicable to a broader scope of aggregation processes.
Han, Lu; Liang, WanZhen; Zhao, Yi; Zhong, Xinxin
2014-06-07
The time-dependent wavepacket diffusive method [X. Zhong and Y. Zhao, J. Chem. Phys. 138, 014111 (2013)] is extended to investigate the energy relaxation and separation of a hot electron-hole pair in organic aggregates with incorporation of Coulomb interaction and electron-phonon coupling. The pair initial condition generated by laser pulse is represented by a Gaussian wavepacket with a central momentum. The results reveal that the hot electron energy relaxation is very well described by two rate processes with the fast rate much larger than the slow one, consistent with experimental observations, and an efficient electron-hole separation is accomplished accompanying the fast energy relaxation. Furthermore, although the extra energy indeed helps the separation by overcoming the Coulomb interaction, the width of initial wavepacket is much sensitive to the separation efficiency and the narrower wavepacket generates the more separated charges. This behavior may be useful to understand the experimental controversy of the hot carrier effect on charge separation.
A limit on the effect of rectified diffusion in volcanic systems
NASA Astrophysics Data System (ADS)
Ichihara, M.; Brodsky, E. E.
2006-01-01
Forced oscillations can push dissolved volatiles into bubbles by a process called rectified diffusion. In engineering applications, the pumping action of rectified diffusion makes bubbles grow. In the geosciences, rectified diffusion is a suggested mechanism to trigger volcanic eruptions with seismic waves generated by distant earthquakes. Previous geoscience studies adopted the engineering results and proposed that in a confined system like a magma chamber, rectified diffusion causes pressure increase rather than bubble growth. However, the volcanic application is fundamentally different than engineering applications in that solubility continually changes with increasing pressure in the confined system. Here we present the first self-consistent treatment of rectified diffusion in a confined system. Evolving solubility has a significant effect. The new solution demonstrates that previous work significantly overestimated the effect of rectified diffusion in magmatic systems. For reasonable seismic wave amplitudes, the pressure change is at the most 2 × 10-9 of its initial value.
TREM2-mediated early microglial response limits diffusion and toxicity of amyloid plaques
Wang, Yaming; Ulland, Tyler K.; Ulrich, Jason D.; Song, Wilbur; Tzaferis, John A.; Hole, Justin T.; Yuan, Peng; Mahan, Thomas E.; Shi, Yang; Gilfillan, Susan; Cella, Marina; Grutzendler, Jaime; DeMattos, Ronald B.; Cirrito, John R.; Holtzman, David M.
2016-01-01
Triggering receptor expressed on myeloid cells 2 (TREM2) is a microglial receptor that recognizes changes in the lipid microenvironment, which may occur during amyloid β (Aβ) accumulation and neuronal degeneration in Alzheimer’s disease (AD). Rare TREM2 variants that affect TREM2 function lead to an increased risk of developing AD. In murine models of AD, TREM2 deficiency prevents microglial clustering around Aβ deposits. However, the origin of myeloid cells surrounding amyloid and the impact of TREM2 on Aβ accumulation are a matter of debate. Using parabiosis, we found that amyloid-associated myeloid cells derive from brain-resident microglia rather than from recruitment of peripheral blood monocytes. To determine the impact of TREM2 deficiency on Aβ accumulation, we examined Aβ plaques in the 5XFAD model of AD at the onset of Aβ-related pathology. At this early time point, Aβ accumulation was similar in TREM2-deficient and -sufficient 5XFAD mice. However, in the absence of TREM2, Aβ plaques were not fully enclosed by microglia; they were more diffuse, less dense, and were associated with significantly greater neuritic damage. Thus, TREM2 protects from AD by enabling microglia to surround and alter Aβ plaque structure, thereby limiting neuritic damage. PMID:27091843
Sooting Limits Of Diffusion Flames With Oxygen-Enriched Air And Diluted Fuel
NASA Technical Reports Server (NTRS)
Sunderland, P. B.; Urban, D. L.; Stocker, D. P.; Chao, B. H.; Axelbaum, R. L.
2003-01-01
Oxygen-enhanced combustion permits certain benefits and flexibility that are not otherwise available in the design of practical combustors, as discussed by Baukal. The cost of pure and enriched oxygen has declined to the point that oxygen-enhanced combustion is preferable to combustion in air for many applications. Carbon sequestration is greatly facilitated by oxygen enrichment because nitrogen can be eliminated from the product stream. For example, when natural gas (or natural gas diluted with CO2) is burned in pure oxygen, the only significant products are water and CO2. Oxygen-enhanced combustion also has important implications for soot formation, as explored in this work. We propose that soot inception in nonpremixed flames requires a region where C/O ratio, temperature, and residence time are above certain critical values. Soot does not form at low temperatures, with the threshold in nonpremixed flames ranging from about 1250-1650 K, a temperature referred to here as the critical temperature for soot inception, Tc. Soot inception also can be suppressed when residence time is short (equivalently, when the strain rate in counterflow flames is high). Soot induction times of 0.8-15 ms were reported by Tesner and Shurupov for acetylene/nitrogen mixtures at 1473 K. Burner stabilized spherical microgravity flames are employed in this work for two main reasons. First, this configuration offers unrestricted control over convection direction. Second, in steady state these flames are strain-free and thus can yield intrinsic sooting limits in diffusion flames, similar to the way past work in premixed flames has provided intrinsic values of C/O ratio associated with soot inception limits.
Gómez-Cuervo, S; Hernández, J; Omil, F
2016-08-01
There is growing international concern about the increasing levels of greenhouse gases in the atmosphere, particularly CO2 and methane. The emissions of methane derived from human activities are associated with large flows and very low concentrations, such as those emitted from landfills and wastewater treatment plants, among others. The present work was focused on the biological methane degradation at diffuse concentrations (0.2% vv(-1)) in a conventional biofilter using a mixture of compost, perlite and bark chips as carrier. An extensive characterization of the process was carried out at long-term operation (250 days) in a fully monitored pilot plant, achieving stable conditions during the entire period. Operational parameters such as waterings, nitrogen addition and inlet loads and contact time influences were evaluated. Obtained results indicate that empty bed residence times within 4-8 min are crucial to maximize elimination rates. Waterings and the type of nitrogen supplied in the nutrient solution (ammonia or nitrate) have a strong impact on the biofilter performance. The better results compatible with a stable operation were achieved using nitrate, with elimination capacities up to 7.6 ± 1.1 g CH4 m(-3 )h(-1). The operation at low inlet concentrations (IC) implied that removal rates obtained were quite limited (ranging 3-8 g CH4 m(-3 )h(-1)); however, these results could be significantly increased (up to 20.6 g CH4 m(-3) h(-1)) at higher IC, which indicates that the mass transfer from the gas to the liquid layer surrounding the biofilm is a key limitation of the process.
Echavarria-Heras, Héctor; Leal-Ramirez, Cecilia; Castillo, Oscar
2014-05-22
Although the process of endocytosis of the low density lipoprotein (LDL) macromolecule and its receptor have been the subject of intense experimental research and modeling, there are still conflicting hypotheses and even conflicting data regarding the way receptors are transported to coated pits, the manner by which receptors are inserted before they aggregate in coated pits, and the display of receptors on the cell surface. At first it was considered that LDL receptors in human fibroblasts are inserted at random locations and then transported by diffusion toward coated pits. But experiments have not ruled out the possibility that the true rate of accumulation of LDL receptors in coated pits might be faster than predicted on the basis of pure diffusion and uniform reinsertion over the entire cell surface. It has been claimed that recycled LDL receptors are inserted preferentially in regions where coated pits form, with display occurring predominantly as groups of loosely associated units. Another mechanism that has been proposed by experimental cell biologists which might affect the accumulation of receptors in coated pits is a retrograde membrane flow. This is essentially linked to a polarized receptor insertion mode and also to the capping phenomenon, characterized by the formation of large patches of proteins that passively flow away from the regions of membrane exocytosis. In this contribution we calculate the mean travel time of LDL receptors to coated pits as determined by the ratio of flow strength to diffusion-coefficient, as well as by polarized-receptor insertion. We also project the resulting display of unbound receptors on the cell membrane. We found forms of polarized insertion that could potentially reduce the mean capture time of LDL receptors by coated pits which is controlled by diffusion and uniform insertion. Our results show that, in spite of its efficiency as a possible device for enhancement of the rate of receptor trapping, polarized
2014-01-01
Although the process of endocytosis of the low density lipoprotein (LDL) macromolecule and its receptor have been the subject of intense experimental research and modeling, there are still conflicting hypotheses and even conflicting data regarding the way receptors are transported to coated pits, the manner by which receptors are inserted before they aggregate in coated pits, and the display of receptors on the cell surface. At first it was considered that LDL receptors in human fibroblasts are inserted at random locations and then transported by diffusion toward coated pits. But experiments have not ruled out the possibility that the true rate of accumulation of LDL receptors in coated pits might be faster than predicted on the basis of pure diffusion and uniform reinsertion over the entire cell surface. It has been claimed that recycled LDL receptors are inserted preferentially in regions where coated pits form, with display occurring predominantly as groups of loosely associated units. Another mechanism that has been proposed by experimental cell biologists which might affect the accumulation of receptors in coated pits is a retrograde membrane flow. This is essentially linked to a polarized receptor insertion mode and also to the capping phenomenon, characterized by the formation of large patches of proteins that passively flow away from the regions of membrane exocytosis. In this contribution we calculate the mean travel time of LDL receptors to coated pits as determined by the ratio of flow strength to diffusion-coefficient, as well as by polarized-receptor insertion. We also project the resulting display of unbound receptors on the cell membrane. We found forms of polarized insertion that could potentially reduce the mean capture time of LDL receptors by coated pits which is controlled by diffusion and uniform insertion. Our results show that, in spite of its efficiency as a possible device for enhancement of the rate of receptor trapping, polarized
Understanding Limitations in the Determination of the Diffuse Galactic Gamma-ray Emission
Moskalenko, Igor V.; Digel, S.W.; Porter, T.A.; Reimer, O.; Strong, A.W.; /Garching, Max Planck Inst., MPE
2006-10-03
We discuss uncertainties and possible sources of errors associated with the determination of the diffuse Galactic {gamma}-ray emission using the EGRET data. Most of the issues will be relevant also in the GLAST era. The focus here is on issues that impact evaluation of dark matter annihilation signals against the diffuse {gamma}-ray emission of the Milky Way.
Scale Invariance and Dynamic Phase Transitions in Diffusion-Limited Reactions
NASA Astrophysics Data System (ADS)
Täuber, Uwe C.
Many systems that can be described in terms of diffusion-limited `chemical' reactions display non-equilibrium continuous transitions separating active from inactive, absorbing states, where stochastic fluctuations cease entirely. Their critical properties can be analyzed via a path-integral representation of the corresponding classical master equation, and the dynamical renormalization group. An overview over the ensuing universality classes in single-species processes is given, and generalizations to reactions with multiple particle species are discussed as well. The generic case is represented by the processes A <==> A + A, and A -> 0, which map onto Reggeon field theory with the critical exponents of directed percolation (DP). For branching and annihilating random walks (BARW) A -> (m+1) A and A + A -> 0, the mean-field rate equation predicts an active state only. Yet BARW with odd m display a DP transition for d ≤2. For even offspring number m, the particle number parity is conserved locally. Below dc ≈ 4/3, this leads to the emergence of an inactive phase that is characterized by the power laws of the pair annihilation process. The critical exponents at the transition are those of the `parity-conserving' (PC) universality class. For local processes without memory, competing pair or triplet annihilation and fission reactions k A -> (k - l) A, k A -> (k+m)A with k=2,3 appear to yield the only other universality classes not described by mean-field theory. In these reactions, site occupation number restrictions play a crucial role.
Observed Limits on Charge Exchange Contributions to the Diffuse X-Ray Background
NASA Technical Reports Server (NTRS)
Crowder, S. G.; Barger, K. A.; Brandl, D. E.; Eckart, M. E.; Galeazzi, M.; Kelley, R. L.; Kilbourne, C. A.; McCammon, D.; Pfendner, C. G.; Porter, F. S.; Rocks, L.; Szymkowiak, A. E.; Teplin, I. M.
2012-01-01
We present a high-resolution spectrum of the diffuse X-ray background from 0.1 to 1 keV for an approximately 1 sr region of the sky centered at l = 90 degrees b = +60 degrees using a 36 pixel array of microcalorimeters flown on a sounding rocket. With an energy resolution of 11 eV FWHM below 1 keV, the spectrum s observed line ratios help separate charge exchange contributions originating within the heliosphere from thermal emission of hot gas in the interstellar medium. The X-ray sensitivity below 1 keV was reduced by about a factor of four from contamination that occurred early in the flight, limiting the significance of the results. The observed centroid of helium-like O VII is 568 (sup +2 (sub -3) eV at 90% confidence. Since the centroid expected for thermal emission is 568.4 eV and for charge exchange is 564.2 eV, thermal emission appears to dominate for this line complex. The dominance of thermal emission is consistent with much of the high-latitude O VII emission originating in 2-3 x 10(exp 6) K gas in the Galactic halo. On the other hand, the observed ratio of C VI Lygamma to Lyalpha is 0.3 plus or minus 0.2. The expected ratios are 0.04 for thermal emission and 0.24 for charge exchange, indicating that charge exchange must contribute strongly to this line and therefore potentially to the rest of the ROSAT R12 band usually associated with 10(sup 6) K emission from the Local Hot Bubble. The limited statistics of this experiment and systematic uncertainties due to the contamination require only greater than 32% thermal emission for O VII and greater than 20% from charge exchange for C VI at the 90% confidence level. An experimental gold coating on the silicon substrate of the array greatly reduced extraneous signals induced on nearby pixels from cosmic rays passing through the substrate, reducing the triggered event rate by a factor of 15 from a previous flight of the instrument.
Brantley, P S
2006-08-08
The double spherical harmonics angular approximation in the lowest order, i.e. double P{sub 0} (DP{sub 0}), is developed for the solution of time-dependent non-equilibrium grey radiative transfer problems in planar geometry. Although the DP{sub 0} diffusion approximation is expected to be less accurate than the P{sub 1} diffusion approximation at and near thermodynamic equilibrium, the DP{sub 0} angular approximation can more accurately capture the complicated angular dependence near a non-equilibrium radiation wave front. In addition, the DP{sub 0} approximation should be more accurate in non-equilibrium optically thin regions where the positive and negative angular domains are largely decoupled. We develop an adaptive angular technique that locally uses either the DP{sub 0} or P{sub 1} flux-limited diffusion approximation depending on the degree to which the radiation and material fields are in thermodynamic equilibrium. Numerical results are presented for two test problems due to Su and Olson and to Ganapol and Pomraning for which semi-analytic transport solutions exist. These numerical results demonstrate that the adaptive P{sub 1}-DP{sub 0} diffusion approximation can yield improvements in accuracy over the standard P{sub 1} diffusion approximation, both without and with flux-limiting, for non-equilibrium grey radiative transfer.
Brantley, P S
2005-12-13
The double spherical harmonics angular approximation in the lowest order, i.e. double P{sub 0} (DP{sub 0}), is developed for the solution of time-dependent non-equilibrium grey radiative transfer problems in planar geometry. Although the DP{sub 0} diffusion approximation is expected to be less accurate than the P{sub 1} diffusion approximation at and near thermodynamic equilibrium, the DP{sub 0} angular approximation can more accurately capture the complicated angular dependence near a non-equilibrium radiation wave front. In addition, the DP{sub 0} approximation should be more accurate in non-equilibrium optically thin regions where the positive and negative angular domains are largely decoupled. We develop an adaptive angular technique that locally uses either the DP{sub 0} or P{sub 1} flux-limited diffusion approximation depending on the degree to which the radiation and material fields are in thermodynamic equilibrium. Numerical results are presented for two test problems due to Su and Olson and to Ganapol and Pomraning for which semi-analytic transport solutions exist. These numerical results demonstrate that the adaptive P{sub 1}-DP{sub 0} diffusion approximation can yield improvements in accuracy over the standard P{sub 1} diffusion approximation, both without and with flux-limiting, for non-equilibrium grey radiative transfer.
2010-01-01
Background The goal of physiologically based pharmacokinetics (PBPK) is to predict drug kinetics from an understanding of the organ/blood exchange. The standard approach is to assume that the organ is "flow limited" which means that the venous blood leaving the organ equilibrates with the well-stirred tissue compartment. Although this assumption is valid for most solutes, it has been shown to be incorrect for several very highly fat soluble compounds which appear to be "diffusion limited". This paper describes the physical basis of this adipose diffusion limitation and its quantitative dependence on the blood/water (Kbld-wat) and octanol/water (Kow) partition coefficient. Methods Experimental measurements of the time dependent rat blood and adipose concentration following either intravenous or oral input were used to estimate the "apparent" adipose perfusion rate (FA) assuming that the tissue is flow limited. It is shown that the ratio of FA to the anatomic perfusion rate (F) provides a measure of the diffusion limitation. A quantitative relationship between this diffusion limitation and Kbld-wat and Kow is derived. This analysis was applied to previously published data, including the Oberg et. al. measurements of the rat plasma and adipose tissue concentration following an oral dose of a mixture of 13 different polychlorinated biphenyls. Results Solutes become diffusion limited at values of log Kow greater than about 5.6, with the adipose-blood exchange rate reduced by a factor of about 30 for a solute with a log Kow of 7.36. Quantitatively, a plot of FA/F versus Kow is well described assuming an adipose permeability-surface area product (PS) of 750/min. This PS corresponds to a 0.14 micron aqueous layer separating the well-stirred blood from the adipose lipid. This is approximately equal to the thickness of the rat adipose capillary endothelium. Conclusions These results can be used to quantitate the adipose-blood diffusion limitation as a function of Kow. This
Eddy diffusion coefficients and their upper limits based on application of the similarity theory
NASA Astrophysics Data System (ADS)
Vlasov, M. N.; Kelley, M. C.
2015-07-01
The equation for the diffusion velocity in the mesosphere and the lower thermosphere (MLT) includes the terms for molecular and eddy diffusion. These terms are very similar. For the first time, we show that, by using the similarity theory, the same formula can be obtained for the eddy diffusion coefficient as the commonly used formula derived by Weinstock (1981). The latter was obtained by taking, as a basis, the integral function for diffusion derived by Taylor (1921) and the three-dimensional Kolmogorov kinetic energy spectrum. The exact identity of both formulas means that the eddy diffusion and heat transport coefficients used in the equations, both for diffusion and thermal conductivity, must meet a criterion that restricts the outer eddy scale to being much less than the scale height of the atmosphere. This requirement is the same as the requirement that the free path of molecules must be much smaller than the scale height of the atmosphere. A further result of this criterion is that the eddy diffusion coefficients Ked, inferred from measurements of energy dissipation rates, cannot exceed the maximum value of 3.2 × 106 cm2 s-1 for the maximum value of the energy dissipation rate of 2 W kg-1 measured in the mesosphere and the lower thermosphere (MLT). This means that eddy diffusion coefficients larger than the maximum value correspond to eddies with outer scales so large that it is impossible to use these coefficients in eddy diffusion and eddy heat transport equations. The application of this criterion to the different experimental data shows that some reported eddy diffusion coefficients do not meet this criterion. For example, the large values of these coefficients (1 × 107 cm2 s-1) estimated in the Turbulent Oxygen Mixing Experiment (TOMEX) do not correspond to this criterion. The Ked values inferred at high latitudes by Lübken (1997) meet this criterion for summer and winter polar data, but the Ked values for summer at low latitudes are larger than the
Fractal dimension and mechanism of aggregation of apple juice particles.
Benítez, E I; Lozano, J E; Genovese, D B
2010-04-01
Turbidity of freshly squeezed apple juice is produced by a polydisperse suspension of particles coming from the cellular tissue. After precipitation of coarse particles by gravity, only fine-colloidal particles remain in suspension. Aggregation of colloidal particles leads to the formation of fractal structures. The fractal dimension is a measure of the internal density of these aggregates and depends on their mechanism of aggregation. Digitized images of primary particles and aggregates of depectinized, diafiltered cloudy apple juice were obtained by scanning electron microscopy (SEM). Average radius of the primary particles was found to be a = 40 ± 11 nm. Maximum radius of the aggregates, R(L), ranged between 250 and 7750 nm. Fractal dimension of the aggregates was determined by analyzing SEM images with the variogram method, obtaining an average value of D(f) = 2.3 ± 0.1. This value is typical of aggregates formed by rapid flocculation or diffusion limited aggregation. Diafiltration process was found to reduce the average size and polydispersity of the aggregates, determined by photon correlation spectroscopy. Average gyration radius of the aggregates before juice diafiltration was found to be R(g) = 629 ± 87 nm. Average number of primary particles per aggregate was calculated to be N = 1174. PMID:21339133
Knott, Brandon C; Duff, Nathan; Doherty, Michael F; Peters, Baron
2009-12-14
In the high friction limit of Kramers' theory, the diffusion coefficient for motion along the reaction coordinate is a crucial parameter in determining reaction rates from mean first passage times. The Einstein relation between mean squared displacement, time, and diffusivity is inaccurate at short times because of ballistic motion and inaccurate at long times because trajectories drift away from maxima in the potential of mean force. Starting from the Smoluchowski equation for a downward parabolic barrier, we show how drift induced by the potential of mean force can be included in estimating the diffusivity. A modified relation between mean squared displacement, time, and diffusivity now also includes a dependence on the barrier curvature. The new relation provides the diffusivity at the top of the barrier from a linear regression that is analogous to the procedure commonly used with Einstein's relation. The new approach has particular advantages over previous approaches when evaluations of the reaction coordinate are costly or when the reaction coordinate cannot be differentiated to compute restraining forces or velocities. We use the new method to study the dynamics of barrier crossing in a Potts lattice gas model of nucleation from solution. Our analysis shows that some current hypotheses about laser-induced nucleation mechanisms lead to a nonzero threshold laser pulse duration below which a laser pulse will not affect nucleation. We therefore propose experiments that might be used to test these hypotheses. PMID:20001029
Güvenç, B Haluk; Ekingen, Gülşen; Tuzlaci, Ayşe; Senel, Ufuk
2005-07-01
Complete excision of diffuse abdominal lymphangiomatosis in the newborn is next to impossible. A 3-day-old female infant was found to have diffuse abdominal lymphangiomatosis predominantly in the left mesocolon and retroperitoneum. Initial management was by marsupialization, which was complicated by chylous ascites requiring periodic paracentesis and nutritional support. At the age of 45 days, left hemicolectomy and partial excision of the retroperitoneal cysts were performed together with intracystic injection of OK-432 into the residual cysts. The patient's progress after the second operation was satisfactory. Initial marsupialization followed by delayed partial resection together with injection of OK-432 into the residual cysts is an effective method of managing diffuse abdominal lymphangiomatosis in the newborn.
Number of propagating modes of a diffusive periodic waveguide in the semiclassical limit.
Barra, Felipe; Maurel, Agnes; Pagneux, Vincent; Zuñiga, Jaime
2010-06-01
We study the number of propagating Bloch modes N(B) of an infinite periodic billiard chain. The asymptotic semiclassical behavior of this quantity depends on the phase-space dynamics of the unit cell, growing linearly with the wave number k in systems with a non-null measure of ballistic trajectories and going like ∼square root of k in diffusive systems. We have calculated numerically N(B) for a waveguide with cosine-shaped walls exhibiting strongly diffusive dynamics. The semiclassical prediction for diffusive systems is verified to good accuracy and a connection between this result and the universality of the parametric variation of energy levels is presented. PMID:20866504
Validity conditions for stochastic chemical kinetics in diffusion-limited systems
Gillespie, Daniel T.; Petzold, Linda R.; Seitaridou, Effrosyni
2014-01-01
The chemical master equation (CME) and the mathematically equivalent stochastic simulation algorithm (SSA) assume that the reactant molecules in a chemically reacting system are “dilute” and “well-mixed” throughout the containing volume. Here we clarify what those two conditions mean, and we show why their satisfaction is necessary in order for bimolecular reactions to physically occur in the manner assumed by the CME and the SSA. We prove that these conditions are closely connected, in that a system will stay well-mixed if and only if it is dilute. We explore the implications of these validity conditions for the reaction-diffusion (or spatially inhomogeneous) extensions of the CME and the SSA to systems whose containing volumes are not necessarily well-mixed, but can be partitioned into cubical subvolumes (voxels) that are. We show that the validity conditions, together with an additional condition that is needed to ensure the physical validity of the diffusion-induced jump probability rates of molecules between voxels, require the voxel edge length to have a strictly positive lower bound. We prove that if the voxel edge length is steadily decreased in a way that respects that lower bound, the average rate at which bimolecular reactions occur in the reaction-diffusion CME and SSA will remain constant, while the average rate of diffusive transfer reactions will increase as the inverse square of the voxel edge length. We conclude that even though the reaction-diffusion CME and SSA are inherently approximate, and cannot be made exact by shrinking the voxel size to zero, they should nevertheless be useful in many practical situations. PMID:24511926
Validity conditions for stochastic chemical kinetics in diffusion-limited systems
NASA Astrophysics Data System (ADS)
Gillespie, Daniel T.; Petzold, Linda R.; Seitaridou, Effrosyni
2014-02-01
The chemical master equation (CME) and the mathematically equivalent stochastic simulation algorithm (SSA) assume that the reactant molecules in a chemically reacting system are "dilute" and "well-mixed" throughout the containing volume. Here we clarify what those two conditions mean, and we show why their satisfaction is necessary in order for bimolecular reactions to physically occur in the manner assumed by the CME and the SSA. We prove that these conditions are closely connected, in that a system will stay well-mixed if and only if it is dilute. We explore the implications of these validity conditions for the reaction-diffusion (or spatially inhomogeneous) extensions of the CME and the SSA to systems whose containing volumes are not necessarily well-mixed, but can be partitioned into cubical subvolumes (voxels) that are. We show that the validity conditions, together with an additional condition that is needed to ensure the physical validity of the diffusion-induced jump probability rates of molecules between voxels, require the voxel edge length to have a strictly positive lower bound. We prove that if the voxel edge length is steadily decreased in a way that respects that lower bound, the average rate at which bimolecular reactions occur in the reaction-diffusion CME and SSA will remain constant, while the average rate of diffusive transfer reactions will increase as the inverse square of the voxel edge length. We conclude that even though the reaction-diffusion CME and SSA are inherently approximate, and cannot be made exact by shrinking the voxel size to zero, they should nevertheless be useful in many practical situations.
Zhou, Shouming; Shiraiwa, Manabu; McWhinney, Robert D; Pöschl, Ulrich; Abbatt, Jonathan P D
2013-01-01
The potential for aerosol physical properties, such as phase, morphology and viscosity/ diffusivity, to affect particle reactivity remains highly uncertain. We report here a study of the effect of bulk diffusivity of polycyclic aromatic hydrocarbons (PAHs) in secondary organic aerosol (SOA) on the kinetics of the heterogeneous reaction of particle-borne benzo[a]pyrene (BaP) with ozone. The experiments were performed by coating BaP-ammonium sulfate particles with multilayers of SOA formed from ozonolysis of alpha-pinene, and by subsequently investigating the kinetics of BaP loss via reaction with excess ozone using an aerosol flow tube coupled to an Aerodyne Aerosol Mass Spectrometer (AMS). All reactions exhibit pseudo-first order kinetics and are empirically well described by a Langmuir-Hinshelwood (L-H) mechanism. The results show that under dry conditions (RH < 5%) diffusion through the SOA coating can lead to significant mass transfer constraints on the kinetics, with behavior between that previously observed by our group for solid and liquid organic coats. The reactivity of BaP was enhanced at -50% relative humidity (RH) suggesting that water uptake lowers the viscosity of the SOA, hence lifting the mass transfer constraint to some degree. The kinetics for -70% RH were similar to results obtained without SOA coats, indicating that the SOA had sufficiently low viscosity and was sufficiently liquid-like that reactants could rapidly diffuse through the coat. A kinetic multi-layer model for aerosol surface and bulk chemistry was applied to simulate the kinetics, yielding estimates for the diffusion coefficients (in cm2 s(-1)) for BaP in alpha-pinene SOA of 2 x 10(-14), 8 x 10(-14) and > 1 x 10(-12) for dry (RH < 5%), 50% RH and 70% RH conditions, respectively. These results clearly indicate that slow diffusion of reactants through SOA coats under specific conditions can provide shielding from gas-phase oxidants, enabling the long-range atmospheric transport of
Zhou, Shouming; Shiraiwa, Manabu; McWhinney, Robert D; Pöschl, Ulrich; Abbatt, Jonathan P D
2013-01-01
The potential for aerosol physical properties, such as phase, morphology and viscosity/ diffusivity, to affect particle reactivity remains highly uncertain. We report here a study of the effect of bulk diffusivity of polycyclic aromatic hydrocarbons (PAHs) in secondary organic aerosol (SOA) on the kinetics of the heterogeneous reaction of particle-borne benzo[a]pyrene (BaP) with ozone. The experiments were performed by coating BaP-ammonium sulfate particles with multilayers of SOA formed from ozonolysis of alpha-pinene, and by subsequently investigating the kinetics of BaP loss via reaction with excess ozone using an aerosol flow tube coupled to an Aerodyne Aerosol Mass Spectrometer (AMS). All reactions exhibit pseudo-first order kinetics and are empirically well described by a Langmuir-Hinshelwood (L-H) mechanism. The results show that under dry conditions (RH < 5%) diffusion through the SOA coating can lead to significant mass transfer constraints on the kinetics, with behavior between that previously observed by our group for solid and liquid organic coats. The reactivity of BaP was enhanced at -50% relative humidity (RH) suggesting that water uptake lowers the viscosity of the SOA, hence lifting the mass transfer constraint to some degree. The kinetics for -70% RH were similar to results obtained without SOA coats, indicating that the SOA had sufficiently low viscosity and was sufficiently liquid-like that reactants could rapidly diffuse through the coat. A kinetic multi-layer model for aerosol surface and bulk chemistry was applied to simulate the kinetics, yielding estimates for the diffusion coefficients (in cm2 s(-1)) for BaP in alpha-pinene SOA of 2 x 10(-14), 8 x 10(-14) and > 1 x 10(-12) for dry (RH < 5%), 50% RH and 70% RH conditions, respectively. These results clearly indicate that slow diffusion of reactants through SOA coats under specific conditions can provide shielding from gas-phase oxidants, enabling the long-range atmospheric transport of
Limited subsolidus diffusion in type B1 CAI: Evidence from Ti distribution in spinel
NASA Technical Reports Server (NTRS)
Meeker, G. P.; Quick, J. E.; Paque, Julie M.
1993-01-01
Most models of calcium aluminum-rich inclusions (CAI) have focused on early stages of formation by equilibrium crystallization of a homogeneous liquid. Less is known about the subsolidus cooling history of CAI. Chemical and isotopic heterogeneties on a scale of tens to hundreds of micrometers (e.g. MacPherson et al. (1989) and Podosek, et al. (1991)) suggest fairly rapid cooling with a minimum of subsolidus diffusion. However, transmission electron microscopy indicates that solid state diffusion may have been an important process at a smaller scale (Barber et al. 1984). If so, chemical evidence for diffusion could provide constraints on cooling times and temperatures. With this in mind, we have begun an investigation of the Ti distribution in spinels from two type B1 CAI from Allende to determine if post-crystallization diffusion was a significant process. The type B1 CAIs, 3529Z and 5241 have been described by Podosek et al. (1991) and by El Goresy et al. (1985) and MacPherson et al. (1989). We have analyzed spinels in these inclusions using the electron microprobe. These spinels are generally euhedral, range in size from less than 10 to 15 micron and are poikilitically enclosed by millimeter-sized pyroxene, melilite, and anorthite. Analyses were obtained from both the mantles and cores of the inclusions. Compositions of pyroxene in the vicinity of individual spinel grains were obtained by analyzing at least two points on opposite sides of the spinel and averaging the compositions. The pyroxene analyses were obtained within 15 microns of the spinel-pyroxene interface. No compositional gradients were observed within single spinel crystals. Ti concentrations in spinels included within pyroxene, melilite, and anorthite are presented.
Lin, Wei-Ching; Chen, Jeon-Hor
2015-06-01
Adequately selecting a therapeutic approach for bladder cancer depends on accurate grading and staging. Substantial inaccuracy of clinical staging with bimanual examination, cystoscopy, and transurethral resection of bladder tumor has facilitated the increasing utility of magnetic resonance imaging to evaluate bladder cancer. Diffusion-weighted imaging (DWI) is a noninvasive functional magnetic resonance imaging technique. The high tissue contrast between cancers and surrounding tissues on DWI is derived from the difference of water molecules motion. DWI is potentially a useful tool for the detection, characterization, and staging of bladder cancers; it can also monitor posttreatment response and provide information on predicting tumor biophysical behaviors. Despite advancements in DWI techniques and the use of quantitative analysis to evaluate the apparent diffusion coefficient values, there are some inherent limitations in DWI interpretation related to relatively poor spatial resolution, lack of cancer specificity, and lack of standardized image acquisition protocols and data analysis procedures that restrict the application of DWI and reproducibility of apparent diffusion coefficient values. In addition, inadequate bladder distension, artifacts, thinness of bladder wall, cancerous mimickers of normal bladder wall and benign lesions, and variations in the manifestation of bladder cancer may interfere with diagnosis and monitoring of treatment. Recognition of these pitfalls and limitations can minimize their impact on image interpretation, and carefully applying the analyzed results and combining with pathologic grading and staging to clinical practice can contribute to the selection of an adequate treatment method to improve patient care.
Mieussens, Luc
2013-11-15
The unified gas kinetic scheme (UGKS) of K. Xu et al. (2010) [37], originally developed for multiscale gas dynamics problems, is applied in this paper to a linear kinetic model of radiative transfer theory. While such problems exhibit purely diffusive behavior in the optically thick (or small Knudsen) regime, we prove that UGKS is still asymptotic preserving (AP) in this regime, but for the free transport regime as well. Moreover, this scheme is modified to include a time implicit discretization of the limit diffusion equation, and to correctly capture the solution in case of boundary layers. Contrary to many AP schemes, this method is based on a standard finite volume approach, it does neither use any decomposition of the solution, nor staggered grids. Several numerical tests demonstrate the properties of the scheme.
NASA Astrophysics Data System (ADS)
Laginha Silva, Patricia; Martins, Flávio A.; Boski, Tomász; Sampath, Dissanayake M. R.
2010-05-01
processes. In this viewpoint the system is broken down into its fundamental components and processes and the model is build up by selecting the important processes regardless of its time and space scale. This viewpoint was only possible to pursue in the recent years due to improvement in system knowledge and computer power (Paola, 2000). The primary aim of this paper is to demonstrate that it is possible to simulate the evolution of the sediment river bed, traditionally studied with synthetic models, with a process-based hydrodynamic, sediment transport and morphodynamic model, solving explicitly the mass and momentum conservation equations. With this objective, a comparison between two mathematical models for alluvial rivers is made to simulate the evolution of the sediment river bed of a conceptual 1D embayment for periods in the order of a thousand years: the traditional synthetic basin infilling aggregate diffusive type model based on the diffusion equation (Paola, 2000), used in the "synthesist" viewpoint and the process-based model MOHID (Miranda et al., 2000). The simulation of the sediment river bed evolution achieved by the process-based model MOHID is very similar to those obtained by the diffusive type model, but more complete due to the complexity of the process-based model. In the MOHID results it is possible to observe a more comprehensive and realistic results because this type of model include processes that is impossible to a synthetic model to describe. At last the combined effect of tide, sea level rise and river discharges was investigated in the process based model. These effects cannot be simulated using the diffusive type model. The results demonstrate the feasibility of using process based models to perform studies in scales of 10000 years. This is an advance relative to the use of synthetic models, enabling the use of variable forcing. REFERENCES • Briggs, L.I. and Pollack, H.N., 1967. Digital model of evaporate sedimentation. Science, 155, 453
Villarreal, Miguel L.; Van Riper, Charles; Petrakis, Roy E.
2013-01-01
Riparian vegetation provides important wildlife habitat in the Southwestern United States, but limited distributions and spatial complexity often leads to inaccurate representation in maps used to guide conservation. We test the use of data conflation and aggregation on multiple vegetation/land-cover maps to improve the accuracy of habitat models for the threatened western yellow-billed cuckoo (Coccyzus americanus occidentalis). We used species observations (n = 479) from a state-wide survey to develop habitat models from 1) three vegetation/land-cover maps produced at different geographic scales ranging from state to national, and 2) new aggregate maps defined by the spatial agreement of cover types, which were defined as high (agreement = all data sets), moderate (agreement ≥ 2), and low (no agreement required). Model accuracies, predicted habitat locations, and total area of predicted habitat varied considerably, illustrating the effects of input data quality on habitat predictions and resulting potential impacts on conservation planning. Habitat models based on aggregated and conflated data were more accurate and had higher model sensitivity than original vegetation/land-cover, but this accuracy came at the cost of reduced geographic extent of predicted habitat. Using the highest performing models, we assessed cuckoo habitat preference and distribution in Arizona and found that major watersheds containing high-probably habitat are fragmented by a wide swath of low-probability habitat. Focus on riparian restoration in these areas could provide more breeding habitat for the threatened cuckoo, offset potential future habitat losses in adjacent watershed, and increase regional connectivity for other threatened vertebrates that also use riparian corridors.
NASA Technical Reports Server (NTRS)
Pellett, Gerald L.; Guerra, Rosemary; Wilson, Lloyd G.; Reeves, Ronald N.; Northam, G. Burton
1987-01-01
Combustion of H2/hydrocarbon (HC) fuel mixtures may be considered in certain volume-limited supersonic airbreathing propulsion applications. Effects of HC addition to H2 were evaluated, using a recent argon-bathed, coaxial, tubular opposed jet burner (OJB) technique to measure the extinction limits of counterflow diffusion flames. The OJB flames were formed by a laminar jet of (N2 and/or HC)-diluted H2 mixture opposed by a similar jet of air at ambient conditions. The OJB data, derived from respective binary mixtures of H2 and methane, ethylene, or propane HCs, were used to characterize BLOWOFF and RESTORE. BLOWOFF is a sudden breaking of the dish-shaped OJB flame to a stable torus or ring shape, and RESTORE marks sudden restoration of the central flame by radial inward flame propagation. BLOWOFF is a measure of kinetically-limited flame reactivity/speed under highly stretched, but relatively ideal impingement flow conditions. RESTORE measures inward radial flame propagation rate, which is sensitive to ignition processes in the cool central core. It is concluded that relatively small molar amounts of added HC greatly reduce the reactivity characteristics of counterflow hydrogen-air diffusion flames, for ambient initial conditions.
Laser-induced heat diffusion limited tissue coagulation as a laser therapy mode
NASA Astrophysics Data System (ADS)
Lubashevsky, Ihor A.; Priezzhev, Alexander V.; Gafiychuk, Vasyl V.
2000-11-01
Previously we have developed a free boundary model for local thermal coagulation induced by laser light absorption when the tissue region affected directly by laser light is sufficiently small and heat diffusion into the surrounding tissue governs the necrosis growth. In the present paper keeping in mind the obtained results we state the point of view on the necrosis formation under these conditions as the basis of an individual laser therapy mode exhibiting specific properties. In particular, roughly speaking, the size of the resulting necrosis domain is determined by the physical characteristics of the tissue and its response to local heating, and by the applicator form rather than the treatment duration and the irradiation power.
Laser-induced heat diffusion limited tissue coagulation as a laser therapy mode
NASA Astrophysics Data System (ADS)
Lubashevsky, Ihor A.; Priezzhev, Alexander V.; Gafiychuk, Vasyl V.
2000-06-01
Previously we have developed a free boundary model for local thermal coagulation induced by laser light absorption when the tissue region affected directly by laser light is sufficiently small and heat diffusion into the surrounding tissue governs the necrosis growth. In the present paper keeping in mind the obtained results we state the point of view on the necrosis formation under these conditions as the basis of an individual layer therapy mode exhibiting specific properties. In particular, roughly speaking, the size of the resulting necrosis domain is determined by the physical characteristics of the tissue and its response to local heating, and by the applicator form rather than the treatment duration and the irradiation power.
Liedl, Rudolf; Ptak, Thomas
2003-11-01
A new reactive transport modelling approach and examples of its application are presented, dealing with the impact of sorption/desorption kinetics on the spreading of solutes, e.g. organic contaminants, in groundwater. Slow sorption/desorption is known from the literature to be strongly responsible for the retardation of organic contaminants. The modelling concept applied in this paper quantifies sorption/desorption kinetics by an intra-particle diffusion approach. According to this idea, solute uptake by or release from the aquifer material is modelled at small scale by a "slow" diffusion process where the diffusion coefficient is reduced as compared to the aqueous diffusion coefficient due to (i) the size and shape of intra-particle pores and (ii) retarded transport of solutes within intra-particle pores governed by a nonlinear sorption isotherm. This process-based concept has the advantage of requiring only measurable model parameters, thus avoiding fitting parameters like first-order rate coefficients. In addition, the approach presented here allows for modelling of slow sorption/desorption in lithologically nonuniform media. Therefore, it accounts for well-known experimental findings indicating that sorptive properties depend on (i) the grain size distribution of the aquifer material and (ii) the lithological composition (e.g. percentage of quartz, sandstone, limestone, etc.) of each grain size fraction. The small-scale physico-chemical model describing sorption/desorption is coupled to a large-scale model of groundwater flow and solute transport. Consequently, hydraulic heterogeneities may also be considered by the overall model. This coupling is regarded as an essential prerequisite for simulating field-scale scenarios which will be addressed by a forthcoming publication. This paper focuses on mathematical model formulation, implementation of the numerical code and lab-scale model applications highlighting the sorption and desorption behavior of an organic
Electrokinetics of diffuse soft interfaces. 1. Limit of low Donnan potentials.
Duval, Jérôme F L; van Leeuwen, Herman P
2004-11-01
The current theoretical approaches to electrokinetics of gels or polyelectrolyte layers are based on the assumption that the position of the very interface between the aqueous medium and the gel phase is well defined. Within this assumption, spatial profiles for the volume fraction of polymer segments (phi), the density of fixed charges in the porous layer (rho fix), and the coefficient modeling the friction to hydrodynamic flow (k) follow a step-function. In reality, the "fuzzy" nature of the charged soft layer is intrinsically incompatible with the concept of a sharp interface and therefore necessarily calls for more detailed spatial representations for phi, rho fix, and k. In this paper, the notion of diffuse interface is introduced. For the sake of illustration, linear spatial distributions for phi and rho fix are considered in the interfacial zone between the bulk of the porous charged layer and the bulk electrolyte solution. The corresponding distribution for k is inferred from the Brinkman equation, which for low phi reduces to Stokes' equation. Linear electrostatics, hydrodynamics, and electroosmosis issues are analytically solved within the context of streaming current and streaming potential of charged surface layers in a thin-layer cell. The hydrodynamic analysis clearly demonstrates the physical incorrectness of the concept of a discrete slip plane for diffuse interfaces. For moderate to low electrolyte concentrations and nanoscale spatial transition of phi from zero (bulk electrolyte) to phi o (bulk gel), the electrokinetic properties of the soft layer as predicted by the theory considerably deviate from those calculated on the basis of the discontinuous approximation by Ohshima. PMID:15518532
NASA Astrophysics Data System (ADS)
Whitehouse, Stuart C.; Bate, Matthew R.; Monaghan, Joe J.
2005-12-01
We describe a new, faster implicit algorithm for solving the radiation hydrodynamics equations in the flux-limited diffusion approximation for smoothed particle hydrodynamics. This improves on the method elucidated in Whitehouse and Bate by using a Gauss-Seidel iterative method rather than iterating over the exchange of energy between pairs of particles. The new algorithm is typically many thousands of times faster than the old one, which will enable more complex problems to be solved. The new algorithm is tested using the same tests performed by Turner and Stone for ZEUS-2D, and repeated by Whitehouse and Bate.
Zbik, Marek S; Frost, Ray L; Song, Yen-Fang
2008-03-01
This paper reports new application of new transmission X-ray microscopy powered by a synchrotron source for the study of aqueous based clay suspensions. This paper delineates the advantages and limitations of this method. The tested transmission X-ray microscopy (TXM) technique has shown good agreement with the cryo-stage SEM technique. The spacial resolution of this TXM technique is 60 nm and clay particles with diameter below 500 nm are clearly visible and their pseudohexagonal symmetry is recognizable in detail. It is clearly demonstrated the methodology of implementing TXM to study aqueous based clay suspensions that are close to approximately 60 nm tomographic resolution. The technique enables us to study discrete structure of clay suspensions in water and within aggregates. This has never been previously possible. Larger crystals, more compact aggregates and less colloidal fraction present in kaolinite from Georgia has impact on faster settling and gelling in denser suspension than for Birdwood kaolinite in which colloidal particles create gel-like networking in less dense aqueous suspension. PMID:18067907
A theoretical analysis of the extinction limits of a methane-air opposed-jet diffusion flame
NASA Technical Reports Server (NTRS)
Olson, S. L.; T'Ien, J. S.
1987-01-01
A theoretical analysis is described for a methane-air diffusion flame stabilized in the forward stagnation region of a porous metal cylinder in a forced convective flow. The analysis includes effects of radiative heat loss from the porous metal surface and finite rate kinetics but neglects the effects of gravity. The theoretically predicted extinction limits compare well with experimentally observed extinction limits from the literature. After the predicted limits compared well with the experimental limits, a parametric study of the effect of fuel surface emissivity and Lewis number was conducted with the numerical model. It was found that the computed blowoff limit is independent of radiative heat loss for high fuel blowing velocities but is a strong function of Lewis number. At low fuel blowing velocities, the extinction limit varies with both radiative heat loss and Lewis number. It is discovered, however, that even if thermal losses from the fuel surface are absent, the flame can extinguish at the fuel surface independently of Lewis number due to excessive reaction zone thinning.
NASA Astrophysics Data System (ADS)
Fouvry, J. B.; Pichon, C.; Chavanis, P. H.
2015-09-01
The secular evolution of an infinitely thin tepid isolated galactic disc made of a finite number of particles is described using the inhomogeneous Balescu-Lenard equation. Assuming that only tightly wound transient spirals are present in the disc, a WKB approximation provides a simple and tractable quadrature for the corresponding drift and diffusion coefficients. It provides insight into the physical processes at work during the secular diffusion of a self-gravitating discrete disc and makes quantitative predictions on the initial variations of the distribution function in action space. When applied to the secular evolution of an isolated stationary self-gravitating Mestel disc, this formalism predicts the initial importance of the corotation resonance in the inner regions of the disc leading to a regime involving radial migration and heating. It predicts in particular the formation of a ridge-like feature in action space, in agreement with simulations, but over-estimates the timescale involved in its appearance. Swing amplification is likely needed to resolve this discrepancy. In astrophysics, the inhomogeneous Balescu-Lenard equation and its WKB limit may also describe the secular diffusion of giant molecular clouds in galactic discs, the secular migration and segregation of planetesimals in proto-planetary discs, or even the long-term evolution of population of stars within the Galactic centre. Appendices are available in electronic form at http://www.aanda.org
Scattering and propagation of terahertz pulses in random soot aggregate systems
NASA Astrophysics Data System (ADS)
Li, Hai-Ying; Wu, Zhen-Sen; Bai, Lu; Li, Zheng-Jun
2014-05-01
Scattering and propagation of terahertz pulses in random soot aggregate systems are studied by using the generalized multi-particle Mie-solution (GMM) and the pulse propagation theory. Soot aggregates are obtained by the diffusion-limited aggregation (DLA) model. For a soot aggregate in soot aggregate systems, scattering characteristics are analyzed by using the GMM. Scattering intensities versus scattering angles are given. The effects of different positions of the aggregate on the scattering intensities, scattering cross sections, extinction cross sections, and absorption cross sections are computed and compared. Based on pulse propagation in random media, the transmission of terahertz pulses in random soot aggregate systems is determined by the two-frequency mutual coherence function. Numerical simulations and analysis are given for terahertz pulses (0.7956 THz).
Acoustic phonon-limited diffusion thermopower in monolayer MoS{sub 2}
Patil, S. B.; Sankeshwar, N. S. Kubakaddi, S. S.
2015-06-24
Diffusion thermopower S{sub d} is investigated, theoretically, as a function of temperature, T and electron concentration, n{sub s} in a n-type monolayer molebdenum disulfide (MoS{sub 2}). Electron scattering due to unscreened deformation potential (DP) coupling of TA phonons, screened DP coupling of LA phonons, and screened piezoelectric (PE) coupling of LA and TA phonons is considered. Total S{sub d} is dominated by electron scattering by TA phonons via unscreened DP coupling. S{sub d} is found to increase (decrease) with increasing T (n{sub s}). At low T and for high n{sub s}, S{sub d} ∼ T and n{sub s}{sup −1} as found from the Mott formula. At a given T and for given ns, S{sub d} in MoS{sub 2} is much larger than that in GaAs, due to the larger electron effective mass in the former.
Modeling ant battles by means of a diffusion-limited Gillespie algorithm.
Martelloni, Gianluca; Santarlasci Alisa; Bagnoli, Franco; Santini, Giacomo
2014-01-01
We propose two modeling approaches to describe the dynamics of ant battles, starting from laboratory experiments on the behavior of two ant species, the invasive Lasius neglectus and the authocthonus Lasius paralienus. This work is mainly motivated by the need to have realistic models to predict the interaction dynamics of invasive species. The two considered species exhibit different fighting strategies. In order to describe the observed battle dynamics, we start by building a chemical model considering the ants and the fighting groups (for instance two ants of a species and one of the other one) as a chemical species. From the chemical equations we deduce a system of differential equations, whose parameters are estimated by minimizing the difference between the experimental data and the model output. We model the fluctuations observed in the experiments by means of a standard Gillespie algorithm. In order to better reproduce the observed behavior, we adopt a spatial agent-based model, in which ants not engaged in fighting groups move randomly (diffusion) among compartments, and the Gillespie algorithm is used to model the reactions inside a compartment.
Liu, Jing; Xu, Zaoli; Cheng, Zhe; Xu, Shen; Wang, Xinwei
2015-12-16
Crystalline ultrahigh molecular weight polyethylene (UHMWPE) has the highest reported thermal conductivity at room temperature: 104 W/(m·K), while theoretical predictions proposed an even higher value of 300 W/(m·K). Defects and amorphous fraction in practical UHMWPE fibers significantly reduces the thermal conductivity from the ideal value. Although the amorphous effect can be readily analyzed based on the effective medium theory, the defect effects are poorly understood. This work reports on the temperature-dependent behavior (down to 22 K) of thermal diffusivity and conductivity of UHMWPE fibers in anticipation of observing the reduction in phonon density and scattering rate against temperature and of freezing out high-momentum phonons to clearly observe the defect effects. By studying the temperature-dependent behavior of thermal reffusivity (Θ, inverse of thermal diffusivity) of UHMWPE fibers, we are able to quantify the defect effects on thermal conductivity. After taking out the amorphous region's effect, the residual thermal reffusivities (Θ0) for the studied two samples at the 0 K limit are determined as 3.45 × 10(4) and 2.95 × 10(4) s/m(2), respectively. For rare-/no-defects crystalline materials, Θ0 should be close to zero at the 0 K limit. The defect-induced low-momentum phonon mean free paths are determined as 8.06 and 9.42 nm for the two samples. They are smaller than the crystallite size in the (002) direction (19.7 nm) determined by X-ray diffraction. This strongly demonstrates the diffuse phonon scattering at the grain boundaries. The grain boundary thermal conductance (G) can be evaluated as G ≈ βρc(p)v with sound accuracy. At room temperature, G is around 3.73 GW/(m(2)·K) for S2, comparable to that of interfaces with tight atomic bonding.
Liu, Jing; Xu, Zaoli; Cheng, Zhe; Xu, Shen; Wang, Xinwei
2015-12-16
Crystalline ultrahigh molecular weight polyethylene (UHMWPE) has the highest reported thermal conductivity at room temperature: 104 W/(m·K), while theoretical predictions proposed an even higher value of 300 W/(m·K). Defects and amorphous fraction in practical UHMWPE fibers significantly reduces the thermal conductivity from the ideal value. Although the amorphous effect can be readily analyzed based on the effective medium theory, the defect effects are poorly understood. This work reports on the temperature-dependent behavior (down to 22 K) of thermal diffusivity and conductivity of UHMWPE fibers in anticipation of observing the reduction in phonon density and scattering rate against temperature and of freezing out high-momentum phonons to clearly observe the defect effects. By studying the temperature-dependent behavior of thermal reffusivity (Θ, inverse of thermal diffusivity) of UHMWPE fibers, we are able to quantify the defect effects on thermal conductivity. After taking out the amorphous region's effect, the residual thermal reffusivities (Θ0) for the studied two samples at the 0 K limit are determined as 3.45 × 10(4) and 2.95 × 10(4) s/m(2), respectively. For rare-/no-defects crystalline materials, Θ0 should be close to zero at the 0 K limit. The defect-induced low-momentum phonon mean free paths are determined as 8.06 and 9.42 nm for the two samples. They are smaller than the crystallite size in the (002) direction (19.7 nm) determined by X-ray diffraction. This strongly demonstrates the diffuse phonon scattering at the grain boundaries. The grain boundary thermal conductance (G) can be evaluated as G ≈ βρc(p)v with sound accuracy. At room temperature, G is around 3.73 GW/(m(2)·K) for S2, comparable to that of interfaces with tight atomic bonding. PMID:26593380
Abraham, J.; De La Vega, G.; Garcia, B.; Videla, M.; Abreu, P.; Andringa, S.; Assis, P.; Brogueira, P.; Conceicao, R.; Goncalves, P.; Pimenta, M.; Santo, C. E.; Tome, B.; Aglietta, M.; Bonino, R.; Castellina, A.; Chiavassa, A.; Fulgione, W.; Gorgi, A.; Lucero, A.
2009-05-15
Data collected at the Pierre Auger Observatory are used to establish an upper limit on the diffuse flux of tau neutrinos in the cosmic radiation. Earth-skimming {nu}{sub {tau}} may interact in the Earth's crust and produce a {tau} lepton by means of charged-current interactions. The {tau} lepton may emerge from the Earth and decay in the atmosphere to produce a nearly horizontal shower with a typical signature, a persistent electromagnetic component even at very large atmospheric depths. The search procedure to select events induced by {tau} decays against the background of normal showers induced by cosmic rays is described. The method used to compute the exposure for a detector continuously growing with time is detailed. Systematic uncertainties in the exposure from the detector, the analysis, and the involved physics are discussed. No {tau} neutrino candidates have been found. For neutrinos in the energy range 2x10{sup 17} eV
Decker, E R; Levitt, D G
1988-01-01
The addition of 2 M formic acid at pH 3.75 increased the single channel H+ ion conductance of gramicidin channels 12-fold at 200 mV. Other weak acids (acetic, lactic, oxalic) produce a similar, but smaller increase. Formic acid (and other weak acids) also blocks the K+ conductance at pH 3.75, but not at pH 6.0 when the anion form predominates. This increased H+ conductance and K+ block can be explained by formic acid (HF) binding to the mouth of the gramicidin channel (Km = 1 M) and providing a source of H+ ions. A kinetic model is derived, based on the equilibrium binding of formic acid to the channel mouth, that quantitatively predicts the conductance for different mixtures of H+, K+, and formic acid. The binding of the neutral formic acid to the mouth of the gramicidin channel is directly supported by the observation that a neutral molecule with a similar structure, formamide (and malonamide and acrylamide), blocks the K+ conductance at pH 6.0. The H+ conductance in the presence of formic acid provides a lower bound for the intrinsic conductance of the gramicidin channel when there is no diffusion limitation at the channel mouth. The 12-fold increase in conductance produced by formic acid suggests that greater than 90% of the total resistance to H+ results from diffusion limitation in the bulk solution. PMID:2449253
Fu, Juanjuan; Gates, Roger N; Xu, Yuefei; Hu, Tianming
2016-10-01
We studied the effects of cold stress (5°C) and re-warming (25°C) on gas exchange, photosystem II, key photosynthetic enzyme activities, gene expression, and carbohydrate metabolite concentrations in two Elymus nutans genotypes differing in cold resistance (DX, cold-tolerant and ZD, cold-sensitive). Cold stress led to irreversible reductions in photosynthetic rate. This reduction was accompanied by declining stomatal and mesophyll conductance (gs and gm), transpiration rate (Tr) and photochemical efficiency in both genotypes, however there were smaller decreases in DX than in ZD. Cold-tolerant DX maintained higher photosynthetic enzyme activities and transcript levels, as well as higher reducing sugar concentrations and sucrose accumulation. The relationship between Pn and internal leaf CO2 concentration (Pn/Ci curve) during cold and re-warming was analyzed to estimate the relative influence of stomatal and non-stomatal components on photosynthesis. Stomatal limitation, non-stomatal limitation, and CO2 compensation point (CP) increased in both genotypes under cold stress, but to a lesser extent in DX. Maximum CO2 assimilation rate (Pmax), and carboxylation efficiency (CE) declined, but DX had significantly higher levels of Pmax and CE than ZD. Following cold-stress recovery, the maximum quantum yield of PSII (Fv/Fm), apparent electron transport rate (ETR), Rubisco activity, Rubisco activation state and CE in DX resumed to the control levels. In contrast, Pn, Pmax, gs, gm, and Tr recovered only partially for DX, suggesting that incomplete recovery of photosynthesis in DX may be mainly related to diffusion limitations. Higher Rubisco large subunit (RbcL) and Rubisco activase (RCA) transcript levels, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity, and carbohydrate accumulation contributed to higher photosynthetic recovery in DX. These results indicate that the maintenance of higher Pn and Pmax under cold stress and recovery in cold-tolerant DX could be
Kappa Distribution in a Homogeneous Medium: Adiabatic Limit of a Super-diffusive Process?
NASA Astrophysics Data System (ADS)
Roth, I.
2015-12-01
The classical statistical theory predicts that an ergodic, weakly interacting system like charged particles in the presence of electromagnetic fields, performing Brownian motions (characterized by small range deviations in phase space and short-term microscopic memory), converges into the Gibbs-Boltzmann statistics. Observation of distributions with a kappa-power-law tails in homogeneous systems contradicts this prediction and necessitates a renewed analysis of the basic axioms of the diffusion process: characteristics of the transition probability density function (pdf) for a single interaction, with a possibility of non-Markovian process and non-local interaction. The non-local, Levy walk deviation is related to the non-extensive statistical framework. Particles bouncing along (solar) magnetic field with evolving pitch angles, phases and velocities, as they interact resonantly with waves, undergo energy changes at undetermined time intervals, satisfying these postulates. The dynamic evolution of a general continuous time random walk is determined by pdf of jumps and waiting times resulting in a fractional Fokker-Planck equation with non-integer derivatives whose solution is given by a Fox H-function. The resulting procedure involves the known, although not frequently used in physics fractional calculus, while the local, Markovian process recasts the evolution into the standard Fokker-Planck equation. Solution of the fractional Fokker-Planck equation with the help of Mellin transform and evaluation of its residues at the poles of its Gamma functions results in a slowly converging sum with power laws. It is suggested that these tails form the Kappa function. Gradual vs impulsive solar electron distributions serve as prototypes of this description.
NASA Technical Reports Server (NTRS)
Sunderland, P. B.; Urban, D. L.; Stocker, D. P.; Chao, B.-H.; Axelbaum, Richard L.; Salzman, Jack (Technical Monitor)
2001-01-01
Limiting conditions for soot-particle inception were studied in microgravity spherical diffusion flames burning ethylene at atmospheric pressure. Nitrogen was supplied in the fuel and/or oxidizer to obtain the broadest range of stoichiometric mixture fraction. Both normal flames (oxygen in ambience) and inverted flames (fuel in ambience) were considered. Microgravity was obtained in the NASA Glenn 2.2-second drop tower. The flames were observed with a color video camera and sooting conditions were defined as conditions for which yellow emission was present throughout the duration of the drop. Sooting limit results were successfully correlated in terms of adiabatic flame temperature and stoichiometric mixture fraction. Soot free conditions were favored by increased stoichiometric mixture fractions. No statistically significant effect of convection direction on sooting limits was observed. The relationship between adiabatic flame temperature and stoichiometric mixture fraction at the sooting limits was found to be in qualitative agreement with a simple theory based on the assumption that soot inception can occur only where temperature and local C/O ratio exceed threshold values (circa 1250 K and 1, respectively).
Ion-specific aggregation of hydrophobic particles.
López-León, Teresa; Ortega-Vinuesa, Juan Luis; Bastos-González, Delfina
2012-06-18
This work shows that colloidal stability and aggregation kinetics of hydrophobic polystyrene (PS) nanospheres are extremely sensitive to the nature of the salt used to coagulate them. Three PS latices and four aggregating electrolytes, which all share the same cation (Na(+)) but have various anions located at different positions in the classical Hofmeister series depending on their kosmotropic or chaotropic character, are used. The present study focuses on analyzing different aggregating parameters, such as critical coagulation concentrations (CCC), cluster size distributions (CSD), initial kinetic constants K(11), and fractal dimensions of the aggregates d(f). While aggregation induced by SO(4)(2-) and Cl(-) behaved according to the predictions of the classical Derjaguin-Landau-Verwey-Overbeek theory, important discrepancies are found with NO(3)(-), which become dramatic when using SCN(-). These discrepancies among the anions were far more significant when they acted as counterions rather than as co-ions. While SO(4)(2-) and Cl(-) trigger fast diffusion-limited aggregation, SCN(-) gives rise to a stationary cluster size distribution in a few aggregation times when working with cationic PS particles. Clear differences are found among all analyzed parameters (CCC, CSD, K(11), and d(f)), and the experimental findings show that particles aggregate in potential wells whose depth is controlled by the chaotropic character of the anion. This paper presents new experimental evidence that may help to understand the microscopic origin of Hofmeister effects, as the observations are consistent with appealing theoretical models developed in the last few years. PMID:22556130
Colloidal aggregation and dynamics in anisotropic fluids
Mondiot, Frédéric; Botet, Robert; Snabre, Patrick; Mondain-Monval, Olivier; Loudet, Jean-Christophe
2014-01-01
We present experiments and numerical simulations to investigate the collective behavior of submicrometer-sized particles immersed in a nematic micellar solution. We use latex spheres with diameters ranging from 190 to 780 nm and study their aggregation properties due to the interplay of the various colloidal forces at work in the system. We found that the morphology of aggregates strongly depends on the particle size, with evidence for two distinct regimes: the biggest inclusions clump together within minutes into either compact clusters or V-like structures that are completely consistent with attractive elastic interactions. On the contrary, the smallest particles form chains elongated along the nematic axis, within comparable timescales. In this regime, Monte Carlo simulations, based on a modified diffusion-limited cluster aggregation model, strongly suggest that the anisotropic rotational Brownian motion of the clusters combined with short-range depletion interactions dominate the system coarsening; elastic interactions no longer prevail. The simulations reproduce the sharp transition between the two regimes on increasing the particle size. We provide reasonable estimates to interpret our data and propose a likely scenario for colloidal aggregation. These results emphasize the growing importance of the diffusion of species at suboptical-wavelength scales and raise a number of fundamental issues. PMID:24715727
Cohen, S.A.; Hosea, J.C.; Timberlake, J.R.
1984-10-19
A limiter with a specially contoured front face is provided. The front face of the limiter (the plasma-side face) is flat with a central indentation. In addition, the limiter shape is cylindrically symmetric so that the limiter can be rotated for greater heat distribution. This limiter shape accommodates the various power scrape-off distances lambda p, which depend on the parallel velocity, V/sub parallel/, of the impacting particles.
ERIC Educational Resources Information Center
Schumacher, Sally
This study of the first year of an inter-organizational diffusion effort by a national educational laboratory, a state department of education, and nine local school districts focuses on the implementation phase within the Research, Development and Diffusion (RD and D) strategy of an aesthetic education program which uses the arts as the…
Tjepkema, John D.; Cartica, Robert J.
1982-01-01
Parasponia is the first non-legume genus proven to form nitrogen-fixing root nodules induced by rhizobia. Infiltration with India ink demonstrated that intercellular air spaces are lacking in the inner layers of the nodule cortex. Oxygen must diffuse through these layers to reach the cells containing the rhizobia, and it was calculated that most of the gradient in O2 partial pressure between the atmosphere and rhizobia occurs at the inner cortex. This was confirmed by O2 microelectrode measurements which showed that the O2 partial pressure was much lower in the zone of infected cells than in the cortex. Measurements of nitrogenase activity and O2 uptake as a function of temperature and partial pressure of O2 were consistent with diffusion limitation of O2 uptake by the inner cortex. In spite of the presumed absence of leghemoglobin in nodules of Parasponia rigida Merr. and Perry, energy usage for nitrogen fixation was similar to that in legume nodules. The results demonstrate that O2 regulation in legume and Parasponia nodules is very similar and differs from O2 regulation in actionorhizal nodules. Images PMID:16662284
Evaluation of Nanoparticle Tracking for Characterization of Fibrillar Protein Aggregates
Yang, Dennis T.; Lu, Xiaomeng; Fan, Yamin; Murphy, Regina M.
2015-01-01
Amyloidogenesis is the process of formation of protein aggregates with fibrillar morphology. Because amyloidogenesis is linked to neurodegenerative disease, there is interest in understanding the mechanism of fibril growth. Kinetic models of amyloidogenesis require data on the number concentration and size distribution of aggregates, but this information is difficult to obtain using conventional methods. Nanoparticle tracking analysis (NTA) is a relatively new technique that may be uniquely suited for obtaining these data. In NTA, the two-dimensional (2-D) trajectory of individual particles is tracked, from which the diffusion coefficient, and, hence, hydrodynamic radius is obtained. Here we examine the validity of NTA in tracking number concentration and size of DNA, as a model of a fibrillar macromolecule. We use NTA to examine three amyloidogenic materials: beta-amyloid, transthyretin, and polyglutamine-containing peptides. Our results are instructive in demonstrating the advantages and some limitations of single-particle diffusion measurements for investigating aggregation in protein systems. PMID:25843955
Rijal Upadhaya, Ajeet; Capetillo-Zarate, Estibaliz; Kosterin, Irina; Abramowski, Dorothee; Kumar, Sathish; Yamaguchi, Haruyasu; Walter, Jochen; Fändrich, Marcus; Staufenbiel, Matthias; Thal, Dietmar Rudolf
2012-11-01
Soluble amyloid β-protein (Aβ) aggregates have been identified in the Alzheimer's disease (AD) brain. Dispersed Aβ aggregates in the brain parenchyma are different from soluble, membrane-associated and plaque-associated solid aggregates. They are in mixture with the extra- or intracellular fluid but can be separated from soluble proteins by ultracentrifugation. To clarify the role of dispersible Aβ aggregates for neurodegeneration we analyzed 2 different amyloid precursor protein (APP)-transgenic mouse models. APP23 mice overexpress human mutant APP with the Swedish mutation. APP51/16 mice express high levels of human wild type APP. Both mice develop Aβ-plaques. Dendritic degeneration, neuron loss, and loss of asymmetric synapses were seen in APP23 but not in APP51/16 mice. The soluble and dispersible fractions not separated from one another were received as supernatant after centrifugation of native forebrain homogenates at 14,000 × g. Subsequent ultracentrifugation separated the soluble, i.e., the supernatant, from the dispersible fraction, i.e., the resuspended pellet. The major biochemical difference between APP23 and APP51/16 mice was that APP23 mice exhibited higher levels of dispersible Aβ oligomers, protofibrils and fibrils precipitated with oligomer (A11) and protofibril/fibril (B10AP) specific antibodies than APP51/16 mice. These differences, rather than soluble Aβ and Aβ plaque pathology were associated with dendritic degeneration, neuron, and synapse loss in APP23 mice in comparison with APP51/16 mice. Immunoprecipitation of dispersible Aβ oligomers, protofibrils, and fibrils revealed that they were associated with APP C-terminal fragments (APP-CTFs). These results indicate that dispersible Aβ oligomers, protofibrils, and fibrils represent an important pool of Aβ aggregates in the brain that critically interact with membrane-associated APP C-terminal fragments. The concentration of dispersible Aβ aggregates, thereby, presumably determines
Cohen, Samuel A.; Hosea, Joel C.; Timberlake, John R.
1986-01-01
A limiter with a specially contoured front face accommodates the various power scrape-off distances .lambda..sub.p, which depend on the parallel velocity, V.sub..parallel., of the impacting particles. The front face of the limiter (the plasma-side face) is flat with a central indentation. In addition, the limiter shape is cylindrically symmetric so that the limiter can be rotated for greater heat distribution.
Boerner, A. J.; Maldonado, D. G.; Hansen, Tom
2012-09-01
Environmental assessments and remediation activities are being conducted by the U.S. Department of Energy (DOE) at the Paducah Gaseous Diffusion Plant (PGDP), Paducah, Kentucky. The Oak Ridge Institute for Science and Education (ORISE), a DOE prime contractor, was contracted by the DOE Portsmouth/Paducah Project Office (DOE-PPPO) to conduct radiation dose modeling analyses and derive single radionuclide soil guidelines (soil guidelines) in support of the derivation of Authorized Limits (ALs) for 'DOE-Owned Property Outside the Limited Area' ('Property') at the PGDP. The ORISE evaluation specifically included the area identified by DOE restricted area postings (public use access restrictions) and areas licensed by DOE to the West Kentucky Wildlife Management Area (WKWMA). The licensed areas are available without restriction to the general public for a variety of (primarily) recreational uses. Relevant receptors impacting current and reasonably anticipated future use activities were evaluated. In support of soil guideline derivation, a Conceptual Site Model (CSM) was developed. The CSM listed radiation and contamination sources, release mechanisms, transport media, representative exposure pathways from residual radioactivity, and a total of three receptors (under present and future use scenarios). Plausible receptors included a Resident Farmer, Recreational User, and Wildlife Worker. single radionuclide soil guidelines (outputs specified by the software modeling code) were generated for three receptors and thirteen targeted radionuclides. These soil guidelines were based on satisfying the project dose constraints. For comparison, soil guidelines applicable to the basic radiation public dose limit of 100 mrem/yr were generated. Single radionuclide soil guidelines from the most limiting (restrictive) receptor based on a target dose constraint of 25 mrem/yr were then rounded and identified as the derived soil guidelines. An additional evaluation using the derived soil
NASA Astrophysics Data System (ADS)
Ehrenreich, Philipp; Birkhold, Susanne T.; Zimmermann, Eugen; Hu, Hao; Kim, Kwang-Dae; Weickert, Jonas; Pfadler, Thomas; Schmidt-Mende, Lukas
2016-09-01
Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. It is a common method to apply the H-aggregate model to UV/Vis or photoluminescence spectra in order to analyze polymer aggregation. In this work we present strategies to obtain reliable and conclusive information on polymer aggregation and morphology based on the application of an H-aggregate analysis on UV/Vis and photoluminescence spectra. We demonstrate, with P3HT as model system, that thickness dependent reflection behavior can lead to misinterpretation of UV/Vis spectra within the H-aggregate model. Values for the exciton bandwidth can deviate by a factor of two for polymer thicknesses below 150 nm. In contrast, photoluminescence spectra are found to be a reliable basis for characterization of polymer aggregation due to their weaker dependence on the wavelength dependent refractive index of the polymer. We demonstrate this by studying the influence of surface characteristics on polymer aggregation for spin-coated thin-films that are commonly used in organic and hybrid solar cells.
Ehrenreich, Philipp; Birkhold, Susanne T.; Zimmermann, Eugen; Hu, Hao; Kim, Kwang-Dae; Weickert, Jonas; Pfadler, Thomas; Schmidt-Mende, Lukas
2016-01-01
Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. It is a common method to apply the H-aggregate model to UV/Vis or photoluminescence spectra in order to analyze polymer aggregation. In this work we present strategies to obtain reliable and conclusive information on polymer aggregation and morphology based on the application of an H-aggregate analysis on UV/Vis and photoluminescence spectra. We demonstrate, with P3HT as model system, that thickness dependent reflection behavior can lead to misinterpretation of UV/Vis spectra within the H-aggregate model. Values for the exciton bandwidth can deviate by a factor of two for polymer thicknesses below 150 nm. In contrast, photoluminescence spectra are found to be a reliable basis for characterization of polymer aggregation due to their weaker dependence on the wavelength dependent refractive index of the polymer. We demonstrate this by studying the influence of surface characteristics on polymer aggregation for spin-coated thin-films that are commonly used in organic and hybrid solar cells. PMID:27582091
Ehrenreich, Philipp; Birkhold, Susanne T; Zimmermann, Eugen; Hu, Hao; Kim, Kwang-Dae; Weickert, Jonas; Pfadler, Thomas; Schmidt-Mende, Lukas
2016-01-01
Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. It is a common method to apply the H-aggregate model to UV/Vis or photoluminescence spectra in order to analyze polymer aggregation. In this work we present strategies to obtain reliable and conclusive information on polymer aggregation and morphology based on the application of an H-aggregate analysis on UV/Vis and photoluminescence spectra. We demonstrate, with P3HT as model system, that thickness dependent reflection behavior can lead to misinterpretation of UV/Vis spectra within the H-aggregate model. Values for the exciton bandwidth can deviate by a factor of two for polymer thicknesses below 150 nm. In contrast, photoluminescence spectra are found to be a reliable basis for characterization of polymer aggregation due to their weaker dependence on the wavelength dependent refractive index of the polymer. We demonstrate this by studying the influence of surface characteristics on polymer aggregation for spin-coated thin-films that are commonly used in organic and hybrid solar cells. PMID:27582091
In situ liquid-cell electron microscopy of colloid aggregation and growth dynamics.
Grogan, Joseph M; Rotkina, Lolita; Bau, Haim H
2011-06-01
We report on real-time observations of the aggregation of gold nanoparticles using a custom-made liquid cell that allows for in situ electron microscopy. Process kinetics and fractal dimension of the aggregates are consistent with three-dimensional cluster-cluster diffusion-limited aggregation, even for large aggregates, for which confinement effects are expected. This apparent paradox was resolved through in situ observations of the interactions between individual particles as well as clusters at various stages of the aggregation process that yielded the large aggregates. The liquid cell described herein facilitates real-time observations of various processes in liquid media with the high resolution of the electron microscope. PMID:21797362
Tepordei, V.V.
1995-01-01
Part of the 1994 Industrial Minerals Review. The production, consumption, and applications of construction aggregates are reviewed. In 1994, the production of construction aggregates, which includes crushed stone and construction sand and gravel combined, increased 7.7 percent to 2.14 Gt compared with the previous year. These record production levels are mostly a result of funding for highway construction work provided by the Intermodal Surface Transportation Efficiency Act of 1991. Demand is expected to increase for construction aggregates in 1995.
NASA Astrophysics Data System (ADS)
Batalioto, F.; Barbero, G.; Figueiredo Neto, A. M.
2007-11-01
We analyze the influence of Ohmic electrodes on the impedance spectroscopy of an electrolytic cell in the shape of a slab. The electrolyte is assumed completely dissociated. The positive and negative ions have different diffusion coefficients. We show that in the very low frequency limit, the electrical impedance of the cell reduces to a pure resistance, whose value depends on the diffusion coefficients and on the conductivity of the electrodes. The ratio between the diffusion coefficients determines the numerical value of the plateaus of the resistance, and the position and amplitude of the local minimum of the reactance of the cell.
Scaling of transverse nuclear magnetic relaxation due to magnetic nanoparticle aggregation.
Brown, Keith A; Vassiliou, Christophoros C; Issadore, David; Berezovsky, Jesse; Cima, Michael J; Westervelt, R M
2010-10-01
The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles decreases the transverse nuclear magnetic resonance (NMR) relaxation time T2CP of adjacent water molecules measured by a Carr-Purcell-Meiboom-Gill (CPMG) pulse-echo sequence. This effect is commonly used to measure the concentrations of a variety of small molecules. We perform extensive Monte Carlo simulations of water diffusing around SPIO nanoparticle aggregates to determine the relationship between T2CP and details of the aggregate. We find that in the motional averaging regime T2CP scales as a power law with the number N of nanoparticles in an aggregate. The specific scaling is dependent on the fractal dimension d of the aggregates. We find T2CP∝N-0.44 for aggregates with d = 2.2, a value typical of diffusion limited aggregation. We also find that in two-nanoparticle systems, T2CP is strongly dependent on the orientation of the two nanoparticles relative to the external magnetic field, which implies that it may be possible to sense the orientation of a two-nanoparticle aggregate. To optimize the sensitivity of SPIO nanoparticle sensors, we propose that it is best to have aggregates with few nanoparticles, close together, measured with long pulse-echo times. PMID:20689678
Hauser, M.G.; Arendt, R.G.; Kelsall, T.; Dwek, E.; Odegard, N.; Weiland, J.L.; Freudenreich, H.T.; Reach, W.T.; Pei, Y.C.; Lubin, P.; Mather, J.C.; Shafer, R.A.; Smoot, G.F.; Weiss, R.; Wilkinson, D.T.; Wright, E.L.
1998-11-01
The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer ({ital COBE}) spacecraft was designed primarily to conduct a systematic search for an isotropic cosmic infrared background (CIB) in 10 photometric bands from 1.25 to 240 {mu}m. The results of that search are presented here. Conservative limits on the CIB are obtained from the minimum observed brightness in all-sky maps at each wavelength, with the faintest limits in the DIRBE spectral range being at 3.5 {mu}m ({nu}{ital I}{sub {nu}} {lt} 64 nW m{sup {minus}2} sr{sup {minus}1}, 95{percent} confidence level) and at 240 {mu}m ({nu}{ital I}{sub {nu}} {lt} 28 nW m{sup {minus}2} sr{sup {minus}1}, 95{percent} confidence level). The bright foregrounds from interplanetary dust scattering and emission, stars, and interstellar dust emission are the principal impediments to the DIRBE measurements of the CIB. These foregrounds have been modeled and removed from the sky maps. Assessment of the random and systematic uncertainties in the residuals and tests for isotropy show that only the 140 and 240 {mu}m data provide candidate detections of the CIB. The residuals and their uncertainties provide CIB upper limits more restrictive than the dark sky limits at wavelengths from 1.25 to 100 {mu}m. No plausible solar system or Galactic source of the observed 140 and 240 {mu}m residuals can be identified, leading to the conclusion that the CIB has been detected at levels of {nu}{ital I}{sub {nu}} = 25 {plus_minus} 7 and 14 {plus_minus} 3 nW m{sup {minus}2} sr{sup {minus}1} at 140 and 240 {mu}m, respectively. The integrated energy from 140 to 240 {mu}m, 10.3 nW m{sup {minus}2} sr{sup {minus}1}, is about twice the integrated optical light from the galaxies in the Hubble Deep Field, suggesting that star formation might have been heavily enshrouded by dust at high redshift. The detections and upper limits reported here provide new constraints on models of the history of energy-releasing processes and dust
Colloidal aggregation in microgravity by critical Casimir forces
NASA Astrophysics Data System (ADS)
Veen, Sandra; Schall, Peter; Antoniuk, Oleg; Potenza, Marco; Alaimo, Matteo; Mazzoni, Stefano; Wegdam, Gerard
2012-02-01
We study aggregation and crystal growth of spherical Teflon colloids in binary liquid mixtures in microgravity by the critical Casimir effect. The critical Casimir effect induces interactions between colloids due to the confinement of bulk fluctuations (density or concentration) near the critical point of liquids. The strength and range of the interaction depends on the length scale of these fluctuations which increase as one approaches the critical point. The interaction potential can thus be tuned with temperature. We follow the growth of structures in real time with Near Field Scattering. Measurements are performed in microgravity in order to study pure diffusion limited aggregation, without disturbance by sedimentation or flow.
Gill, Daniel Fury
2007-05-01
The objective of this work is to investigate the thick diffusion limit of various spatial discretizations of the one-dimensional, steady-state, monoenergetic, discrete ordinates neutron transport equation. This work specifically addresses the two lowest order nodal methods, AHOT-N0 and AHOT-N1, as well as reconsiders the asymptotic limit of the Diamond Difference method. The asymptotic analyses of the AHOT-N0 and AHOT-N1 nodal methods show that AHOT-N0 does not possess the thick diffusion limit for cell edge or cell average fluxes except under very limiting conditions, which is to be expected considering the AHOT-N0 method limits to the Step method in the thick diffusion limit. The AHOT-N1 method, which uses a linear in-cell representation of the flux, was shown to possess the thick diffusion limit for both cell average and cell edge fluxes. The thick diffusion limit of the DD method, including the boundary conditions, was derived entirely in terms of cell average scalar fluxes. It was shown that, for vacuum boundaries, only when σ_{t}, h, and Q are constant and σ_{a} = 0 is the asymptotic limit of the DD method close to the finite-differenced diffusion equation in the system interior, and that the boundary conditions between the systems will only agree in the absence of an external source. For a homogeneous medium an effective diffusion coefficient was shown to be present, which was responsible for causing numeric diffusion in certain cases. A technique was presented to correct the numeric diffusion in the interior by altering certain problem parameters. Numerical errors introduced by the boundary conditions and material interfaces were also explored for a two-region problem using the Diamond Difference method. A discrete diffusion solution which exactly solves the one-dimensional diffusion equation in a homogeneous region with constant cross sections and a uniform external source was also developed and shown to be equal to the finite
LI, BO; LIU, YUAN
2015-01-01
A phase-field free-energy functional for the solvation of charged molecules (e.g., proteins) in aqueous solvent (i.e., water or salted water) is constructed. The functional consists of the solute volumetric and solute-solvent interfacial energies, the solute-solvent van der Waals interaction energy, and the continuum electrostatic free energy described by the Poisson–Boltzmann theory. All these are expressed in terms of phase fields that, for low free-energy conformations, are close to one value in the solute phase and another in the solvent phase. A key property of the model is that the phase-field interpolation of dielectric coefficient has the vanishing derivative at both solute and solvent phases. The first variation of such an effective free-energy functional is derived. Matched asymptotic analysis is carried out for the resulting relaxation dynamics of the diffused solute-solvent interface. It is shown that the sharp-interface limit is exactly the variational implicit-solvent model that has successfully captured capillary evaporation in hydrophobic confinement and corresponding multiple equilibrium states of underlying biomolecular systems as found in experiment and molecular dynamics simulations. Our phase-field approach and analysis can be used to possibly couple the description of interfacial fluctuations for efficient numerical computations of biomolecular interactions. PMID:26877556
NASA Astrophysics Data System (ADS)
Lane, Taylor K.; McClarren, Ryan G.
2013-09-01
This work presents semi-analytic solutions to a radiation-hydrodynamics problem of a radiation source driving an initially cold medium. Our solutions are in the equilibrium diffusion limit, include material motion and allow for radiation-dominated situations where the radiation energy is comparable to (or greater than) the material internal energy density. As such, this work is a generalization of the classical Marshak wave problem that assumes no material motion and that the radiation energy is negligible. Including radiation energy density in the model serves to slow down the wave propagation. The solutions provide insight into the impact of radiation energy and material motion, as well as present a novel verification test for radiation transport packages. As a verification test, the solution exercises the radiation-matter coupling terms and their v/c treatment without needing a hydrodynamics solve. An example comparison between the self-similar solution and a numerical code is given. Tables of the self-similar solutions are also provided.
NASA Astrophysics Data System (ADS)
Hu, Pengxiang; Jiang, Zhaoxia; Liu, Qingsong; Heslop, David; Roberts, Andrew P.; Torrent, José; Barrón, Vidal
2016-06-01
Hematite and goethite in soils are often aluminum (Al) substituted, which can dramatically change their reflectance and magnetic properties and bias abundance estimates using diffuse reflectance spectroscopy (DRS) and magnetic techniques. In this study, synthetic Al-substituted hematites and goethites and two Chinese loess/paleosol sequences were investigated to test the feasibility and limitations of estimating Al-hematite and Al-goethite concentration. When Al substitution is limited (Al/(Al + Fe) molar ratio < ~8%), the reflectance spectrum provides a reliable estimate of the goethite/hematite concentration ratio. New empirical relationships between the DRS band intensity ratio and the true concentration goethite/hematite ratio are estimated as goethite/hematite = 1.56 × (I425 nm/I535 nm) or goethite/hematite = 6.32 × (I480 nm/I535 nm), where I425 nm, I480 nm, and I535 nm are the amplitudes of DRS second-derivative curves for characteristic bands at ~425 nm, ~480 nm, and ~535 nm, respectively. High Al substitution (> ~8%) reduces DRS band intensity, which leads to biased estimates of mineral concentration. Al substitution and grain size exert a control on coercivity distributions of hematite and goethite and, thus, affect the hard isothermal remanent magnetization. By integrating DRS and magnetic methods, we suggest a way to constrain hematite and goethite Al substitution in natural loess. Results indicate that hematite and goethite in Chinese loess have Al contents lower than ~8% and, thus, that DRS can be used to trace hematite and goethite concentration variations.
Aggregate-mediated charge transport in ionomeric electrolytes
NASA Astrophysics Data System (ADS)
Lu, Keran; Maranas, Janna; Milner, Scott
Polymers such PEO can conduct ions, and have been studied as possible replacements for organic liquid electrolytes in rechargeable metal-ion batteries. More generally, fast room-temperature ionic conduction has been reported for a variety of materials, from liquids to crystalline solids. Unfortunately, polymer electrolytes generally have limited conductivity; these polymers are too viscous to have fast ion diffusion like liquids, and too unstructured to promote cooperative transport like crystalline solids. Ionomers are polymer electrolytes in which ionic groups are covalently bound to the polymer backbone, neutralized by free counterions. These materials also conduct ions, and can exhibit strong ionic aggregation. Using coarse-grained molecular dynamics, we explore the forces driving ionic aggregation, and describe the role ion aggregates have in mediating charge transport. The aggregates are string-like such that ions typically have two neighbors. We find ion aggregates self-assemble like worm-like micelles. Excess charge, or free ions, occasionally coordinate with aggregates and are transported along the chain in a Grotthuss-like mechanism. We propose that controlling ionomer aggregate structure through materials design can enhance cooperative ion transport.
Bonnefous, Y.C.; Gadgil, A.J.; Revzan, K.L.; Fisk, W.J.; Riley, W.J.
1993-01-01
A subslab aggregate layer can increase the radon entry rate into a building by up to a factor of 5. We use a previously tested numerical technique to investigate and confirm this phenomenon. Then we demonstrate that a sub-aggregate membrane has the potential to significantly reduce the increase in radon entry rate due to the aggregate layer, even when a gap exists between the perimeter of the membrane and the footer. Such membranes greatly reduce diffusion of radon from the soil into the aggregate and are impermeable to flow. Radon entry through the basement floor slab is limited to radon entry through the holes in the membrane. In addition, a sub-aggregate membrane is predicted to improve the performance of active sub-slab ventilation systems and makes passive systems more promising.
Induced growth of dendrite gold nanostructure by controlling self-assembly aggregation dynamics.
Abdellatif, M H; Abdelrasoul, G N; Scarpellini, A; Marras, S; Diaspro, A
2015-11-15
Self-assembly of gold nanoparticles (AuNPs) is an important growth mode for fabricating functional materials. In this work we report a dendrite structure formed by slowing down the aggregation dynamics of AuNPs self-assembly. The obtained results show that the aggregation dynamics is dominated by the Reaction Limited Aggregation Model (RLA) more than the Diffusion Limited Aggregation Model (DLA). In which the repulsion due to electrostatic forces is dominant by the Van Der Walls attraction forces, and low sticking probability of nanoparticles. The aggregation dynamics of AuNPs can be slowed down if the water evaporation of the drop casted colloidal AuNPs on a quartz substrate is slowed. Slowing down the evaporation allows electrostatic repulsion forces to decrease gradually. At certain point, the attraction forces become higher than the electrostatic repulsion and hence cluster aggregation take place slowly. The slow aggregation dynamics allows the nanoparticles to sample all possible orientation in the sticking site, searching for the lowest energy configuration. The size distribution of the nanoparticles in liquid is confirmed using dynamic light scattering based on Stokes-Einstein equation for diffusion coefficient in water. X-ray and photoluminescence (PL) spectra of the sample after aggregation showed a shift which is related to the aggregation compared with non-aggregated colloidal nanoparticles in the solution. The study shows that dendrite self similar structure can be formed by slowing down the aggregation dynamics of nanoparticles as a result of minimizing the Helmholtz free surface energy of the system. PMID:26233557
Colloidal Aggregate Structure under Shear by USANS
NASA Astrophysics Data System (ADS)
Chatterjee, Tirtha; van Dyk, Antony K.; Ginzburg, Valeriy V.; Nakatani, Alan I.
2015-03-01
Paints are complex formulations of polymeric binders, inorganic pigments, dispersants, surfactants, colorants, rheology modifiers, and other additives. A commercially successful paint exhibits a desired viscosity profile over a wide shear rate range from 10-5 s-1 for settling to >104 s-1 for rolling, and spray applications. Understanding paint formulation structure is critical as it governs the paint viscosity profile. However, probing paint formulation structure under shear is a challenging task due to the formulation complexity containing structures with different hierarchical length scales and their alterations under the influence of an external flow field. In this work mesoscale structures of paint formulations under shear are investigated using Ultra Small-Angle Neutron Scattering (rheo-USANS). Contrast match conditions were utilized to independently probe the structure of latex binder particle aggregates and the TiO2 pigment particle aggregates. Rheo-USANS data revealed that the aggregates are fractal in nature and their self-similarity dimensions and correlations lengths depend on the chemistry of the binder particles, the type of rheology modifier present and the shear stress imposed upon the formulation. These results can be explained in the framework of diffusion and reaction limited transient aggregates structure evolution under simple shear.
Bower, C; Washington, C; Purewal, T S
1996-04-01
The effect of increasing solid phase concentration on the morphology and flocculation rate of model aerosol suspensions has been investigated. Suspensions of micronized salbutamol sulphate and lactose in trichlorotrifluoroethane (P113) were studied under conditions of increasing shear stress. By use of image analysis techniques, measurement of aggregate size, fractal dimension and rate of aggregation was performed. The effect of the surfactant sorbitan monooleate on morphology and flocculation rate was also studied. Increased solid phase concentration caused an increase in the rate of aggregation and average aggregate size at a given value of shear stress. Surfactant addition retarded the aggregation rate, and caused a shift from a diffusion-limited cluster aggregation to a reaction-limited cluster aggregation mechanism. The aggregate profiles showed a corresponding change from rugged and crenellated without surfactant, to increasingly smooth and Euclidian with increasing surfactant concentration. The morphological changes were characterized by a decrease in the average boundary fractal dimension which also correlated well with the corresponding reduction in aggregation rate.
Langer, W.H.; Tepordei, V.V.; Bolen, W.P.
2000-01-01
Construction aggregates consist primarily of crushed stone and construction sand and gravel. Total estimated production of construction aggregates increased in 1999 by about 2% to 2.39 Gt (2.64 billion st) compared with 1998. This record production level continued an expansion that began in 1992. By commodities, crushed stone production increased 3.3%, while sand and gravel production increased by about 0.5%.
Tepordei, V.V.
1994-01-01
Part of a special section on industrial minerals in 1993. The 1993 production of construction aggregates increased 6.3 percent over the 1992 figure, to reach 2.01 Gt. This represents the highest estimated annual production of combined crushed stone and construction sand and gravel ever recorded in the U.S. The outlook for construction aggregates and the issues facing the industry are discussed.
On the radiative properties of soot aggregates part 1: Necking and overlapping
NASA Astrophysics Data System (ADS)
Yon, J.; Bescond, A.; Liu, F.
2015-09-01
There is a strong interest in accurately modelling the radiative properties of soot aggregates (also known as black carbon particles) emitted from combustion systems and fires to gain improved understanding of the role of black carbon to global warming. This study conducted a systematic investigation of the effects of overlapping and necking between neighbouring primary particles on the radiative properties of soot aggregates using the discrete dipole approximation. The degrees of overlapping and necking are quantified by the overlapping and necking parameters. Realistic soot aggregates were generated numerically by constructing overlapping and necking to fractal aggregates formed by point-touch primary particles simulated using a diffusion-limited cluster aggregation algorithm. Radiative properties (differential scattering, absorption, total scattering, specific extinction, asymmetry factor and single scattering albedo) were calculated using the experimentally measured soot refractive index over the spectral range of 266-1064 nm for 9 combinations of the overlapping and necking parameters. Overlapping and necking affect significantly the absorption and scattering properties of soot aggregates, especially in the near UV spectrum due to the enhanced multiple scattering effects within an aggregate. By using correctly modified aggregate properties (fractal dimension, prefactor, primary particle radius, and the number of primary particle) and by accounting for the effects of multiple scattering, the simple Rayleigh-Debye-Gans theory for fractal aggregates can reproduce reasonably accurate radiative properties of realistic soot aggregates.
Fractal structure and the dynamics of aggregation of synthetic melanin in low pH aqueous solutions
Huang, J.S.; Sung, J.; Eisner, M.; Moss, S.C.; Gallas, J.
1989-01-01
We have used static and dynamic light scattering to study the dynamics of aggregation of synthetic melanin, an amorphous biopolymeric substance, in low pH aqueous solution. We have found that, depending on the final pH value of the solutions, there existed two regimes of the aggregation kinetics, one corresponding to diffusion limited aggregation (DLA), and the other corresponding to reaction limited aggregation (RLA). The precipitates formed in these two regimes can be characterized by fractal structures. We have found fractal dimensions of d/sub f/ = 1.8 for the DLA clusters and d/sub f/ = 2.2 for the RLA clusters. These results agree well with the proposed limits of the fractal dimensions of the gold aggregates formed in aqueous solutions by Weitz et al.
Moinzadeh, Pia; Aberer, Elisabeth; Ahmadi-Simab, Keihan; Blank, Norbert; Distler, Joerg H W; Fierlbeck, Gerhard; Genth, Ekkehard; Guenther, Claudia; Hein, Ruediger; Henes, Joerg; Herich, Lena; Herrgott, Ilka; Koetter, Ina; Kreuter, Alexander; Krieg, Thomas; Kuhr, Kathrin; Lorenz, Hanns-Martin; Meier, Florian; Melchers, Inga; Mensing, Hartwig; Mueller-Ladner, Ulf; Pfeiffer, Christiane; Riemekasten, Gabriela; Sárdy, Miklós; Schmalzing, Marc; Sunderkoetter, Cord; Susok, Laura; Tarner, Ingo H; Vaith, Peter; Worm, Margitta; Wozel, Gottfried; Zeidler, Gabriele; Hunzelmann, Nicolas; Ahrazoglu, Nil Mona
2015-01-01
Background Systemic sclerosis (SSc)-overlap syndromes are a very heterogeneous and remarkable subgroup of SSc-patients, who present at least two connective tissue diseases (CTD) at the same time, usually with a specific autoantibody status. Objectives To determine whether patients, classified as overlap syndromes, show a disease course different from patients with limited SSc (lcSSc) or diffuse cutaneous SSc (dcSSc). Methods The data of 3240 prospectively included patients, registered in the database of the German Network for Systemic Scleroderma and followed between 2003 and 2013, were analysed. Results Among 3240 registered patients, 10% were diagnosed as SSc-overlap syndrome. Of these, 82.5% were female. SSc-overlap patients had a mean age of 48±1.2 years and carried significantly more often ‘other antibodies’ (68.0%; p<0.0001), including anti-U1RNP, -PmScl, -Ro, -La, as well as anti-Jo-1 and -Ku antibodies. These patients developed musculoskeletal involvement earlier and more frequently (62.5%) than patients diagnosed as lcSSc (32.2%) or dcSSc (43.3%) (p<0.0001). The onset of lung fibrosis and heart involvement in SSc-overlap patients was significantly earlier than in patients with lcSSc and occurred later than in patients with dcSSc. Oesophagus, kidney and PH progression was similar to lcSSc patients, whereas dcSSc patients had a significantly earlier onset. Conclusions These data support the concept that SSc-overlap syndromes should be regarded as a separate SSc subset, distinct from lcSSc and dcSSc, due to a different progression of the disease, different proportional distribution of specific autoantibodies, and of different organ involvement. PMID:24389298
NASA Astrophysics Data System (ADS)
Boness, D. A.; Terrell-Martinez, B.
2010-12-01
As part of an ongoing undergraduate research project of light scattering calculations involving fractal carbonaceous soot aggregates relevant to current anthropogenic and natural sources in Earth's atmosphere, we have read with interest a recent paper [E.T. Wolf and O.B Toon,Science 328, 1266 (2010)] claiming that the Faint Young Sun paradox discussed four decades ago by Carl Sagan and others can be resolved without invoking heavy CO2 concentrations as a greenhouse gas warming the early Earth enough to sustain liquid water and hence allow the origin of life. Wolf and Toon report that a Titan-like Archean Earth haze, with a fractal haze aggregate nature due to nitrogen-methane photochemistry at high altitudes, should block enough UV light to protect the warming greenhouse gas NH3 while allowing enough visible light to reach the surface of the Earth. To test this hypothesis, we have employed a rigorous T-Matrix arbitrary-particle light scattering technique, to avoid the simplifications inherent in Mie-sphere scattering, on haze fractal aggregates at UV and visible wavelenths of incident light. We generate these model aggregates using diffusion-limited cluster aggregation (DLCA) algorithms, which much more closely fit actual haze fractal aggregates than do diffusion-limited aggregation (DLA) algorithms.
Naglič, Peter; Pernuš, Franjo; Likar, Boštjan; Bürmen, Miran
2015-10-01
Light propagation models often simplify the interface between the optical fiber probe tip and tissue to a laterally uniform boundary with mismatched refractive indices. Such simplification neglects the precise optical properties of the commonly used probe tip materials, e.g. stainless steel or black epoxy. In this paper, we investigate the limitations of the laterally uniform probe-tissue interface in Monte Carlo simulations of diffuse reflectance. In comparison to a realistic probe-tissue interface that accounts for the layout and properties of the probe tip materials, the simplified laterally uniform interface is shown to introduce significant errors into the simulated diffuse reflectance.
Naglič, Peter; Pernuš, Franjo; Likar, Boštjan; Bürmen, Miran
2015-01-01
Light propagation models often simplify the interface between the optical fiber probe tip and tissue to a laterally uniform boundary with mismatched refractive indices. Such simplification neglects the precise optical properties of the commonly used probe tip materials, e.g. stainless steel or black epoxy. In this paper, we investigate the limitations of the laterally uniform probe-tissue interface in Monte Carlo simulations of diffuse reflectance. In comparison to a realistic probe-tissue interface that accounts for the layout and properties of the probe tip materials, the simplified laterally uniform interface is shown to introduce significant errors into the simulated diffuse reflectance. PMID:26504647
Palamakumbura, A H; Foshay, M C; Vitello, L B; Erman, J E
1999-11-23
The rate of the reaction between p-nitroperoxybenzoic acid and cytochrome c peroxidase (CcP) has been investigated as a function of pH and ionic strength. The pH dependence of the reaction between CcP and peracetic acid has also been determined. The rate of the reactions are influenced by two heme-linked ionizations in the protein. The enzyme is active when His-52 (pK(a) 3.8 +/- 0.1) is unprotonated and an unknown group with a pK(a) of 9.8 +/- 0.1 is protonated. The bimolecular rate constant for the reaction between peracetic acid and CcP and between p-nitroperoxybenzoic acid and CcP are (1.8 +/- 0.1) x 10(7) and (1.6 +/- 0.2) x 10(7) M(-)(1) s(-)(1), respectively. These rates are about 60% slower than the reaction between hydrogen peroxide and CcP. A critical comparison of the pH dependence of the reactions of hydrogen peroxide, peracetic acid, and p-nitroperoxybenzoic acid with CcP provides evidence that both the neutral and anionic forms of the two peroxyacids react directly with the enzyme. The peracetate and p-nitroperoxybenzoate anions react with CcP with rates of (1.5 +/- 0.1) x 10(6) and (1.6 +/- 0.2) x 10(6) M(-)(1) s(-)(1), respectively, about 10 times slower than the neutral peroxyacids. These data indicate that CcP discriminates between the neutral peroxyacids and their negatively charged ions. However, the apparent bimolecular rate constant for reaction between p-nitroperoxybenzoate and CcP is independent of ionic strength in the range of 0.01-1.0 M, suggesting that electrostatic repulsion between the anion and CcP is not the cause of the lower reactivity for the peroxybenzoate anion. The data are consistent with the hypothesis that the rate-limiting step for the oxidation of CcP to compound I by both neutral peroxyacid and the negatively charged peroxide ion is diffusion of the reactants through the protein matrix, from the surface of the protein to the distal heme pocket. PMID:10569950
Forget, Nathalie L; Kim Juniper, S
2013-04-01
We systematically studied free-living bacterial diversity within aggregations of the vestimentiferan tubeworm Ridgeia piscesae sampled from two contrasting flow regimes (High Flow and Low Flow) in the Endeavour Hydrothermal Vents Marine Protected Area (MPA) on the Juan de Fuca Ridge (Northeast Pacific). Eight samples of particulate detritus were recovered from paired tubeworm grabs from four vent sites. Most sequences (454 tag and Sanger methods) were affiliated to the Epsilonproteobacteria, and the sulfur-oxidizing genus Sulfurovum was dominant in all samples. Gammaproteobacteria were also detected, mainly in Low Flow sequence libraries, and were affiliated with known methanotrophs and decomposers. The cooccurrence of sulfur reducers from the Deltaproteobacteria and the Epsilonproteobacteria suggests internal sulfur cycling within these habitats. Other phyla detected included Bacteroidetes, Actinobacteria, Chloroflexi, Firmicutes, Planctomycetes, Verrucomicrobia, and Deinococcus-Thermus. Statistically significant relationships between sequence library composition and habitat type suggest a predictable pattern for High Flow and Low Flow environments. Most sequences significantly more represented in High Flow libraries were related to sulfur and hydrogen oxidizers, while mainly heterotrophic groups were more represented in Low Flow libraries. Differences in temperature, available energy for metabolism, and stability between High Flow and Low Flow habitats potentially explain their distinct bacterial communities.
Forget, Nathalie L; Kim Juniper, S
2013-04-01
We systematically studied free-living bacterial diversity within aggregations of the vestimentiferan tubeworm Ridgeia piscesae sampled from two contrasting flow regimes (High Flow and Low Flow) in the Endeavour Hydrothermal Vents Marine Protected Area (MPA) on the Juan de Fuca Ridge (Northeast Pacific). Eight samples of particulate detritus were recovered from paired tubeworm grabs from four vent sites. Most sequences (454 tag and Sanger methods) were affiliated to the Epsilonproteobacteria, and the sulfur-oxidizing genus Sulfurovum was dominant in all samples. Gammaproteobacteria were also detected, mainly in Low Flow sequence libraries, and were affiliated with known methanotrophs and decomposers. The cooccurrence of sulfur reducers from the Deltaproteobacteria and the Epsilonproteobacteria suggests internal sulfur cycling within these habitats. Other phyla detected included Bacteroidetes, Actinobacteria, Chloroflexi, Firmicutes, Planctomycetes, Verrucomicrobia, and Deinococcus-Thermus. Statistically significant relationships between sequence library composition and habitat type suggest a predictable pattern for High Flow and Low Flow environments. Most sequences significantly more represented in High Flow libraries were related to sulfur and hydrogen oxidizers, while mainly heterotrophic groups were more represented in Low Flow libraries. Differences in temperature, available energy for metabolism, and stability between High Flow and Low Flow habitats potentially explain their distinct bacterial communities. PMID:23401293
Forget, Nathalie L; Kim Juniper, S
2013-01-01
We systematically studied free-living bacterial diversity within aggregations of the vestimentiferan tubeworm Ridgeia piscesae sampled from two contrasting flow regimes (High Flow and Low Flow) in the Endeavour Hydrothermal Vents Marine Protected Area (MPA) on the Juan de Fuca Ridge (Northeast Pacific). Eight samples of particulate detritus were recovered from paired tubeworm grabs from four vent sites. Most sequences (454 tag and Sanger methods) were affiliated to the Epsilonproteobacteria, and the sulfur-oxidizing genus Sulfurovum was dominant in all samples. Gammaproteobacteria were also detected, mainly in Low Flow sequence libraries, and were affiliated with known methanotrophs and decomposers. The cooccurrence of sulfur reducers from the Deltaproteobacteria and the Epsilonproteobacteria suggests internal sulfur cycling within these habitats. Other phyla detected included Bacteroidetes, Actinobacteria, Chloroflexi, Firmicutes, Planctomycetes, Verrucomicrobia, and Deinococcus–Thermus. Statistically significant relationships between sequence library composition and habitat type suggest a predictable pattern for High Flow and Low Flow environments. Most sequences significantly more represented in High Flow libraries were related to sulfur and hydrogen oxidizers, while mainly heterotrophic groups were more represented in Low Flow libraries. Differences in temperature, available energy for metabolism, and stability between High Flow and Low Flow habitats potentially explain their distinct bacterial communities. PMID:23401293
Aggregation in charged nanoparticles solutions induced by different interactions
NASA Astrophysics Data System (ADS)
Abbas, S.; Kumar, Sugam; Aswal, V. K.; Kohlbrecher, J.
2016-05-01
Small-angle neutron scattering (SANS) has been used to study the aggregation of anionic silica nanoparticles as induced through different interactions. The nanoparticle aggregation is induced by addition of salt (NaCl), cationic protein (lysozyme) and non-ionic surfactant (C12E10) employing different kind of interactions. The results show that the interaction in presence of salt can be explained using DLVO theory whereas non-DLVO forces play important role for interaction of nanoparticles with protein and surfactant. The presence of salt screens the repulsion between charged nanoparticles giving rise to a net attraction in the DLVO potential. On the other hand, strong electrostatic attraction between nanoparticle and oppositely charged protein leads to protein-mediated nanoparticle aggregation. In case of non-ionic surfactant, the relatively long-range attractive depletion interaction is found to be responsible for the particle aggregation. Interestingly, the completely different interactions lead to similar kind of aggregate morphology. The nanoparticle aggregates formed are found to have mass fractal nature having a fractal dimension (~2.5) consistent with diffusion limited type of fractal morphology in all three cases.
Rosspeintner, Arnulf; Angulo, Gonzalo; Vauthey, Eric
2014-02-01
To access the intrinsic, diffusion free, rate constant of bimolecular photoinduced electron transfer reactions, fluorescence quenching experiments have been performed with 14 donor/acceptor pairs, covering a driving-force range going from 0.6 to 2.4 eV, using steady-state and femtosecond time-resolved emission, and applying a diffusion-reaction model that accounts for the static and transient stages of the quenching for the analysis. The intrinsic electron transfer rate constants are up to 2 orders of magnitude larger than the diffusion rate constant in acetonitrile. Above ∼1.5 eV, a slight decrease of the rate constant is observed, pointing to a much weaker Marcus inverted region than those reported for other types of electron transfer reactions, such as charge recombination. Despite this, the driving force dependence can be rationalized in terms of Marcus theory. PMID:24400958
Wei, Jingsong; Wang, Rui
2014-03-28
In this work, the resolving limit of maskless direct laser writing is overcome by cooperative manipulation from nonlinear reverse saturation absorption and thermal diffusion, where the nonlinear reverse saturation absorption can induce the formation of below diffraction-limited energy absorption spot, and the thermal diffusion manipulation can make the heat quantity at the central region of energy absorption spot propagate along the thin film thickness direction. The temperature at the central region of energy absorption spot transiently reaches up to melting point and realizes nanolithography. The sample “glass substrate/AgInSbTe” is prepared, where AgInSbTe is taken as nonlinear reverse saturation absorption thin film. The below diffraction-limited energy absorption spot is simulated theoretically and verified experimentally by near-field spot scanning method. The “glass substrate/Al/AgInSbTe” sample is prepared, where the Al is used as thermal conductive layer to manipulate the thermal diffusion channel because the thermal diffusivity coefficient of Al is much larger than that of AgInSbTe. The direct laser writing is conducted by a setup with a laser wavelength of 650 nm and a converging lens of NA=0.85, the lithographic marks with a size of about 100 nm are obtained, and the size is only about 1/10 the incident focused spot. The experimental results indicate that the cooperative manipulation from nonlinear reverse saturation absorption and thermal diffusion is a good method to realize nanolithography in maskless direct laser writing with visible light.
Pabst, Breana; Pitts, Betsey; Lauchnor, Ellen; Stewart, Philip S
2016-10-01
An experimental model that mimicked the structure and characteristics of in vivo biofilm infections, such as those occurring in the lung or in dermal wounds where no biomaterial surface is present, was developed. In these infections, microbial biofilm forms as cell aggregates interspersed in a layer of mucus or host matrix material. This structure was modeled by filling glass capillary tubes with an agarose gel that had been seeded with Staphylococcus aureus bacteria and then incubating the gel biofilm in medium for up to 30 h. Confocal microscopy showed that the bacteria formed in discrete pockets distributed throughout the gel matrix. These aggregates enlarged over time and also developed a size gradient, with the clusters being larger near the nutrient- and oxygen-supplied interface and smaller at greater depths. Bacteria entrapped in gels for 24 h grew slowly (specific growth rate, 0.06 h(-1)) and were much less susceptible to oxacillin, minocycline, or ciprofloxacin than planktonic cells. Microelectrode measurements showed that the oxygen concentration decreased with depth into the gel biofilm, falling to values less than 3% of air saturation at depths of 500 μm. An anaerobiosis-responsive green fluorescent protein reporter gene for lactate dehydrogenase was induced in the region of the gel where the measured oxygen concentrations were low, confirming biologically relevant hypoxia. These results show that the gel biofilm model captures key features of biofilm infection in mucus or compromised tissue: formation of dense, distinct aggregates, reduced specific growth rates, local hypoxia, and antibiotic tolerance. PMID:27503656
Rajagopal, Karthikan; Wood, Joseph; Tran, Benjamin; Patapoff, Thomas W; Nivaggioli, Thierry
2013-08-01
Polymer implants are promising systems for sustained release applications but their utility for protein delivery has been hindered because of concerns over drug stability at elevated temperatures required for processing. Using bovine serum albumin (BSA) as a model, we have assessed whether proteins can be formulated for processing at elevated temperatures. Specifically, the effect of trehalose and histidine-HCl buffer on BSA stability in a spray-dried formulation has been investigated at temperatures ranging from 80°C to 110°C. When both the sugar and buffer are present, aggregation is suppressed even when exposed to 100°C, the extrusion temperature of poly(lactide-co-glycolide) (PLGA), a bioresorbable polymer. Estimation of aggregation rate constants (k) indicate that though both trehalose and histidine-HCl buffer contribute to BSA stability, the effect because of trehalose alone is more pronounced. BSA-loaded PLGA implants were prepared using hot-melt extrusion process and in vitro release was conducted in phosphate buffered saline at 37°C. Comparison of drug released from implants prepared using four different formulations confirmed that maximal release was achieved from the formulation in which BSA was least aggregated. These studies demonstrate that when trehalose and histidine-HCl buffer are included in spray-dried formulations, BSA stability is maintained both during processing at 100°C and long-term residence within implants.
Meng, Zhiyong; Hashmi, Sara M; Elimelech, Menachem
2013-02-15
The time-evolutions of nanoparticle hydrodynamic radius and aggregate fractal dimension during the aggregation of fullerene (C(60)) nanoparticles (FNPs) were measured via simultaneous multiangle static and dynamic light scattering. The FNP aggregation behavior was determined as a function of monovalent (NaCl) and divalent (CaCl(2)) electrolyte concentration, and the impact of addition of dissolved natural organic matter (humic acid) to the solution was also investigated. In the absence of humic acid, the fractal dimension decreased over time with monovalent and divalent salts, suggesting that aggregates become slightly more open and less compact as they grow. Although the aggregates become slightly more open, the magnitude of the fractal dimension suggests intermediate aggregation between the diffusion- and reaction-limited regimes. We observed different aggregation behavior with monovalent and divalent salts upon the addition of humic acid to the solution. For NaCl-induced aggregation, the introduction of humic acid significantly suppressed the aggregation rate of FNPs at NaCl concentrations lower than 150mM. In this case, the aggregation was intermediate or reaction-limited even at NaCl concentrations as high as 500mM, giving rise to aggregates with a fractal dimension of 2.0. For CaCl(2)-induced aggregation, the introduction of humic acid enhanced the aggregation of FNPs at CaCl(2) concentrations greater than about 5mM due to calcium complexation and bridging effects. Humic acid also had an impact on the FNP aggregate structure in the presence of CaCl(2), resulting in a fractal dimension of 1.6 for the diffusion-limited aggregation regime. Our results with CaCl(2) indicate that in the presence of humic acid, FNP aggregates have a more open and loose structure than in the absence of humic acid. The aggregation results presented in this paper have important implications for the transport, chemical reactivity, and toxicity of engineered nanoparticles in aquatic
Aggregation and sedimentation in gas-fluidized beds of cohesive powders.
Castellanos, A; Valverde, J M; Quintanilla, M A
2001-10-01
We present measurements on the settling velocity of gas-fluidized beds of fine cohesive powders. In the solidlike regime (solid volume fraction straight phi>straight phi(c)) particles are static, sustained by enduring contacts. The settling is hindered by interparticle contacts and is a very slow process. In the fluidlike regime (straight phi
Nelson, T.I.; Bolen, W.P.
2007-01-01
Construction aggregates, primarily stone, sand and gravel, are recovered from widespread naturally occurring mineral deposits and processed for use primarily in the construction industry. They are mined, crushed, sorted by size and sold loose or combined with portland cement or asphaltic cement to make concrete products to build roads, houses, buildings, and other structures. Much smaller quantities are used in agriculture, cement manufacture, chemical and metallurgical processes, glass production and many other products.
Tepordei, V.V.
1996-01-01
Part of the Annual Commodities Review 1995. Production of construction aggregates such as crushed stone and construction sand and gravel showed a marginal increase in 1995. Most of the 1995 increases were due to funding for highway construction work. The major areas of concern to the industry included issues relating to wetlands classification and the classification of crystalline silica as a probable human carcinogen. Despite this, an increase in demand is anticipated for 1996.
Tepordei, V.V.
1993-01-01
Part of a special section on the market performance of industrial minerals in 1992. Production of construction aggregates increased by 4.6 percent in 1992. This increase was due, in part, to the increased funding for transportation and infrastructure projects. The U.S. produced about 1.05 Gt of crushed stone and an estimated 734 Mt of construction sand and gravel in 1992. Demand is expected to increase by about 5 percent in 1993.
Reinforcement of rubber by fractal aggregates
NASA Astrophysics Data System (ADS)
Witten, T. A.; Rubinstein, M.; Colby, R. H.
1993-03-01
Rubber is commonly reinforced with colloidal aggregates of carbon or silica, whose structure has the scale invariance of a fractal object. Reinforced rubbers support large stresses, which often grow faster than linearly with the strain. We argue that under strong elongation the stress arises through lateral compression of the aggregates, driven by the large bulk modulus of the rubber. We derive a power-law relationship between stress and elongation λ when λgg 1. The predicted power p depends on the fractal dimension D and a second structural scaling exponent C. For diffusion-controlled aggregates this power p should lie beween 0.9 and 1.1 ; for reaction-controlled aggregates p should lie between 1.8 and 2.4. For uniaxial compression the analogous powers lie near 4. Practical rubbers filled with fractal aggregates should approach the conditions of validity for these scaling laws. On renforce souvent le caoutchouc avec des agrégats de carbone ou de silice dont la structure a l'invariance par dilatation d'un objet fractal. Les caoutchoucs ainsi renforcés supportent de grandes contraintes qui croissent souvent plus vite que l'élongation. Nous prétendons que, sous élongation forte, cette contrainte apparaît à cause d'une compression latérale des agrégats induite par le module volumique important du caoutchouc. Nous établissons une loi de puissance reliant la contrainte et l'élongation λ quand λgg 1. Cet exposant p dépend de la dimension fractale D et d'un deuxième exposant structural C. Pour des agrégats dont la cinétique de formation est limitée par diffusion, p vaut entre 0,9 et 1,1. Si la cinétique est limitée par le soudage local des particules, p vaut entre 1,8 et 2,4. Sous compression uniaxiale, les puissances homologues valent environ 4. Des caoutchoucs pratiques chargés de tels agrégats devraient approcher des conditions où ces lois d'échelle sont valables.
Aggregation kinetics and dissolution of coated silver nanoparticles.
Li, Xuan; Lenhart, John J; Walker, Harold W
2012-01-17
Determining the fate of manufactured nanomaterials in the environment is contingent upon understanding how stabilizing agents influence the stability of nanoparticles in aqueous systems. In this study, the aggregation and dissolution tendencies of uncoated silver nanoparticles and the same particles coated with three common coating agents, trisodium citrate, sodium dodecyl sulfate (SDS), and Tween 80 (Tween), were evaluated. Early stage aggregation kinetics of the uncoated and coated silver nanoparticles were assessed by dynamic light scattering over a range of electrolyte types (NaCl, NaNO(3), and CaCl(2)) and concentrations that span those observed in natural waters. Although particle dissolution was observed, aggregation of all particle types was still consistent with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation of citrate-coated particles and SDS-coated particles were very similar to that for the uncoated particles, as the critical coagulation concentrations (CCC) of the particles in different electrolytes were all approximately the same (40 mM NaCl, 30 mM NaNO(3), and 2 mM CaCl(2)). The Tween-stabilized particles were significantly more stable than the other particles, however, and in NaNO(3) aggregation was not observed up to an electrolyte concentration of 1 M. Differences in the rate of aggregation under diffusion-limited aggregation conditions at high electrolyte concentrations for the SDS and Tween-coated particles, in combination with the moderation of their electrophoretic mobilities, suggest SDS and Tween imparted steric interactions to the particles. The dissolution of the silver nanoparticles was inhibited by the SDS and Tween coatings, but not by the citrate coating, and in chloride-containing electrolytes a secondary precipitate of AgCl was observed bridging the individual particles. These results indicate that coating agents could significant influence the fate of silver nanoparticles in aquatic systems, and in some
Surface fractals in liposome aggregation.
Roldán-Vargas, Sándalo; Barnadas-Rodríguez, Ramon; Quesada-Pérez, Manuel; Estelrich, Joan; Callejas-Fernández, José
2009-01-01
In this work, the aggregation of charged liposomes induced by magnesium is investigated. Static and dynamic light scattering, Fourier-transform infrared spectroscopy, and cryotransmission electron microscopy are used as experimental techniques. In particular, multiple intracluster scattering is reduced to a negligible amount using a cross-correlation light scattering scheme. The analysis of the cluster structure, probed by means of static light scattering, reveals an evolution from surface fractals to mass fractals with increasing magnesium concentration. Cryotransmission electron microscopy micrographs of the aggregates are consistent with this interpretation. In addition, a comparative analysis of these results with those previously reported in the presence of calcium suggests that the different hydration energy between lipid vesicles when these divalent cations are present plays a fundamental role in the cluster morphology. This suggestion is also supported by infrared spectroscopy data. The kinetics of the aggregation processes is also analyzed through the time evolution of the mean diffusion coefficient of the aggregates. PMID:19257067
Surface fractals in liposome aggregation.
Roldán-Vargas, Sándalo; Barnadas-Rodríguez, Ramon; Quesada-Pérez, Manuel; Estelrich, Joan; Callejas-Fernández, José
2009-01-01
In this work, the aggregation of charged liposomes induced by magnesium is investigated. Static and dynamic light scattering, Fourier-transform infrared spectroscopy, and cryotransmission electron microscopy are used as experimental techniques. In particular, multiple intracluster scattering is reduced to a negligible amount using a cross-correlation light scattering scheme. The analysis of the cluster structure, probed by means of static light scattering, reveals an evolution from surface fractals to mass fractals with increasing magnesium concentration. Cryotransmission electron microscopy micrographs of the aggregates are consistent with this interpretation. In addition, a comparative analysis of these results with those previously reported in the presence of calcium suggests that the different hydration energy between lipid vesicles when these divalent cations are present plays a fundamental role in the cluster morphology. This suggestion is also supported by infrared spectroscopy data. The kinetics of the aggregation processes is also analyzed through the time evolution of the mean diffusion coefficient of the aggregates.
On the radiative properties of soot aggregates - Part 2: Effects of coating
NASA Astrophysics Data System (ADS)
Liu, Fengshan; Yon, Jérôme; Bescond, Alexandre
2016-03-01
The effects of weakly absorbing material coating on soot have attracted considerable research attention in recent years due to the significant influence of such coating on soot radiative properties and the large differences predicted by different numerical models. Soot aggregates were first numerically generated using the diffusion limited cluster aggregation algorithm to produce fractal aggregates formed by log-normally distributed polydisperse spherical primary particles in point-touch. These aggregates were then processed by adding a certain amount of primary particle overlapping and necking to simulate the soot morphology observed from transmission electron microscopy images. After this process, a layer of WAM coating of different thicknesses was added to these more realistic soot aggregates. The radiative properties of these coated soot aggregates over the spectral range of 266-1064 nm were calculated by the discrete dipole approximation (DDA) using the spectrally dependent refractive index of soot for four aggregates containing Np=1, 20, 51 and 96 primary particles. The considered coating thicknesses range from 0% (no coating) up to 100% coating in terms of the primary particle diameter. Coating enhances both the particle absorption and scattering cross sections, with much stronger enhancement to the scattering one, as well as the asymmetry factor and the single scattering albedo. The absorption enhancement is stronger in the UV than in the visible and the near infrared. The simple corrections to the Rayleigh-Debye-Gans fractal aggregates theory for uncoated soot aggregates are found not working for coated soot aggregates. The core-shell model significantly overestimates the absorption enhancement by coating in the visible and the near infrared compared to the DDA results of the coated soot particle. Treating an externally coated soot aggregate as an aggregate formed by individually coated primary particles significantly underestimates the absorption
Bolen, W.P.; Tepordei, V.V.
2001-01-01
The estimated production during 2000 of construction aggregates, crushed stone, and construction sand and gravel increased by about 2.6% to 2.7 Gt (3 billion st), compared with 1999. The expansion that started in 1992 continued with record production levels for the ninth consecutive year. By commodity, construction sand and gravel production increased by 4.5% to 1.16 Gt (1.28 billion st), while crushed stone production increased by 1.3% to 1.56 Gt (1.72 billion st).
Delayed plastic relaxation limit in SiGe islands grown by Ge diffusion from a local source
Vanacore, G. M.; Zani, M.; Tagliaferri, A.; Nicotra, G.; Bollani, M.; Bonera, E.; Montalenti, F.; Picco, A.; Boioli, F.; Capellini, G.; Isella, G.; Osmond, J.
2015-03-14
The hetero-epitaxial strain relaxation in nano-scale systems plays a fundamental role in shaping their properties. Here, the elastic and plastic relaxation of self-assembled SiGe islands grown by surface-thermal-diffusion from a local Ge solid source on Si(100) are studied by atomic force and transmission electron microscopies, enabling the simultaneous investigation of the strain relaxation in different dynamical regimes. Islands grown by this technique remain dislocation-free and preserve a structural coherence with the substrate for a base width as large as 350 nm. The results indicate that a delay of the plastic relaxation is promoted by an enhanced Si-Ge intermixing, induced by the surface-thermal-diffusion, which takes place already in the SiGe overlayer before the formation of a critical nucleus. The local entropy of mixing dominates, leading the system toward a thermodynamic equilibrium, where non-dislocated, shallow islands with a low residual stress are energetically stable. These findings elucidate the role of the interface dynamics in modulating the lattice distortion at the nano-scale, and highlight the potential use of our growth strategy to create composition and strain-controlled nano-structures for new-generation devices.
Delayed plastic relaxation limit in SiGe islands grown by Ge diffusion from a local source
NASA Astrophysics Data System (ADS)
Vanacore, G. M.; Nicotra, G.; Zani, M.; Bollani, M.; Bonera, E.; Montalenti, F.; Capellini, G.; Isella, G.; Osmond, J.; Picco, A.; Boioli, F.; Tagliaferri, A.
2015-03-01
The hetero-epitaxial strain relaxation in nano-scale systems plays a fundamental role in shaping their properties. Here, the elastic and plastic relaxation of self-assembled SiGe islands grown by surface-thermal-diffusion from a local Ge solid source on Si(100) are studied by atomic force and transmission electron microscopies, enabling the simultaneous investigation of the strain relaxation in different dynamical regimes. Islands grown by this technique remain dislocation-free and preserve a structural coherence with the substrate for a base width as large as 350 nm. The results indicate that a delay of the plastic relaxation is promoted by an enhanced Si-Ge intermixing, induced by the surface-thermal-diffusion, which takes place already in the SiGe overlayer before the formation of a critical nucleus. The local entropy of mixing dominates, leading the system toward a thermodynamic equilibrium, where non-dislocated, shallow islands with a low residual stress are energetically stable. These findings elucidate the role of the interface dynamics in modulating the lattice distortion at the nano-scale, and highlight the potential use of our growth strategy to create composition and strain-controlled nano-structures for new-generation devices.
Ranger, Jochen; Kuhn, Jörg-Tobias; Szardenings, Carsten
2016-05-01
Psychological tests are usually analysed with item response models. Recently, some alternative measurement models have been proposed that were derived from cognitive process models developed in experimental psychology. These models consider the responses but also the response times of the test takers. Two such models are the Q-diffusion model and the D-diffusion model. Both models can be calibrated with the diffIRT package of the R statistical environment via marginal maximum likelihood (MML) estimation. In this manuscript, an alternative approach to model calibration is proposed. The approach is based on weighted least squares estimation and parallels the standard estimation approach in structural equation modelling. Estimates are determined by minimizing the discrepancy between the observed and the implied covariance matrix. The estimator is simple to implement, consistent, and asymptotically normally distributed. Least squares estimation also provides a test of model fit by comparing the observed and implied covariance matrix. The estimator and the test of model fit are evaluated in a simulation study. Although parameter recovery is good, the estimator is less efficient than the MML estimator.
Okorie, Chinonye K; Ogbole, Godwin I; Owolabi, Mayowa O; Ogun, Olufunmilola; Adeyinka, Abiodun; Ogunniyi, Adesola
2015-01-01
A variety of imaging modalities exist for the diagnosis of stroke. Several studies have been carried out to ascertain their contribution to the management of acute stroke and to compare the benefits and limitations of each modality. Diffusion-weighted imaging (DWI) has been described as the optimal imaging technique for diagnosing acute ischemic stroke, yet limited evidence is available on the value of DWI in the management of ischemic stroke with low-field magnetic resonance (MR) systems. Although high-field MR imaging (MRI) is desirable for DWI, low-field scanners provide an acceptable clinical compromise which is of importance to developing countries posed with the challenge of limited availability of high-field units. The purpose of this paper was to systematically review the literature on the usefulness of DWI in acute stroke management with low-field MRI scanners and present the experience in Nigeria. PMID:26709342
Colloidal Recycling: Reconfiguration of Random Aggregates into Patchy Particles.
Meester, Vera; Verweij, Ruben W; van der Wel, Casper; Kraft, Daniela J
2016-04-26
The key ingredients to the successful bottom-up construction of complex materials are believed to be colloids with anisotropic shapes and directional, or patchy, interactions. We present an approach for creating such anisotropic patchy particles based on reconfiguring randomly shaped aggregates of colloidal spheres. While colloidal aggregates are often undesirable in colloidal dispersions due to their random shapes, we exploit them as a starting point to synthesize patchy particles. By a deliberate destabilization of the colloidal particles, diffusion-limited aggregation is induced which partitions the particles into randomly shaped aggregates with controlled size distribution. We achieve a reconfiguration of the aggregates into uniform structures by swelling the polymer spheres with an apolar solvent. The swelling lowers the attractive van der Waals forces, lubricates the contact area between the spheres, and drives the reorganization through minimization of the interfacial energy of the swollen polymer network. This reorganization process yields patchy particles whose patch arrangement is uniform for up to five patches. For particles with more patches, we find that the patch orientation depends on the degree of phase separation between the spheres and the monomer. This enables the synthesis of patchy particles with unprecedented patch arrangements. We demonstrate the broad applicability of this recycling strategy for making patchy particles as well as clusters of spheres by varying the swelling ratio, swelling solvent, surfactant concentration, and swelling time. PMID:27014995
Aggregation and dendritic growth in a magnetic granular system
NASA Astrophysics Data System (ADS)
González-Gutiérrez, J.; Carrillo-Estrada, J. L.; Ruiz-Suárez, J. C.
2013-12-01
We experimentally study the aggregation of non-Brownian paramagnetic beads in a vibrofluidized system induced by an external magnetic dipole. A dendritic growth is observed in real time, particle by particle, and with the naked eye. Two aggregation stages are observed, where tip, tip-split and side-branching growths are differentiated. We found clusters morphologically similar to those generated by a diffusion limited aggregation algorithm (DLA). However, in our case, due to the finite range of the magnetic field, the clusters reach a finite size and their structures exhibit different rates of aggregation. These are revealed by the existence of two different scaling relations of the mass with the gyration radius, and the nature of the radial mass distribution function. The structures of the clusters are fractal objects with an effective mass fractal dimension of around 1.8. We found that an exponential function describes the aggregation phenomenon as a function of time. This exponential behavior is independent of the final state of the morphology (shape and length) of the agglomerates.
Colloidal Recycling: Reconfiguration of Random Aggregates into Patchy Particles.
Meester, Vera; Verweij, Ruben W; van der Wel, Casper; Kraft, Daniela J
2016-04-26
The key ingredients to the successful bottom-up construction of complex materials are believed to be colloids with anisotropic shapes and directional, or patchy, interactions. We present an approach for creating such anisotropic patchy particles based on reconfiguring randomly shaped aggregates of colloidal spheres. While colloidal aggregates are often undesirable in colloidal dispersions due to their random shapes, we exploit them as a starting point to synthesize patchy particles. By a deliberate destabilization of the colloidal particles, diffusion-limited aggregation is induced which partitions the particles into randomly shaped aggregates with controlled size distribution. We achieve a reconfiguration of the aggregates into uniform structures by swelling the polymer spheres with an apolar solvent. The swelling lowers the attractive van der Waals forces, lubricates the contact area between the spheres, and drives the reorganization through minimization of the interfacial energy of the swollen polymer network. This reorganization process yields patchy particles whose patch arrangement is uniform for up to five patches. For particles with more patches, we find that the patch orientation depends on the degree of phase separation between the spheres and the monomer. This enables the synthesis of patchy particles with unprecedented patch arrangements. We demonstrate the broad applicability of this recycling strategy for making patchy particles as well as clusters of spheres by varying the swelling ratio, swelling solvent, surfactant concentration, and swelling time.
Aggregation of a hydrophobically modified poly(propylene imine) dendrimer
NASA Astrophysics Data System (ADS)
Tan, Susheng; Su, Aihua; Ford, Warren T.
2008-10-01
The poly(propylene imine) dendrimer DAB-dendr- (NH2)8 was hydrophobically modified with dodecanoyl end groups. The modified dendrimer was deposited onto mica by adsorption from solution and observed by atomic force microscopy. With the decrease of adsorption time, the modified dendrimer varied from continuous film to scattered islands. For the adsorption time of 20s the dendrimer formed a sub-monolayer thin film that contained many fractal aggregates of fractal dimension 1.80 that were > 1 μm in diameter and no more than 0.8nm thick. After 5 months at 1#1 , the initial fractal aggregates transformed into disks and other less-branched shapes with average heights of the domains of 0.6nm and 0.4nm, respectively. Formation of the fractal aggregates is explained by diffusion-limited aggregation. The slow reorganization of dendrimer molecules in the fractal aggregates occurs at a temperature well above the Tg of the dendrimer.
Aggregation of liposomes in presence of La3+: a study of the fractal dimension.
Sabín, Juan; Prieto, Gerardo; Ruso, Juan M; Messina, Paula; Sarmiento, Félix
2007-07-01
A study of the fractal dimension of the aggregation of three different types of large unilamellar vesicles, formed by egg yolk phosphatidylcholine (EYPC), dimyristoyl-phosphocholine (DMPC), and dipalmitoyl-phosphocholine (DPPC), in the presence of La3+, is presented. Aggregate liposome fractal dimensions were calculated by two methods, aggregation kinetics, using the approaches diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA) and angle-scattering light dispersion. Electrophoretic measurements show a similar variation of the zeta potential (zeta potential) for EYPC and DPPC, with a small increase of initial positive values. However, the zeta potential of DMPC changes from a initial negative value to near zero with increasing La3+ concentration. The evolution of the aggregate sizes was followed by light scattering. DPPC and DMPC show a RLCA regimen growth at low La3+ concentrations and a DLCA regimen at higher concentrations. In the case of EYPC, the final size of aggregation strongly depends on La3+ concentration. The calculated fractal dimension is in the range 1.8 to 2.1.
Size dependent fractal aggregation mediated through surfactant in silica nanoparticle solution
NASA Astrophysics Data System (ADS)
Kumar, Sugam; Aswal, V. K.; Kohlbrecher, J.
2012-06-01
Small-angle neutron scattering (SANS) has been used to study aggregation of anionic silica nanoparticles in presence of cationic surfactant (DTAB) in aqueous solution. The measurements were carried out for different sizes of nanoparticles (8.2, 16.4 and 26.4 nm) at fixed (1 wt%) nanoparticles and surfactant concentration. It is found that the adsorption of surfactant micelles on the silica nanoparticles leads to the aggregation of nanoparticles, which is characterized by a fractal structure. The number of adsorbed micelles on nanoparticle increases from 7 to 152 with the increase in the size of the nanoparticle from 8.2 to 26.4 nm, whereas interestingly the fractal dimension remains same. The aggregate morphology in these systems is expected to be governed by the diffusion limited aggregation.
De Jesus, M.C.; Weiss, R.A.
1996-10-01
Surface conductive polymer was developed by in situ oxidative polymerization of pyrrole in lightly sulfonated polystyrene ionomers (SPS) using a chemical oxidative process. The polymerization only occurred where the monomer and FeCl{sub 3} oxidizing agent could mutually diffuse which resulted in a reaction confined to a surface layer of the host polymer. The amount of PPy incorporated during the polymerization as well as the surface conductivity attained were function of the sulfonation level. The mechanical properties of the composite were significantly improved above Tg, which is believed to be a combination of two effects: formation of a physical network due to intermolecular interactions between the two polymers and mechanical reinforcement due to the rigidity of the PPy.
Effects of water-contaminated air on blowoff limits of opposed jet hydrogen-air diffusion flames
NASA Technical Reports Server (NTRS)
Pellett, Gerald L.; Jentzen, Marilyn E.; Wilson, Lloyd G.; Northam, G. Burton
1988-01-01
The effects of water-contaminated air on the extinction and flame restoration of the central portion of N2-diluted H2 versus air counterflow diffusion flames are investigated using a coaxial tubular opposed jet burner. The results show that the replacement of N2 contaminant in air by water on a mole for mole basis decreases the maximum sustainable H2 mass flow, just prior to extinction, of the flame. This result contrasts strongly with the analogous substitution of water for N2 in a relatively hot premixed H2-O2-N2 flame, which was shown by Koroll and Mulpuru (1986) to lead to a significant, kinetically controlled increase in laminar burning velocity.
NASA Astrophysics Data System (ADS)
Gorodetskyi, O.; Giona, M.; Anderson, P. D.
2012-07-01
This paper extends the mapping matrix formalism to include the effects of molecular diffusion in the analysis of mixing processes in chaotic flows. The approach followed is Lagrangian, by considering the stochastic formulation of advection-diffusion processes via the Langevin equation for passive fluid particle motion. In addition, the inclusion of diffusional effects in the mapping matrix formalism permits to frame the spectral properties of mapping matrices in the purely convective limit in a quantitative way. Specifically, the effects of coarse graining can be quantified by means of an effective Péclet number that scales as the second power of the linear lattice size. This simple result is sufficient to predict the scaling exponents characterizing the behavior of the eigenvalue spectrum of the advection-diffusion operator in chaotic flows as a function of the Péclet number, exclusively from purely kinematic data, by varying the grid resolution. Simple but representative model systems and realistic physically realizable flows are considered under a wealth of different kinematic conditions-from the presence of large quasi-periodic islands intertwined by chaotic regions, to almost globally chaotic conditions, to flows possessing "sticky islands"-providing a fairly comprehensive characterization of the different numerical phenomenologies that may occur in the quantitative analysis of mapping matrices, and ultimately of chaotic mixing processes.
Fuchs, P C; Barry, A L; Thornsberry, C; Gavan, T L; Jones, R N
1983-01-01
Augmentin (Beecham Laboratories, Bristol, Tenn.), a combination drug consisting of two parts amoxicillin to one part clavulanic acid and a potent beta-lactamase inhibitor, was evaluated in vitro in comparison with ampicillin or amoxicillin or both for its inhibitory and bactericidal activities against selected clinical isolates. Regression analysis was performed and tentative disk diffusion susceptibility breakpoints were determined. A multicenter performance study of the disk diffusion test was conducted with three quality control organisms to determine tentative quality control limits. All methicillin-susceptible staphylococci and Haemophilus influenzae isolates were susceptible to Augmentin, although the minimal inhibitory concentrations for beta-lactamase-producing strains of both groups were, on the average, fourfold higher than those for enzyme-negative strains. Among the Enterobacteriaceae, Augmentin exhibited significantly greater activity than did ampicillin against Klebsiella pneumoniae, Citrobacter diversus, Proteus vulgaris, and about one-third of the Escherichia coli strains tested. Bactericidal activity usually occurred at the minimal inhibitory concentration. There was a slight inoculum concentration effect on the Augmentin minimal inhibitory concentrations. On the basis of regression and error rate-bounded analyses, the suggested interpretive disk diffusion susceptibility breakpoints for Augmentin are: susceptible, greater than or equal to 18 mm; resistant, less than or equal to 13 mm (gram-negative bacilli); and susceptible, greater than or equal to 20 mm (staphylococci and H. influenzae). The use of a beta-lactamase-producing organism, such as E. coli Beecham 1532, is recommended for quality assurance of Augmentin susceptibility testing. PMID:6625554
Development of Arsenic and Iron Biogeochemical Gradients upon Anaerobiosis at Soil Aggregate Scale
NASA Astrophysics Data System (ADS)
Masue-Slowey, Y.; Pallud, C.; Bedore, P.; Tufano, K.; Fendorf, S.
2008-12-01
In aerated soils, As release is limited due to the strong interaction between As(V) and soil minerals. However, under anaerobic conditions, As desorption is stimulated by As(V) reduction to As(III) and reductive dissolution/transformation of Fe (hydr)oxides, common hosts of As. The effect of As(V) and Fe(III) reduction on As release has been extensively studied in laboratory batch and column systems; correlation of apparent Fe and As reduction, with concomitant release to pore water, has also been noted under field conditions. What remains unresolved is the coupling of biogeochemical and physical processes that ultimately control As transport within structured media such as soils. Soils are heterogeneous porous media that are comprised of individual aggregates having pores that are dominated by diffusive (aggregate interiors) or advective (aggregate exteriors) transport. As a consequence of physical and chemical differences in the interior and the exterior of aggregates, As(III,V) and Fe(II,III) chemical gradients develop. Here, we examine As release from constructed aggregates exposed to fluctuating redox conditions. Artificial aggregates were made with As(V) adsorbed ferrihydrite-coated sand homogeneously inoculated with Shewanella sp. ANA-3 (model As(V) and Fe(III) reducer) and then fused using an agarose binder into spheres. Aggregates were placed in a flow reactor and saturated flow of aerobic or anaerobic artificial groundwater media was initiated. Redox fluctuated in select systems to examine changes in chemical gradient under changing aeration status. Our results show that within aerated solutions, oxidized aggregate exteriors provide a "gprotective barrier"h against As release despite anoxia within diffusively constrained aggregate interiors. During a transition to anaerobic conditions in advective zones, however, As is released and transport is promoted. Our study illustrates the microscale variation in biogeoechemical processes within soils and the
Fractal aggregates induced by liposome-liposome interaction in the presence of Ca2+.
Sabín, J; Prieto, G; Ruso, J M; Sarmiento, F
2007-10-01
We present a study of the fractal dimension of clusters of large unilamellar vesicles (LUVs) formed by egg yolk phosphatidylcholine (EYPC), dimyristoylphosphocholine (DMPC) and dipalmitoylphosphocholine (DPPC) induced by Ca2+ . Fractal dimensions were calculated by application of two methods, measuring the angular dependency of the light scattered by the clusters and following the evolution of the cluster size. In all cases, the fractal dimensions fell in the range from 2.1 to 1.8, corresponding to two regimes: diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA). Whereas DMPC clusters showed a typical transition from the RLCA to the DLCA aggregation, EYPC exhibited an unusual behaviour, since the aggregation was limited for a higher concentration than the critical aggregation concentration. The behaviour of DPPC was intermediate, with a transition from the RLCA to the DLCA regimes with cluster sizes depending on Ca2+ concentration. Studies on the reversibility of the aggregates show that EYPC and DPPC clusters can be re-dispersed by dilution with water. DMPC does not present reversibility. Reversibility is evidence of the existence of secondary minima in the DLVO potential between two liposomes. To predict these secondary minima, a correction of the DLVO model was necessary taking into account a repulsive force of hydration.
Characterizing unsaturated diffusion in porous tuff gravel
Hu, Qinhong; Kneafsey, Timothy J.; Roberts, Jeffery J.; Tomutsa, Liviu; Wang, Joseph, S.Y.
2003-11-12
Evaluation of solute diffusion in unsaturated porous gravel is very important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (for example, the potential high-level nuclear waste repository at Yucca Mountain, Nevada). For a porous aggregate medium such as granular tuff, the total water content is comprised of surface water and interior water. The surface water component (water film around grains and pendular water between the grain contacts) could serve as a predominant diffusion pathway. To investigate the extent to which surface water films and contact points affect solute diffusion in unsaturated gravel, we examined the configuration of water using x-ray computed tomography in partially saturated gravel, and made quantitative measurements of diffusion at multiple water contents using two different techniques. In the first, diffusion coefficients of potassium chloride in 2-4 mm granular tuff at multiple water contents were calculated from electrical conductivity measurements using the Nernst-Einstein equation. In the second, we used laser ablation with inductively coupled plasma-mass spectrometry to perform micro-scale mapping, allowing the measurement of diffusion coefficients for a mixture of chemical tracers for tuff cubes and tetrahedrons having two contact geometries (cube-cube and cube-tetrahedron). The x-ray computed tomography images show limited contact between grains, and this could hinder the pathways for diffusive transport. Experimental results show the critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water content of 1.5%, the measured solute diffusion coefficient is as low as 1.5 x 10{sup -14} m{sup 2}/s for tuff gravel. Currently used diffusion models relating diffusion coefficients to total volumetric water content inadequately describe unsaturated diffusion behavior in porous gravel at very low water contents.
Characterizing Unsaturated Diffusion in Porous Tuff Gravel
Hu, Q; Kneafsey, T J; Roberts, J J; Tomutsa, L; Wang, J S
2003-11-12
Evaluation of solute diffusion in unsaturated porous gravel is very important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (e.g., the potential high-level nuclear waste repository at Yucca Mountain, Nevada). For a porous aggregate medium such as granular tuff, the total water content is comprised of surface water and interior water. The surface water component (water film around grains and pendular water between the grain contacts) could serve as a predominant diffusion pathway. To investigate the extent of surface water films and contact points affect solute diffusion in unsaturated gravel, we examined the configuration of water using x-ray computed tomography in partially saturated gravel, and made quantitative measurements of diffusion at multiple water contents using two different techniques. In the first, diffusion coefficients of potassium chloride in 2-4 mm granular tuff at multiple water contents are calculated from electrical conductivity measurements using the Nernst-Einstein equation. In the second, we used laser ablation with inductively coupled plasma-mass spectrometry to perform micro-scale mapping, allowing the measurement of diffusion coefficients for a mixture of chemical tracers for tuff cubes and tetrahedrons having two contact geometries (cube-cube and cube-tetrahedron). The x-ray computed tomography images show limited contact between grains, and this could significantly hinder the pathways for diffusive transport. Experimental results show the critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water content of 1.5%, the measured solute diffusion coefficient is as low as 1.5 x 10{sup -14} m{sup 2}/s. Currently used diffusion models relating diffusion coefficients to total volumetric water content inadequately describe unsaturated diffusion behavior in porous gravel.
Mimouni, N.; Chikh, S.; Rahli, O.; Bennacer, R.
2014-07-15
Two-dimensional (2D) and three-dimensional (3D) numerical simulations of double diffusion natural convection in an elongated enclosure filled with a binary fluid saturating a porous medium are carried out in the present work. The Boussinesq approximation is made in the formulation of the problem, and Neumann boundary conditions for temperature and concentration are adopted, respectively, on vertical and horizontal walls of the cavity. The used numerical method is based on the control volume approach, with the third order quadratic upstream interpolation scheme in approximating the advection terms. A semi implicit method algorithm is used to handle the velocity-pressure coupling. To avoid the excessively high computer time inherent to the solution of 3D natural convection problems, full approximation storage with full multigrid method is used to solve the problem. A wide range of the controlling parameters (Rayleigh-Darcy number Ra, lateral aspect ratio Ay, Lewis number Le, and the buoyancy ration N) is investigated. We clearly show that increasing the depth of the cavity (i.e., the lateral aspect ratio) has an important effect on the flow patterns. The 2D perfect parallel flows obtained for small lateral aspect ratio are drastically destabilized by increasing the cavity lateral dimension. This yields a 3D fluid motion with a much more complex flow pattern and the usually considered 2D parallel flow model cannot be applied.
NASA Technical Reports Server (NTRS)
Mahoney, W. A.; Ling, J. C.; Jacobson, A. S.; Lingenfelter, R. E.
1982-01-01
Data obtained during a two-week period in the fall of 1979 with the HEAO 3 gamma-ray spectroscopy experiment have been searched for diffuse galactic plane gamma-ray line emission expected t4 result from the decay of nucleosynthetic Fe-60, Al-26, and Na-22. With the possible exception of the 1809 keV line from Al-26 decay, for which a 2.6-omicron cosmic excess of (6.0 + or - 2.3) x 0.0001 photons/sq cm per sec per rad was measured, no positive detection was made. However, new limits ranging from 1.8 to 11 times 0.0001 photons/sq cm per sec per rad, at the 3-omicron level of confidence, have been placed on diffuse emission in these lines from the vicinity of the galactic center (between -30 and 30 deg). These limits are lower than some theories predict and thus place new constraints on the yields of these radionuclides in explosive nucleosynthesis and on the present rate of galactic nucleosynthesis.
NASA Astrophysics Data System (ADS)
Onuma, Kazuo; Kanzaki, Noriko
2005-11-01
The aggregation of laminin-1 in a physiological solution was observed using time-resolved static light scattering. In a solution containing 150 mM of NaCl and 1 mM of CaCl 2, with a pH of 7.2 buffered by 50 mM Tris, and maintained at 25 °C, the weight-averaged mass (molecular weight) Mw, and radius of gyration Rg, of the aggregate were measured at 10 s intervals. The aggregation kinetics changed from reaction-limited cluster aggregation (RLCA) to diffusion-limited cluster aggregation (DLCA) over time. The fractal dimension estimated in the DLCA regime was 1.71 from the M˜Rgd relationship, which is consistent with the df of 1.75 obtained from an R˜t plot. Direct calculation of df using the scattering intensity and scattering vector revealed that df gradually increased over time in the DLCA regime, suggesting that spontaneous restructuring of the aggregate had occurred. This restructuring would have been caused by hydrophobic contact in the aggregate. The form factor of the aggregate was well fitted by a linear random coil model and not by a simple spherical model.
Early stage fractal growth in thin films below the percolation limit
NASA Astrophysics Data System (ADS)
Batabyal, R.; Mahato, J. C.; Das, Debolina; Dev, B. N.
2013-02-01
We demonstrate the fractal growth of epitaxial Ag thin films on Si(111) surfaces using scanning tunneling microscopy (STM). The initial stage growth of Ag thin films provides islands of compact shape. These compact-shaped two-dimensional (2D) islands follow the Euclidian dimension 2. As the islands grow they become fractal in nature. The fractal (Hausdorff) dimension of the islands depends on the coverage of the Ag thin films. The mechanism responsible for this fractal nature of the Ag nanostructures varies from diffusion limited aggregation (DLA) to diffusion limited cluster aggregation (DLCA).
Yu, Jeong Il; Nam, Heerim; Ahn, Yong Chan; Kim, Won Seog; Park, Keunchil; Kim, Seok Jin
2010-10-01
Purpose: To report treatment outcomes after combined-modality therapy in patients with Stage I/II head-and-neck (HN) diffuse large B cell lymphoma (DLBL). Methods and Materials: Eighty-six eligible patients received sequential chemotherapy and involved-lesion radiation therapy from 1995 to 2006. After a median of four cycles of CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) or rituximab-plus-CHOP chemotherapy, a median of 41.4 Gy was delivered to the known initial gross lesion with adequate margin (2 to 3 cm). Results: After a median follow-up of 57 months, eight treatment failures were observed: distant metastasis in 8 patients; and locoregional failure in 4 patients. Among the 4 patients with locoregional failure, 3 presented with in-field failures, and 1 both in-field and out-of-field failure (contralateral neck). Rates of overall survival (OS) and freedom from progression (FFP) at 10 years were 74.1% and 88.9%, respectively. There was no severe side effect except 1 patient with Grade 3 mucositis during and after completion of radiation therapy. Multivariate analyses showed that absence of B symptom (p = 0.022) and normal lactate dehydrogenase (p = 0.017) were related to favorable OS, age >60 years (p = 0.033) was related to favorable FFP, and international prognostic index of 0 or 1 was related to favorable OS (p = 0.003) and FFP (p = 0.03). Conclusion: This study demonstrated that patients with Stage I/II HN DLBL did not need whole-neck irradiation. Involved-lesion radiation therapy might reduce radiation toxicity with favorable treatment results.
Waves and aggregation patterns in myxobacteria
NASA Astrophysics Data System (ADS)
Igoshin, Oleg A.; Welch, Roy; Kaiser, Dale; Oster, George
2004-03-01
Under starvation conditions, a population of myxobacteria aggregates to build a fruiting body whose shape is species-specific and within which the cells sporulate. Early in this process, cells often pass through a "ripple phase" characterized by traveling linear, concentric, and spiral waves. These waves are different from the waves observed during slime mold aggregation that depend on diffusible morphogens, because myxobacteria communicate by direct contact. The difference is most dramatic when waves collide: rather than annihilating one another, myxobacterial waves appear to pass through one another unchanged. Under certain conditions, the spacing and location of the nascent fruiting bodies is determined by the wavelength and pattern of the waves. Later in fruiting body development, waves are replaced by streams of cells that circulate around small initial aggregates enlarging and rounding them. Still later, pairs of motile aggregates coalesce to form larger aggregates that develop into fruiting bodies. Here we present a mathematical model that quantitatively explains these wave and aggregation phenomena.
Fractal dimension of alumina aggregates grown in two dimensions
NASA Technical Reports Server (NTRS)
Larosa, Judith L.; Cawley, James D.
1992-01-01
The concepts of fractal geometry are applied to the analysis of 0.4-micron alumina constrained to agglomerate in two dimensions. Particles were trapped at the bottom surface of a drop of a dilute suspension, and the agglomeration process was directly observed, using an inverted optical microscope. Photographs were digitized and analyzed, using three distinct approaches. The results indicate that the agglomerates are fractal, having a dimension of approximately 1.5, which agrees well with the predictions of the diffusion-limited cluster-cluster aggregation model.
Oxygen Transport and Stem Cell Aggregation in Stirred-Suspension Bioreactor Cultures
Wu, Jincheng; Rostami, Mahboubeh Rahmati; Cadavid Olaya, Diana P.; Tzanakakis, Emmanuel S.
2014-01-01
Stirred-suspension bioreactors are a promising modality for large-scale culture of 3D aggregates of pluripotent stem cells and their progeny. Yet, cells within these clusters experience limitations in the transfer of factors and particularly O2 which is characterized by low solubility in aqueous media. Cultured stem cells under different O2 levels may exhibit significantly different proliferation, viability and differentiation potential. Here, a transient diffusion-reaction model was built encompassing the size distribution and ultrastructural characteristics of embryonic stem cell (ESC) aggregates. The model was coupled to experimental data from bioreactor and static cultures for extracting the effective diffusivity and kinetics of consumption of O2 within mouse (mESC) and human ESC (hESC) clusters. Under agitation, mESC aggregates exhibited a higher maximum consumption rate than hESC aggregates. Moreover, the reaction-diffusion model was integrated with a population balance equation (PBE) for the temporal distribution of ESC clusters changing due to aggregation and cell proliferation. Hypoxia was found to be negligible for ESCs with a smaller radius than 100 µm but became appreciable for aggregates larger than 300 µm. The integrated model not only captured the O2 profile both in the bioreactor bulk and inside ESC aggregates but also led to the calculation of the duration that fractions of cells experience a certain range of O2 concentrations. The approach described in this study can be employed for gaining a deeper understanding of the effects of O2 on the physiology of stem cells organized in 3D structures. Such frameworks can be extended to encompass the spatial and temporal availability of nutrients and differentiation factors and facilitate the design and control of relevant bioprocesses for the production of stem cell therapeutics. PMID:25032842
Oxygen transport and stem cell aggregation in stirred-suspension bioreactor cultures.
Wu, Jincheng; Rostami, Mahboubeh Rahmati; Cadavid Olaya, Diana P; Tzanakakis, Emmanuel S
2014-01-01
Stirred-suspension bioreactors are a promising modality for large-scale culture of 3D aggregates of pluripotent stem cells and their progeny. Yet, cells within these clusters experience limitations in the transfer of factors and particularly O2 which is characterized by low solubility in aqueous media. Cultured stem cells under different O2 levels may exhibit significantly different proliferation, viability and differentiation potential. Here, a transient diffusion-reaction model was built encompassing the size distribution and ultrastructural characteristics of embryonic stem cell (ESC) aggregates. The model was coupled to experimental data from bioreactor and static cultures for extracting the effective diffusivity and kinetics of consumption of O2 within mouse (mESC) and human ESC (hESC) clusters. Under agitation, mESC aggregates exhibited a higher maximum consumption rate than hESC aggregates. Moreover, the reaction-diffusion model was integrated with a population balance equation (PBE) for the temporal distribution of ESC clusters changing due to aggregation and cell proliferation. Hypoxia was found to be negligible for ESCs with a smaller radius than 100 µm but became appreciable for aggregates larger than 300 µm. The integrated model not only captured the O2 profile both in the bioreactor bulk and inside ESC aggregates but also led to the calculation of the duration that fractions of cells experience a certain range of O2 concentrations. The approach described in this study can be employed for gaining a deeper understanding of the effects of O2 on the physiology of stem cells organized in 3D structures. Such frameworks can be extended to encompass the spatial and temporal availability of nutrients and differentiation factors and facilitate the design and control of relevant bioprocesses for the production of stem cell therapeutics.
Monte Carlo simulation of ramified aggregates on hetero-substrates
NASA Astrophysics Data System (ADS)
Qian, Chang-Ji; Li, Hong; Zhong, Rui; Luo, Meng-Bo; Ye, Gao-Xiang
2009-05-01
We have studied the aggregation of particles on a hetero-substrate consisting of two different substrates A and B with finite surface barriers EAB and EBA between the AB and BA boundaries, respectively. With the diffusion energy limited aggregation (DELA) model, we find that the number of clusters and the mean radius of gyration of the clusters are dependent on the surface barriers EAB and EBA. For the case with a constant of EBA, a series of minima are summarized as EAB = (E0 - kBAEBA)/kAB with kAB and kBA being two integers, for main minima (kBA = kAB - 1) and two local minima (kBA = kAB and kBA = kAB + 1) between two neighbouring main minima.
Fractal aggregation of ZnO nanoparticles under different aqueous solution chemistries
NASA Astrophysics Data System (ADS)
Zhou, D.; Keller, A. A.
2009-12-01
The aggregation of ZnO nanoparticles influences not only their environmental transport but also their toxicity. In natural aquatic systems, the ubiquitous presence of natural organic matter (NOM) can lead to interactions with released ZnO nanoparticles (NPs) and influence their transport. In this study, the aggregation kinetics of ZnO with and without NOM under different ionic strength and pH were examined by both time-resolved dynamic light scattering and sedimentation experiments. ZnO aggregates faster as the pH approaches its point of zero charge. The role of ionic strength role was examined by determining the reaction-limited cluster regime (RLCR) and diffusion-limited cluster regime (DLCR). The critical coagulation concentration (CCC) was determined as 25mM using NaCl as the electrolyte at pH 8. A higher initial ZnO concentration leads to faster aggregation. DLVO calculations agreed well with the experimental data. At high NOM concentration, ZnO aggregation was significantly slowed. However, at low NOM concentration, bridging effect was observed. Finally, the fractal dimensions of ZnO aggregates under different conditions were determined by static light scattering (SLS).
Hayes, T.A.; Rosen, R.S.; Kassner, M.E.
1999-12-01
Interim dry storage of spent nuclear fuel (SNF) rods is of critical concern because a shortage of existing SNF wet storage capacity combined with delays in the availability of a permanent disposal repository has led to an increasing number of SNF rods being placed into interim dry storage. Safe interim dry storage must be maintained for a minimum of twenty years according to the Standard Review Plan for Dry Cask Storage Systems [1] and the Code of Federal Regulations, 10 CFR Part 72 [2]. Interim dry storage licensees must meet certain safety conditions when storing SNF rods to ensure that there is a ''very low probability (e.g. 0.5%) of cladding breach during long-term storage'' [1]. Commercial SNF typically consists of uranium oxide pellets surrounded by a thin cladding. The cladding is usually an {alpha}-zirconium based alloy know as ''Zircaloy''. In dry storage, the SNF rods are confined in one of several types of cask systems approved by the Nuclear Regulatory Commission (NRC). ''The cask system must be designed to prevent degradation of fuel cladding that results in a type of cladding breach, such as axial-splits or ductile fracture, where irradiated UO{sub 2} particles may be released. In addition, the fuel cladding should not degrade to the point where more than one percent of the fuel rods suffer pinhole or hairline crack type failure under normal storage conditions [1].'' The NRC has approved two models [3,4] for use by proposed dry storage licensees to determine the maximum initial temperature limit for nuclear fuel rods in dry storage that supposedly meet the above criteria and yield consistent temperature limits. Though these two models are based on the same fundamental failure theory, different assumptions have been made including the choice of values for material constants in the failure equation. This report will examine and compare the similarities and inconsistencies of these two models. It will illustrate some of the shortcomings of the current
NASA Technical Reports Server (NTRS)
Jameson, Antony
1994-01-01
The theory of non-oscillatory scalar schemes is developed in this paper in terms of the local extremum diminishing (LED) principle that maxima should not increase and minima should not decrease. This principle can be used for multi-dimensional problems on both structured and unstructured meshes, while it is equivalent to the total variation diminishing (TVD) principle for one-dimensional problems. A new formulation of symmetric limited positive (SLIP) schemes is presented, which can be generalized to produce schemes with arbitrary high order of accuracy in regions where the solution contains no extrema, and which can also be implemented on multi-dimensional unstructured meshes. Systems of equations lead to waves traveling with distinct speeds and possibly in opposite directions. Alternative treatments using characteristic splitting and scalar diffusive fluxes are examined, together with modification of the scalar diffusion through the addition of pressure differences to the momentum equations to produce full upwinding in supersonic flow. This convective upwind and split pressure (CUSP) scheme exhibits very rapid convergence in multigrid calculations of transonic flow, and provides excellent shock resolution at very high Mach numbers.
Henneberg, Anders; Sorrell, Brian K; Brix, Hans
2012-11-01
Aerenchymatous plants can transport methane (CH(4) ) from the root zone to the atmosphere, bypassing the surface-oxidizing layers of the soil, yet morphological and anatomical factors that govern the transport of methane have rarely been critically tested in manipulative experiments. Here, we investigated the methane transport capacity of hydroponically grown Juncus effusus, in experiments with roots submerged in nutrient solutions sparged with methane (1.16 mmol CH(4) l(-1)). Through a range of manipulations of the above- and below-ground plant parts, we tested the contradictory claims in the literature regarding which sites provide the greatest resistance to gas transport. Root manipulations had the greatest effect on methane transport. Removing root material reduced methane transport significantly, and especially the lateral roots and the root tips were important. Cutting of the shoots, with or without subsequent sealing, did not alter methane transport significantly. We confirm modelling predictions that the limiting factor for methane transport in the tussock forming wetland graminoid, J. effusus, is the amount of permeable root surface, estimated using the proxy measurement of root length. The aerial tissues do not provide any significant resistance to methane transport, and the methane is emitted from the lower 50 mm of the shoots.
NASA Astrophysics Data System (ADS)
Pierre Auger Collaboration; Abreu, P.; Aglietta, M.; Ahlers, M.; Ahn, E. J.; Albuquerque, I. F. M.; Allard, D.; Allekotte, I.; Allen, J.; Allison, P.; Almela, A.; Alvarez Castillo, J.; Alvarez-Muñiz, J.; Alves Batista, R.; Ambrosio, M.; Aminaei, A.; Anchordoqui, L.; Andringa, S.; Antiči'c, T.; Aramo, C.; Arganda, E.; Arqueros, F.; Asorey, H.; Assis, P.; Aublin, J.; Ave, M.; Avenier, M.; Avila, G.; Badescu, A. M.; Balzer, M.; Barber, K. B.; Barbosa, A. F.; Bardenet, R.; Barroso, S. L. C.; Baughman, B.; Bäuml, J.; Baus, C.; Beatty, J. J.; Becker, K. H.; Bellétoile, A.; Bellido, J. A.; BenZvi, S.; Berat, C.; Bertou, X.; Biermann, P. L.; Billoir, P.; Blanch-Bigas, O.; Blanco, F.; Blanco, M.; Bleve, C.; Blümer, H.; Boháčová, M.; Boncioli, D.; Bonifazi, C.; Bonino, R.; Borodai, N.; Brack, J.; Brancus, I.; Brogueira, P.; Brown, W. C.; Bruijn, R.; Buchholz, P.; Bueno, A.; Buroker, L.; Burton, R. E.; Caballero-Mora, K. S.; Caccianiga, B.; Caramete, L.; Caruso, R.; Castellina, A.; Catalano, O.; Cataldi, G.; Cazon, L.; Cester, R.; Chauvin, J.; Cheng, S. H.; Chiavassa, A.; Chinellato, J. A.; Chirinos Diaz, J.; Chudoba, J.; Cilmo, M.; Clay, R. W.; Cocciolo, G.; Collica, L.; Coluccia, M. R.; Conceição, R.; Contreras, F.; Cook, H.; Cooper, M. J.; Coppens, J.; Cordier, A.; Coutu, S.; Covault, C. E.; Creusot, A.; Criss, A.; Cronin, J.; Curutiu, A.; Dagoret-Campagne, S.; Dallier, R.; Daniel, B.; Dasso, S.; Daumiller, K.; Dawson, B. R.; de Almeida, R. M.; De Domenico, M.; De Donato, C.; de Jong, S. J.; De La Vega, G.; de Mello Junior, W. J. M.; de Mello Neto, J. R. T.; De Mitri, I.; de Souza, V.; de Vries, K. D.; del Peral, L.; del Río, M.; Deligny, O.; Dembinski, H.; Dhital, N.; Di Giulio, C.; Díaz Castro, M. L.; Diep, P. N.; Diogo, F.; Dobrigkeit, C.; Docters, W.; D'Olivo, J. C.; Dong, P. N.; Dorofeev, A.; dos Anjos, J. C.; Dova, M. T.; D'Urso, D.; Dutan, I.; Ebr, J.; Engel, R.; Erdmann, M.; Escobar, C. O.; Espadanal, J.; Etchegoyen, A.; Facal San Luis, P.; Falcke, H.; Farrar, G.; Fauth, A. C.; Fazzini, N.; Ferguson, A. P.; Fick, B.; Figueira, J. M.; Filevich, A.; Filipčič, A.; Fliescher, S.; Fracchiolla, C. E.; Fraenkel, E. D.; Fratu, O.; Fröhlich, U.; Fuchs, B.; Gaior, R.; Gamarra, R. F.; Gambetta, S.; García, B.; Garcia Roca, S. T.; Garcia-Gamez, D.; Garcia-Pinto, D.; Gascon Bravo, A.; Gemmeke, H.; Ghia, P. L.; Giller, M.; Gitto, J.; Glass, H.; Gold, M. S.; Golup, G.; Gomez Albarracin, F.; Gómez Berisso, M.; Gómez Vitale, P. F.; Gonçalves, P.; Gonzalez, J. G.; Gookin, B.; Gorgi, A.; Gouffon, P.; Grashorn, E.; Grebe, S.; Griffith, N.; Grigat, M.; Grillo, A. F.; Guardincerri, Y.; Guarino, F.; Guedes, G. P.; Hansen, P.; Harari, D.; Harrison, T. A.; Harton, J. L.; Haungs, A.; Hebbeker, T.; Heck, D.; Herve, A. E.; Hojvat, C.; Hollon, N.; Holmes, V. C.; Homola, P.; Hörandel, J. R.; Horvath, P.; Hrabovský, M.; Huber, D.; Huege, T.; Insolia, A.; Ionita, F.; Italiano, A.; Jansen, S.; Jarne, C.; Jiraskova, S.; Josebachuili, M.; Kadija, K.; Kampert, K. H.; Karhan, P.; Kasper, P.; Katkov, I.; Kégl, B.; Keilhauer, B.; Keivani, A.; Kelley, J. L.; Kemp, E.; Kieckhafer, R. M.; Klages, H. O.; Kleifges, M.; Kleinfeller, J.; Knapp, J.; Koang, D.-H.; Kotera, K.; Krohm, N.; Krömer, O.; Kruppke-Hansen, D.; Kuempel, D.; Kulbartz, J. K.; Kunka, N.; La Rosa, G.; Lachaud, C.; LaHurd, D.; Latronico, L.; Lauer, R.; Lautridou, P.; Le Coz, S.; Leão, M. S. A. B.; Lebrun, D.; Lebrun, P.; Leigui de Oliveira, M. A.; Letessier-Selvon, A.; Lhenry-Yvon, I.; Link, K.; López, R.; Lopez Agüera, A.; Louedec, K.; Lozano Bahilo, J.; Lu, L.; Lucero, A.; Ludwig, M.; Lyberis, H.; Maccarone, M. C.; Macolino, C.; Maldera, S.; Maller, J.; Mandat, D.; Mantsch, P.; Mariazzi, A. G.; Marin, J.; Marin, V.; Maris, I. C.; Marquez Falcon, H. R.; Marsella, G.; Martello, D.; Martin, L.; Martinez, H.; Martínez Bravo, O.; Martraire, D.; Masías Meza, J. J.; Mathes, H. J.; Matthews, J.; Matthews, J. A. J.; Matthiae, G.; Maurel, D.; Maurizio, D.; Mazur, P. O.; Medina-Tanco, G.; Melissas, M.; Melo, D.; Menichetti, E.; Menshikov, A.; Mertsch, P.; Meurer, C.; Meyhandan, R.; Mi'canovi'c, S.; Micheletti, M. I.; Minaya, I. A.; Miramonti, L.; Molina-Bueno, L.; Mollerach, S.; Monasor, M.; Monnier Ragaigne, D.; Montanet, F.; Morales, B.; Morello, C.; Moreno, E.; Moreno, J. C.; Mostafá, M.; Moura, C. A.; Muller, M. A.; Müller, G.; Münchmeyer, M.; Mussa, R.; Navarra, G.; Navarro, J. L.; Navas, S.; Necesal, P.; Nellen, L.; Nelles, A.; Neuser, J.; Nhung, P. T.; Niechciol, M.; Niemietz, L.; Nierstenhoefer, N.; Nitz, D.; Nosek, D.; Nožka, L.; Oehlschläger, J.; Olinto, A.; Ortiz, M.; Pacheco, N.; Pakk Selmi-Dei, D.; Palatka, M.; Pallotta, J.; Palmieri, N.; Parente, G.; Parizot, E.; Parra, A.; Pastor, S.; Paul, T.; Pech, M.; Peķala, J.; Pelayo, R.; Pepe, I. M.; Perrone, L.; Pesce, R.; Petermann, E.; Petrera, S.; Petrolini, A.; Petrov, Y.; Pfendner, C.; Piegaia, R.; Pierog, T.; Pieroni, P.; Pimenta, M.; Pirronello, V.; Platino, M.; Plum, M.; Ponce, V. H.; Pontz, M.; Porcelli, A.; Privitera, P.; Prouza, M.; Quel, E. J.; Querchfeld, S.; Rautenberg, J.; Ravel, O.; Ravignani, D.; Revenu, B.; Ridky, J.; Riggi, S.; Risse, M.; Ristori, P.; Rivera, H.; Rizi, V.; Roberts, J.; Rodrigues de Carvalho, W.; Rodriguez, G.; Rodriguez Cabo, I.; Rodriguez Martino, J.; Rodriguez Rojo, J.; Rodríguez-Frías, M. D.; Ros, G.; Rosado, J.; Rossler, T.; Roth, M.; Rouillé-d'Orfeuil, B.; Roulet, E.; Rovero, A. C.; Rühle, C.; Saftoiu, A.; Salamida, F.; Salazar, H.; Salesa Greus, F.; Salina, G.; Sánchez, F.; Santo, C. E.; Santos, E.; Santos, E. M.; Sarazin, F.; Sarkar, B.; Sarkar, S.; Sato, R.; Scharf, N.; Scherini, V.; Schieler, H.; Schiffer, P.; Schmidt, A.; Scholten, O.; Schoorlemmer, H.; Schovancova, J.; Schovánek, P.; Schröder, F.; Schulte, S.; Schuster, D.; Sciutto, S. J.; Scuderi, M.; Segreto, A.; Settimo, M.; Shadkam, A.; Shellard, R. C.; Sidelnik, I.; Sigl, G.; Silva Lopez, H. H.; Sima, O.; 'Smiałkowski, A.; Šmída, R.; Snow, G. R.; Sommers, P.; Sorokin, J.; Spinka, H.; Squartini, R.; Srivastava, Y. N.; Stanic, S.; Stapleton, J.; Stasielak, J.; Stephan, M.; Stutz, A.; Suarez, F.; Suomijärvi, T.; Supanitsky, A. D.; Šuša, T.; Sutherland, M. S.; Swain, J.; Szadkowski, Z.; Szuba, M.; Tapia, A.; Tartare, M.; Taşcău, O.; Tcaciuc, R.; Thao, N. T.; Thomas, D.; Tiffenberg, J.; Timmermans, C.; Tkaczyk, W.; Todero Peixoto, C. J.; Toma, G.; Tomankova, L.; Tomé, B.; Tonachini, A.; Travnicek, P.; Tridapalli, D. B.; Tristram, G.; Trovato, E.; Tueros, M.; Ulrich, R.; Unger, M.; Urban, M.; Valdés Galicia, J. F.; Valiño, I.; Valore, L.; van Aar, G.; van den Berg, A. M.; van Vliet, A.; Varela, E.; Vargas Cárdenas, B.; Vázquez, J. R.; Vázquez, R. A.; Veberič, D.; Verzi, V.; Vicha, J.; Videla, M.; Villaseñor, L.; Wahlberg, H.; Wahrlich, P.; Wainberg, O.; Walz, D.; Watson, A. A.; Weber, M.; Weidenhaupt, K.; Weindl, A.; Werner, F.; Westerhoff, S.; Whelan, B. J.; Widom, A.; Wieczorek, G.; Wiencke, L.; Wilczyńska, B.; Wilczyński, H.; Will, M.; Williams, C.; Winchen, T.; Wommer, M.; Wundheiler, B.; Yamamoto, T.; Yapici, T.; Younk, P.; Yuan, G.; Yushkov, A.; Zamorano Garcia, B.; Zas, E.; Zavrtanik, D.; Zavrtanik, M.; Zaw, I.; Zepeda, A.; Zhou, J.; Zhu, Y.; Zimbres Silva, M.; Ziolkowski, M.
2012-08-01
The surface detector array of the Pierre Auger Observatory can detect neutrinos with energy E ν between 1017 eV and 1020 eV from point-like sources across the sky south of +55° and north of -65° declinations. A search has been performed for highly inclined extensive air showers produced by the interaction of neutrinos of all flavors in the atmosphere (downward-going neutrinos), and by the decay of tau leptons originating from tau neutrino interactions in Earth's crust (Earth-skimming neutrinos). No candidate neutrinos have been found in data up to 2010 May 31. This corresponds to an equivalent exposure of ~3.5 years of a full surface detector array for the Earth-skimming channel and ~2 years for the downward-going channel. An improved upper limit on the diffuse flux of tau neutrinos has been derived. Upper limits on the neutrino flux from point-like sources have been derived as a function of the source declination. Assuming a differential neutrino flux k PS · E -2 ν from a point-like source, 90% confidence level upper limits for k PS at the level of ≈5 × 10-7 and 2.5 × 10-6 GeV cm-2 s-1 have been obtained over a broad range of declinations from the searches for Earth-skimming and downward-going neutrinos, respectively.
Stamatakis, Michail; Vlachos, Dionisios G
2011-12-14
Well-mixed and lattice-based descriptions of stochastic chemical kinetics have been extensively used in the literature. Realizations of the corresponding stochastic processes are obtained by the Gillespie stochastic simulation algorithm and lattice kinetic Monte Carlo algorithms, respectively. However, the two frameworks have remained disconnected. We show the equivalence of these frameworks whereby the stochastic lattice kinetics reduces to effective well-mixed kinetics in the limit of fast diffusion. In the latter, the lattice structure appears implicitly, as the lumped rate of bimolecular reactions depends on the number of neighbors of a site on the lattice. Moreover, we propose a mapping between the stochastic propensities and the deterministic rates of the well-mixed vessel and lattice dynamics that illustrates the hierarchy of models and the key parameters that enable model reduction.
Fisher, Gregory L; Szakal, Christopher; Wetteland, Christopher J; Winograd, Nicholas
2006-02-01
The chemical evolution of poly(tetrafluoroethylene) (PTFE) that is brought about by increasing levels of irradiation with alpha particles is accompanied by the emergence and proliferation of functionalized moieties. Families of reaction products specifically identified in the alpha-irradiated polymer matrix include hydride-, hydroxide-, and oxide-functionalized fluorocarbons. The data also indicate the emergence of hydrogen peroxide (H2O2) and hydrazine (N2H4), but no distinct evidence suggesting the formation of perfluorinated amines, amides, or cyanogens is found. In this article we substantiate the speciation of emergent species and reveal evidence of intercalated molecular gases with which alpha particle-generated radicals may react to form the observed products. Furthermore, we present evidence to suggest that the kinetics of alpha particle-induced reaction is limited by the diffusion of radicals within the polymer matrix. That is to say, chemical additives in the polymer matrix are shown to be scavengers of H*, O*, and F* radicals and limit the rates of reaction that produce functionalized fluorocarbon moieties. Above a threshold dose of alpha particles, the concentration of radicals exceeds that of the scavenger species, and free radical diffusion commences as evidenced by a sudden increase in the yield of reaction products. Samples of PTFE were irradiated to alpha doses in the range of 10(7) to 5 x 10(10) rad with 5.5 MeV 4He2+ ions from a tandem accelerator. Residual gas analysis (RGA) was utilized to monitor the liberation of molecular gases from PTFE during alpha particle irradiation of samples in vacuum. Static time-of-flight SIMS (TOF-SIMS), equipped with a 20 keV C60+ source, was employed to probe chemical changes as a function of alpha particle irradiation. Chemical images and high-resolution mass spectra were collected in both the positive and negative polarities.
Aggregation-structure-elasticity relationship of gels
NASA Astrophysics Data System (ADS)
Ma, Hang-Shing
Aerogel is a mesoporous, low-density material which is desirable for applications like thermal insulation and low-k interlayer dielectric. However, its lack of mechanical integrity hinders its development. Experiments have shown that aerogels exhibit a scaling relationship E ∝ rho m between modulus E and density rho, with the exponent m usually between 3 and 4. The objective of the dissertation is to use computer modeling to understand how the random aggregation process accounts for the fractal structure and the compliant nature of aerogels. Model gels were created by the diffusion-limited cluster-cluster aggregation (DLCA), which simulates random aggregation leading to the sol-gel transition. Then each resulting structure was modeled as an elastic beam network and numerically compressed using the finite element method (FEM). Analyses showed that the DLCA gels reproduced the scaling relationship after trimming the non-contributive dangling branches from the mechanically efficient looped networks. The dangling bond deflection (DEF) model was therefore developed to model the random rotational movement of the dangling branches and the subsequent loop structure formation. Model gels with extensive loops and negligible dangling branches were simulated by combining the DLCA and DEF models. Representation of the aerogel networks by the DLCADEF models was validated for the resemblance of the fractal geometry and elastic behavior. The lack of mechanical integrity in aerogels is a natural consequence of the random aggregation and the resulting fractal structure. Fractal clusters are created in the early stage of aggregation, each of which is characterized by a dense core and sparse perimeter. These clusters grow in size until they percolate at the gel point by knitting together at the perimeters. The gel structure possesses a "blob-and-link" architecture, with the blobs representing the rigid cores of the fractal clusters, and the links corresponding to the tenuous chains
What favors convective aggregation and why?
NASA Astrophysics Data System (ADS)
Muller, Caroline; Bony, Sandrine
2015-07-01
The organization of convection is ubiquitous, but its physical understanding remains limited. One particular type of organization is the spatial self-aggregation of convection, taking the form of cloud clusters, or tropical cyclones in the presence of rotation. We show that several physical processes can give rise to self-aggregation and highlight the key features responsible for it, using idealized simulations. Longwave radiative feedbacks yield a "radiative aggregation." In that case, sufficient spatial variability of radiative cooling rates yields a low-level circulation, which induces the upgradient energy transport and radiative-convective instability. Not only do vertically integrated radiative budgets matter but the vertical profile of cooling is also crucial. Convective aggregation is facilitated when downdrafts below clouds are weak ("moisture-memory aggregation"), and this is sufficient to trigger aggregation in the absence of longwave radiative feedbacks. These results shed some light on the sensitivity of self-aggregation to various parameters, including resolution or domain size.
NASA Astrophysics Data System (ADS)
Chang, W.; Akbari, A.; Frigon, D.; Ghoshal, S.
2011-12-01
Petroleum hydrocarbon contamination of soils and groundwater is an environmental concern. Bioremediation has been frequently considered a cost-effective, less disruptive remedial technology. Formation of soil aggregate fractions in unsaturated soils is generally believed to hinder aerobic hydrocarbon biodegradation due to the slow intra-pore diffusion of nutrients and oxygen within the aggregate matrix and to the reduced bioavailability of hydrocarbons. On the other hand, soil aggregates may harbour favourable niches for indigenous bacteria, providing protective microsites against various in situ environmental stresses. The size of the soil aggregates is likely to be a critical factor for these processes and could be interpreted as a relevant marker for biodegradation assessment. There have been only limited attempts in the past to assess petroleum hydrocarbon biodegradation in unsaturated soils as a function of aggregate size. This study is aimed at investigating the roles of aggregate sizes and aggregate microstructures on biodegradation activity. Field-aged, contaminated, clayey soils were shipped from Norman Wells, Canada. Attempts were made to stimulate indigenous microbial activity by soil aeration and nutrient amendments in a pilot-scale biopile tank (1m L×0.65m W×0.3 m H). A control biopile was maintained without the nutrient amendment but was aerated. The initial concentrations of petroleum hydrocarbons in the field-contaminated soils increased with increasing aggregate sizes, which were classified in three fractions: micro- (<250 μm), meso- (>250-2000 μm) and macro-aggregates (>2000 μm). Compared to the TPH analyses at whole-soil level, the petroleum hydrocarbon analyses based on the aggregate-size levels demonstrated more clearly the extent of biodegradation of non-volatile, heavier hydrocarbons (C16-C34) in the soil. The removal of the C16-C34 hydrocarbons was 44% in macro-aggregates, but only 13% in meso-aggregates. The increased protein
NASA Astrophysics Data System (ADS)
Deng, Yan-Hong; Ye, Chao; Yuan, Yuan; Liu, Hui-Min; Cui, Jin
2011-04-01
We investigate the effect of silicone oil viscosity on the aggregation behavior of C:F clusters deposited on silicone oil liquid substrates with viscous coefficients of 100, 350 and 500mm2/s by C4F8 dual-frequency capacitively coupled plasma. The aggregated C:F clusters all exhibit a branch-like fractal structure. However, the fractal dimension decreases from 1.67 to 1.45 with the silicone oil viscous coefficient increasing from 100mm2/s to 500 mm2/s. Owing to the fractal dimension of 1.67 and 1.45, corresponding to the diffusion-limited-aggregation (DLA) model and the cluster-cluster-aggregation (CCA) model respectively, the results show that the increase of silicone oil viscosity can lead to the change of C:F clusters aggregating on a silicone oil liquid substrate from DLA to CCA growth.
Directional sensing and streaming in Dictyostelium aggregation.
Almeida, Sofia; Dilão, Rui
2016-05-01
We merge the Kessler-Levine simple discrete model for Dictyostelium cyclic adenosine monophosphate (cAMP) production and diffusion with the Dilão-Hauser directional sensing aggregation mechanism. The resulting compound model describes all the known transient patterns that emerge during Dictyostelium aggregation, which include the spontaneous formation of cAMP self-sustained target and spiral waves and streaming. We show that the streaming patterns depend on the speed of the amoebae, on the relaxation time for the production of cAMP, on the cAMP degradation rate, and on directional sensing. Moreover, we show that different signaling centers emerge during Dictyostelium aggregation. PMID:27300919
Directional sensing and streaming in Dictyostelium aggregation
NASA Astrophysics Data System (ADS)
Almeida, Sofia; Dilão, Rui
2016-05-01
We merge the Kessler-Levine simple discrete model for Dictyostelium cyclic adenosine monophosphate (cAMP) production and diffusion with the Dilão-Hauser directional sensing aggregation mechanism. The resulting compound model describes all the known transient patterns that emerge during Dictyostelium aggregation, which include the spontaneous formation of cAMP self-sustained target and spiral waves and streaming. We show that the streaming patterns depend on the speed of the amoebae, on the relaxation time for the production of cAMP, on the cAMP degradation rate, and on directional sensing. Moreover, we show that different signaling centers emerge during Dictyostelium aggregation.
Directional sensing and streaming in Dictyostelium aggregation.
Almeida, Sofia; Dilão, Rui
2016-05-01
We merge the Kessler-Levine simple discrete model for Dictyostelium cyclic adenosine monophosphate (cAMP) production and diffusion with the Dilão-Hauser directional sensing aggregation mechanism. The resulting compound model describes all the known transient patterns that emerge during Dictyostelium aggregation, which include the spontaneous formation of cAMP self-sustained target and spiral waves and streaming. We show that the streaming patterns depend on the speed of the amoebae, on the relaxation time for the production of cAMP, on the cAMP degradation rate, and on directional sensing. Moreover, we show that different signaling centers emerge during Dictyostelium aggregation.
Koscher, Brent A; Bronstein, Noah D; Olshansky, Jacob H; Bekenstein, Yehonadav; Alivisatos, A Paul
2016-09-21
Ion-exchange transformations allow access to nanocrystalline materials with compositions that are inaccessible via direct synthetic routes. However, additional mechanistic insight into the processes that govern these reactions is needed. We present evidence for the presence of two distinct mechanisms of exchange during anion exchange in CsPbX3 nanocrystals (NCs), ranging in size from 6.5 to 11.5 nm, for transformations from CsPbBr3 to CsPbCl3 or CsPbI3. These NCs exhibit bright luminescence throughout the exchange, allowing their optical properties to be observed in real time, in situ. The iodine exchange presents surface-reaction-limited exchanges allowing all anionic sites within the NC to appear chemically identical, whereas the chlorine exchange presents diffusion-limited exchanges proceeding through a more complicated exchange mechanism. Our results represent the first steps toward developing a microkinetic description of the anion exchange, with implications not only for understanding the lead halide perovskites but also for nanoscale ion exchange in general. PMID:27606934
Pavlova, Anna; Cheng, Chi-Yuan; Kinnebrew, Maia; Lew, John; Dahlquist, Frederick W.; Han, Songi
2016-01-01
Protein aggregation plays a critical role in the pathogenesis of neurodegenerative diseases, and the mechanism of its progression is poorly understood. Here, we examine the structural and dynamic characteristics of transiently evolving protein aggregates under ambient conditions by directly probing protein surface water diffusivity, local protein segment dynamics, and interprotein packing as a function of aggregation time, along the third repeat domain and C terminus of Δtau187 spanning residues 255–441 of the longest isoform of human tau. These measurements were achieved with a set of highly sensitive magnetic resonance tools that rely on site-specific electron spin labeling of Δtau187. Within minutes of initiated aggregation, the majority of Δtau187 that is initially homogeneously hydrated undergoes structural transformations to form partially structured aggregation intermediates. This is reflected in the dispersion of surface water dynamics that is distinct around the third repeat domain, found to be embedded in an intertau interface, from that of the solvent-exposed C terminus. Over the course of hours and in a rate-limiting process, a majority of these aggregation intermediates proceed to convert into stable β-sheet structured species and maintain their stacking order without exchanging their subunits. The population of β-sheet structured species is >5% within 5 min of aggregation and gradually grows to 50–70% within the early stages of fibril formation, while they mostly anneal block-wisely to form elongated fibrils. Our findings suggest that the formation of dynamic aggregation intermediates constitutes a major event occurring in the earliest stages of tau aggregation that precedes, and likely facilitates, fibril formation and growth. PMID:26712030
DeJong, Stephanie A; Lu, Zhenyu; Cassidy, Brianna M; O'Brien, Wayne L; Morgan, Stephen L; Myrick, Michael L
2015-09-01
Detection limits (DL) for blood on four fabric types were estimated for calibrations derived using partial least squares regression applied to infrared (IR) diffuse reflection spectra. Samples were prepared by dip-coating acrylic, cotton, nylon, and polyester fabrics from solutions of diluted rat blood. While DLs often appear in terms of dilution factor in the forensic community, mass percentage, coverage (mass per unit area), or film thickness are often more relevant when comparing experimental methods. These alternate DL units are related to one another and presented here. The best IR diffuse reflection DLs for blood on acrylic and cotton fabrics were in the mid-IR spectral window corresponding to the protein Amide I/II absorption bands. These DLs were dilution by a factor of 2300 (0.019% w/w blood solids) for acrylic and a factor of 610 (0.055% w/w blood solids) for cotton. The best DL for blood on polyester was found in the mid-IR spectral window corresponding to the protein Amide A absorption band at dilution by a factor of 900 (0.034% w/w blood solids). Because of the similarity between the IR spectra of blood solids and nylon fabrics, no satisfactory IR DLs were determined for the calibration of blood on nylon. We compare our values to DLs reported for blood detection using the standard luminol method. The most commonly reported luminol DLs are of the order of 1000-fold dilution, which we estimate are a factor of 2-7 lower than our reported IR DLs on a coverage basis.
Owczarz, Marta; Motta, Anna C; Morbidelli, Massimo; Arosio, Paolo
2015-07-14
We apply a kinetic analysis platform to study the intermolecular interactions underlying the colloidal stability of dispersions of charged amyloid fibrils consisting of a model amphiphilic peptide (RADA 16-I). In contrast to the aggregation mechanisms observed in the large majority of proteins and peptides, where several elementary reactions involving both monomers and fibrils are present simultaneously, the system selected in this work allows the specific investigation of the fibril-fibril aggregation process. We examine the intermolecular interactions driving the aggregation reaction at pH 2.0 by changing the buffer composition in terms of salt concentration, type of ion as well as type and concentration of organic solvent. The aggregation kinetics are followed by dynamic light scattering, and the experimental data are simulated by Smoluchowski population balance equations, which allow to estimate the energy barrier between two colliding fibrils in terms of the Fuchs stability ratio (W). When normalized on a dimensionless time weighted on the Fuchs stability ratio, the aggregation profiles under a broad range of conditions collapse on a single master curve, indicating that the buffer composition modifies the aggregation kinetics without affecting the aggregation mechanism. Our results show that the aggregation process does not occur under diffusion-limited conditions. Rather, the reaction rate is limited by the presence of an activation energy barrier that is largely dominated by electrostatic repulsive interactions. Such interactions could be reduced by increasing the concentration of salt, which induces charge screening, or the concentration of organic solvent, which affects the dielectric constant. It is remarkable that the dependence of the activation energy on the ionic strength can be described quantitatively in terms of charge screening effects in the frame of the DLVO theory, although specific anion and cation effects are also observed. While anion
Reaction-diffusion modelling of bacterial colony patterns
NASA Astrophysics Data System (ADS)
Mimura, Masayasu; Sakaguchi, Hideo; Matsushita, Mitsugu
2000-07-01
It is well known from experiments that bacterial species Bacillus subtilis exhibit various colony patterns. These are essentially classified into five types in the morphological diagram, depending on the substrate softness and nutrient concentration. (A) diffusion-limited aggregation-like; (B) Eden-like; (C) concentric ring-like; (D) disk-like; and (E) dense branching morphology-like. There arises the naive question of whether the diversity of colony patterns observed in experiments is caused by different effects or governed by the same underlying principles. Our research has led us to propose reaction-diffusion models to describe the morphological diversity of colony patterns except for Eden-like ones.
Abreu, P.; Andringa, S.; Aglietta, M.; Ahlers, M.; Ahn, E. J.; Albuquerque, I. F. M.; Allard, D.; Allekotte, I.; Allen, J.; Allison, P.; Almela, A.; Alvarez Castillo, J.; Alvarez-Muniz, J.; Alves Batista, R.; Ambrosio, M.; Aramo, C.; Aminaei, A.; Anchordoqui, L.; Antici'c, T.; Collaboration: Pierre Auger Collaboration; and others
2012-08-10
The surface detector array of the Pierre Auger Observatory can detect neutrinos with energy E{sub {nu}} between 10{sup 17} eV and 10{sup 20} eV from point-like sources across the sky south of +55 Degree-Sign and north of -65 Degree-Sign declinations. A search has been performed for highly inclined extensive air showers produced by the interaction of neutrinos of all flavors in the atmosphere (downward-going neutrinos), and by the decay of tau leptons originating from tau neutrino interactions in Earth's crust (Earth-skimming neutrinos). No candidate neutrinos have been found in data up to 2010 May 31. This corresponds to an equivalent exposure of {approx}3.5 years of a full surface detector array for the Earth-skimming channel and {approx}2 years for the downward-going channel. An improved upper limit on the diffuse flux of tau neutrinos has been derived. Upper limits on the neutrino flux from point-like sources have been derived as a function of the source declination. Assuming a differential neutrino flux k{sub PS} {center_dot} E {sup -2}{sub {nu}} from a point-like source, 90% confidence level upper limits for k{sub PS} at the level of Almost-Equal-To 5 Multiplication-Sign 10{sup -7} and 2.5 Multiplication-Sign 10{sup -6} GeV cm{sup -2} s{sup -1} have been obtained over a broad range of declinations from the searches for Earth-skimming and downward-going neutrinos, respectively.
Kwon, Jeanny; Kim, Il Han; Kim, Byoung Hyuck; Kim, Tae Min; Heo, Dae Seog
2015-05-01
Purpose: The purpose of this study was to evaluate the role of involved-lesion radiation therapy (ILRT) after rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) chemotherapy in limited stage diffuse large B-cell lymphoma (DLBCL) by comparing outcomes of R-CHOP therapy alone with R-CHOP followed by ILRT. Methods and Materials: We identified 198 patients treated with R-CHOP (median, 6 cycles) for pathologically confirmed DLBCL of limited stage from July 2004 to December 2012. Clinical characteristics of these patients were 33% with stage I and 66.7% with stage II; 79.8% were in the low or low-intermediate risk group; 13.6% had B symptoms; 29.8% had bulky tumors (≥7 cm); and 75.3% underwent ≥6 cycles of R-CHOP therapy. RT was given to 43 patients (21.7%) using ILRT technique, which included the prechemotherapy tumor volume with a median margin of 2 cm (median RT dose: 36 Gy). Results: After a median follow-up of 40 months, 3-year progression-free survival (PFS) and overall survival (OS) were 85.8% and 88.9%, respectively. Multivariate analysis showed ≥6 cycles of R-CHOP (PFS, P=.004; OS, P=.004) and ILRT (PFS, P=.021; OS, P=.014) were favorable prognosticators of PFS and OS. A bulky tumor (P=.027) and response to R-CHOP (P=.012) were also found to be independent factors of OS. In subgroup analysis, the effect of ILRT was prominent in patients with a bulky tumor (PFS, P=.014; OS, P=.030) or an elevated level of serum lactate dehydrogenase (LDH; PFS, P=.004; OS, P=.012). Conclusions: Our results suggest that ILRT after R-CHOP therapy improves PFS and OS in patients with limited stage DLBCL, especially in those with bulky disease or an elevated serum LDH level.
Stability and aggregation of metal oxide nanoparticles in natural aqueous matrices.
Keller, Arturo A; Wang, Hongtao; Zhou, Dongxu; Lenihan, Hunter S; Cherr, Gary; Cardinale, Bradley J; Miller, Robert; Ji, Zhaoxia
2010-03-15
There is a pressing need for information on the mobility of nanoparticles in the complex aqueous matrices found in realistic environmental conditions. We dispersed three different metal oxide nanoparticles (TiO(2), ZnO and CeO(2)) in samples taken from eight different aqueous media associated with seawater, lagoon, river, and groundwater, and measured their electrophoretic mobility, state of aggregation, and rate of sedimentation. The electrophoretic mobility of the particles in a given aqueous media was dominated by the presence of natural organic matter (NOM) and ionic strength, and independent of pH. NOM adsorbed onto these nanoparticles significantly reduces their aggregation, stabilizing them under many conditions. The transition from reaction to diffusion limited aggregation occurs at an electrophoretic mobility from around -2 to -0.8 microm s(-1) V(-1) cm. These results are key for designing and interpreting nanoparticle ecotoxicity studies in various environmental conditions. PMID:20151631
Symbiotic Cell Differentiation and Cooperative Growth in Multicellular Aggregates
Yamagishi, Jumpei F; Saito, Nen; Kaneko, Kunihiko
2016-01-01
As cells grow and divide under a given environment, they become crowded and resources are limited, as seen in bacterial biofilms and multicellular aggregates. These cells often show strong interactions through exchanging chemicals, as evident in quorum sensing, to achieve mutualism and division of labor. Here, to achieve stable division of labor, three characteristics are required. First, isogenous cells differentiate into several types. Second, this aggregate of distinct cell types shows better growth than that of isolated cells without interaction and differentiation, by achieving division of labor. Third, this cell aggregate is robust with respect to the number distribution of differentiated cell types. Indeed, theoretical studies have thus far considered how such cooperation is achieved when the ability of cell differentiation is presumed. Here, we address how cells acquire the ability of cell differentiation and division of labor simultaneously, which is also connected with the robustness of a cell society. For this purpose, we developed a dynamical-systems model of cells consisting of chemical components with intracellular catalytic reaction dynamics. The reactions convert external nutrients into internal components for cellular growth, and the divided cells interact through chemical diffusion. We found that cells sharing an identical catalytic network spontaneously differentiate via induction from cell-cell interactions, and then achieve division of labor, enabling a higher growth rate than that in the unicellular case. This symbiotic differentiation emerged for a class of reaction networks under the condition of nutrient limitation and strong cell-cell interactions. Then, robustness in the cell type distribution was achieved, while instability of collective growth could emerge even among the cooperative cells when the internal reserves of products were dominant. The present mechanism is simple and general as a natural consequence of interacting cells with
Murphy, Ryan J; Pristinski, Denis; Migler, Kalman; Douglas, Jack F; Prabhu, Vivek M
2010-05-21
There are many important processes where the stability of nanoparticles can change due to changes in solution environment. These processes are often difficult to study under controlled changes to the solution conditions. Dynamic light scattering was used to measure the initial kinetics of aggregation of carboxylated polystyrene nanoparticles after well-defined pH jumps using aqueous solutions of photoacid generator (PAG). With this approach, the pH of the solution was controlled by exposure to ultraviolet (UV) light without the delays from mixing or stirring. The aggregation kinetics of the nanoparticles was extremely sensitive to the solution pH. The UV exposure dose is inversely correlated with the resulting surface charge of the nanoparticles. Decreasing pH decreases the electrostatic repulsion force between particles and leads to aggregation. The reaction-limited or diffusion-limited aggregation kinetics was sensitive to the pH quench depth, relative to the acid-equilibrium constant (pK(a)) of the surface carboxylic acid groups on the nanoparticles. Since numerous PAGs are commercially available, this approach provides a flexible method to study the aggregation of a variety of solvent-dispersed nanoparticle systems. PMID:20499988
Reduce Confusion about Diffusion.
ERIC Educational Resources Information Center
Hebrank, Mary R.
1997-01-01
Presents activities that allow students to explore the fundamental but poorly understood concept of diffusion by appealing to their kinesthetic senses first, then challenging their analytical skills as they try to deduce the mathematical principle involved. Presents a computer simulation of diffusion and discusses diffusion's limitations and…
NASA Astrophysics Data System (ADS)
Liu, Tao; Gao, Xiang; He, Bei-Gang; Yu, Jing-Kun
2016-07-01
The La0.8Sr0.2(Ga1- x Co x )0.8Mg0.2O3- δ (LSGMC x = 0.05, 0.1, 0.15, 0.2, 0.25) and La0.8Sr0.2(Ga1- x Fe x )0.8Mg0.2O3- δ (LSGMF x = 0.1, 0.2, 0.3) samples were prepared by solid-state reaction. The structure, conductivity, thermal expansion behavior, and chemical compatibility were studied by XRD, dilatometry, and four-terminal method. A limiting current oxygen sensor was prepared with La0.8Sr0.2Ga0.83Mg0.17O2.815 as a solid electrolyte and La0.8Sr0.2(Ga0.75Co0.25)0.8Mg0.2O3- δ as a dense diffusion barrier. The oxygen-sensitive characteristic was measured at different oxygen concentrations. The results show that the phase structure of samples is cubic, except La0.8Sr0.2(Ga0.75Co0.25)0.8Mg0.2O3- δ , which has a hexagonal structure. The change in activation energy for electrical conductivity and the increase in thermal expansion coefficient are confirmed to correlate with an increasing concentration of oxygen vacancies. The limiting current oxygen sensor exhibits a good limiting current platform and the limiting current depends linearly on the oxygen concentration: I L(mA) = 12.8519 + 2.2667 x_{{{O}_{{2}} }} (mol%, 0 < x_{{{{O}}_{ 2} }} < 3.31) at 750 °C, I L(mA) = 14.3222 + 3.5180 x_{{{O}_{{2}} }} (mol%, 0 < x_{{{{O}}_{ 2} }} < 4.16) at 800 °C, and I L(mA) = 15.2872 + 5.0269x_{{{O}_{{2}} }}(mol%, 0 < x_{{{{O}}_{ 2} }} < 4.12) at 850 °C. The sensor has the best sensitivity at 850 °C. As the oxygen concentration increases, the interface resistance of the sensor decreases at 850 °C.
NASA Astrophysics Data System (ADS)
Tetsu, Hiroyuki; Nakamoto, Taishi
2016-03-01
Radiation is an important process of energy transport, a force, and a basis for synthetic observations, so radiation hydrodynamics (RHD) calculations have occupied an important place in astrophysics. However, although the progress in computational technology is remarkable, their high numerical cost is still a persistent problem. In this work, we compare the following schemes used to solve the nonlinear simultaneous equations of an RHD algorithm with the flux-limited diffusion approximation: the Newton-Raphson (NR) method, operator splitting, and linearization (LIN), from the perspective of the computational cost involved. For operator splitting, in addition to the traditional simple operator splitting (SOS) scheme, we examined the scheme developed by Douglas & Rachford (DROS). We solve three test problems (the thermal relaxation mode, the relaxation and the propagation of linear waves, and radiating shock) using these schemes and then compare their dependence on the time step size. As a result, we find the conditions of the time step size necessary for adopting each scheme. The LIN scheme is superior to other schemes if the ratio of radiation pressure to gas pressure is sufficiently low. On the other hand, DROS can be the most efficient scheme if the ratio is high. Although the NR scheme can be adopted independently of the regime, especially in a problem that involves optically thin regions, the convergence tends to be worse. In all cases, SOS is not practical.
NASA Astrophysics Data System (ADS)
Fauji, Shantanu
We consider the problem of energy efficient and fault tolerant in--network aggregation for wireless sensor networks (WSNs). In-network aggregation is the process of aggregation while collecting data from sensors to the base station. This process should be energy efficient due to the limited energy at the sensors and tolerant to the high failure rates common in sensor networks. Tree based in--network aggregation protocols, although energy efficient, are not robust to network failures. Multipath routing protocols are robust to failures to a certain degree but are not energy efficient due to the overhead in the maintenance of multiple paths. We propose a new protocol for in-network aggregation in WSNs, which is energy efficient, achieves high lifetime, and is robust to the changes in the network topology. Our protocol, gossip--based protocol for in-network aggregation (GPIA) is based on the spreading of information via gossip. GPIA is not only adaptive to failures and changes in the network topology, but is also energy efficient. Energy efficiency of GPIA comes from all the nodes being capable of selective message reception and detecting convergence of the aggregation early. We experimentally show that GPIA provides significant improvement over some other competitors like the Ridesharing, Synopsis Diffusion and the pure version of gossip. GPIA shows ten fold, five fold and two fold improvement over the pure gossip, the synopsis diffusion and Ridesharing protocols in terms of network lifetime, respectively. Further, GPIA retains gossip's robustness to failures and improves upon the accuracy of synopsis diffusion and Ridesharing.
Thermodynamics of Protein Aggregation
NASA Astrophysics Data System (ADS)
Osborne, Kenneth L.; Barz, Bogdan; Bachmann, Michael; Strodel, Birgit
Amyloid protein aggregation characterizes many neurodegenerative disorders, including Alzheimer's, Parkinson's, and Creutz- feldt-Jakob disease. Evidence suggests that amyloid aggregates may share similar aggregation pathways, implying simulation of full-length amyloid proteins is not necessary for understanding amyloid formation. In this study we simulate GNNQQNY, the N-terminal prion-determining domain of the yeast protein Sup35 to investigate the thermodynamics of structural transitions during aggregation. We use a coarse-grained model with replica-exchange molecular dynamics to investigate the association of 3-, 6-, and 12-chain GNNQQNY systems and we determine the aggregation pathway by studying aggregation states of GN- NQQNY. We find that the aggregation of the hydrophilic GNNQQNY sequence is mainly driven by H-bond formation, leading to the formation of /3-sheets from the very beginning of the assembly process. Condensation (aggregation) and ordering take place simultaneously, which is underpinned by the occurrence of a single heat capacity peak only.
Biological framework for soil aggregation: Implications for ecological functions.
NASA Astrophysics Data System (ADS)
Ghezzehei, Teamrat; Or, Dani
2016-04-01
Soil aggregation is heuristically understood as agglomeration of primary particles bound together by biotic and abiotic cementing agents. The organization of aggregates is believed to be hierarchical in nature; whereby primary particles bond together to form secondary particles and subsequently merge to form larger aggregates. Soil aggregates are not permanent structures, they continuously change in response to internal and external forces and other drivers, including moisture, capillary pressure, temperature, biological activity, and human disturbances. Soil aggregation processes and the resulting functionality span multiple spatial and temporal scales. The intertwined biological and physical nature of soil aggregation, and the time scales involved precluded a universally applicable and quantifiable framework for characterizing the nature and function of soil aggregation. We introduce a biophysical framework of soil aggregation that considers the various modes and factors of the genesis, maturation and degradation of soil aggregates including wetting/drying cycles, soil mechanical processes, biological activity and the nature of primary soil particles. The framework attempts to disentangle mechanical (compaction and soil fragmentation) from in-situ biophysical aggregation and provides a consistent description of aggregate size, hierarchical organization, and life time. It also enables quantitative description of biotic and abiotic functions of soil aggregates including diffusion and storage of mass and energy as well as role of aggregates as hot spots of nutrient accumulation, biodiversity, and biogeochemical cycles.
A Coarse-Grained Model for Polyglutamine Aggregation Modulated by Amphipathic Flanking Sequences
Ruff, Kiersten M.; Khan, Siddique J.; Pappu, Rohit V.
2014-01-01
The aggregation of proteins with expanded polyglutamine (polyQ) tracts is directly relevant to the formation of neuronal intranuclear inclusions in Huntington’s disease. In vitro studies have uncovered the effects of flanking sequences as modulators of the driving forces and mechanisms of polyQ aggregation in sequence segments associated with HD. Specifically, a seventeen-residue amphipathic stretch (N17) that is directly N-terminal to the polyQ tract in huntingtin decreases the overall solubility, destabilizes nonfibrillar aggregates, and accelerates fibril formation. Published results from atomistic simulations showed that the N17 module reduces the frequency of intermolecular association. Our reanalysis of these simulation results demonstrates that the N17 module also reduces interchain entanglements between polyQ domains. These two effects, which are observed on the smallest lengthscales, are incorporated into phenomenological pair potentials and used in coarse-grained Brownian dynamics simulations to investigate their impact on large-scale aggregation. We analyze the results from Brownian dynamics simulations using the framework of diffusion-limited cluster aggregation. When entanglements prevail, which is true in the absence of N17, small spherical clusters and large linear aggregates form on distinct timescales, in accord with in vitro experiments. Conversely, when entanglements are quenched and a barrier to intermolecular associations is introduced, both of which are attributable to N17, the timescales for forming small species and large linear aggregates become similar. Therefore, the combination of a reduction of interchain entanglements through homopolymeric polyQ and barriers to intermolecular associations appears to be sufficient for providing a minimalist phenomenological rationalization of in vitro observations regarding the effects of N17 on polyQ aggregation. PMID:25185558
Wax crystallization and aggregation in a model crude oil
NASA Astrophysics Data System (ADS)
Vignati, Emanuele; Piazza, Roberto; Visintin, Ruben F. G.; Lapasin, Romano; D'Antona, Paolo; Lockhart, Thomas P.
2005-11-01
The high-molecular-weight paraffinic ('wax') fraction separates from crude oils at low temperatures, a process that can lead to a sol-gel transition when the mass of wax solids exceeds 1-2%. Attractive interactions between the micron-size wax solids suspended in the non-polar medium have been suggested to be responsible for gel formation. The present study reports an optically transparent model oil system, based on a mixture of linear and branched paraffins. Rheological measurements and optical microscopy show that the model system reproduces essential features of crude oil gels. Small-angle light scattering studies conducted at temperatures intermediate between the cloud point (58 °C) and sol-gel transition (39 °C) show that phase separation and wax solid aggregation are rapid processes, leading to the formation of dynamically arrested structures well above the sol-gel transition determined rheologically. Analysis of gravity settling effects has provided a rough estimate for the yield stress of the wax particle network formed (greater than 0.7 Pa at 45 °C and 0.07 Pa at 55 °C). Clusters formed by the aggregated wax solids possess a fractal dimension of about 1.8, consistent with diffusion-limited cluster-cluster aggregation.
Individual based and mean-field modeling of direct aggregation
Burger, Martin; Haškovec, Jan; Wolfram, Marie-Therese
2013-01-01
We introduce two models of biological aggregation, based on randomly moving particles with individual stochasticity depending on the perceived average population density in their neighborhood. In the first-order model the location of each individual is subject to a density-dependent random walk, while in the second-order model the density-dependent random walk acts on the velocity variable, together with a density-dependent damping term. The main novelty of our models is that we do not assume any explicit aggregative force acting on the individuals; instead, aggregation is obtained exclusively by reducing the individual stochasticity in response to higher perceived density. We formally derive the corresponding mean-field limits, leading to nonlocal degenerate diffusions. Then, we carry out the mathematical analysis of the first-order model, in particular, we prove the existence of weak solutions and show that it allows for measure-valued steady states. We also perform linear stability analysis and identify conditions for pattern formation. Moreover, we discuss the role of the nonlocality for well-posedness of the first-order model. Finally, we present results of numerical simulations for both the first- and second-order model on the individual-based and continuum levels of description. PMID:24926113
Aggregation of a dibenzo[b,def]chrysene based organic photovoltaic material in solution.
Simonov, Alexandr N; Kemppinen, Peter; Pozo-Gonzalo, Cristina; Boas, John F; Bilic, Ante; Scully, Andrew D; Attia, Adel; Nafady, Ayman; Mashkina, Elena A; Winzenberg, Kevin N; Watkins, Scott E; Bond, Alan M
2014-06-19
Detailed electrochemical studies have been undertaken on molecular aggregation of the organic semiconductor 7,14-bis((triisopropylsilyl)-ethynyl)dibenzo[b,def]chrysene (TIPS-DBC), which is used as an electron donor material in organic solar cells. Intermolecular association of neutral TIPS-DBC molecules was established by using (1)H NMR spectroscopy as well as by the pronounced dependence of the color of TIPS-DBC solutions on concentration. Diffusion limited current data provided by near steady-state voltammetry also reveal aggregation. Furthermore, variation of concentration produces large changes in shapes of transient DC and Fourier transformed AC (FTAC) voltammograms for oxidation of TIPS-DBC in dichloromethane. Subtle effects of molecular aggregation on the reduction of TIPS-DBC are also revealed by the highly sensitive FTAC voltammetric method. Simulations of FTAC voltammetric data provide estimates of the kinetic and thermodynamic parameters associated with oxidation and reduction of TIPS-DBC. Significantly, aggregation of TIPS-DBC facilitates both one-electron oxidation and reduction by shifting the reversible potentials to less and more positive values, respectively. EPR spectroscopy is used to establish the identity of one-electron oxidized and reduced forms of TIPS-DBC. Implications of molecular aggregation on the HOMO energy level in solution are considered with respect to efficiency of organic photovoltaic devices utilizing TIPS-DBC as an electron donor material.
Dynamics of proteins aggregation. I. Universal scaling in unbounded media
NASA Astrophysics Data System (ADS)
Zheng, Size; Javidpour, Leili; Shing, Katherine S.; Sahimi, Muhammad
2016-10-01
It is well understood that in some cases proteins do not fold correctly and, depending on their environment, even properly-folded proteins change their conformation spontaneously, taking on a misfolded state that leads to protein aggregation and formation of large aggregates. An important factor that contributes to the aggregation is the interactions between the misfolded proteins. Depending on the aggregation environment, the aggregates may take on various shapes forming larger structures, such as protein plaques that are often toxic. Their deposition in tissues is a major contributing factor to many neuro-degenerative diseases, such as Alzheimer's, Parkinson's, amyotrophic lateral sclerosis, and prion. This paper represents the first part in a series devoted to molecular simulation of protein aggregation. We use the PRIME, a meso-scale model of proteins, together with extensive discontinuous molecular dynamics simulation to study the aggregation process in an unbounded fluid system, as the first step toward MD simulation of the same phenomenon in crowded cellular environments. Various properties of the aggregates have been computed, including dynamic evolution of aggregate-size distribution, mean aggregate size, number of peptides that contribute to the formation of β sheets, number of various types of hydrogen bonds formed in the system, radius of gyration of the aggregates, and the aggregates' diffusivity. We show that many of such quantities follow dynamic scaling, similar to those for aggregation of colloidal clusters. In particular, at long times the mean aggregate size S(t) grows with time as, S(t) ˜ tz, where z is the dynamic exponent. To our knowledge, this is the first time that the qualitative similarity between aggregation of proteins and colloidal aggregates has been pointed out.
Formation of Tethers from Spreading Cellular Aggregates.
Beaune, Grégory; Winnik, Françoise M; Brochard-Wyart, Françoise
2015-12-01
Membrane tubes are commonly extruded from cells and vesicles when a point-like force is applied on the membrane. We report here the unexpected formation of membrane tubes from lymph node cancer prostate (LNCaP) cell aggregates in the absence of external applied forces. The spreading of LNCaP aggregates deposited on adhesive glass substrates coated with fibronectin is very limited because cell-cell adhesion is stronger than cell-substrate adhesion. Some cells on the aggregate periphery are very motile and try to escape from the aggregate, leading to the formation of membrane tubes. Tethered networks and exchange of cargos between cells were observed as well. Growth of the tubes is followed by either tube retraction or tube rupture. Hence, even very cohesive cells are successful in escaping aggregates, which may lead to epithelial mesenchymal transition and tumor metastasis. We interpret the dynamics of formation and retraction of tubes in the framework of membrane mechanics. PMID:26509898
Messer, O.E.; Mezzacappa, A. |; Bruenn, S.W.; Guidry, M.W. |
1998-11-01
We compare Newtonian three-flavor multigroup Boltzmann (MGBT) and (Bruenn`s) multigroup flux-limited diffusion (MGFLD) neutrino transport in postbounce core-collapse supernova environments. We focus our study on quantities central to the postbounce neutrino heating mechanism for reviving the stalled shock. Stationary-state three-flavor neutrino distributions are developed in thermally and hydrodynamically frozen time slices obtained from core collapse and bounce simulations that implement Lagrangian hydrodynamics and MGFLD neutrino transport. We obtain distributions for time slices at 106 and 233 ms after core bounce for the core of a 15 {ital M}{sub {circle_dot}} progenitor, and at 156 ms after core bounce for a 25 {ital M}{sub {circle_dot}} progenitor. For both transport methods, the electron neutrino and antineutrino luminosities, rms energies, and mean inverse flux factors, all of which enter the neutrino heating rates, are computed as functions of radius and compared. The net neutrino heating rates are also computed as functions of radius and compared. Notably, we find significant differences in neutrino luminosities and mean inverse flux factors between the two transport methods for both precollapse models and for all three time slices. In each case, the luminosities for each transport method begin to diverge above the neutrinospheres, where the MGBT luminosities become larger than their MGFLD counterparts, finally settling to a constant difference maintained to the edge of the core. We find that the mean inverse flux factors, which describe the degree of forward peaking in the neutrino radiation field, also differ significantly between the two transport methods, with MGBT providing more isotropic radiation fields in the gain region. Most important, for a region above the gain radius we find net heating rates for MGBT that are as much as {approximately}2 times the corresponding MGFLD rates, and we find net cooling rates below the gain radius that are
Higgs, Paul G
2016-06-08
A long-standing problem for the origins of life is that polymerization of many biopolymers, including nucleic acids and peptides, is thermodynamically unfavourable in aqueous solution. If bond making and breaking is reversible, monomers and very short oligomers predominate. Recent experiments have shown that wetting and drying cycles can overcome this problem and drive the formation of longer polymers. In the dry phase, bond formation is favourable, but diffusion is restricted, and bonds only form between monomers that are initially close together. In the wet phase, some of the bonds are hydrolyzed. However, repositioning of the molecules allows new bonds to form in the next dry phase, leading to an increase in mean polymer length. Here, we consider a simple theoretical model that explains the effect of cycling. There is an equilibrium length distribution with a high mean length that could be achieved if diffusion occurred freely in the dry phase. This equilibrium is inaccessible without diffusion. A single dry cycle without diffusion leads to mean lengths much shorter than this. Repeated cycling leads to a significant increase in polymerization relative to a single cycle. In the most favourable case, cycling leads to the same equilibrium length distribution as would be achieved if free diffusion were possible in the dry phase. These results support the RNA World scenario by explaining a potential route to synthesis of long RNAs; however, they also imply that cycling would be beneficial to the synthesis of other kinds of polymers, including peptides, where bond formation involves a condensation reaction.
Critical space scales for aggregation-mediated carbon export from ocean fertilization
NASA Astrophysics Data System (ADS)
Waite, Anya M.; Johnson, David
2003-07-01
Several experiments have attempted to test Martin's hypothesis that addition of soluble iron to iron-limited ocean regions will lead to an increase in carbon sequestration via phytoplankton growth and sedimentation. However, artificially iron-induced blooms do not always trigger increases in vertical carbon export. Here we show that to trigger export, a patch must be larger than a threshold size. Phytoplankton sink en masse only after reaching a critical concentration for aggregation, and concentration is dependent on the competitive balance between growth and horizontal turbulent diffusion, which in turn varies with patch length scale. We summarize this balance using a non-dimensional parameter, Q, and use a simple 2D model solving a growth-diffusion-export equation to show that export flux occurs from a fertilized patch when Q < 1, and that flux is maximized at a value of Q below this critical point. A simple nutrient limitation model generates predictions of particle export from patch fertilization experiments.
NASA Astrophysics Data System (ADS)
Virk, Amninder; Stait-Gardner, Timothy; Willis, Scott; Torres, Allan; Price, William
2015-02-01
Molecular crowding occurs when the total concentration of macromolecular species in a solution is so high that a considerable proportion of the volume is physically occupied and therefore not accessible to other molecules. This results in significant changes in the solution properties of the molecules in such systems. Macromolecular crowding is ubiquitous in biological systems due to the generally high intracellular protein concentrations. The major hindrance to understanding crowding is the lack of direct comparison of experimental data with theoretical or simulated data. Self-diffusion is sensitive to changes in the molecular weight and shape of the diffusing species, and the available diffusion space (i.e., diffusive obstruction). Consequently, diffusion measurements are a direct means for probing crowded systems including the self-association of molecules. In this work, nuclear magnetic resonance measurements of the self-diffusion of four amino acids (glycine, alanine, valine and phenylalanine) up to their solubility limit in water were compared directly with molecular dynamics simulations. The experimental data were then analyzed using various models of aggregation and obstruction. Both experimental and simulated data revealed that the diffusion of both water and the amino acids were sensitive to the amino acid concentration. The direct comparison of the simulated and experimental data afforded greater insights into the aggregation and obstruction properties of each amino acid.
Formation of Interfacial Patterns in Aggregation and Viscous Flows
NASA Astrophysics Data System (ADS)
Vicsek, Tamás
1987-01-01
Computer simulations and experiments on viscous fingering are used to investigate the effects of fluctuations, driving force and anisotropy on the growth of two dimensional unstable interfaces. It is demonstrated that variations of the diffusion-limited aggregation model capture many of the most important features of Laplacian pattern formation. In the viscous fingering experiments carried out in a radial Hele-Shaw cell with nematic or smectic liquid crystals a number of unexpected morphological phase transitions can be observed including crossovers from tip splitting to dendritic growth and from fractal to homogeneous structures. The investigations reviewed here suggest that the role of noise, driving force and anisotropy is crucial in the formation of patterns and it is the complex interplay of these factors which produces the great variety of morphologies found in nature.
Zhang, Li-Chen; Risoul, Véronique; Latifi, Amel; Christie, John M; Zhang, Cheng-Cai
2013-09-01
In the filamentous heterocyst-forming cyanobacterium Anabaena PCC 7120, vegetative cells and heterocysts are interdependent on each other and engaged in exchanges of metabolites for survival when grown under diazotrophic conditions. In this organism, the periplasm appears to be continuous along each filament, with a shared outer membrane; however, barriers exist preventing free diffusion of the fluorescent protein GFP (27 kDa) targeted into the periplasmic space. Here we expressed a smaller fluorescent protein iLOV (≈ 13 kDa) fused to the All3333 (a putative homologue of NrtA) signal sequence corresponding to those recognized by the TAT protein translocation system, which exports iLOV to the periplasm of either heterocysts or vegetative cells. Fluorescence microscopy and immunoblot analysis indicated that the iLOV protein is translocated into the periplasm of the producing cell and properly processed, but does not diffuse to neighboring cells via the periplasm. Thus, periplasmic barriers appear to block diffusion of molecules with a size of 13 kDa, the minimum size tested thus far. Assuming that the physical barrier is the peptidoglycan sacculus, its pores might allow diffusion of molecules within the size range between the PatS pentapeptide and iLOV, thus between 0.53 kDa and 13 kDa.
Zhang, Li-Chen; Risoul, Véronique; Latifi, Amel; Christie, John M; Zhang, Cheng-Cai
2013-09-01
In the filamentous heterocyst-forming cyanobacterium Anabaena PCC 7120, vegetative cells and heterocysts are interdependent on each other and engaged in exchanges of metabolites for survival when grown under diazotrophic conditions. In this organism, the periplasm appears to be continuous along each filament, with a shared outer membrane; however, barriers exist preventing free diffusion of the fluorescent protein GFP (27 kDa) targeted into the periplasmic space. Here we expressed a smaller fluorescent protein iLOV (≈ 13 kDa) fused to the All3333 (a putative homologue of NrtA) signal sequence corresponding to those recognized by the TAT protein translocation system, which exports iLOV to the periplasm of either heterocysts or vegetative cells. Fluorescence microscopy and immunoblot analysis indicated that the iLOV protein is translocated into the periplasm of the producing cell and properly processed, but does not diffuse to neighboring cells via the periplasm. Thus, periplasmic barriers appear to block diffusion of molecules with a size of 13 kDa, the minimum size tested thus far. Assuming that the physical barrier is the peptidoglycan sacculus, its pores might allow diffusion of molecules within the size range between the PatS pentapeptide and iLOV, thus between 0.53 kDa and 13 kDa. PMID:23748014
NASA Astrophysics Data System (ADS)
Pallud, C.; Masue-Slowey, Y.; Fendorf, S.
2010-05-01
Iron (hydr)oxides are ubiquitous in soils and sediments and play a dominant role in the geochemistry of surface and subsurface environments. Their fate depends on local environmental conditions, which in structured soils may vary significantly over short distances due to mass-transfer limitations on solute delivery and metabolite removal. In the present study, artificial soil aggregates were used to investigate the coupling of physical and biogeochemical processes affecting the spatial distribution of iron (Fe) phases resulting from reductive transformation of ferrihydrite. Spherical aggregates made of ferrihydrite-coated sand were inoculated with the dissimilatory Fe-reducing bacterium Shewanella putrefaciens strain CN-32, and placed into a flow reactor, the reaction cell simulates a diffusion-dominated soil aggregate surrounded by an advective flow domain. The spatial and temporal evolution of secondary mineralization products resulting from dissimilatory Fe reduction of ferrihydrite were followed within the aggregates in response to a range of flow rates and lactate concentrations. Strong radial variations in the distribution of secondary phases were observed owing to diffusively controlled delivery of lactate and efflux of Fe(II) and bicarbonate. In the aggregate cortex, only limited formation of secondary Fe phases were observed over 30 d of reaction, despite high rates of ferrihydrite reduction. Under all flow conditions tested, ferrihydrite transformation was limited in the cortex (70-85 mol.% Fe remained as ferrihydrite) because metabolites such as Fe(II) and bicarbonate were efficiently removed in outflow solutes. In contrast, within the inner fractions of the aggregate, limited mass-transfer results in metabolite (Fe(II) and bicarbonate) build-up and the consummate transformation of ferrihydrite - only 15-40 mol.% Fe remained as ferrihydrite after 30 d of reaction. Goethite/lepidocrocite, and minor amounts of magnetite, formed in the aggregate mid
ERIC Educational Resources Information Center
Liffen, C. L.; Hunter, M.
1980-01-01
Described is a school project to investigate aggregations in flatworms which may be influenced by light intensity, temperature, and some form of chemical stimulus released by already aggregating flatworms. Such investigations could be adopted to suit many educational levels of science laboratory activities. (DS)
Higgs, Paul G
2016-01-01
A long-standing problem for the origins of life is that polymerization of many biopolymers, including nucleic acids and peptides, is thermodynamically unfavourable in aqueous solution. If bond making and breaking is reversible, monomers and very short oligomers predominate. Recent experiments have shown that wetting and drying cycles can overcome this problem and drive the formation of longer polymers. In the dry phase, bond formation is favourable, but diffusion is restricted, and bonds only form between monomers that are initially close together. In the wet phase, some of the bonds are hydrolyzed. However, repositioning of the molecules allows new bonds to form in the next dry phase, leading to an increase in mean polymer length. Here, we consider a simple theoretical model that explains the effect of cycling. There is an equilibrium length distribution with a high mean length that could be achieved if diffusion occurred freely in the dry phase. This equilibrium is inaccessible without diffusion. A single dry cycle without diffusion leads to mean lengths much shorter than this. Repeated cycling leads to a significant increase in polymerization relative to a single cycle. In the most favourable case, cycling leads to the same equilibrium length distribution as would be achieved if free diffusion were possible in the dry phase. These results support the RNA World scenario by explaining a potential route to synthesis of long RNAs; however, they also imply that cycling would be beneficial to the synthesis of other kinds of polymers, including peptides, where bond formation involves a condensation reaction. PMID:27338479
Higgs, Paul G.
2016-01-01
A long-standing problem for the origins of life is that polymerization of many biopolymers, including nucleic acids and peptides, is thermodynamically unfavourable in aqueous solution. If bond making and breaking is reversible, monomers and very short oligomers predominate. Recent experiments have shown that wetting and drying cycles can overcome this problem and drive the formation of longer polymers. In the dry phase, bond formation is favourable, but diffusion is restricted, and bonds only form between monomers that are initially close together. In the wet phase, some of the bonds are hydrolyzed. However, repositioning of the molecules allows new bonds to form in the next dry phase, leading to an increase in mean polymer length. Here, we consider a simple theoretical model that explains the effect of cycling. There is an equilibrium length distribution with a high mean length that could be achieved if diffusion occurred freely in the dry phase. This equilibrium is inaccessible without diffusion. A single dry cycle without diffusion leads to mean lengths much shorter than this. Repeated cycling leads to a significant increase in polymerization relative to a single cycle. In the most favourable case, cycling leads to the same equilibrium length distribution as would be achieved if free diffusion were possible in the dry phase. These results support the RNA World scenario by explaining a potential route to synthesis of long RNAs; however, they also imply that cycling would be beneficial to the synthesis of other kinds of polymers, including peptides, where bond formation involves a condensation reaction. PMID:27338479
Aggregation behaviour of biohybrid microgels from elastin-like recombinamers.
Singh, Smriti; Demco, Dan Eugen; Rahimi, Khosrow; Fechete, Radu; Rodriguez-Cabello, José Carlos; Möller, Martin
2016-07-20
Investigation of the aggregation behavior of biohybrid microgels, which can potentially be used as drug carriers, is an important topic, because aggregation not only causes loss of activity, but also toxicity and immunogenicity. To study this effect we synthesized microgels from elastin-like recombinamers (ELRs) using the miniemulsion technique. The existence of aggregation for such biohybrid microgels at different concentrations and temperatures was studied by different methods which include dynamic light scattering (DLS), (1)H high-resolution magic angle sample spinning (HRMAS) NMR spectroscopy, relaxometry and diffusometry. A hysteresis effect was detected in the process of aggregation by DLS as a function of temperature that strongly depends on ELR microgel concentration. The aggregation process was further quantitatively analyzed by the concentration dependence of the (1)H amino-acid residue chemical shifts and microgel diffusivity measured by NMR methods using the population balance kinetic aggregation model. PMID:27378252
Protein aggregation in salt solutions
Kastelic, Miha; Kalyuzhnyi, Yurij V.; Hribar-Lee, Barbara; Dill, Ken A.; Vlachy, Vojko
2015-01-01
Protein aggregation is broadly important in diseases and in formulations of biological drugs. Here, we develop a theoretical model for reversible protein–protein aggregation in salt solutions. We treat proteins as hard spheres having square-well-energy binding sites, using Wertheim’s thermodynamic perturbation theory. The necessary condition required for such modeling to be realistic is that proteins in solution during the experiment remain in their compact form. Within this limitation our model gives accurate liquid–liquid coexistence curves for lysozyme and γ IIIa-crystallin solutions in respective buffers. It provides good fits to the cloud-point curves of lysozyme in buffer–salt mixtures as a function of the type and concentration of salt. It than predicts full coexistence curves, osmotic compressibilities, and second virial coefficients under such conditions. This treatment may also be relevant to protein crystallization. PMID:25964322
Protein aggregation in salt solutions.
Kastelic, Miha; Kalyuzhnyi, Yurij V; Hribar-Lee, Barbara; Dill, Ken A; Vlachy, Vojko
2015-05-26
Protein aggregation is broadly important in diseases and in formulations of biological drugs. Here, we develop a theoretical model for reversible protein-protein aggregation in salt solutions. We treat proteins as hard spheres having square-well-energy binding sites, using Wertheim's thermodynamic perturbation theory. The necessary condition required for such modeling to be realistic is that proteins in solution during the experiment remain in their compact form. Within this limitation our model gives accurate liquid-liquid coexistence curves for lysozyme and γ IIIa-crystallin solutions in respective buffers. It provides good fits to the cloud-point curves of lysozyme in buffer-salt mixtures as a function of the type and concentration of salt. It than predicts full coexistence curves, osmotic compressibilities, and second virial coefficients under such conditions. This treatment may also be relevant to protein crystallization.
Insights into asphaltene aggregation in the Na-montmorillonite interlayer.
Zhu, Xinzhe; Chen, Daoyi; Wu, Guozhong
2016-10-01
This study aimed to provide insights into the diffusion and aggregation of asphaltenes in the Na-montmorillonite (MMT) interlayer with different water saturation, salinity, interlayer space and humic substances. The molecular configuration, density profile, diffusion coefficient and aggregation intensity were determined by molecular dynamic simulation, while the 3D topography and particle size of the aggregates were characterized by atomic force microscopy. Results indicated that the diffusivity of asphaltenes was up to 5-fold higher in the MMT interlayer filled with fresh water than with saline water (salinity: 35‰). However, salinity had little impact on the asphaltene aggregation. This study also showed a marked decrease in the mobility of asphaltenes with decrease in the pore water content and the interlayer space of MMT. This was more pronounced in the organo-MMT where the humic substances were present. The co-aggregation process resulted in the sequestration of asphaltenes in the hollow cone-shaped cavity of humic substances in the MMT interlayer, which decreased the asphaltene diffusion by up to one-order of magnitude and increased the asphaltene aggregation by about 33%. These findings have important ramifications for evaluating the fate and transport of heavy fractions of the residual oil in the contaminated soils. PMID:27362529
Insights into asphaltene aggregation in the Na-montmorillonite interlayer.
Zhu, Xinzhe; Chen, Daoyi; Wu, Guozhong
2016-10-01
This study aimed to provide insights into the diffusion and aggregation of asphaltenes in the Na-montmorillonite (MMT) interlayer with different water saturation, salinity, interlayer space and humic substances. The molecular configuration, density profile, diffusion coefficient and aggregation intensity were determined by molecular dynamic simulation, while the 3D topography and particle size of the aggregates were characterized by atomic force microscopy. Results indicated that the diffusivity of asphaltenes was up to 5-fold higher in the MMT interlayer filled with fresh water than with saline water (salinity: 35‰). However, salinity had little impact on the asphaltene aggregation. This study also showed a marked decrease in the mobility of asphaltenes with decrease in the pore water content and the interlayer space of MMT. This was more pronounced in the organo-MMT where the humic substances were present. The co-aggregation process resulted in the sequestration of asphaltenes in the hollow cone-shaped cavity of humic substances in the MMT interlayer, which decreased the asphaltene diffusion by up to one-order of magnitude and increased the asphaltene aggregation by about 33%. These findings have important ramifications for evaluating the fate and transport of heavy fractions of the residual oil in the contaminated soils.
Thermal Aggregation of Recombinant Protective Antigen: Aggregate Morphology and Growth Rate
Belton, Daniel J.; Miller, Aline F.
2013-01-01
The thermal aggregation of the biopharmaceutical protein recombinant protective antigen (rPA) has been explored, and the associated kinetics and thermodynamic parameters have been extracted using optical and environmental scanning electron microscopies (ESEMs) and ultraviolet light scattering spectroscopy (UV-LSS). Visual observations and turbidity measurements provided an overall picture of the aggregation process, suggesting a two-step mechanism. Microscopy was used to examine the structure of aggregates, revealing an open morphology formed by the clustering of the microscopic aggregate particles. UV-LSS was used and developed to elucidate the growth rate of these particles, which formed in the first stage of the aggregation process. Their growth rate is observed to be high initially, before falling to converge on a final size that correlates with the ESEM data. The results suggest that the particle growth rate is limited by rPA monomer concentration, and by obtaining data over a range of incubation temperatures, an approach was developed to model the aggregation kinetics and extract the rate constants and the temperature dependence of aggregation. In doing so, we quantified the susceptibility of rPA aggregation under different temperature and environmental conditions and moreover demonstrated a novel use of UV spectrometry to monitor the particle aggregation quantitatively, in situ, in a nondestructive and time-resolved manner. PMID:23476645
Fuentealba, Rodrigo A.; Marasa, Jayne; Diamond, Marc I.; Piwnica-Worms, David; Weihl, Conrad C.
2012-01-01
Intracellular protein aggregation is a common pathologic feature in neurodegenerative diseases such as Huntington' disease, amyotrophic lateral sclerosis and Parkinson' disease. Although progress towards understanding protein aggregation in vitro has been made, little of this knowledge has translated to patient therapy. Moreover, mechanisms controlling aggregate formation and catabolism in cellulo remain poorly understood. One limitation is the lack of tools to quantitatively monitor protein aggregation and disaggregation. Here, we developed a protein-aggregation reporter that uses huntingtin exon 1 containing 72 glutamines fused to the N-terminal end of firefly luciferase (httQ72-Luc). httQ72-Luc fails to aggregate unless seeded by a non-luciferase-containing polyglutamine (polyQ) protein such as Q80-cfp. Upon co-aggregation, httQ72-luc becomes insoluble and loses its enzymatic activity. Using httQ72-Luc with Q80(CFP/YFP) as seeds, we screened the Johns Hopkins Clinical Compound Library and identified leflunomide, a dihydroorotate dehydrogenase inhibitor with immunosuppressive and anti-psoriatic activities, as a novel drug that prevents polyQ aggregation. Leflunomide and its active metabolite teriflunomide inhibited protein aggregation independently of their known role in pyrimidine biosynthesis, since neither uridine treatment nor other pyrimidine biosynthesis inhibitors affected polyQ aggregation. Inducible cell line and cycloheximide-chase experiments indicate that these drugs prevent incorporation of expanded polyQ into an aggregate. This study demonstrates the usefulness of luciferase-based protein aggregate reporters for high-throughput screening applications. As current trials are under-way for teriflunomide in the treatment of multiple sclerosis, we propose that this drug be considered a possible therapeutic agent for polyQ diseases. PMID:22052286
Langer, William H.; Drew, Lawrence J.; Sachs, J.S.
2004-01-01
This book is designed to help you understand our aggregate resources-their importance, where they come from, how they are processed for our use, the environmental concerns related to their mining and processing, how those concerns are addressed, and the policies and regulations designed to safeguard workers, neighbors, and the environment from the negative impacts of aggregate mining. We hope this understanding will help prepare you to be involved in decisions that need to be made-individually and as a society-to be good stewards of our aggregate resources and our living planet.
Protein Colloidal Aggregation Project
NASA Technical Reports Server (NTRS)
Oliva-Buisson, Yvette J. (Compiler)
2014-01-01
To investigate the pathways and kinetics of protein aggregation to allow accurate predictive modeling of the process and evaluation of potential inhibitors to prevalent diseases including cataract formation, chronic traumatic encephalopathy, Alzheimer's Disease, Parkinson's Disease and others.
NASA Astrophysics Data System (ADS)
Cherniak, D. J.; Watson, E. B.
2010-09-01
Diffusion of Li under anhydrous conditions at 1 atm and under fluid-present elevated pressure (1.0-1.2 GPa) conditions has been measured in natural zircon. The source of diffusant for 1-atm experiments was ground natural spodumene, which was sealed under vacuum in silica glass capsules with polished slabs of zircon. An experiment using a Dy-bearing source was also conducted to evaluate possible rate-limiting effects on Li diffusion of slow-diffusing REE+3 that might provide charge balance. Diffusion experiments performed in the presence of H2O-CO2 fluid were run in a piston-cylinder apparatus, using a source consisting of a powdered mixture of spodumene, quartz and zircon with oxalic acid added to produce H2O-CO2 fluid. Nuclear reaction analysis (NRA) with the resonant nuclear reaction 7Li(p,γ)8Be was used to measure diffusion profiles for the experiments. The following Arrhenius parameters were obtained for Li diffusion normal to the c-axis over the temperature range 703-1.151°C at 1 atm for experiments run with the spodumene source: D_{text{Li}} = 7.17 × 10^{ - 7} { exp }( - 275 ± 11 {text{kJmol}}^{ - 1} /{text{RT}}){text{m}}2 {text{s}}^{ - 1}. Diffusivities are similar for transport parallel to the c-axis, indicating little anisotropy for Li diffusion in zircon. Similar Li diffusivities were also found for experiments run under fluid-present conditions and for the experiment run with the Dy-bearing source. Li diffusion is considerably faster than diffusion of other cations in zircon, with a smaller activation energy for diffusion. Although Li diffusion in zircon is comparatively rapid, zircons will be moderately retentive of Li signatures at mid-crustal metamorphic temperatures, but they are unlikely to retain this information for geologically significant times under high-grade metamorphism.
Cluster-Cluster Aggregation Calculations of Fractal Haze Particles: Titan and the Early Earth
NASA Astrophysics Data System (ADS)
Terrell-Martinez, Bernice; Boness, David
2010-10-01
The atmosphere of the Archean Earth (3.8 to 2.5 billion years ago) is thought to have been dominated by a thick hydrocarbon haze similar to that of Titan's current atmosphere. To understand radiative transport in the atmospheres of the early Earth and of Titan, it is necessary to compute light scattering in UV, visible, and IR wavelength ranges for realistic fractal aggregate hydrocarbon aerosol particles. We report preliminary work on MATLAB, True BASIC, and Fortran programs to simulate the growth of fractal aggregate aerosols through diffusion limited aggregation (DLA) and cluster-cluster aggregation (CCA) physical processes. The results of these computations are being used with a T-Matrix light scattering program to test recently published, widely-reported conclusions about the early Earth and the faint young Sun paradox [E. T. Wolf and O. B. Toon, Science 328, 1266 (2010)]. This modeling is also relevant to understanding atmospheric carbonaceous soot aerosol anthropogenic and natural effects on climate change of Earth today.
NASA Technical Reports Server (NTRS)
Huddleston, D. E.; Neugebauer, M.; Goldstein, B. E.
1994-01-01
The shape of the velocity distribution of water group ions observed by the Giotto ion mass spectrometer on its approach to comet Halley is modeled to derive empirical values for the rates of ionization, energy diffusion, and loss in the midcometosheath. The model includes the effect of rapid pitch angle scattering into a bispherical shell distribution as well as the effect of the magnetization of the plasma on the charge exchange loss rate. It is found that the average rate of ionization of cometary neutrals in this region of the cometosheath appears to be of the order of a factor 3 faster than the `standard' rates approx. 1 x 10(exp -6)/s that are generally assumed to model the observations in most regions of the comet environment. For the region of the coma studied in the present work (approx. 1 - 2 x 10(exp 5) km from the nucleus), the inferred energy diffusion coefficient is D(sub 0) approx. equals 0.0002 to 0.0005 sq km/cu s, which is generally lower than values used in other models. The empirically obtained loss rate appears to be about an order of magnitude greater than can be explained by charge exchange with the `standard' cross section of approx. 2 x 10(exp -15)sq cm. However such cross sections are not well known and for water group ion/water group neutral interactions, rates as high as 8 x 10(exp -15) sq cm have previously been suggested in the literature. Assuming the entire loss rate is due to charge exchange yields a rate of creation of fast neutral atoms of the order of approx. 10(exp -4)/s or higher, depending on the level of velocity diffusion. The fast neutrals may, in turn, be partly responsible for the higher-than-expected ionization rate.
Cell aggregation and sedimentation.
Davis, R H
1995-01-01
The aggregation of cells into clumps or flocs has been exploited for decades in such applications as biological wastewater treatment, beer brewing, antibiotic fermentation, and enhanced sedimentation to aid in cell recovery or retention. More recent research has included the use of cell aggregation and sedimentation to selectively separate subpopulations of cells. Potential biotechnological applications include overcoming contamination, maintaining plasmid-bearing cells in continuous fermentors, and selectively removing nonviable hybridoma cells from perfusion cultures.
Interference of diffusive light waves.
Schmitt, J M; Knüttel, A; Knutson, J R
1992-10-01
We examine interference effects resulting from the superposition of photon-density waves produced by coherently modulated light incident upon a turbid medium. Photon-diffusion theory is used to derive expressions for the ac magnitude and phase of the aggregate diffusive wave produced in full- and half-space volumes by two sources. Using a frequency-domain spectrometer operating at 410 MHz, we verify interference patterns predicted by the model in scattering samples having optical properties similar to those of skin tissue. Potential imaging applications of interfering diffusive waves are discussed in the context of the theoretical and experimental results.
NASA Astrophysics Data System (ADS)
Faliagas, A. C.
2016-03-01
Maxwell's theory of multicomponent diffusion and subsequent extensions are based on systems of mass and momentum conservation equations. The partial stress tensor, which is involved in these equations, is expressed in terms of the gradients of velocity fields by statistical and continuum mechanical methods. We propose a method for the solution of Maxwell's equations of diffusion coupled with Müller's expression for the partial stress tensor. The proposed method consists in a singular perturbation process, followed by a weak (finite element) analysis of the resulting PDE systems. The singularity involved in the obtained equations was treated by a special technique, by which lower-order systems were supplemented by proper combinations of higher-order equations. The method proved particularly efficient for the solution of the Maxwell-Müller system, eventually reducing the number of unknown fields to that of the classical Navier-Stokes/Fick system. It was applied to the classical Stefan tube problem and the Hagen-Poiseuille flow in a hollow-fiber membrane tube. Numerical results for these problems are presented, and compared with the Navier-Stokes/Fick approximation. It is shown that the 0-th order term of the Maxwell-Müller equations differs from a properly formulated Navier-Stokes/Fick system, by a numerically insignificant amount. Numerical results for 1st-order terms indicate a good agreement of the classical approximation (with properly formulated Navier-Stokes and Fick's equations) with the Maxwell-Müller system, in the studied cases.
Laser light scattering as a probe of fractal colloid aggregates
NASA Technical Reports Server (NTRS)
Weitz, David A.; Lin, M. Y.
1989-01-01
The extensive use of laser light scattering is reviewed, both static and dynamic, in the study of colloid aggregation. Static light scattering enables the study of the fractal structure of the aggregates, while dynamic light scattering enables the study of aggregation kinetics. In addition, both techniques can be combined to demonstrate the universality of the aggregation process. Colloidal aggregates are now well understood and therefore represent an excellent experimental system to use in the study of the physical properties of fractal objects. However, the ultimate size of fractal aggregates is fundamentally limited by gravitational acceleration which will destroy the fractal structure as the size of the aggregates increases. This represents a great opportunity for spaceborne experimentation, where the reduced g will enable the growth of fractal structures of sufficient size for many interesting studies of their physical properties.
Multispecies diffusion models: A study of uranyl species diffusion
Liu, Chongxuan; Shang, Jianying; Zachara, John M.
2011-12-14
Rigorous numerical description of multi-species diffusion requires coupling of species, charge, and aqueous and surface complexation reactions that collectively affect diffusive fluxes. The applicability of a fully coupled diffusion model is, however, often constrained by the availability of species self-diffusion coefficients, as well as by computational complication for imposing charge conservation. In this study, several diffusion models with variable complexity in charge and species coupling were formulated and compared to describe reactive multi-species diffusion in groundwater. Diffusion of uranyl [U(VI)] species was used as an example in demonstrating the effectiveness of the models in describing multi-species diffusion. Numerical simulations found that a diffusion model with a single, common diffusion coefficient for all species was sufficient to describe multi-species U(VI) diffusion under steady-state condition of major chemical composition, but not under transient chemical conditions. Simulations revealed that a fully coupled diffusion model can be well approximated by a component-based diffusion model, which considers difference in diffusion coefficients between chemical components, but not between the species within each chemical component. This treatment significantly enhanced computational efficiency at the expense of minor charge conservation. The charge balance in the component-based diffusion model can be rigorously enforced, if necessary, by adding an artificial kinetic reaction term induced by the charge separation. The diffusion models were applied to describe U(VI) diffusive mass transfer in intragranular domains in two sediments collected from US Department of Energy's Hanford 300A where intragrain diffusion is a rate-limiting process controlling U(VI) adsorption and desorption. The grain-scale reactive diffusion model was able to describe U(VI) adsorption/desorption kinetics that has been described using a semi-empirical, multi-rate model
Fibronectin Aggregation and Assembly
Ohashi, Tomoo; Erickson, Harold P.
2011-01-01
The mechanism of fibronectin (FN) assembly and the self-association sites are still unclear and contradictory, although the N-terminal 70-kDa region (I1–9) is commonly accepted as one of the assembly sites. We previously found that I1–9 binds to superfibronectin, which is an artificial FN aggregate induced by anastellin. In the present study, we found that I1–9 bound to the aggregate formed by anastellin and a small FN fragment, III1–2. An engineered disulfide bond in III2, which stabilizes folding, inhibited aggregation, but a disulfide bond in III1 did not. A gelatin precipitation assay showed that I1–9 did not interact with anastellin, III1, III2, III1–2, or several III1–2 mutants including III1–2KADA. (In contrast to previous studies, we found that the III1–2KADA mutant was identical in conformation to wild-type III1–2.) Because I1–9 only bound to the aggregate and the unfolding of III2 played a role in aggregation, we generated a III2 domain that was destabilized by deletion of the G strand. This mutant bound I1–9 as shown by the gelatin precipitation assay and fluorescence resonance energy transfer analysis, and it inhibited FN matrix assembly when added to cell culture. Next, we introduced disulfide mutations into full-length FN. Three disulfide locks in III2, III3, and III11 were required to dramatically reduce anastellin-induced aggregation. When we tested the disulfide mutants in cell culture, only the disulfide bond in III2 reduced the FN matrix. These results suggest that the unfolding of III2 is one of the key factors for FN aggregation and assembly. PMID:21949131
50 CFR 20.61 - Importation limits.
Code of Federal Regulations, 2011 CFR
2011-10-01
... migratory game birds in excess of the following importation limits: (a) Doves and pigeons. (1) From any..., singly or in the aggregate of all species, and 10 pigeons, singly or in the aggregate of all species....
50 CFR 20.61 - Importation limits.
Code of Federal Regulations, 2010 CFR
2010-10-01
... migratory game birds in excess of the following importation limits: (a) Doves and pigeons. (1) From any..., singly or in the aggregate of all species, and 10 pigeons, singly or in the aggregate of all species....
50 CFR 20.61 - Importation limits.
Code of Federal Regulations, 2014 CFR
2014-10-01
... migratory game birds in excess of the following importation limits: (a) Doves and pigeons. (1) From any..., singly or in the aggregate of all species, and 10 pigeons, singly or in the aggregate of all species....
50 CFR 20.61 - Importation limits.
Code of Federal Regulations, 2013 CFR
2013-10-01
... migratory game birds in excess of the following importation limits: (a) Doves and pigeons. (1) From any..., singly or in the aggregate of all species, and 10 pigeons, singly or in the aggregate of all species....
50 CFR 20.61 - Importation limits.
Code of Federal Regulations, 2012 CFR
2012-10-01
... migratory game birds in excess of the following importation limits: (a) Doves and pigeons. (1) From any..., singly or in the aggregate of all species, and 10 pigeons, singly or in the aggregate of all species....
Three-dimensional chemotaxis-driven aggregation of tumor cells
Puliafito, Alberto; De Simone, Alessandro; Seano, Giorgio; Gagliardi, Paolo Armando; Di Blasio, Laura; Chianale, Federica; Gamba, Andrea; Primo, Luca; Celani, Antonio
2015-01-01
One of the most important steps in tumor progression involves the transformation from a differentiated epithelial phenotype to an aggressive, highly motile phenotype, where tumor cells invade neighboring tissues. Invasion can occur either by isolated mesenchymal cells or by aggregates that migrate collectively and do not lose completely the epithelial phenotype. Here, we show that, in a three-dimensional cancer cell culture, collective migration of cells eventually leads to aggregation in large clusters. We present quantitative measurements of cluster velocity, coalescence rates, and proliferation rates. These results cannot be explained in terms of random aggregation. Instead, a model of chemotaxis-driven aggregation – mediated by a diffusible attractant – is able to capture several quantitative aspects of our results. Experimental assays of chemotaxis towards culture conditioned media confirm this hypothesis. Theoretical and numerical results further suggest an important role for chemotactic-driven aggregation in spreading and survival of tumor cells. PMID:26471876
Different Strategies for Aggregation in Social Amoeba Colonies
NASA Astrophysics Data System (ADS)
Franck, Carl; Monaghan, Ryan; Bae, Albert; Loh, Duane; Bodenschatz, Eberhard
2007-03-01
When confronted by starvation, collections of the amoeba Dictyostelium discoideum seek to aggregate in order to form genome-preserving stalk and spore structures. We have been interested in the means by which individual cells unite for this purpose. It has long been recognized that communication by means of diffusion of small molecules affords one such strategy: periodic chemical wave signaling can direct individual cells to an aggregation site. By employing thin layer substrates that presumably alter the propagation characteristics of such waves, we have shifted the colonial aggregation strategies to modes that rely on adhesive interactions for initial stages of multicellular assembly. Besides relentless aggregation of individual cells into large scale streams, these substrates reveal remarkable structures composed of only a few cells which we call ``squads'' that search for each other in order to achieve sufficient aggregation mass in sparse populations.
NASA Astrophysics Data System (ADS)
Villar, Ricardo G.; Pelayo, Jigg L.; Mozo, Ray Mari N.; Salig, James B., Jr.; Bantugan, Jojemar
2016-06-01
Leaning on the derived results conducted by Central Mindanao University Phil-LiDAR 2.B.11 Image Processing Component, the paper attempts to provides the application of the Light Detection and Ranging (LiDAR) derived products in arriving quality Landcover classification considering the theoretical approach of data analysis principles to minimize the common problems in image classification. These are misclassification of objects and the non-distinguishable interpretation of pixelated features that results to confusion of class objects due to their closely-related spectral resemblance, unbalance saturation of RGB information is a challenged at the same time. Only low density LiDAR point cloud data is exploited in the research denotes as 2 pts/m2 of accuracy which bring forth essential derived information such as textures and matrices (number of returns, intensity textures, nDSM, etc.) in the intention of pursuing the conditions for selection characteristic. A novel approach that takes gain of the idea of object-based image analysis and the principle of allometric relation of two or more observables which are aggregated for each acquisition of datasets for establishing a proportionality function for data-partioning. In separating two or more data sets in distinct regions in a feature space of distributions, non-trivial computations for fitting distribution were employed to formulate the ideal hyperplane. Achieving the distribution computations, allometric relations were evaluated and match with the necessary rotation, scaling and transformation techniques to find applicable border conditions. Thus, a customized hybrid feature was developed and embedded in every object class feature to be used as classifier with employed hierarchical clustering strategy for cross-examining and filtering features. This features are boost using machine learning algorithms as trainable sets of information for a more competent feature detection. The product classification in this
Optics of metal nanoparticle aggregates with light induced motion.
Drachev, Vladimir P; Perminov, Sergey V; Rautian, Sergey G
2007-07-01
Light-induced forces between metal nanoparticles change the geometry of the aggregates and affect their optical properties. Light absorption, scattering and scattering of a probe beam are numerically studied with Newton's equations and the coupled dipole equations for penta-particle aggregates. The relative changes in optical responses are large compared with the linear, low-intensity limit and relatively fast with nanosecond characteristic times. Time and intensity dependencies are shown to be sensitive to the initial potential of the aggregation forces.
Multiparticle sintering dynamics: from fractal-like aggregates to compact structures.
Eggersdorfer, Max L; Kadau, Dirk; Herrmann, Hans J; Pratsinis, Sotiris E
2011-05-17
Multiparticle sintering is encountered in almost all high temperature processes for material synthesis (titania, silica, and nickel) and energy generation (e.g., fly ash formation) resulting in aggregates of primary particles (hard- or sinter-bonded agglomerates). This mechanism of particle growth is investigated quantitatively by mass and energy balances during viscous sintering of amorphous aerosol materials (e.g., SiO(2) and polymers) that typically have a distribution of sizes and complex morphology. This model is validated at limited cases of sintering between two (equally or unequally sized) particles, and chains of particles. The evolution of morphology, surface area and radii of gyration of multiparticle aggregates are elucidated for various sizes and initial fractal dimension. For each of these structures that had been generated by diffusion limited (DLA), cluster-cluster (DLCA), and ballistic particle-cluster agglomeration (BPCA) the surface area evolution is monitored and found to scale differently than that of the radius of gyration (moment of inertia). Expressions are proposed for the evolution of fractal dimension and the surface area of aggregates undergoing viscous sintering. These expressions are important in design of aerosol processes with population balance equations (PBE) and/or fluid dynamic simulations for material synthesis or minimization and even suppression of particle formation. PMID:21488641
NASA Astrophysics Data System (ADS)
Fukuyama, Hidenao
Recent advances of magnetic resonance imaging have been described, especially stressed on the diffusion sequences. We have recently applied the diffusion sequence to functional brain imaging, and found the appropriate results. In addition to the neurosciences fields, diffusion weighted images have improved the accuracies of clinical diagnosis depending upon magnetic resonance images in stroke as well as inflammations.
Wilson, C.; Swan, C.
2007-07-01
New technology carried out at Tufts University and the University of Massachusetts on synthetic lightweight aggregate has created material from various qualities of fly ash from coal-fired power plants for use in different engineered applications. In pilot scale manufacturing tests an 'SLA' containing 80% fly ash and 20% mixed plastic waste from packaging was produced by 'dry blending' mixed plastic with high carbon fly ash. A trial run was completed to produce concrete masonry unit (CMU) blocks at a full-scale facility. It has been shown that SLA can be used as a partial substitution of a traditional stone aggregate in hot asphalt mix. 1 fig., 2 photos.
Tracking Hypoxic Signaling within Encapsulated Cell Aggregates
Skiles, Matthew L.; Sahai, Suchit; Blanchette, James O.
2011-01-01
nutrients, notably oxygen, is therefore reduced and limited by diffusion. This reduced oxygen availability may especially impact β-cells whose insulin secretory function is highly dependent on oxygen11-13. Capsule composition and geometry will also impact diffusion rates and lengths for oxygen. Therefore, we also describe a technique for identifying hypoxic cells within our PEG capsules. Infection of the cells with a recombinant adenovirus allows for a fluorescent signal to be produced when intracellular hypoxia-inducible factor (HIF) pathways are activated14. As HIFs are the primary regulators of the transcriptional response to hypoxia, they represent an ideal target marker for detection of hypoxic signaling15. This approach allows for easy and rapid detection of hypoxic cells. Briefly, the adenovirus has the sequence for a red fluorescent protein (Ds Red DR from Clontech) under the control of a hypoxia-responsive element (HRE) trimer. Stabilization of HIF-1 by low oxygen conditions will drive transcription of the fluorescent protein (Figure 1). Additional details on the construction of this virus have been published previously15. The virus is stored in 10% glycerol at -80° C as many 150 μL aliquots in 1.5 mL centrifuge tubes at a concentration of 3.4 x 1010 pfu/mL. Previous studies in our lab have shown that MIN6 cells encapsulated as aggregates maintain their viability throughout 4 weeks of culture in 20% oxygen. MIN6 aggregates cultured at 2 or 1% oxygen showed both signs of necrotic cells (still about 85-90% viable) by staining with ethidium bromide as well as morphological changes relative to cells in 20% oxygen. The smooth spherical shape of the aggregates displayed at 20% was lost and aggregates appeared more like disorganized groups of cells. While the low oxygen stress does not cause a pronounced drop in viability, it is clearly impacting MIN6 aggregation and function as measured by glucose-stimulated insulin secretion15. Western blot analysis of encapsulated
Tracking hypoxic signaling within encapsulated cell aggregates.
Skiles, Matthew L; Sahai, Suchit; Blanchette, James O
2011-01-01
, is therefore reduced and limited by diffusion. This reduced oxygen availability may especially impact β-cells whose insulin secretory function is highly dependent on oxygen. Capsule composition and geometry will also impact diffusion rates and lengths for oxygen. Therefore, we also describe a technique for identifying hypoxic cells within our PEG capsules. Infection of the cells with a recombinant adenovirus allows for a fluorescent signal to be produced when intracellular hypoxia-inducible factor (HIF) pathways are activated. As HIFs are the primary regulators of the transcriptional response to hypoxia, they represent an ideal target marker for detection of hypoxic signaling. This approach allows for easy and rapid detection of hypoxic cells. Briefly, the adenovirus has the sequence for a red fluorescent protein (Ds Red DR from Clontech) under the control of a hypoxia-responsive element (HRE) trimer. Stabilization of HIF-1 by low oxygen conditions will drive transcription of the fluorescent protein (Figure 1). Additional details on the construction of this virus have been published previously. The virus is stored in 10% glycerol at -80° C as many 150 μL aliquots in 1.5 mL centrifuge tubes at a concentration of 3.4 x 10(10) pfu/mL. Previous studies in our lab have shown that MIN6 cells encapsulated as aggregates maintain their viability throughout 4 weeks of culture in 20% oxygen. MIN6 aggregates cultured at 2 or 1% oxygen showed both signs of necrotic cells (still about 85-90% viable) by staining with ethidium bromide as well as morphological changes relative to cells in 20% oxygen. The smooth spherical shape of the aggregates displayed at 20% was lost and aggregates appeared more like disorganized groups of cells. While the low oxygen stress does not cause a pronounced drop in viability, it is clearly impacting MIN6 aggregation and function as measured by glucose-stimulated insulin secretion. Western blot analysis of encapsulated cells in 20% and 1% oxygen also
Aggregates, broccoli and cauliflower
NASA Astrophysics Data System (ADS)
Grey, Francois; Kjems, Jørgen K.
1989-09-01
Naturally grown structures with fractal characters like broccoli and cauliflower are discussed and compared with DLA-type aggregates. It is suggested that the branching density can be used to characterize the growth process and an experimental method to determine this parameter is proposed.
An Aggregate IRT Procedure for Exploratory Factor Analysis
ERIC Educational Resources Information Center
Camilli, Gregory; Fox, Jean-Paul
2015-01-01
An aggregation strategy is proposed to potentially address practical limitation related to computing resources for two-level multidimensional item response theory (MIRT) models with large data sets. The aggregate model is derived by integration of the normal ogive model, and an adaptation of the stochastic approximation expectation maximization…
NASA Astrophysics Data System (ADS)
Wynn, J. G.; Chelladurai, J. L.
2009-12-01
A large body of terrestrial paleoenvironmental proxy research hinges on the well-established diffusion-production-equilibrium model of soil CO2. Although soil CO2 is the linchpin between the isotopic composition of pedogenic carbonate and the terrestrial biomass, it is not preserved directly in paleosols, and the model makes important boundary condition assumptions on processes that control its isotopic composition. The standard model assumes two-end member diffusional mixing between soil-respired CO2 and atmospheric CO2, and equilibrium precipitation of soil CaCO3 in an open system. We modified the boundary conditions of the standard model to specifically account for the flux and stable isotope fractionation of CO2 into the CaCO3 reservoir under partially closed-system conditions. Model calculations show that diversion of a relatively small fraction of the CO2 flux into CaCO3 results in a large change in the isotopic composition of soil CO2 and hence of CaCO3 at the sites of pedogenic carbonate accumulation. We also incubated two types of biomass (derived from C3 and C4 photosynthesis) in synthetic profiles with controlled conditions designed to test this modified model hypothesis. While soil CO2 from the surface horizons validated two-end member mixing, the δ13C values of CO2 from developing carbonate horizons was consistently 13C-depleted by several per mil compared to model predictions. This negative offset from the standard model mixing diagram (a Keeling Plot) is only consistent with the positive fractionation factor for carbonate precipitation in partially-closed system conditions. We also consider the conditions under which terrestrial paleoenvironmental proxy studies may be compromised by inappropriate boundary condition assumptions, in an effort to provide more robust model validation and application to terrestrial proxy data.
De Luca, A.; Texier, M.; Burle, N.; Oison, V.; Pichaud, B.; Portavoce, A.; Grosjean, C.
2014-01-07
Two doses (10{sup 13} and 10{sup 15} cm{sup −2}) of tungsten (W) atoms were implanted in different Si(001) wafers in order to study W diffusion in Si. The samples were annealed or oxidized at temperatures between 776 and 960 °C. The diffusion profiles were measured by secondary ion mass spectrometry, and defect formation was studied by transmission electron microscopy and atom probe tomography. W is shown to reduce Si recrystallization after implantation and to exhibit, in the temperature range investigated, a solubility limit close to 0.15%–0.2%, which is higher than the solubility limit of usual metallic impurities in Si. W diffusion exhibits unusual linear diffusion profiles with a maximum concentration always located at the Si surface, slower kinetics than other metals in Si, and promotes vacancy accumulation close to the Si surface, with the formation of hollow cavities in the case of the higher W dose. In addition, Si self-interstitial injection during oxidation is shown to promote W-Si clustering. Taking into account these observations, a diffusion model based on the simultaneous diffusion of interstitial W atoms and W-Si atomic pairs is proposed since usual models used to model diffusion of metallic impurities and dopants in Si cannot reproduce experimental observations.
Contrasting effects of nanoparticle-protein attraction on amyloid aggregation
Radic, Slaven; Davis, Thomas P; Ke, Pu Chun; Ding, Feng
2015-01-01
Nanoparticles (NPs) have been experimentally found to either promote or inhibit amyloid aggregation of proteins, but the molecular mechanisms for such complex behaviors remain unknown. Using coarse-grained molecular dynamics simulations, we investigated the effects of varying the strength of nonspecific NP-protein attraction on amyloid aggregation of a model protein, the amyloid-beta peptide implicated in Alzheimer's disease. Specifically, with increasing NP-peptide attraction, amyloid aggregation on the NP surface was initially promoted due to increased local protein concentration on the surface and destabilization of the folded state. However, further increase of NP-peptide attraction decreased the stability of amyloid fibrils and reduced their lateral diffusion on the NP surface necessary for peptide conformational changes and self-association, thus prohibiting amyloid aggregation. Moreover, we found that the relative concentration between protein and NPs also played an important role in amyloid aggregation. With a high NP/protein ratio, NPs that intrinsically promote protein aggregation may display an inhibitive effect by depleting the proteins in solution while having a low concentration of the proteins on each NP's surface. Our coarse-grained molecular dynamics simulation study offers a molecular mechanism for delineating the contrasting and seemingly conflicting effects of NP-protein attraction on amyloid aggregation and highlights the potential of tailoring anti-aggregation nanomedicine against amyloid diseases. PMID:26989481
Theory of diffusion controlled growth
NASA Astrophysics Data System (ADS)
Ball, R. C.; Somfai, E.
2003-03-01
We present a new theoretical framework for Diffusion Limited Aggregation and associated Dielectric Breakdown Models (DBM) in two dimensions [R. C. Ball and E. Somfai; PRL 89, 135503 (2002); and cond-mat/0210598]. The first key step is to understand how these models interrelate when the ultra-violet cut-off strategy is changed, leading us to propose a new set of equivalence relations between generalisations of the original DBM. This enables us to approach these models through the dynamics of conformal maps with a fixed charge cut-off. Using logarithmic field variables then leads to coupled moment equations which appear to be renormalisable. Within the simplest, Gaussian, truncation of mode-mode coupling, all properties can be calculated. The agreement with prior knowledge from simulations is encouraging, and a new superuniversality of the tip scaling exponent is discussed. We find angular resonances relatable to the cone angle theory, and we are led to predict a new Screening Transition in the DBM at large η.
Boerner, A. J.; Maldonado, D. G.
2012-06-01
This report contains the technical basis in support of the DOE?s derivation of Authorized Limits (ALs) for the DOE Paducah C-746-U Landfill. A complete description of the methodology, including an assessment of the input parameters, model inputs, and results is provided in this report. This report also provides initial recommendations on applying the derived soil guidelines. The ORISE-derived soil guidelines are specifically applicable to the Landfill at the end of its operational life. A suggested 'upper bound' multiple of the derived soil guidelines for individual shipments is provided.
DOSY-NMR and raman investigations on the self-aggregation and cyclodextrin complexation of vanillin.
Ferrazza, Ruggero; Rossi, Barbara; Guella, Graziano
2014-06-26
Vanillin (4-hydroxy-3-methoxybenzaldehyde) is a phenolic aldehyde with limited solubility in water; in this work, we investigate its self-aggregation, as well as its complexation equilibria with β-cyclodextrin by using nuclear magnetic resonance (NMR) and vibrational spectroscopy. In particular, diffusion-ordered NMR (DOSY) measurements allowing to detect diffusional changes caused by aggregation/inclusion phenomena lead to a reliable estimate of the equilibrium constants of these processes, while Raman spectroscopy was used to further characterize some structural details of vanillin self-aggregates and inclusion complexes. Although the self-association binding constant of vanillin in water was found to be low (K(a) ∼10), dimeric species are not negligible within the investigated range of concentration (3-65 mM); on the other hand, formation of β-cyclodextrin self-aggregates was not detected by DOSY measurements on aqueous solutions of β-cyclodextrin at different concentrations (2-12 mM). Finally, the binding of vanillin with β-cyclodextrin, as measured by the DOSY technique within a narrow range of concentrations (2-15 mM) by assuming the existence of only the monomeric 1:1 vanillin/β-CD complex, was about an order of magnitude higher (K(c) ∼ 90) than self-aggregation. However, the value of the equilibrium constant for this complexation was found to be significantly affected by the analytical concentrations of the host and guest system, thus indicating that K(c) is an "apparent" equilibrium constant.
The Harrison Diffusion Kinetics Regimes in Solute Grain Boundary Diffusion
Belova, Irina; Fiedler, T; Kulkarni, Nagraj S; Murch, Prof. Graeme
2012-01-01
Knowledge of the limits of the principal Harrison kinetics regimes (Type-A, B and C) for grain boundary diffusion is very important for the correct analysis of the depth profiles in a tracer diffusion experiment. These regimes for self-diffusion have been extensively studied in the past by making use of the phenomenological Lattice Monte Carlo (LMC) method with the result that the limits are now well established. The relationship of those self-diffusion limits to the corresponding ones for solute diffusion in the presence of solute segregation to the grain boundaries remains unclear. In the present study, the influence of solute segregation on the limits is investigated with the LMC method for the well-known parallel grain boundary slab model by showing the equivalence of two diffusion models. It is shown which diffusion parameters are useful for identifying the limits of the Harrison kinetics regimes for solute grain boundary diffusion. It is also shown how the measured segregation factor from the diffusion experiment in the Harrison Type-B kinetics regime may differ from the global segregation factor.
Multispecies diffusion models: A study of uranyl species diffusion
NASA Astrophysics Data System (ADS)
Liu, Chongxuan; Shang, Jianying; Zachara, John M.
2011-12-01
, where intragranular diffusion is a rate-limiting process controlling U(VI) adsorption and desorption. The grain-scale reactive diffusion model was able to describe U(VI) adsorption/desorption kinetics that had been previously described using a semiempirical, multirate model. Compared with the multirate model, the diffusion models have the advantage to provide spatiotemporal speciation evolution within the diffusion domains.
Characterization of Ovine Dermal Papilla Cell Aggregation
Sari, Agnes Rosarina Prita; Rufaut, Nicholas Wolfgang; Jones, Leslie Norman; Sinclair, Rodney Daniel
2016-01-01
Context: The dermal papilla (DP) is a condensation of mesenchymal cells at the proximal end of the hair follicle, which determines hair shaft size and regulates matrix cell proliferation and differentiation. DP cells have the ability to regenerate new hair follicles. These cells tend to aggregate both in vitro and in vivo. This tendency is associated with the ability of papilla cells to induce hair growth. However, human papilla cells lose their hair-inducing activity in later passage number. Ovine DP cells are different from human DP cells since they do not lose their aggregative behavior or hair-inducing activity in culture. Nonetheless, our understanding of ovine DP cells is still limited. Aim: The aim of this study was to observe the expression of established DP markers in ovine cells and their association with aggregation. Subjects and Methods: Ovine DP cells from three different sheep were compared. Histochemistry, immunoflourescence, and polymerase chain reaction experiments were done to analyze the DP markers. Results: We found that ovine DP aggregates expressed all the 16 markers evaluated, including alkaline phosphatase and versican. Expression of the versican V0 and V3 isoforms, neural cell adhesion molecule, and corin was increased significantly with aggregation, while hey-1 expression was significantly decreased. Conclusions: Overall, the stable expression of numerous markers suggests that aggregating ovine DP cells have a similar phenotype to papillae in vivo. The stability of their molecular phenotype is consistent with their robust aggregative behavior and retained follicle-inducing activity after prolonged culture. Their phenotypic stability in culture contrasts with DP cells from other species, and suggests that a better understanding of ovine DP cells might provide opportunities to improve the hair-inducing activity and therapeutic potential of human cells. PMID:27625564
Characterization of Ovine Dermal Papilla Cell Aggregation
Sari, Agnes Rosarina Prita; Rufaut, Nicholas Wolfgang; Jones, Leslie Norman; Sinclair, Rodney Daniel
2016-01-01
Context: The dermal papilla (DP) is a condensation of mesenchymal cells at the proximal end of the hair follicle, which determines hair shaft size and regulates matrix cell proliferation and differentiation. DP cells have the ability to regenerate new hair follicles. These cells tend to aggregate both in vitro and in vivo. This tendency is associated with the ability of papilla cells to induce hair growth. However, human papilla cells lose their hair-inducing activity in later passage number. Ovine DP cells are different from human DP cells since they do not lose their aggregative behavior or hair-inducing activity in culture. Nonetheless, our understanding of ovine DP cells is still limited. Aim: The aim of this study was to observe the expression of established DP markers in ovine cells and their association with aggregation. Subjects and Methods: Ovine DP cells from three different sheep were compared. Histochemistry, immunoflourescence, and polymerase chain reaction experiments were done to analyze the DP markers. Results: We found that ovine DP aggregates expressed all the 16 markers evaluated, including alkaline phosphatase and versican. Expression of the versican V0 and V3 isoforms, neural cell adhesion molecule, and corin was increased significantly with aggregation, while hey-1 expression was significantly decreased. Conclusions: Overall, the stable expression of numerous markers suggests that aggregating ovine DP cells have a similar phenotype to papillae in vivo. The stability of their molecular phenotype is consistent with their robust aggregative behavior and retained follicle-inducing activity after prolonged culture. Their phenotypic stability in culture contrasts with DP cells from other species, and suggests that a better understanding of ovine DP cells might provide opportunities to improve the hair-inducing activity and therapeutic potential of human cells.
Comets as porous aggregates of interstellar dust.
Greenberg, J M; Remo, J L
1997-05-30
A comet model is derived based on the interstellar dust chemical composition in dense molecular and diffuse clouds resulting from their subsequent chemical interactions and UV photoprocessing. The collapsing presolar nebula leads to a porous aggregate model for comet nuclei, from which is derived certain physical properties that include thermal conductivity and tensile strength. The porous morphological structure is also shown to imply anomalous (expansion rather than contraction) behavior when subjected to strong shock waves, which is supported by recent shock experiments on (porous) carbonaceous chondrite material.
Collisional Aggregation Due to Turbulence
NASA Astrophysics Data System (ADS)
Pumir, Alain; Wilkinson, Michael
2016-03-01
Collisions between particles suspended in a fluid play an important role in many physical processes. As an example, collisions of microscopic water droplets in clouds are a necessary step in the production of macroscopic raindrops. Collisions of dust grains are also conjectured to be important for planet formation in the gas surrounding young stars and to play a role in the dynamics of sand storms. In these processes, collisions are favored by fast turbulent motions. Here we review recent advances in the understanding of collisional aggregation due to turbulence. We discuss the role of fractal clustering of particles and caustic singularities of their velocities. We also discuss limitations of the Smoluchowski equation for modeling such processes. These advances lead to a semiquantitative understanding on the influence of turbulence on collision rates and point to deficiencies in the current understanding of rainfall and planet formation.
Meneely, Kathleen M; Luo, Qianyi; Dhar, Prajnaparamita; Lamb, Audrey L
2013-10-01
The isochorismate synthase from Pseudomonas aeruginosa (PchA) catalyzes the conversion of chorismate to isochorismate, which is subsequently converted by a second enzyme (PchB) to salicylate for incorporation into the salicylate-capped siderophore pyochelin. PchA is a member of the MST family of enzymes, which includes the structurally homologous isochorismate synthases from Escherichia coli (EntC and MenF) and salicylate synthases from Yersinia enterocolitica (Irp9) and Mycobacterium tuberculosis (MbtI). The latter enzymes generate isochorismate as an intermediate before generating salicylate and pyruvate. General acid-general base catalysis has been proposed for isochorismate synthesis in all five enzymes, but the residues required for the isomerization are a matter of debate, with both lysine221 and glutamate313 proposed as the general base (PchA numbering). This work includes a classical characterization of PchA with steady state kinetic analysis, solvent kinetic isotope effect analysis and by measuring the effect of viscosogens on catalysis. The results suggest that isochorismate production from chorismate by the MST enzymes is the result of general acid-general base catalysis with a lysine as the base and a glutamic acid as the acid, in reverse protonation states. Chemistry is determined to not be rate limiting, favoring the hypothesis of a conformational or binding step as the slow step. PMID:23942051
Meneely, Kathleen M.; Luo, Qianyi; Dhar, Prajnaparamita; Lamb, Audrey L.
2013-01-01
The isochorismate synthase from Pseudomonas aeruginosa (PchA) catalyzes the conversion of chorismate to isochorismate, which is subsequently converted by a second enzyme (PchB) to salicylate for incorporation into the salicylate-capped siderophore pyochelin. PchA is a member of the MST family of enzymes, which includes the structurally homologous isochorismate synthases from E. coli (EntC and MenF) and salicylate synthases from Yersinia enterocolitica (Irp9) and Mycobacterium tuberculosis (MbtI). The latter enzymes generate isochorismate as an intermediate before generating salicylate and pyruvate. General acid – general base catalysis has been proposed for isochorismate synthesis in all five enzymes, but the residues required for the isomerization are a matter of debate, with both lysine221 and glutamate313 proposed as the general base (PchA numbering). This work includes a classical characterization of PchA with steady state kinetic analysis, solvent kinetic isotope effect analysis and by measuring the effect of viscosogens on catalysis. The results suggest that isochorismate production from chorismate by the MST enzymes is the result of general acid – general base catalysis with a lysine as the base and a glutamic acid as the acid, in reverse protonation states. Chemistry is determined to not be rate limiting, favoring the hypothesis of a conformational or binding step as the slow step. PMID:23942051
Chun, J.-S.; Desjardins, P.; Lavoie, C.; Shin, C.-S.; Cabral, C.; Petrov, I.; Greene, J. E.
2001-06-15
Single-crystal TiN(111) layers, 45 nm thick, were grown on MgO(111) by ultrahigh vacuum reactive magnetron sputter deposition in pure N{sub 2} discharges at T{sub s}=700{degree}C. Epitaxial Al(111) overlayers, 160 nm thick, were then deposited at T{sub s}=100{degree}C in Ar without breaking vacuum. Interfacial reactions and changes in bilayer microstructure due to annealing at 620 and 650{degree}C were investigated using x-ray diffraction and transmission electron microscopy (TEM). The interfacial regions of samples annealed at 620{degree}C consist of continuous {approx_equal}7-nm-thick epitaxial wurtzite-structure AlN(0001) layers containing a high density of stacking faults, with {approx_equal}22 nm thick tetragonal Al{sub 3}Ti(112) overlayers. Surprisingly, samples annealed at the higher temperature are more stable against Al{sub 3}Ti formation. TEM analyses of bilayers annealed at 650{degree}C (10{degree}C below the Al melting point!) reveal only the self-limited growth of an {approx_equal}3-nm-thick interfacial layer of perfect smooth epitaxial wurtzite-structure AlN(0001) which serves as an extremely effective deterrent for preventing further interlayer reactions. {copyright} 2001 American Institute of Physics.
Meneely, Kathleen M; Luo, Qianyi; Dhar, Prajnaparamita; Lamb, Audrey L
2013-10-01
The isochorismate synthase from Pseudomonas aeruginosa (PchA) catalyzes the conversion of chorismate to isochorismate, which is subsequently converted by a second enzyme (PchB) to salicylate for incorporation into the salicylate-capped siderophore pyochelin. PchA is a member of the MST family of enzymes, which includes the structurally homologous isochorismate synthases from Escherichia coli (EntC and MenF) and salicylate synthases from Yersinia enterocolitica (Irp9) and Mycobacterium tuberculosis (MbtI). The latter enzymes generate isochorismate as an intermediate before generating salicylate and pyruvate. General acid-general base catalysis has been proposed for isochorismate synthesis in all five enzymes, but the residues required for the isomerization are a matter of debate, with both lysine221 and glutamate313 proposed as the general base (PchA numbering). This work includes a classical characterization of PchA with steady state kinetic analysis, solvent kinetic isotope effect analysis and by measuring the effect of viscosogens on catalysis. The results suggest that isochorismate production from chorismate by the MST enzymes is the result of general acid-general base catalysis with a lysine as the base and a glutamic acid as the acid, in reverse protonation states. Chemistry is determined to not be rate limiting, favoring the hypothesis of a conformational or binding step as the slow step.
Proteins aggregation and human diseases
NASA Astrophysics Data System (ADS)
Hu, Chin-Kun
2015-04-01
Many human diseases and the death of most supercentenarians are related to protein aggregation. Neurodegenerative diseases include Alzheimer's disease (AD), Huntington's disease (HD), Parkinson's disease (PD), frontotemporallobar degeneration, etc. Such diseases are due to progressive loss of structure or function of neurons caused by protein aggregation. For example, AD is considered to be related to aggregation of Aβ40 (peptide with 40 amino acids) and Aβ42 (peptide with 42 amino acids) and HD is considered to be related to aggregation of polyQ (polyglutamine) peptides. In this paper, we briefly review our recent discovery of key factors for protein aggregation. We used a lattice model to study the aggregation rates of proteins and found that the probability for a protein sequence to appear in the conformation of the aggregated state can be used to determine the temperature at which proteins can aggregate most quickly. We used molecular dynamics and simple models of polymer chains to study relaxation and aggregation of proteins under various conditions and found that when the bending-angle dependent and torsion-angle dependent interactions are zero or very small, then protein chains tend to aggregate at lower temperatures. All atom models were used to identify a key peptide chain for the aggregation of insulin chains and to find that two polyQ chains prefer anti-parallel conformation. It is pointed out that in many cases, protein aggregation does not result from protein mis-folding. A potential drug from Chinese medicine was found for Alzheimer's disease.
Concrete Waste Recycling Process for High Quality Aggregate
Ishikura, Takeshi; Fujii, Shin-ichi
2008-01-15
Large amount of concrete waste generates during nuclear power plant (NPP) dismantling. Non-contaminated concrete waste is assumed to be disposed in a landfill site, but that will not be the solution especially in the future, because of decreasing tendency of the site availability and natural resources. Concerning concrete recycling, demand for roadbeds and backfill tends to be less than the amount of dismantled concrete generated in a single rural site, and conventional recycled aggregate is limited of its use to non-structural concrete, because of its inferior quality to ordinary natural aggregate. Therefore, it is vital to develop high quality recycled aggregate for general uses of dismantled concrete. If recycled aggregate is available for high structural concrete, the dismantling concrete is recyclable as aggregate for industry including nuclear field. Authors developed techniques on high quality aggregate reclamation for large amount of concrete generated during NPP decommissioning. Concrete of NPP buildings has good features for recycling aggregate; large quantity of high quality aggregate from same origin, record keeping of the aggregate origin, and little impurities in dismantled concrete such as wood and plastics. The target of recycled aggregate in this development is to meet the quality criteria for NPP concrete as prescribed in JASS 5N 'Specification for Nuclear Power Facility Reinforced Concrete' and JASS 5 'Specification for Reinforced Concrete Work'. The target of recycled aggregate concrete is to be comparable performance with ordinary aggregate concrete. The high quality recycled aggregate production techniques are assumed to apply for recycling for large amount of non-contaminated concrete. These techniques can also be applied for slightly contaminated concrete dismantled from radiological control area (RCA), together with free release survey. In conclusion: a technology on dismantled concrete recycling for high quality aggregate was developed
Amino acid induced fractal aggregation of gold nanoparticles: Why and how.
Doyen, Matthieu; Goole, Jonathan; Bartik, Kristin; Bruylants, Gilles
2016-02-15
Gold colloids are the object of many studies as they are reported to have potential biological sensing, imaging and drug delivery applications. In the presence of certain amino acids the aggregation of the gold nanoparticles into linear structures is observed, as highlighted by the appearance of a second plasmon band in the UV-Vis spectra of the colloid. The mechanism behind this phenomenon is still under debate. In order to help elucidate this issue, the interaction between gold colloids and different amino acids, modified amino acids and molecules mimicking their side-chain was monitored by UV-Vis absorption, DLS and TEM. The results show that phenomenon can be rationalized in terms of the Diffusion Limited Colloid Aggregation (DLCA) model which gives rise to the fractal aggregation colloids. The global charge of the compound, which influences the ionic strength of the solution, and the ease with which the compound can interact with the GNPs and affect their surface potential, are, the two parameters which control the DLCA regime. Calculations based on the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory confirm all the experimental observations. PMID:26613335
Amino acid induced fractal aggregation of gold nanoparticles: Why and how.
Doyen, Matthieu; Goole, Jonathan; Bartik, Kristin; Bruylants, Gilles
2016-02-15
Gold colloids are the object of many studies as they are reported to have potential biological sensing, imaging and drug delivery applications. In the presence of certain amino acids the aggregation of the gold nanoparticles into linear structures is observed, as highlighted by the appearance of a second plasmon band in the UV-Vis spectra of the colloid. The mechanism behind this phenomenon is still under debate. In order to help elucidate this issue, the interaction between gold colloids and different amino acids, modified amino acids and molecules mimicking their side-chain was monitored by UV-Vis absorption, DLS and TEM. The results show that phenomenon can be rationalized in terms of the Diffusion Limited Colloid Aggregation (DLCA) model which gives rise to the fractal aggregation colloids. The global charge of the compound, which influences the ionic strength of the solution, and the ease with which the compound can interact with the GNPs and affect their surface potential, are, the two parameters which control the DLCA regime. Calculations based on the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory confirm all the experimental observations.
Puertas, Antonio M; Odriozola, Gerardo
2007-05-24
We have studied the link between the kinetics of clustering and the phase behavior of dilute colloids with short range attractions of moderate strength. This was done by means of computer simulations and a theoretical kinetic model originally developed to deal with reversible colloidal aggregation. Three different regions of the phase diagram were accessed. For weak attractions, a gas phase of small clusters in equilibrium forms in the system. For intermediate attractions, the system undergoes liquid-gas separation, which is signatured by the formation of a few large droplike aggregates, a gas phase of small clusters, and an overall kinetics where a few seeds succeed in explosively growing at long times, after a lag time. Finally, for very strong attractions, fractal unbreakable clusters form and grow following DLCA-like (diffusion limited cluster aggregation) kinetics; liquid-gas separation is prevented by the strength of the bonds, which do not allow restructuration. Good qualitative and quantitative agreement is found between the dynamic simulations and the kinetic model in all the three regions.
N /A
2002-08-06
The US Department of Energy (DOE) has completed an environmental assessment (DOE/EA-1414) for the proposed implementation of the authorized limits process for waste acceptance at the C-746-U Landfill at the Paducah Gaseous Diffusion Plant (PGDP) in Paducah, Kentucky. Based on the results of the impact analysis reported in the EA, which is incorporated herein by this reference, DOE has determined that the proposed action is not a major Federal action that would significantly affect the quality of the human environment within the context of the ''National Environmental Policy Act of 1969'' (NEPA). Therefore preparation of an environmental impact statement is not necessary, and DOE is issuing this Finding of No Significant Impact (FONSI).
Dynamics of cell aggregates fusion: Experiments and simulations
NASA Astrophysics Data System (ADS)
Thomas, Gilberto L.; Mironov, Vladimir; Nagy-Mehez, Agnes; Mombach, José C. M.
2014-02-01
Fusion of cell tissues is an ubiquitous phenomenon and has important technological applications including tissue biofabrication. In this work we present experimental results of aggregates fusion using adipose derived stem cells (ADSC) and a three dimensional computer simulation of the process using the cellular Potts model with aggregates reaching 10,000 cells. We consider fusion of round aggregates and monitor the dimensionless neck area of contact between the two aggregates to characterize the process, as done for the coalescence of liquid droplets and polymers. Both experiments and simulations show that the evolution of this quantity obeys a power law in time. We also study quantitatively individual cell motion with the simulation and it corresponds to an anomalous diffusion.
12 CFR 24.4 - Investment limits.
Code of Federal Regulations, 2011 CFR
2011-01-01
... ENTITIES, COMMUNITY DEVELOPMENT PROJECTS, AND OTHER PUBLIC WELFARE INVESTMENTS § 24.4 Investment limits. (a) Limits on aggregate outstanding investments. A national bank's aggregate outstanding investments under... 12 Banks and Banking 1 2011-01-01 2011-01-01 false Investment limits. 24.4 Section 24.4 Banks...
12 CFR 24.4 - Investment limits.
Code of Federal Regulations, 2010 CFR
2010-01-01
... ENTITIES, COMMUNITY DEVELOPMENT PROJECTS, AND OTHER PUBLIC WELFARE INVESTMENTS § 24.4 Investment limits. (a) Limits on aggregate outstanding investments. A national bank's aggregate outstanding investments under... 12 Banks and Banking 1 2010-01-01 2010-01-01 false Investment limits. 24.4 Section 24.4 Banks...
12 CFR 24.4 - Investment limits.
Code of Federal Regulations, 2013 CFR
2013-01-01
... 12 Banks and Banking 1 2013-01-01 2013-01-01 false Investment limits. 24.4 Section 24.4 Banks and... ENTITIES, COMMUNITY DEVELOPMENT PROJECTS, AND OTHER PUBLIC WELFARE INVESTMENTS § 24.4 Investment limits. (a) Limits on aggregate outstanding investments. A national bank's aggregate outstanding investments...
12 CFR 24.4 - Investment limits.
Code of Federal Regulations, 2012 CFR
2012-01-01
... 12 Banks and Banking 1 2012-01-01 2012-01-01 false Investment limits. 24.4 Section 24.4 Banks and... ENTITIES, COMMUNITY DEVELOPMENT PROJECTS, AND OTHER PUBLIC WELFARE INVESTMENTS § 24.4 Investment limits. (a) Limits on aggregate outstanding investments. A national bank's aggregate outstanding investments...
12 CFR 24.4 - Investment limits.
Code of Federal Regulations, 2014 CFR
2014-01-01
... 12 Banks and Banking 1 2014-01-01 2014-01-01 false Investment limits. 24.4 Section 24.4 Banks and... ENTITIES, COMMUNITY DEVELOPMENT PROJECTS, AND OTHER PUBLIC WELFARE INVESTMENTS § 24.4 Investment limits. (a) Limits on aggregate outstanding investments. A national bank's aggregate outstanding investments...
Miller, Maria A; Rodrigues, Miguel A; Glass, Matthew A; Singh, Satish K; Johnston, Keith P; Maynard, Jennifer A
2013-04-01
Freezing of protein solutions perturbs protein conformation, potentially leading to aggregate formation during long-term storage in the frozen state. Macroscopic protein concentration profiles in small cylindrical vessels were determined for a monoclonal antibody frozen in a trehalose-based formulation for various freezing protocols. Slow cooling rates led to concentration differences between outer edges of the tank and the center, up to twice the initial concentration. Fast cooling rates resulted in much smaller differences in protein distribution, likely due to the formation of dendritic ice, which traps solutes in micropockets, limiting their transport by convection and diffusion. Analysis of protein stability after more than 6 months storage at either -10°C or -20°C [above glass transition temperature (T'g )] or -80°C (below T'g ) revealed that aggregation correlated with the cooling rate. Slow-cooled vessels stored above T'g exhibited increased aggregation with time. In contrast, fast-cooled vessels and those stored below T'g showed small to no increase in aggregation at any position. Rapid entrapment of protein in a solute matrix by fast freezing results in improved stability even when stored above T'g . © 2013 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 102:1194-1208, 2013.
Dynamics of fire ant aggregations
NASA Astrophysics Data System (ADS)
Tennenbaum, Michael; Hu, David; Fernandez-Nieves, Alberto
Fire ant aggregations are an inherently active system. Each ant harvests its own energy and can convert it into motion. The motion of individual ants contributes non-trivially to the bulk material properties of the aggregation. We have measured some of these properties using plate-plate rheology, where the response to an applied external force or deformation is measured. In this talk, we will present data pertaining to the aggregation behavior in the absence of any external force. We quantify the aggregation dynamics by monitoring the rotation of the top plate and by measuring the normal force. We then compare the results with visualizations of 2D aggregations.
NASA Astrophysics Data System (ADS)
Wu, Yu; Cheng, Tianhai; Zheng, Lijuan; Chen, Hao
2016-10-01
Light absorption enhancement of aged soot aerosols is highly sensitive to the morphologies and mixing states of soot aggregates and their non-absorbing coatings, such as organic materials. The quantification of these effects on the optical properties of thinly coated soot aerosols is simulated using an effective model with fixed volume fractions. Fractal aggregated soot was simulated using the diffusion limited aggregation (DLA) algorithm and discretized into soot dipoles. The dipoles of non-absorbing aerosols, whose number was fixed by the volume fraction, were further generated from the neighboring random edge dipoles. Their optical properties were calculated using the discrete dipole approximation (DDA) method and were compared with other commonly used models. The optical properties of thinly coated soot calculated using the fixed volume fraction model are close to (less than ~10% difference) the results of the fixed coating thickness model, except their asymmetry parameters (up to ~25% difference). In the optical simulations of thinly coated soot aerosols, this relative difference of asymmetry parameters and phase functions between these realistic models may be notable. The realizations of the fixed volume fraction model may introduce smaller variation of optical results than those of the fixed coating thickness model. Moreover, the core-shell monomers model and homogeneous aggregated spheres model with the Maxwell-Garnett (MG) theory may underestimate (up to ~20%) the cross sections of thinly coated soot aggregates. The single core-shell sphere model may largely overestimate (up to ~150%) the cross sections and single scattering albedo of thinly coated soot aggregates, and it underestimated (up to ~60%) their asymmetry parameters. It is suggested that the widely used single core-shell sphere approximation may not be suitable for the single scattering calculations of thinly coated soot aerosols.
Multinomial Diffusion Equation
Balter, Ariel I.; Tartakovsky, Alexandre M.
2011-06-01
We have developed a novel stochastic, space/time discrete representation of particle diffusion (e.g. Brownian motion) based on discrete probability distributions. We show that in the limit of both very small time step and large concentration, our description is equivalent to the space/time continuous stochastic diffusion equation. Being discrete in both time and space, our model can be used as an extremely accurate, efficient, and stable stochastic finite-difference diffusion algorithm when concentrations are so small that computationally expensive particle-based methods are usually needed. Through numerical simulations, we show that our method can generate realizations that capture the statistical properties of particle simulations. While our method converges converges to both the correct ensemble mean and ensemble variance very quickly with decreasing time step, but for small concentration, the stochastic diffusion PDE does not, even for very small time steps.
Aggregate-scale heterogeneity in iron (hydr)oxide reductive transformations
Tufano, K.J.; Benner, S.G.; Mayer, K.U.; Marcus, M.A.; Nico, P.S.; Fendorf, S.
2009-06-15
There is growing awareness of the complexity of potential reaction pathways and the associated solid-phase transformations during the reduction of Fe (hydr)oxides, especially ferrihydrite. An important observation in static and advective-dominated systems is that microbially produced Fe(II) accelerates Ostwald ripening of ferrihydrite, thus promoting the formation of thermodynamically more stable ferric phases (lepidocrocite and goethite) and, at higher Fe(II) surface loadings, the precipitation of magnetite; high Fe(II) levels can also lead to green rust formation, and with high carbonate levels siderite may also be formed. This study expands this emerging conceptual model to a diffusion-dominated system that mimics an idealized micropore of a ferrihydrite-coated soil aggregate undergoing reduction. Using a novel diffusion cell, coupled with micro-x-ray fluorescence and absorption spectroscopies, we determined that diffusion-controlled gradients in Fe{sup 2+}{sub (aq)} result in a complex array of spatially distributed secondary mineral phases. At the diffusive pore entrance, where Fe{sup 2+} concentrations are highest, green rust and magnetite are the dominant secondary Fe (hydr)oxides (30 mol% Fe each). At intermediate distances from the inlet, green rust is not observed and the proportion of magnetite decreases from approximately 30 to <10%. Across this same transect, the proportion of goethite increases from undetectable up to >50%. At greater distances from the advective-diffusive boundary, goethite is the dominant phase, comprising between 40 and 95% of the Fe. In the presence of magnetite, lepidocrocite forms as a transient-intermediate phase during ferrihydrite-to-goethite conversion; in the absence of magnetite, conversion to goethite is more limited. These experimental observations, coupled with results of reactive transport modeling, confirm the conceptual model and illustrate the potential importance of diffusion-generated concentration gradients in
Impact of aggregate formation on the viscosity of protein solutions.
Nicoud, Lucrèce; Lattuada, Marco; Yates, Andrew; Morbidelli, Massimo
2015-07-21
Gaining knowledge on the stability and viscosity of concentrated therapeutic protein solutions is of great relevance to the pharmaceutical industry. In this work, we borrow key concepts from colloid science to rationalize the impact of aggregate formation on the changes in viscosity of a concentrated monoclonal antibody solution. In particular, we monitor the kinetics of aggregate growth under thermal stress by static and dynamic light scattering, and we follow the rise in solution viscosity by measuring the diffusion coefficient of tracer nanoparticles with dynamic light scattering. Moreover, we characterize aggregate morphology in the frame of the fractal geometry. We show that the curves of the increase in viscosity with time monitored at three different protein concentrations collapse on one single master curve when the reaction profiles are normalized based on an effective volume fraction occupied by the aggregates, which depends on the aggregate size, concentration and morphology. Importantly, we find that the viscosity of an aggregate sample is lower than the viscosity of a monomeric sample of a similar occupied volume fraction due to the polydispersity of the aggregate distribution.
Solute effects on the irreversible aggregation of serum albumin.
Bagger, Heidi L; Øgendal, Lars H; Westh, Peter
2007-10-01
Thermal stress on bovine serum albumin (BSA) promotes protein aggregation through the formation of intermolecular beta-sheets. We have used light scattering and chromatography to study effects of (<1 M) Na(2)SO(4), NaSCN, sucrose, sorbitol and urea on the rate of the thermal aggregation. Both salts were strong inhibitors of BSA aggregation and they reduced both the size and number (concentration) of aggregate particles compared to non-ionic solutes (or pure buffer). Hence, the salts appear to suppress both nucleation- and growth rate. The non-electrolyte additives reduced the initial aggregation rate (compared to pure buffer), but did not significantly limit the extent of aggregation in samples quenched after 27 min. heat exposure (40-50% aggregation in all samples). The non-electrolytes did, however, modify the aggregation process as they consistently brought about smaller but more concentrated aggregates than pure buffer. The results are discussed along the lines of linkage- and transition state theories. In this framework, the rate of the aggregation process is governed by the equilibrium between a thermally denatured state (D) and the transition state D( not equal). Thus, the effect of a solute relies on its preferential interactions with respectively D and D( not equal). The current results do not show any correlation between the solutes' preferential interactions with native BSA and their effect on the rate of aggregation. This suggests that non-specific, "Hofmeister-type" interactions, which scale with the solvent accessible surface area, are of minor importance. Rather, salt induced suppression of aggregation is suggested to depend on the modulation of specific electrostatic forces in the D( not equal) state.
NASA Astrophysics Data System (ADS)
Senoo, Y.
The influence of vaneless diffusers on flow in centrifugal compressors, particularly on surge, is discussed. A vaneless diffuser can demonstrate stable operation in a wide flow range only if it is installed with a backward leaning blade impeller. The circumferential distortion of flow in the impeller disappears quickly in the vaneless diffuser. The axial distortion of flow at the diffuser inlet does not decay easily. In large specific speed compressors, flow out of the impeller is distorted axially. Pressure recovery of diffusers at distorted inlet flow is considerably improved by half guide vanes. The best height of the vanes is a little 1/2 diffuser width. In small specific speed compressors, flow out of the impeller is not much distorted and pressure recovery can be predicted with one-dimensional flow analysis. Wall friction loss is significant in narrow diffusers. The large pressure drop at a small flow rate can cause the positive gradient of the pressure-flow rate characteristic curve, which may cause surging.
Kinetics of protein aggregation
NASA Astrophysics Data System (ADS)
Knowles, Tuomas
2015-03-01
Aggregation into linear nanostructures, notably amyloid and amyloid-like fibrils, is a common form of behaviour exhibited by a range of peptides and proteins. This process was initially discovered in the context of the aetiology of a range of neurodegenerative diseases, but has recently been recognised to of general significance and has been found at the origin of a number of beneficial functional roles in nature, including as catalytic scaffolds and functional components in biofilms. This talk discusses our ongoing efforts to study the kinetics of linear protein self-assembly by using master equation approaches combined with global analysis of experimental data.
NASA Astrophysics Data System (ADS)
Barr, Stephen; Luijten, Erik
2010-03-01
The aggregation of virus particles is a particular form of colloidal self-assembly, since viruses of a give type are monodisperse and have identical, anisotropic surface charge distributions. In small-angle X-ray scattering experiments, the Qbeta virus was found to organize in different crystal structures in the presence of divalent salt and non-adsorbing polymer. Since a simple isotropic potential cannot explain the occurrence of all observed phases, we employ computer simulations to investigate how the surface charge distribution affects the virus interactions. Using a detailed model of the virus particle, we find an asymmetric ion distribution around the virus which gives rise to the different phases observed.
Hot moments in spawning aggregations: implications for ecosystem-scale nutrient cycling
NASA Astrophysics Data System (ADS)
Archer, Stephanie K.; Allgeier, Jacob E.; Semmens, Brice X.; Heppell, Scott A.; Pattengill-Semmens, Christy V.; Rosemond, Amy D.; Bush, Phillippe G.; McCoy, Croy M.; Johnson, Bradley C.; Layman, Craig A.
2015-03-01
Biogeochemical hot moments occur when a temporary increase in availability of one or more limiting reactants results in elevated rates of biogeochemical reactions. Many marine fish form transient spawning aggregations, temporarily increasing their local abundance and thus nutrients supplied via excretion at the aggregation site. In this way, nutrients released by aggregating fish could create a biogeochemical hot moment. Using a combination of empirical and modeling approaches, we estimate nitrogen and phosphorus supplied by aggregating Nassau grouper ( Epinephelus striatus). Data suggest aggregating grouper supply up to an order-of-magnitude more nitrogen and phosphorus than daily consumer-derived nutrient supply on coral reefs without aggregating fish. Comparing current and historic aggregation-level excretion estimates shows that overfishing reduced nutrients supplied by aggregating fish by up to 87 %. Our study illustrates a previously unrecognized ecosystem viewpoint regarding fish spawning aggregations and provides an additional perspective on the repercussions of their overexploitation.
NASA Technical Reports Server (NTRS)
Takahashi, Fumiaki; Katta, V. R.
2006-01-01
Diffusion flames are commonly used for industrial burners in furnaces and flares. Oxygen/fuel burners are usually diffusion burners, primarily for safety reasons, to prevent flashback and explosion in a potentially dangerous system. Furthermore, in most fires, condensed materials pyrolyze, vaporize, and burn in air as diffusion flames. As a result of the interaction of a diffusion flame with burner or condensed-fuel surfaces, a quenched space is formed, thus leaving a diffusion flame edge, which plays an important role in flame holding in combustion systems and fire spread through condensed fuels. Despite a long history of jet diffusion flame studies, lifting/blowoff mechanisms have not yet been fully understood, compared to those of premixed flames. In this study, the structure and stability of diffusion flames of gaseous hydrocarbon fuels in coflowing air at normal earth gravity have been investigated experimentally and computationally. Measurements of the critical mean jet velocity (U(sub jc)) of methane, ethane, or propane at lifting or blowoff were made as a function of the coflowing air velocity (U(sub a)) using a tube burner (i.d.: 2.87 mm). By using a computational fluid dynamics code with 33 species and 112 elementary reaction steps, the internal chemical-kinetic structures of the stabilizing region of methane and propane flames were investigated. A peak reactivity spot, i.e., reaction kernel, is formed in the flame stabilizing region due to back-diffusion of heat and radical species against an oxygen-rich incoming flow, thus holding the trailing diffusion flame. The simulated flame base moved downstream under flow conditions close to the measured stability limit.
NASA Technical Reports Server (NTRS)
Takahashi, Fumiaki; Katta, Viswanath R.
2007-01-01
Diffusion flames are commonly used for industrial burners in furnaces and flares. Oxygen/fuel burners are usually diffusion burners, primarily for safety reasons, to prevent flashback and explosion in a potentially dangerous system. Furthermore, in most fires, condensed materials pyrolyze, vaporize, and burn in air as diffusion flames. As a result of the interaction of a diffusion flame with burner or condensed-fuel surfaces, a quenched space is formed, thus leaving a diffusion flame edge, which plays an important role in flame holding in combustion systems and fire spread through condensed fuels. Despite a long history of jet diffusion flame studies, lifting/blowoff mechanisms have not yet been fully understood, compared to those of premixed flames. In this study, the structure and stability of diffusion flames of gaseous hydrocarbon fuels in coflowing air at normal earth gravity have been investigated experimentally and computationally. Measurements of the critical mean jet velocity (U(sub jc)) of methane, ethane, or propane at lifting or blowoff were made as a function of the coflowing air velocity (U(sub a)) using a tube burner (i.d.: 2.87 mm) (Fig. 1, left). By using a computational fluid dynamics code with 33 species and 112 elementary reaction steps, the internal chemical-kinetic structures of the stabilizing region of methane and propane flames were investigated (Fig. 1, right). A peak reactivity spot, i.e., reaction kernel, is formed in the flame stabilizing region due to back-diffusion of heat and radical species against an oxygen-rich incoming flow, thus holding the trailing diffusion flame. The simulated flame base moved downstream under flow conditions close to the measured stability limit.
A nonlocal continuum model for biological aggregation.
Topaz, Chad M; Bertozzi, Andrea L; Lewis, Mark A
2006-10-01
We construct a continuum model for biological aggregations in which individuals experience long-range social attraction and short-range dispersal. For the case of one spatial dimension, we study the steady states analytically and numerically. There exist strongly nonlinear states with compact support and steep edges that correspond to localized biological aggregations, or clumps. These steady-state clumps are reached through a dynamic coarsening process. In the limit of large population size, the clumps approach a constant density swarm with abrupt edges. We use energy arguments to understand the nonlinear selection of clump solutions, and to predict the internal density in the large population limit. The energy result holds in higher dimensions as well, and is demonstrated via numerical simulations in two dimensions.
Formation and Growth of Stacking Fault Tetrahedra in Ni via Vacancy Aggregation Mechanism
Aidhy, Dilpuneet S.; Lu, Chenyang; Jin, Ke; Bei, Hongbin; Zhang, Yanwen; Wang, Lumin; Weber, William J.
2015-12-29
Using molecular dynamics simulations, the formation and growth of stacking fault tetrahedra (SFT) are captured by vacancy cluster diffusion and aggregation mechanisms in Ni. The vacancytetrahedron acts as a nucleation point for SFT formation. Simulations show that perfect SFT can grow to the next size perfect SFT via a vacancy aggregation mechanism. The stopping and range of ions in matter (SRIM) calculations and transmission electron microscopy (TEM) observations reveal that SFT can form farther away from the initial cascade-event locations, indicating the operation of diffusion-based vacancy-aggregation mechanism.
Taurine and platelet aggregation
Nauss-Karol, C.; VanderWende, C.; Gaut, Z.N.
1986-03-01
Taurine is a putative neurotransmitter or neuromodulator. The endogenous taurine concentration in human platelets, determined by amino acid analysis, is 15 ..mu..M/g. In spite of this high level, taurine is actively accumulated. Uptake is saturable, Na/sup +/ and temperature dependent, and suppressed by metabolic inhibitors, structural analogues, and several classes of centrally active substances. High, medium and low affinity transport processes have been characterized, and the platelet may represent a model system for taurine transport in the CNS. When platelets were incubated with /sup 14/C-taurine for 30 minutes, then resuspended in fresh medium and reincubated for one hour, essentially all of the taurine was retained within the cells. Taurine, at concentrations ranging from 10-1000 ..mu..M, had no effect on platelet aggregation induced by ADP or epinephrine. However, taurine may have a role in platelet aggregation since 35-39% of the taurine taken up by human platelets appears to be secreted during the release reaction induced by low concentrations of either epinephrine or ADP, respectively. This release phenomenon would imply that part of the taurine taken up is stored directly in the dense bodies of the platelet.
NASA Technical Reports Server (NTRS)
1981-01-01
A diffuse celestial radiation which is isotropic at least on a course scale were measured from the soft X-ray region to about 150 MeV, at which energy the intensity falls below that of the galactic emission for most galactic latitudes. The spectral shape, the intensity, and the established degree of isotropy of this diffuse radiation already place severe constraints on the possible explanations for this radiation. Among the extragalactic theories, the more promising explanations of the isotropic diffuse emission appear to be radiation from exceptional galaxies from matter antimatter annihilation at the boundaries of superclusters of galaxies of matter and antimatter in baryon symmetric big bang models. Other possible sources for extragalactic diffuse gamma radiation are discussed and include normal galaxies, clusters of galaxies, primordial cosmic rays interacting with intergalactic matter, primordial black holes, and cosmic ray leakage from galaxies.
NASA Technical Reports Server (NTRS)
Nicolet, M. A.
1983-01-01
The choice of the metallic film for the contact to a semiconductor device is discussed. One way to try to stabilize a contact is by interposing a thin film of a material that has low diffusivity for the atoms in question. This thin film application is known as a diffusion barrier. Three types of barriers can be distinguished. The stuffed barrier derives its low atomic diffusivity to impurities that concentrate along the extended defects of a polycrystalline layer. Sacrificial barriers exploit the fact that some (elemental) thin films react in a laterally uniform and reproducible fashion. Sacrificial barriers have the advantage that the point of their failure is predictable. Passive barriers are those most closely approximating an ideal barrier. The most-studied case is that of sputtered TiN films. Stuffed barriers may be viewed as passive barriers whose low diffusivity material extends along the defects of the polycrystalline host.
Spreading and spontaneous motility of multicellular aggregates on soft substrates
NASA Astrophysics Data System (ADS)
Brochard-Wyart, Françoise
2013-03-01
We first describe the biomechanics of multicellular aggregates, a model system for tissues and tumors. We first characterize the tissue mechanical properties (surface tension, elasticity, viscosity) by a new pipette aspiration technique. The aggregate exhibits a viscoelastic response but, unlike an inert fluid, we observe aggregate reinforcement with pressure, which for a narrow range of pressures results in pulsed contractions or shivering. We interpret this reinforcement as a mechanosensitive active response of the acto-myosin cortex. Such an active behavior has previously been found to cause tissue pulsation during dorsal closure of Drosophila embryo. We then describe the spreading of aggregates on rigid glass substrates, varying both intercellular and substrate adhesion. We find both partial and complete wetting regimes. For the dynamics, we find a universal spreading law at short time, analogous to that of a viscoelastic drop. At long time, we observe, for strong substrate adhesion, a precursor film spreading around the aggregate. Depending on aggregate cohesion, this precursor film can be a dense cellular monolayer (liquid state) or consist of individual cells escaping from the aggregate body (gas state). The transition from liquid to gas state appears also to be present in the progression of a tumor from noninvasive to metastatic, known as the epithelial-mesenchymal transition. Finally, we describe the effect of the substrate rigidity on the phase diagram of wetting. On soft gels decorated with fibronectin and strongly cohesive aggregates, we have observed a wetting transition induced by the substrate rigidity: on ultra soft gels, below an elastic modulus Ec the aggregates do not spread, whereas above Ec we observe a precursor film expending with a diffusive law. The diffusion coefficient D(E) present a maximum for E =Em. A maximum of mobility versus the substrate rigidity had also been observed for single cells. Near Em, we observe a new phenomenon: a cell
Holographic characterization of protein aggregates
NASA Astrophysics Data System (ADS)
Wang, Chen; Zhong, Xiao; Ruffner, David; Stutt, Alexandra; Philips, Laura; Ward, Michael; Grier, David
Holographic characterization directly measures the size distribution of subvisible protein aggregates in suspension and offers insights into their morphology. Based on holographic video microscopy, this analytical technique records and interprets holograms of individual aggregates in protein solutions as they flow down a microfluidic channel, without requiring labeling or other exceptional sample preparation. The hologram of an individual protein aggregate is analyzed in real time with the Lorenz-Mie theory of light scattering to measure that aggregate's size and optical properties. Detecting, counting and characterizing subvisible aggregates proceeds fast enough for time-resolved studies, and lends itself to tracking trends in protein aggregation arising from changing environmental factors. No other analytical technique provides such a wealth of particle-resolved characterization data in situ. Holographic characterization promises accelerated development of therapeutic protein formulations, improved process control during manufacturing, and streamlined quality assurance during storage and at the point of use. Mrsec and MRI program of the NSF, Spheryx Inc.
Lorenzano-Porras, C.F.; Reeder, D.H.; Annen, M.J.; Carr, P.W.; McCormick, A.V.
1995-08-01
The effects of sintering temperature and duration on the pore structure of chromatographic zirconia particles produced by the controlled polymerization-induced aggregation of 1,000 {angstrom} colloids are studied with an eye toward optimally strengthening the aggregates and eliminating small pores while preserving large pores. Nitrogen adsorption and mercury porosimetry are used to estimate the surface area, pore volume, and pore size distribution. Pulsed field gradient NMR measurements of solvent diffusion are used to estimate the diffusion tortuosity of the pore space. Initially of course, the pore volume and surface area decrease significantly, the decrease being more pronounced at higher temperatures. With prolonged sintering, the pore size, pore volume, and surface area change much more slowly, but the diffusion tortuosity seems to be minimized at a sintering temperature and time at which pores are allowed to redistribute so as to optimize large pores. The aggregates synthesized by this aggregation method apparently produce metastable large pores which are not easily collapsed.
The single scattering properties of the aerosol particles as aggregated spheres
NASA Astrophysics Data System (ADS)
Wu, Y.; Gu, X.; Cheng, T.; Xie, D.; Yu, T.; Chen, H.; Guo, J.
2012-08-01
The light scattering and absorption properties of anthropogenic aerosol particles such as soot aggregates are complicated in the temporal and spatial distribution, which introduce uncertainty of radiative forcing on global climate change. In order to study the single scattering properties of anthorpogenic aerosol particles, the structures of these aerosols such as soot paticles and soot-containing mixtures with the sulfate or organic matter, are simulated using the parallel diffusion limited aggregation algorithm (DLA) based on the transmission electron microscope images (TEM). Then, the single scattering properties of randomly oriented aerosols, such as scattering matrix, single scattering albedo (SSA), and asymmetry parameter (AP), are computed using the superposition T-matrix method. The comparisons of the single scattering properties of these specific types of clusters with different morphological and chemical factors such as fractal parameters, aspect ratio, monomer radius, mixture mode and refractive index, indicate that these different impact factors can respectively generate the significant influences on the single scattering properties of these aerosols. The results show that aspect ratio of circumscribed shape has relatively small effect on single scattering properties, for both differences of SSA and AP are less than 0.1. However, mixture modes of soot clusters with larger sulfate particles have remarkably important effects on the scattering and absorption properties of aggregated spheres, and SSA of those soot-containing mixtures are increased in proportion to the ratio of larger weakly absorbing attachments. Therefore, these complex aerosols come from man made pollution cannot be neglected in the aerosol retrievals. The study of the single scattering properties on these kinds of aggregated spheres is important and helpful in remote sensing observations and atmospheric radiation balance computations.
A modified Stokes-Einstein equation for Aβ aggregation
2011-01-01
Background In all amyloid diseases, protein aggregates have been implicated fully or partly, in the etiology of the disease. Due to their significance in human pathologies, there have been unprecedented efforts towards physiochemical understanding of aggregation and amyloid formation over the last two decades. An important relation from which hydrodynamic radii of the aggregate is routinely measured is the classic Stokes-Einstein equation. Here, we report a modification in the classical Stokes-Einstein equation using a mixture theory approach, in order to accommodate the changes in viscosity of the solvent due to the changes in solute size and shape, to implement a more realistic model for Aβ aggregation involved in Alzheimer’s disease. Specifically, we have focused on validating this model in protofibrill lateral association reactions along the aggregation pathway, which has been experimentally well characterized. Results The modified Stokes-Einstein equation incorporates an effective viscosity for the mixture consisting of the macromolecules and solvent where the lateral association reaction occurs. This effective viscosity is modeled as a function of the volume fractions of the different species of molecules. The novelty of our model is that in addition to the volume fractions, it incorporates previously published reports on the dimensions of the protofibrils and their aggregates to formulate a more appropriate shape rather than mere spheres. The net result is that the diffusion coefficient which is inversely proportional to the viscosity of the system is now dependent on the concentration of the different molecules as well as their proper shapes. Comparison with experiments for variations in diffusion coefficients over time reveals very similar trends. Conclusions We argue that the standard Stokes-Einstein’s equation is insufficient to understand the temporal variations in diffusion when trying to understand the aggregation behavior of Aβ42 proteins. Our
Diffusion processes in general relativistic radiating spheres
Barreto, W.; Herrera, L.; Santos, N.O.; Universidad Central de Venezuela, Caracas; Observatorio Nacional do Brasil, Rio de Janeiro )
1989-09-01
The influence of diffusion processes on the dynamics of general relativistic radiating spheres is systematically studied by means of two examples. Differences between the streaming-out limit and the diffusion limit are exhibited, for both models, through the evolution curves of dynamical variables. In particular it is shown the Bondi mass decreases, for both models, in the diffusion limit as compared with its value at the streaming-out regime. 15 refs.
Barnett, Gregory V; Drenski, Michael; Razinkov, Vladimir; Reed, Wayne F; Roberts, Christopher J
2016-10-15
Parallel temperature initial rates (PTIR) from chromatographic separation of aggregating protein solutions are combined with continuous simultaneous multiple sample light scattering (SMSLS) to make quantitative deductions about protein aggregation kinetics and mechanisms. PTIR determines the rates at which initially monomeric proteins are converted to aggregates over a range of temperatures, under initial-rate conditions. Using SMSLS for the same set of conditions provides time courses of the absolute Rayleigh scattering ratio, IR(t), from which a potentially different measure of aggregation rates can be quantified. The present report compares these measures of aggregation rates across a range of solution conditions that result in different aggregation mechanisms for anti-streptavidin (AS) immunoglobulin gamma-1 (IgG1). The results illustrate how the two methods provide complementary information when deducing aggregation mechanisms, as well as cases where they provide new mechanistic details that were not possible to deduce in previous work. Criteria are presented for when the two techniques are expected to give equivalent results for quantitative rates, the potential limitations when solution non-idealities are large, as well as a comparison of the temperature dependence of AS-IgG1 aggregation rates with published data for other antibodies. PMID:27510552
Peptide aggregation in neurodegenerative disease.
Murphy, Regina M
2002-01-01
In the not-so-distant past, insoluble aggregated protein was considered as uninteresting and bothersome as yesterday's trash. More recently, protein aggregates have enjoyed considerable scientific interest, as it has become clear that these aggregates play key roles in many diseases. In this review, we focus attention on three polypeptides: beta-amyloid, prion, and huntingtin, which are linked to three feared neurodegenerative diseases: Alzheimer's, "mad cow," and Huntington's disease, respectively. These proteins lack any significant primary sequence homology, yet their aggregates possess very similar features, specifically, high beta-sheet content, fibrillar morphology, relative insolubility, and protease resistance. Because the aggregates are noncrystalline, secrets of their structure at nanometer resolution are only slowly yielding to X-ray diffraction, solid-state NMR, and other techniques. Besides structure, the aggregates may possess similar pathways of assembly. Two alternative assembly pathways have been proposed: the nucleation-elongation and the template-assisted mode. These two modes may be complementary, not mutually exclusive. Strategies for interfering with aggregation, which may provide novel therapeutic approaches, are under development. The structural similarities between protein aggregates of dissimilar origin suggest that therapeutic strategies successful against one disease may have broad utility in others. PMID:12117755
Topics in Probabilistic Judgment Aggregation
ERIC Educational Resources Information Center
Wang, Guanchun
2011-01-01
This dissertation is a compilation of several studies that are united by their relevance to probabilistic judgment aggregation. In the face of complex and uncertain events, panels of judges are frequently consulted to provide probabilistic forecasts, and aggregation of such estimates in groups often yield better results than could have been made…
Mineral of the month: aggregates
Tepordei, Valentin V.
2005-01-01
Natural aggregates, consisting of crushed stone, and sand and gravel, are a major contributor to economic health, and have an amazing variety of uses. Aggregates are among the most abundant mineral resources and are major basic raw materials used by construction, agriculture and other industries that employ complex chemical and metallurgical processes.
Sucrose diffusion in aqueous solution.
Price, Hannah C; Mattsson, Johan; Murray, Benjamin J
2016-07-28
The diffusion of sugar in aqueous solution is important both in nature and in technological applications, yet measurements of diffusion coefficients at low water content are scarce. We report directly measured sucrose diffusion coefficients in aqueous solution. Our technique utilises a Raman isotope tracer method to monitor the diffusion of non-deuterated and deuterated sucrose across a boundary between the two aqueous solutions. At a water activity of 0.4 (equivalent to 90 wt% sucrose) at room temperature, the diffusion coefficient of sucrose was determined to be approximately four orders of magnitude smaller than that of water in the same material. Using literature viscosity data, we show that, although inappropriate for the prediction of water diffusion, the Stokes-Einstein equation works well for predicting sucrose diffusion under the conditions studied. As well as providing information of importance to the fundamental understanding of diffusion in binary solutions, these data have technological, pharmaceutical and medical implications, for example in cryopreservation. Moreover, in the atmosphere, slow organic diffusion may have important implications for aerosol growth, chemistry and evaporation, where processes may be limited by the inability of a molecule to diffuse between the bulk and the surface of a particle. PMID:27364512
Sucrose diffusion in aqueous solution
Murray, Benjamin J.
2016-01-01
The diffusion of sugar in aqueous solution is important both in nature and in technological applications, yet measurements of diffusion coefficients at low water content are scarce. We report directly measured sucrose diffusion coefficients in aqueous solution. Our technique utilises a Raman isotope tracer method to monitor the diffusion of non-deuterated and deuterated sucrose across a boundary between the two aqueous solutions. At a water activity of 0.4 (equivalent to 90 wt% sucrose) at room temperature, the diffusion coefficient of sucrose was determined to be approximately four orders of magnitude smaller than that of water in the same material. Using literature viscosity data, we show that, although inappropriate for the prediction of water diffusion, the Stokes–Einstein equation works well for predicting sucrose diffusion under the conditions studied. As well as providing information of importance to the fundamental understanding of diffusion in binary solutions, these data have technological, pharmaceutical and medical implications, for example in cryopreservation. Moreover, in the atmosphere, slow organic diffusion may have important implications for aerosol growth, chemistry and evaporation, where processes may be limited by the inability of a molecule to diffuse between the bulk and the surface of a particle. PMID:27364512
Molecular aggregation of humic substances
Wershaw, R. L.
1999-01-01
Humic substances (HS) form molecular aggregates in solution and on mineral surfaces. Elucidation of the mechanism of formation of these aggregates is important for an understanding of the interactions of HS in soils arid natural waters. The HS are formed mainly by enzymatic depolymerization and oxidation of plant biopolymers. These reactions transform the aromatic and lipid plant components into amphiphilic molecules, that is, molecules that consist of separate hydrophobic (nonpolar) and hydrophilic (polar) parts. The nonpolar parts of the molecules are composed of relatively unaltered segments of plant polymers and the polar parts of carboxylic acid groups. These amphiphiles form membrane-like aggregates on mineral surfaces and micelle-like aggregates in solution. The exterior surfaces of these aggregates are hydrophilic, and the interiors constitute separate hydrophobic liquid-like phases.
Nanoparticle aggregation: principles and modeling.
Zhang, Wen
2014-01-01
The high surface area to volume ratio of nanoparticles usually results in highly reactive and colloidal instability compared to their bulk counterparts. Aggregation as well as many other transformations (e.g., dissolution) in the environment may alter the physiochemical properties, reactivity, fate, transport, and biological interactions (e.g., bioavailability and uptake) of nanoparticles. The unique properties pertinent to nanoparticles, such as shape, size, surface characteristics, composition, and electronic structures, greatly challenge the ability of colloid science to understand nanoparticle aggregation and its environmental impacts. This review briefly introduces fundamentals about aggregation, fractal dimensions, classic and extended Derjaguin-Landau-Verwey-Overbeak (DLVO) theories, aggregation kinetic modeling, experimental measurements, followed by detailed discussions on the major factors on aggregation and subsequent effects on nanomaterial transport and reactivity.
Immunogenicity of Therapeutic Protein Aggregates.
Moussa, Ehab M; Panchal, Jainik P; Moorthy, Balakrishnan S; Blum, Janice S; Joubert, Marisa K; Narhi, Linda O; Topp, Elizabeth M
2016-02-01
Therapeutic proteins have a propensity for aggregation during manufacturing, shipping, and storage. The presence of aggregates in protein drug products can induce adverse immune responses in patients that may affect safety and efficacy, and so it is of concern to both manufacturers and regulatory agencies. In this vein, there is a lack of understanding of the physicochemical determinants of immunological responses and a lack of standardized analytical methods to survey the molecular properties of aggregates associated with immune activation. In this review, we provide an overview of the basic immune mechanisms in the context of interactions with protein aggregates. We then critically examine the literature with emphasis on the underlying immune mechanisms as they relate to aggregate properties. Finally, we highlight the gaps in our current understanding of this issue and offer recommendations for future research. PMID:26869409
Mechanics of fire ant aggregations
NASA Astrophysics Data System (ADS)
Tennenbaum, Michael; Liu, Zhongyang; Hu, David; Fernandez-Nieves, Alberto
2016-01-01
Fire ants link their bodies to form aggregations; these can adopt a variety of structures, they can drip and spread, or withstand applied loads. Here, by using oscillatory rheology, we show that fire ant aggregations are viscoelastic. We find that, at the lowest ant densities probed and in the linear regime, the elastic and viscous moduli are essentially identical over the spanned frequency range, which highlights the absence of a dominant mode of structural relaxation. As ant density increases, the elastic modulus rises, which we interpret by alluding to ant crowding and subsequent jamming. When deformed beyond the linear regime, the aggregation flows, exhibiting shear-thinning behaviour with a stress load that is comparable to the maximum load the aggregation can withstand before individual ants are torn apart. Our findings illustrate the rich, collective mechanical behaviour that can arise in aggregations of active, interacting building blocks.
Mechanics of fire ant aggregations.
Tennenbaum, Michael; Liu, Zhongyang; Hu, David; Fernandez-Nieves, Alberto
2016-01-01
Fire ants link their bodies to form aggregations; these can adopt a variety of structures, they can drip and spread, or withstand applied loads. Here, by using oscillatory rheology, we show that fire ant aggregations are viscoelastic. We find that, at the lowest ant densities probed and in the linear regime, the elastic and viscous moduli are essentially identical over the spanned frequency range, which highlights the absence of a dominant mode of structural relaxation. As ant density increases, the elastic modulus rises, which we interpret by alluding to ant crowding and subsequent jamming. When deformed beyond the linear regime, the aggregation flows, exhibiting shear-thinning behaviour with a stress load that is comparable to the maximum load the aggregation can withstand before individual ants are torn apart. Our findings illustrate the rich, collective mechanical behaviour that can arise in aggregations of active, interacting building blocks. PMID:26501413
Adsorption-induced colloidal aggregation
NASA Astrophysics Data System (ADS)
Law, B. M.; Petit, J.-M.; Beysens, D.
1998-03-01
Reversible colloidal aggregation in binary liquid mixtures has been studied for a number of years. As the phase separation temperature of the liquid mixture is approached the thickness of an adsorption layer around the colloidal particles increases. Beysens and coworkers have demonstrated experimentally that this adsorption layer is intimately connected with the aggregation of the colloidal particles, however, no definitive theory has been available which can explain all of the experimental observations. In this contribution we describe an extension of the Derjaguin, Landau, Verwey, and Overbeek theory of colloidal aggregation which takes into account the presence of the adsorption layer and which more realistically models the attractive dispersion interactions. This modified theory can quantitatively account for many of the observed experimental features such as the characteristics of the aggregated state, the general shape of the aggregation line, and the temperature dependence of the second virial coefficient for a lutidine-water mixture containing a small volume fraction of silica colloidal particles.
NASA Astrophysics Data System (ADS)
Ying, Samantha C.; Masue-Slowey, Yoko; Kocar, Benjamin D.; Griffis, Sarah D.; Webb, Samuel; Marcus, Matthew A.; Francis, Christopher A.; Fendorf, Scott
2013-03-01
The aggregate-based structure of soils imparts physical heterogeneity that gives rise to variation in microbial and chemical processes which influence the speciation and retention of trace elements such as As. To examine the impact of distributed redox conditions on the fate of As in soils, we imposed various redox treatments upon constructed soil aggregates composed of ferrihydrite- and birnessite-coated sands presorbed with As(V) and inoculation with the dissimilatory metal reducing bacterium Shewanella sp. ANA-3. Aeration of the advecting solution surrounding the aggregates was varied to simulate environmental conditions. We find that diffusion-limited transport within high dissolved organic carbon environments allows reducing conditions to persist in the interior of aggregates despite aerated advecting external solutes, causing As, Mn, and Fe to migrate from the reduced aggregate interiors to the aerated exterior region. Upon transitioning to anoxic conditions in the external solutes, pulses of As, Mn and Fe are released into the advecting solution, while, conversely, a transition to aerated conditions in the exterior resulted in a cessation of As, Mn, and Fe release. Importantly, we find that As(III) oxidation by birnessite is appreciable only in the presence of O2; oxidation of As(III) to As(V) by Mn-oxides ceases under anaerobic conditions apparently as a result of microbially mediated Mn(IV/III) reduction. Our results demonstrate the importance of considering redox conditions and the physical complexity of soils in determining As dynamics, where redox transitions can either enhance or inhibit As release due to speciation shifts in both sorbents (solubilization versus precipitation of Fe and Mn oxides) and sorbates (As).
ERIC Educational Resources Information Center
Foy, Barry G.
1977-01-01
Two demonstrations are described. Materials and instructions for demonstrating movement of molecules into cytoplasm using agar blocks, phenolphthalein, and sodium hydroxide are given. A simple method for demonstrating that the rate of diffusion of a gas is inversely proportional to its molecular weight is also presented. (AJ)
NASA Astrophysics Data System (ADS)
Haba, Z.
2009-02-01
We discuss relativistic diffusion in proper time in the approach of Schay (Ph.D. thesis, Princeton University, Princeton, NJ, 1961) and Dudley [Ark. Mat. 6, 241 (1965)]. We derive (Langevin) stochastic differential equations in various coordinates. We show that in some coordinates the stochastic differential equations become linear. We obtain momentum probability distribution in an explicit form. We discuss a relativistic particle diffusing in an external electromagnetic field. We solve the Langevin equations in the case of parallel electric and magnetic fields. We derive a kinetic equation for the evolution of the probability distribution. We discuss drag terms leading to an equilibrium distribution. The relativistic analog of the Ornstein-Uhlenbeck process is not unique. We show that if the drag comes from a diffusion approximation to the master equation then its form is strongly restricted. The drag leading to the Tsallis equilibrium distribution satisfies this restriction whereas the one of the Jüttner distribution does not. We show that any function of the relativistic energy can be the equilibrium distribution for a particle in a static electric field. A preliminary study of the time evolution with friction is presented. It is shown that the problem is equivalent to quantum mechanics of a particle moving on a hyperboloid with a potential determined by the drag. A relation to diffusions appearing in heavy ion collisions is briefly discussed.
ERIC Educational Resources Information Center
Dou, Remy; Hogan, DaNel; Kossover, Mark; Spuck, Timothy; Young, Sarah
2013-01-01
Diffusion has often been taught in science courses as one of the primary ways by which molecules travel, particularly within organisms. For years, classroom teachers have used the same common demonstrations to illustrate this concept (e.g., placing drops of food coloring in a beaker of water). Most of the time, the main contributor to the motion…