Transient Dispersion Research Articles

Microfluidic bolus induced gradient generator for live cell signalling

Major events in cell biology are initiated by the binding of ligands to cell surface receptors and/or their transport into cells. We present a study of a simple microchannel system that integrates a bolus generator and surface-adhered cell culture domains. Our system allows the delivery of small packets or boluses of biomolecules to a cell population. Owing to pressure driven microfluidic flow of the bolus, a gradient of cell surface bound ligands is established along the length of the microchannel. Experimental data for the epidermal growth factor (EGF) binding to its receptor on A431 cells are presented. We highlight the effect of changing Peclet number (or flowrate), bolus shape, bolus volume and ligand concentration on the gradient formed longitudinally in the microchannel. A mathematical model describing the transient convection, diffusion, dispersion and binding of ligands to cell surface receptors is developed. The model provides essential design guidelines for our system with good qualitative agreement with experimental data. The results suggest ways to modulate the amount of bound ligand and the gradient independently. This simple microsystem is suitable for generating longer range gradients involving larger cell populations as compared to existing microfluidic systems.

From random sphere packings to regular pillar arrays: Analysis of transverse dispersion

We study the impact of microscopic order on transverse dispersion in the interstitial void space of bulk (unconfined) chromatographic beds by numerical simulations of incompressible fluid flow and mass transport of a passive tracer. Our study includes polydisperse random sphere packings (computer-generated with particle size distributions of modern core–shell and sub-2μm particles), the macropore space morphology of a physically reconstructed silica monolith, and computer-generated regular pillar arrays. These bed morphologies are analyzed by their velocity probability density distributions, transient dispersion behavior, and the dependence of asymptotic transverse dispersion coefficients on the mobile phase velocity. In our work, the spherical particles, the monolith skeleton, and the cylindrical pillars are all treated as impermeable solid phase (nonporous) and the tracer is unretained, to focus on the impact of microscopic order on flow and (particularly transverse) hydrodynamic dispersion in the interstitial void space. The microscopic order of the pillar arrays causes their velocity probability density distributions to start and end abruptly, their transient dispersion coefficients to oscillate, and the asymptotic transverse dispersion coefficients to plateau out of initial power law behavior. The microscopically disordered beds, by contrast, follow power law behavior over the whole investigated velocity range, for which we present refined equations (i.e., Eq. (13) and the data in Table 2 for the polydisperse sphere packings; Eq. (17) for the silica monolith). The bulk bed morphologies and their intrinsic differences addressed in this work determine how efficient a bed can relax the transverse concentration gradients caused by wall effects, which exist in all confined separation media used in chromatographic practice. Whereas the effect of diffusion on transverse dispersion decreases and ultimately disappears at increasing velocity with the microscopically disordered chromatographic beds, it dominates in the pillar arrays. The pillar arrays therefore become the least forgiving bed morphology with macroscopic heterogeneities and the engendered longitudinal dispersion in chromatographic practice. Wall effects in pillar arrays and the monolith caused by their confinement impact band broadening, which is traditionally observed on a macroscopic scale, more seriously than in the packings.

Equivalent cloud methods used for explosion risk and consequence studies

The reactivity of a flammable gas mixture depends strongly on the concentration. Explosions can only take place between the flammability limits LFL and UFL (5%–14% for methane), with by far the strongest explosions occurring near stoichiometry. When performing explosion studies to evaluate or minimize risk, optimizing design or ways to mitigate, many different approaches exist. Worst-case approaches assuming stoichiometric gas clouds filling the entire facility are often much too conservative and may lead to very expensive solutions. More refined approaches studying release scenarios leading to flammable clouds can give a more precise description of the risk (probabilistic approach) or worst-case consequences (realistic worst-case study).One main challenge with such approaches is that there can be thousands of potential release scenarios to study, e.g. variations of release location, direction, rate-profile, wind direction and strength. For each resulting gas cloud there can further be thousands of explosion scenarios as the transient non-homogeneous gas cloud can be ignited at a number of different locations and times. To reduce the number of explosion scenarios, in early 1990s GexCon developed a concept called Equivalent Stoichiometric Clouds (ESC, initially called Erfac, later modified to Q5 and Q9) to linearize the expected hazards from arbitrary non-homogeneous, dispersed flammable gas clouds. The idea is that the potential explosion consequences from any non-homogeneous gas cloud can be approximated by exploding a smaller gas cloud at stoichiometric concentration.These concepts are in extensive use in explosion risk and consequence assessments. For probabilistic assessments all transient dispersion scenarios modeled may for each time step be given an ignition probability and an equivalent cloud size. For realistic worst-case assessments, the dispersed gas clouds may be ignited at the time when the estimated equivalent gas cloud has its maximum. Compared to alternative simplifications, e.g. applying faster and less accurate consequence models, the equivalent cloud method simplifications keep much of the precision required in an explosion study. Despite the wide acceptance and use of these methods, they have also been criticized for not being conservative enough or for being inaccurate, and some groups prefer a much more conservative approach substituting any predicted flammable gas cloud volume with the most reactive concentration. It is well known that explosion consequences may vary strongly when changing ignition location and other parameters, and one can therefore not expect that an equivalent cloud method will accurately reproduce any ignited gas cloud scenario (which has never been the goal), but rather provide a reasonable estimate of expected explosion consequences when used according to GexCon developed guidance. The currently recommended Q9 method works well for a range of scenarios, and for certain more confined or high reactivity scenarios a more conservative approach is recommended (Q8). For some scenarios the current approach has weaknesses, e.g. aerosols. This paper will describe different equivalent cloud methods, show examples of use and evaluate performance, and discuss weaknesses and potential improvements.

Modeling of dispersion in a polymeric chromatographic monolith

Dispersion in a commercial polymeric monolith was simulated on a sample geometry obtained by direct imaging using high-resolution electron microscopy. A parallelized random walk algorithm, implemented using a velocity field obtained previously by the lattice-Boltzmann method, was used to model mass transfer. Both point particles and probes of finite size were studied. Dispersion simulations with point particles using periodic boundaries resulted in plate heights that varied almost linearly with flow rate, at odds with the weaker dependence suggested by experimental observations and predicted by theory. This discrepancy resulted from the combined effect of the artificial symmetry in the velocity field and the periodic boundaries implemented to emulate macroscopic column lengths. Eliminating periodicity and simulating a single block length instead resulted in a functional dependence of plate heights on flow rate more in accord with experimental trends and theoretical predictions for random media. The lower values of the simulated plate heights than experimental ones are attributed in part to the presence of walls in real systems, an effect not modeled by the algorithm. On the other hand, analysis of transient dispersion coefficients and comparison of lateral particle positions at the entry and exit hinted at non-asymptotic behavior and a strong degree of correlation that was presumably a consequence of preferential high-velocity pathways in the raw sample block. Simulations with finite-sized probes resulted in particle trajectories that frequently terminated at narrow constrictions of the geometry. The amount of entrapment was predicted to increase monotonically with flow rate, evidently due to the relative contributions to transport by convection that carries particles to choke-points and diffusion that dislodges these entrapped particles. The overall effect is very similar to a flow-dependent entrapment phenomenon previously observed experimentally for adenovirus.

Aggregation Kinetics of Single-Walled Carbon Nanotubes in Nonaqueous Solvents: Critical Coagulation Concentrations and Transient Dispersion Stability

In this paper we present the aggregation kinetics of single-walled carbon nanotubes (SWCNTs), when aggregated by NaBr and CaCl2 in the solvents DMF, NMP, and 50/50 DMF/NMP (v/v). We have measured critical coagulation concentrations (CCCs) by finding the diffusion-limited aggregation regime for each salt/solvent combination, as required by DLVO theory. We have observed a “transient stability” effect for dispersions in NMP and DMF/NMP that we attribute to SWCNTs overcoming a torsional energy barrier to bundling that is theoretically predicted for SWCNT aggregation in NMP. Lastly, when comparing our CCCs for pure vs mixed solvents, we do not see any enhancement in dispersion stability for mixed solvents, as was observed in our recently published work studying the significantly lower “onset of aggregation” coagulant concentration range, indicating different aggregation mechanisms in the two regimes.

History recovery and source identification of multiple gaseous contaminants releasing with thermal effects in an indoor environment

Thermal transport and transient dispersion of pollutants emitted from two discrete strips within the displacement ventilation enclosure have been modeled numerically. Following the full numerical simulation of turbulent air flows, the inverse determinations of multiple pollutant sources were conducted by the use of quasi reversibility methodology. Direct simulation together with the turbulent streamlines and turbulent heatlines demonstrate that the enclosure flow pattern, enclosure air thermal level and heat transfer potential will depend on the interactions of external forced flow and thermal buoyancy driven flows, i.e., Reynolds number (2 × 10 3 ⩽ Re ⩽ 10 4) and Grashof number (10 6 ⩽ Gr ⩽ 10 10). In subsequent forward time and backward time modeling of airborne pollutant transports, temporal evolutions of enclosure average concentration and pollutant exhaust are shown to depend on the supplying velocity ( Re), thermal plume ( Gr), pollutant diffusivity (0.1 ⩽ Sc ⩽ 2), and the pitch between both sources (0.2 H ⩽ d PSL = d PSR ⩽ 0.7 H). Reverse time modeling of airborne spread has demonstrated that increasing the spread rate and the concentration sensitivity of airborne pollutants will facilitate the identification of pollutant sources.

Ultrasonic digital signal processing simulation in viscoelastic medium with generalized parametric function

This paper describes a simulation of signal pathway in an ultrasonic A-scan. It includes transient diffraction acoustic field, transducer response, absorption and dispersion in viscoelastic medium with arbitrary distribution function. The approach selected here aims at developing digital computer simulation of the physical process that underlies these effects. The soft tissue is considered as a viscoelastic medium, and the relaxation theory has been used to calculate the absorption and dispersion data in the tissue. A signal processing technique is developed in this paper to estimate the degradation in range of resolution due to wave absorption.

Numerical simulation of surface acoustic wave and detection of surface crack in steel

The surface acoustic wave (SAW) propagating in a sample of steel is simulated by using finite element method (FEM). The waves are excited by a load function with propagation properties such as phase velocity dispersion and wide bandwidth. A two-dimensional model consisting of surface defects loaded with a wideband 50–200 MHz and short time 0.1 μs displacement function is investigated in the time and frequency domains. By transient dynamic analysis, Fourier transform and dispersion calculation, snapshots of propagating wave and responses from sensing points are presented. It is indicated that this supervision approach is sensitive to the surface cracks and reflections.

Effect of tidal fluctuations on transient dispersion of simulated contaminant concentrations in coastal aquifers

Variable-density groundwater models require extensive computational resources, particularly for simulations representing short-term hydrologic variability such as tidal fluctuations. Saltwater-intrusion models usually neglect tidal fluctuations and this may introduce errors in simulated concentrations. The effects of tides on simulated concentrations in a coastal aquifer were assessed. Three analyses are reported: in the first, simulations with and without tides were compared for three different dispersivity values. Tides do not significantly affect the transfer of a hypothetical contaminant into the ocean; however, the concentration difference between tidal and non-tidal simulations could be as much as 15%. In the second analysis, the dispersivity value for the model without tides was increased in a zone near the ocean boundary. By slightly increasing dispersivity in this zone, the maximum concentration difference between the simulations with and without tides was reduced to as low as 7%. In the last analysis, an apparent dispersivity value was calculated for each model cell using the simulated velocity variations from the model with tides. Use of apparent dispersivity values in models with a constant ocean boundary seems to provide a reasonable approach for approximating tidal effects in simulations where explicit representation of tidal fluctuations is not feasible.

Buoyancy driven flow of light gases in atmosphere compared to that of hot gases

The transient formation and subsequent dispersion of the plumes of a fixed mass of lighter than air gases emerging out of open cylindrical enclosures with negligible pressure difference was investigated using 3-D and 2-D CFD models. Subsequently, the dispersion into atmosphere of a similar amount of equally buoyant hot air was also considered. The structure and dynamics of the resulting thermally driven hot air plumes were compared to the corresponding characteristics of the mass-transfer driven isothermal plumes. Some cases were investigated in which the dispersing gases were both lighter than air and at a different temperature from that of the atmosphere. The similarities and differences of these double heat-mass-transfer driven problems with the other cases were discussed. It was shown that a criterion developed previously for judging the validity of the 2-D model relative to the more complex 3-D approach for mass-transfer driven problems could be equally applied for the thermally driven or double heat and mass-transfer driven plume flow characteristics.

Modelling concentration fluctuations and individual exposure in complex urban environments

One of the key problems in coping with deliberate or accidental atmospheric releases, which in many cases are short or/and result in high concentrations, is the ability to reliably predict the individual exposure during the event. Furthermore, for consequence assessment and countermeasures application, it is more realistic to rely on the maximum expected dosage rather than on the actual dosage. Recently, Bartzis et al. (2008) have introduced an approach relating maximum dosage to parameters such as concentration variance and turbulence integral time scale. The need for an estimation of these parameters poses new challenges to CFD models. In the CFD RANS model ADREA, new approaches have been implemented recently, where the parameterization of the dispersion of a pollutant emitted from a point source depends not only on the parameters of turbulence, but also on the pollutant travel times. In this study the new methodology is tested against MUST and FLADIS field experimental data, which consist of high resolution concentration time series enabling the production of short term dosage data. The present comparisons further strengthen the evidence that the applied methodology is capable of dealing properly with complex transient dispersion phenomena.

Dispersive and absorptive properties of a Λ-type atomic system with two fold lower-levels

The dispersion and the absorption properties of a driven four-level Λ-type atomic system is investigated. It is found that the interaction of double-dark states lead to controllable group velocity of the weak probe field by the intensity of driving field and the relative phase between applied fields. Moreover, the transient dispersion, absorption and the group index are also discussed. The required switching time for switching the group velocity of a weak probe field from subluminal to superluminal pulse propagation is then discussed.

Transient analysis of pollutant dispersion in a cylindrical pipe with a nonlinear waste discharge concentration

In this study we investigate the transient dispersion of a pollutant ejected by an external source into a laminar flow of an incompressible fluid in a cylindrical pipe. The Boussinesq assumption is applied and the nonlinear governing equations of momentum and pollutant concentration are obtained. The problem is solved numerically using a semi-implicit finite difference method. Solutions are presented in graphical form and given in terms of fluid velocity, pollutant concentration, skin friction and wall mass transfer rate for various parametric values. The model can be a useful tool for understanding the polluting situations of an improper discharge incident and evaluating the effects of decontaminating measures for the water body.

Statistical analysis of packed beds, the origin of short-range disorder, and its impact on eddy dispersion

We quantified the microstructural disorder of packed beds and correlated it with the resulting eddy dispersion. For this purpose we designed a set of bulk (unconfined) monodisperse random sphere packings with a systematic, protocol-dependent degree of microstructural heterogeneity, covering a porosity range from the random-close to the random-loose packing limit ( ɛ = 0.366–0.46). With the precise knowledge of particle positions, size, and shape we conducted a Voronoï tessellation of all packings and correlated the statistical moments of the Voronoï volume distributions (standard deviation and skewness) with the porosity and the protocol-dependent microstructural disorder. The deviation of the Voronoï volume distributions from the delta function of a crystalline packing describes the origin of short-range disorder of the investigated random packings. Eddy dispersion was simulated over a wide range of reduced velocities (0.5 ≤ ν ≤ 750) and analyzed with the comprehensive Giddings equation. Transient dispersion was found to correlate with the spatial scales of heterogeneity in the packings. The analysis of short-range disorder based on the Voronoï volume distributions revealed a strong correlation with the short-range interchannel contribution to eddy dispersion, whereas transchannel dispersion was relatively little affected. The presented approach defines a strictly scientific route to the key morphology–transport relationships of current and future chromatographic supports, including their morphological reconstruction, statistical analysis, and the correlation with relevant transport phenomena. It also guides us in our understanding, comparison, and optimization of the diverse packing algorithms and protocols used in simulations and experimental studies.

Transient dispersion and absorption in a V-shaped atomic system

We investigate the dynamical behavior of the dispersion and the absorption in a V-type three level atomic system. It is shown that in the presence of decay-induced interference the probe dispersion and absorption are phase dependent. We find that an incoherent pumping field provides an additional control parameter for switching the group velocity of a light pulse. The required switching times for switching the group velocity of a probe field from subluminal to superluminal pulse propagation is then discussed.

Transient Hydrodynamic Dispersion in Rough Open Channels: Theoretical Analysis of Bed-Form Effects

A stochastic Lagrangian approach is proposed to describe dispersion in a two-dimensional low-regime river flow past random bed undulations characterized by the superposition of periodical and stationary exponential correlations, through a suitable time-dependent coefficient. The resulting dimensionless expression depends on the overall resistance factor, which is proportional to the average Peclet number of the process. A graphical analysis shows that the oscillatory transient originating from the periodic component of the bottom elevation pattern is enhanced by reduced global flow resistance, while relatively more intense tracer spreading is associated with wavelike profiles affected by persistent trendless random noise, which also determines the characteristic time needed by the plume to achieve the asymptotic domain. Numerical simulations, validating the first-order analytical approach in a wide range of heterogeneous bed geometries, are also discussed. A semianalytical procedure is finally suggested for the study of depth-averaged transport processes in real three-dimensional streams, based on the use of the derived dispersion coefficient.

Transient electroluminescence and anomalous dispersion of charge carriers in thin polymer films

The electroluminescence and charge carrier transport in an organic light-emitting diode based on a thin polymer film are investigated. The initial kinetics of transient electroluminescence is analyzed theoretically. Both the theoretical and experimental curves are characterized by an anomalous dispersion and universality, which can be considered a consequence of the nonequilibrium field-assisted dispersion of charge carriers. It is demonstrated that the drift mobility of charge carriers should be determined from the time corresponding to the half-height of the steady-state electroluminescence intensity rather than from the delay time of the electroluminescence.

Measurement of Solute Transport in the Endothelial Glycocalyx Using Indicator Dilution Techniques

A new method is presented to quantify changes in permeability of the endothelial glycocalyx to small solutes and fluid flow using techniques of indicator dilution. Following infusion of a bolus of fluorescent solutes (either FITC or FITC conjugated Dextran70) into the rat mesenteric circulation, its transient dispersion through post-capillary venules was recorded and analyzed offline. To represent dispersion of solute as a function of radial position in a microvessel, a virtual transit time (VTT) was calculated from the first moment of fluorescence intensity-time curves. Computer simulations and subsequent in vivo measurements showed that the radial gradient of VTT within the glycocalyx layer (Delta VTT/Delta r) may be related to the hydraulic resistance within the layer along the axial direction in a post-capillary venule and the effective diffusion coefficient within the glycocalyx. Modeling the inflammatory process by superfusion of the mesentery with 10(-7) M fMLP, Delta VTT/Delta r was found to decrease significantly from 0.23 +/- 0.08 SD s/microm to 0.18 +/- 0.09 SD s/microm. Computer simulations demonstrated that Delta VTT/Delta r is principally determined by three independent variables: glycocalyx thickness (delta), hydraulic resistivity (K(r)) and effective diffusion coefficient of the solute (D(eff)) within the glycocalyx. Based upon these simulations, the measured 20% decrease in Delta VTT/Delta r at the endothelial cell surface corresponds to a 20% increase in D(eff) over a broad range in K(r), assuming a constant thickness delta. The absolute magnitude of D(eff) required to match Delta VTT/Delta r between in vivo measurements and simulations was found to be on the order of 2.5 x 10(-3) x D(free), where D(free) is the diffusion coefficient of FITC in aqueous media. Thus the present method may provide a useful tool for elucidating structural and molecular alterations in the glycocalyx as occur with ischemia, metabolic and inflammatory events.

An efficient approach to transient turbulent dispersion modeling by CFD–statistical analysis of a many-puff system

Transient turbulent dispersion phenomena can be found in various practical problems, such as the accidental release of toxic chemical vapor and the airborne transmission of infectious droplets. Computational fluid dynamics (CFD) is an effective tool for analyzing such transient dispersion behaviors. However, the transient CFD analysis is often computationally expensive and time consuming. In the present study, a computationally efficient CFD–statistical hybrid modeling method has been developed for studying transient turbulent dispersion. In this method, the source emission is represented by emissions of many infinitesimal puffs. Statistical analysis is performed to obtain first the statistical properties of the puff trajectories and subsequently the most probable distribution of the puff trajectories that represent the macroscopic dispersion behaviors. In two case studies of ambient dispersion, the numerical modeling results obtained agree reasonably well with both experimental measurements and conventional k–ε modeling results published in the literature. More importantly, the proposed many-puff CFD-statistical hybrid modeling method effectively reduces the computational time by two orders of magnitude.

CDC2/SPDY transiently associates with endoplasmic reticulum exit sites during oocyte maturation.

BackgroundMammalian oocytes acquire competence to be fertilized during meiotic maturation. The protein kinase CDC2 plays a pivotal role in several key maturation events, in part through controlled changes in CDC2 localization. Although CDC2 is involved in initiation of maturation, a detailed analysis of CDC2 localization at the onset of maturation is lacking. In this study, the subcellular distribution of CDC2 and its regulatory proteins cyclin B and SPDY in combination with several organelle markers at the onset of pig oocyte maturation has been investigated.ResultsOur results demonstrate that CDC2 transiently associates with a single domain, identified as a cluster of endoplasmic reticulum (ER) exit sites (ERES) by the presence of SEC23, in the cortex of maturing porcine oocytes prior to germinal vesicle break down. Inhibition of meiosis resumption by forskolin treatment prevented translocation of CDC2 to this ERES cluster. Phosphorylated GM130 (P-GM130), which is a marker for fragmented Golgi, localized to ERES in almost all immature oocytes and was not affected by forskolin treatment. After removal of forskolin from the culture media, the transient translocation of CDC2 to ERES was accompanied by a transient dispersion of P-GM130 into the ER suggesting a role for CDC2 in redistributing Golgi components that have collapsed into ERES further into the ER during meiosis. Finally, we show that SPDY, rather than cyclin B, colocalizes with CDC2 at ERES, suggesting a role for the CDC2/SPDY complex in regulating the secretory pathway during oocyte maturation.ConclusionOur data demonstrate the presence of a novel structure in the cortex of porcine oocytes that comprises ERES and transiently accumulates CDC2 prior to germinal vesicle breakdown. In addition, we show that SPDY, but not cyclin B, localizes to this ERES cluster together with CDC2.