Macro Dispersion Research Articles

PMMA/SWCNT Composites with Very Low Electrical Percolation Threshold by Direct Incorporation and Masterbatch Dilution and Characterization of Electrical and Thermoelectrical Properties.

In the present study, Poly(methyl methacrylate) (PMMA)/single-walled carbon nanotubes (SWCNT) composites were prepared by melt mixing to achieve suitable SWCNT dispersion and distribution and low electrical resistivity, whereby the SWCNT direct incorporation method was compared with masterbatch dilution. An electrical percolation threshold of 0.05-0.075 wt% was found, the lowest threshold value for melt-mixed PMMA/SWCNT composites reported so far. The influence of rotation speed and method of SWCNT incorporation into the PMMA matrix on the electrical properties and the SWCNT macro dispersion was investigated. It was found that increasing rotation speed improved macro dispersion and electrical conductivity. The results showed that electrically conductive composites with a low percolation threshold could be prepared by direct incorporation using high rotation speed. The masterbatch approach leads to higher resistivity values compared to the direct incorporation of SWCNTs. In addition, the thermal behavior and thermoelectric properties of PMMA/SWCNT composites were studied. The Seebeck coefficients vary from 35.8 µV/K to 53.4 µV/K for composites up to 5 wt% SWCNT.

Analytical solutions for contaminant fate and transport in parallel plate fracture-rock matrix systems with poiseuille flow

Modeling contaminant transport in fractured-rock matrix systems often approximates the effect of the parabolic flow field in the fractures (i.e., Poiseuille flow) on transport by adding a dispersion term to the uniform flow field. In this study, an analytical solution is derived to model contaminant transport in a parallel-plate fractured-rock matrix that explicitly simulates Poiseuille flow in the fractures, eliminating the need for the dispersion approximation. In addition to simulating Poiseuille flow in the fracture, the contaminant transport model developed here includes: (1) two-dimensional contaminant diffusion in the fractures and matrix, (2) first-order decay in the aqueous phase, and (3) rate-limited sorption onto matrix solids. It should be noted, however, that this model, much like the commonly employed Taylor dispersion approximation, neglects macro dispersion, thereby limiting the model’s applicability to systems having wide fracture apertures with extremely high flow velocities (Pe > 104). Model equations are analytically solved in the Laplace domain and numerically inverted. In addition, analytical expressions for the zeroth, first, and second spatial moments of the concentration profiles along the fractures are derived for both the new Poiseuille flow model as well as a model that approximates the effect of Poiseuille flow on transport by using a dispersion term. The first and second moment expressions are used to quantify how well the dispersion term approximates the effect of Poiseuille flow. Simulations confirm that the dispersion approximation will be adequate for natural fractures at long times. However, if a modeler is concerned with short-time transport behavior or transport behavior in systems with relatively wide-aperture fractures and high groundwater velocities where macro dispersion can be ignored, such as may be found at engineered geothermal systems and carbon capture and storage sites, there may be significant differences between model simulations that explicitly incorporate Poiseuille flow and those that approximate Poiseuille flow with a dispersion term. The model presented here allows the modeler to analytically quantify these differences, which, depending on the modeling objective, may cause the dispersion approximation to be inadequate. Simulations were also run to examine the effect of adsorption rate on remediation of fractured-rock matrix systems. It was shown that moderate adsorption rate constants could lead to very long remediation times, if remediation success is quantified by achieving low concentrations within the fracture.

Application of Terahertz Spectroscopy to Rubber Products: Evaluation of Vulcanization and Silica Macro Dispersion

Industrial applications of terahertz (THz) technology are becoming more widespread. In particular, novel evaluation methods for essential rubber products are being developed. THz absorbance spectra of various rubber polymers and reagents enable visualization of filler dispersions and vulcanization reactions. Here, improved visualization of the vulcanization reaction in thick rubber samples is discussed. Silica macro-dispersion is also analyzed because it is a general filler in automobile tires and has been difficult to evaluate with conventional techniques.

Spatial moment analysis of solute transport with Langmuir sorption in a fracture–skin–matrix coupled system

Sorption is one of the key processes that plays a major role in the transport of contaminants in fractured porous media. Extensive studies have been conducted on sorption isotherms in fracture matrix coupled system but studies pertaining to sorption in fractured porous media with fracture–skin are very limited. In this study, a numerical model is developed for analysing the influence of sorption intensities on velocity, macro dispersion coefficient and dispersivity using the method of spatial moments. Implicit finite difference numerical technique has been used to solve the coupled non-linear governing equations. A varying grid is adopted at the fracture and skin interface to capture the mass transfer at the interface. Results suggest that the role of non-linear sorption is dominant in comparison with that of advection and dispersion in deciding the final relative concentration within the fracture. The role of sorption partition coefficients is not always enhancing the mixing phenomena which lead to dilution of solutes. Furthermore, the role of sorption partition coefficients is extremely sensitive in the sense that the resultant magnitude of effective dispersivity may either get enhanced or mitigated depending on the magnitude of sorption partition coefficients.

Analysis tools for fibrous nanofiller polymer composites: Macro- and nanoscale dispersion assessments correlated with mechanical and electrical composite properties

Emerging methods to study the nanofiller agglomeration state (macrodispersion) and level of individualized nanofiller (nanodispersion) within polymer nanocomposites have been used separately to characterize microstructure and relate to material properties. However, nanofiller dispersion characteristics on different scales within polymer nanocomposites may affect the composite properties in different ways and their complement has not been studied quantitatively. In this study, the dispersion of carbon nanofiber in polycarbonate, controlled by sonication conditions, was studied by the combination of macro and nanoscale style quantitative dispersion characterization tools. The quantitative dispersion results were then correlated with the electrical and mechanical properties of the nanocomposites. Key quantitative indicators of performance were identified for nanocomposite macrodispersion such as the shape of the agglomerate size distributions and number of agglomerates per volume. However, the nanodispersion style analysis demonstrated more significant differences between similarly macrodispersed systems, providing vital information required to assess nanofiller network formation potential. The results indicate the relevant use of differently scaled image analysis processes and their necessity for a comprehensive understanding of the dispersion as well as the properties of nanocomposites. POLYM. COMPOS., 35:10–18, 2014. © 2013 Society of Plastics Engineers

Hydraulic conductivity fields: Gaussian or not?

Hydraulic conductivity (K) fields are used to parameterize groundwater flow and transport models. Numerical simulations require a detailed representation of the K field, synthesized to interpolate between available data. Several recent studies introduced high resolution K data (HRK) at the Macro Dispersion Experiment (MADE) site, and used ground-penetrating radar (GPR) to delineate the main structural features of the aquifer. This paper describes a statistical analysis of these data, and the implications for K field modeling in alluvial aquifers. Two striking observations have emerged from this analysis. The first is that a simple fractional difference filter can have a profound effect on data histograms, organizing non-Gaussian ln K data into a coherent distribution. The second is that using GPR facies allows us to reproduce the significantly non-Gaussian shape seen in real HRK data profiles, using a simulated Gaussian ln K field in each facies. This illuminates a current controversy in the literature, between those who favor Gaussian ln K models, and those who observe non-Gaussian ln K fields. Both camps are correct, but at different scales.

Hydrostratigraphic analysis of the MADE site with full-resolution GPR and direct-push hydraulic profiling

[1] Full-resolution 3D Ground-Penetrating Radar (GPR) data were combined with high-resolution hydraulic conductivity (K) data from vertical Direct-Push (DP) profiles to characterize a portion of the highly heterogeneous MAcro Dispersion Experiment (MADE) site. This is an important first step to better understand the influence of aquifer heterogeneities on observed anomalous transport. Statistical evaluation of DP data indicates non-normal distributions that have much higher similarity within each GPR facies than between facies. The analysis of GPR and DP data provides high-resolution estimates of the 3D geometry of hydrostratigraphic zones, which can then be populated with stochastic K fields. The lack of such estimates has been a significant limitation for testing and parameterizing a range of novel transport theories at sites where the traditional advection-dispersion model has proven inadequate.

Characterization of the Kinetics of Intercalation/Exfoliation Processes of Nanoclay in Polyamide

Nanoclay dispersion in polyamide (PA) matrix has been investigated using a newly developed online measured electrical conductance (OMEC) method. This is based on measuring the electrical conductivity during the mixing process in the chamber of the internal mixer. The obtained results have shown that the online conductance increases up to 60 × 10−6 S, which corresponds to the dispersion (intercalation/exfoliation) of nanoclay in the PA matrix. The results concerning macro and micro dispersion of nanoclay received from different experimental techniques, like optical microscopy, transmission electron microscopy, SAXS analysis, and stress strain behavior confirm this correlation. The degradation processes of polyamide and surfactants however, restrict the application of the online conductance method at high processing temperature.

Remediating Large Plumes: Overcoming the Scale Challenge

Introduction The remediation industry, now three decades old, continues to evolve. This is driven by many factors: technological developments, a need to address emerging contaminants, regulatory changes, economic factors, and sustainability considerations. These have created both challenges and opportunities at different stages of the industry’s evolution. Although many things have changed with time, one of the most difficult practical challenges remains—namely, how to deal with large dissolved plumes (referred to as “large plumes” throughout this article). Presumptive thinking emphasizes containment and management of these large plumes rather than focusing on mass removal and site closures. The term large plume obviously highlights scale as being the critically defining attribute; however, it is worth identifying other attributes that are discussed in this article:

The future of applied tracers in hydrogeology

Tracingtechniqueshave a broadapplicationinscience andhave demonstrated particular usefulness in hydrogeology.Applied tracers, which are defined as non-natural con-stituents that are intentionally introduced, are especiallypowerful investigative tools because the tracer application(or source term) is controlled and well characterized. Thispermits quantification of transport parameters and mea-surement of subsurface properties in a way often un-matched by standard physical methods. Furthermore,tracer tests directly measure properties in-situ and can beused to investigate very specific processes by selectingtracers with appropriate physicochemical properties. Inmany cases, tracer test methods offer the most accurate orpractical way to measure specific parameters, and in somecases, they are the only reliable investigative technique.Depending on the application, tracer tests can be used tocharacterize properties representative of large subsurfacevolumes or investigate small-scale transport phenomena.Applied tracers have been widely used for centuries tocharacterize flowpaths and estimate groundwater velocities.In fact, Kass (1998) notes that the Jewish historian FlaviusJosephus recorded in approximately 10 A.D. the use ofchaff as a tracer to link the spring source of the JordanRiver to a nearby pond. Quantitative tracer tests usingchloride, fluorescein, and bacteria were first conducted inthe large karst regions in Europe in the late 1800s and early1900s. After World War II, advances in chemical mea-surement technology permitted quantification of signifi-cantly lower tracer breakthrough concentrations and madehigh-frequency sampling economically feasible. Addition-ally, these technological advances lead to a significant in-crease in the diversity of constituents used as tracers.Historically, applied tracers in hydrogeology havebeen used mostly to characterize groundwater flow in kartregions. During the 1960s, benchmark studies exploitedapplied tracers to understand the controls on groundwaterrecharge (Horton and Hawkins 1965; Zimmerman et al.1966) and significantly advanced an understanding of theflowpaths of rainfall to the water table. During the past 30years, applied tracers have been used increasingly to un-derstand solute transport phenomena in porous media andfractured rock aquifers, motivated primarily by environ-mental concerns related to disposal of radioactive andother wastes. For example, the well-known large-scaletracer experiments conducted at the Borden, Cape Cod,and Macro Dispersion Experiment (MADE) sites in the1980s (Sudicky 1986; LeBlanc et al. 1991; Boggs et al.1992, respectively) were designed to compare observedfield-scale solute dispersion to macrodispersion predictedby stochastic analysis of independently-measured aquiferheterogeneity. These experiments have resulted in nu-merous important publications investigating the signifi-cance of local geologic heterogeneity, large-scale hy-draulic conductivity trends, sorption, and rate-limitedprocesses on solute transport. The results of these testshave also been used to evaluate the performance of nu-merical contaminant transport models.Tracing as a hydrogeologic investigative tool hasgrown significantly, and over the past decade, many newapplications of applied tracers have been developed toinvestigate advanced transport phenomena, includingmulti-species reactive transport, colloid-facilitated trans-port, pore-scale mixing, and fracture-matrix control. Thisincrease in tracer research is indicated by the increase inthe number of papers. The increase of papers published inHydrogeology Journal and other groundwater-relatedjournals over the past decade that incorporate either anapplied tracer technique as part of a broader hydrogeo-logical investigation or specifically develop a new use ofapplied tracers in hydrogeology. For example, Aggarwal

Exact transverse macro dispersion coefficients for transport in heterogeneous porous media

We study transport through heterogeneous media. We derive the exact large scale transport equation. The macro dispersion coefficients are determined by additional partial differential equations. In the case of infinite Peclet numbers, we present explicit results for the transverse macro dispersion coefficients. In two spatial dimensions, we demonstrate that the transverse macro dispersion coefficient is zero. The result is not limited on lowest order perturbation theory approximations but is an exact result. However, the situation in three spatial dimensions is very different: The transverse macro dispersion coefficients are finite – a result which is confirmed by numerical simulations we performed.

Post irradiation examination of uranium inclusions in inert matrices.

The inert matrix materials CeO 2, MgO, Y 2O 3, MgAl 2O 4 and Y 3Al 5O 12 were selected as candidates for inert matrices for the EFTTRA 2 2 Experimental Feasibility of Targets for TRAnsmutation, a European project in which CEA, NRG, EdF, FzK, JRC-ITU and JRC-IAM cooperate. -T3 neutron irradiation experiment. Most targets contain 20% enriched 235U fissile inclusions with an average size of roughly 150 μm. The volume fraction of the fissile phase is either 2.5 vol% UO 2 or 19.6 vol% of Y 5.78UO x in the inert matrices. The samples were irradiated for 198.9 full power days in the High Flux Reactor in Petten. The calculated burn-up is between 17.3 and 19.5% FIMA. The temperature of the cladding was kept at 600 ± 25 K. A dimensional change of at least +5 % is measured after neutron irradiation for Y 3Al 5O 12 and MgAl 2O 4 with macro dispersions of UO 2; the other targets with a macro dispersion of UO 2 show a volume change of less than + 1 vol%. The fractional release of the fission gas Xe is more than 40% for the MgAl 2O 4 and Y 3Al 5O 12 matrices with a macro dispersion of UO 2, the other targets show a fractional release of Xe of less than 15%. Cracks are observed in MgO and MgAl 2O 4 targets which is possibly related to the stress caused by swelling of the UO 2 inclusions.

Estimation of groundwater flow parameters using least squares

In this paper, we present several variations on the least squares approach of estimating steady-state flow parameters in a groundwater model. We test the approach on numerically generated data, and also apply it to field data from the MADE (MAcro Dispersion Experiment) data set obtained at Columbus AFB, Mississippi. Our results are based on a least squares cost functional with a finite difference scheme used to solve the steady-state flow equation (Darcy's Law). An implicit filtering optimization algorithm is implemented in parallel using PVM.

The symmetry approximation for nonsymmetric permeability tensors and its consequences for mass transport

It is well known that the permeability has a tensor character. In practical applications, this is accounted for by the introduction of three principal permeabilities — three scalars — and three mutually orthogonal principal axes. In this paper, it is investigated whether this is always the exact way of describing anisotropy and, if not, what the consequences of the principal axes approximation are for flow and transport. First, it is shown that spatial upscaling may result in nonsymmetric large-scale permeability tensors, for which principal axes do not exist. However, it is possible to define generalized principal axes: three principal axes for the flux and three for the pressure gradient, with only three principal permeabilities. Since nonsymmetric permeability tensors are undesirable in practical applications, an approximation method making the nonsymmetric permeability symmetric is introduced. The important conclusion is then that the exact large-scale flux and large-scale pressure gradient do not have the same directions as the approximate flux and approximate pressure gradient. A practical consequence is that the principal axes approximation results in a difference between flux and transport direction. When considering miscible displacement or transport of mass dissolved in groundwater, the velocity component normal to the flux direction may be considered as a contribution to the transverse macro dispersion.