A numerical study on the effect of static magnetic field on the hemodynamics of magnetic fluid in biological porous media

Journal of Thermal Analysis and Calorimetry - High interstitial fluid pressure in the tumor is among the most important barriers to drug delivery. The use of the static magnetic field is one of the...     

The effect of matrix structure on the diffusion of fluids in porous media

The effect of matrix structure on the transport properties of adsorbed fluids is studied using computer simulations and percolation theory. The model system consists of a fluid of hard spheres diffusing in a matrix of hard spheres fixed in space. Three different arrangements of the fixed spheres, random, templated, and polymeric, are investigated. For a given matrix volume fraction the diffusion coefficient of the fluid, D, is sensitive to the manner in which the matrix is constructed, with large differences between the three types of matrices. The matrix is mapped onto an effective lattice composed of vertices and bonds using a Voronoi tessellation method where the connectivity of bonds is determined using a geometric criterion, i.e., a bond is connected if a fluid particle can pass directly between the two pores the bond connects, and disconnected otherwise. The percolation threshold is then determined from the connectivity of the bonds. D displays universal scaling behavior in the reduced volume fraction, i.e., D approximately (1-phi(m)phi(c))(gamma), where phi(m) is the matrix volume fraction and phi(c) is the matrix volume fraction at the percolation threshold. We find that gamma approximately 2.2, independent of matrix type, which is different from the result gamma approximately 1.53 for diffusion in lattice models, but similar to that for conduction in Swiss cheese models. Lattice simulations with biased hopping probabilities are consistent with the continuous-space simulations, and this shows that the universal behavior of diffusion is sensitive to details of local dynamics.     

The rheology of pseudoplastic fluids in porous media using network modeling

This paper considers the rheology of pseudoplastic (shear thinning) fluids in porous media. The central problem studied is the relationship between the viscometric behavior of the polymer solution and its observed behavior in the porous matrix. In the past, a number of macroscopic approaches have been applied, usually based on capillary bundle models of the porous medium. These simplified models have been used along with constitutive equations describing the fluid behavior (usually of power law type) to establish semiempirical macroscopic equations describing the flow of non-Newtonian fluids in porous media. This procedure has been reasonably successful in correlating experimental results on the flow of polymer solutions through both consolidated and unconsolidated porous materials. However, it does not allow an interpretation of polymer flow in porous media in terms of the flows on a microscopic scale; nor does it allow us to predict changes in macroscopic behavior resulting from variations at a microscopic level in the characteristics of the porous medium such as pore size distribution. In this work, we use a network approach to the modeling of non-Newtonian rheology, in order to understand some of the more detailed features of polymjer flow in porous media. This approach provides a mathematical bridge between the behavior of the non-Newtonian fluid in a single capillary and the macroscopic behavior as deduced from the pressure drop-flow rate relation across the whole network model. It demonstrates the importance of flow redistribution within the elements of the capillary network as the overall pressure gradient varies. As an example of a pseudoplastic fluid in a porous medium, we consider the flow of xanthan biopolymer. This polymer is important as a displacing fluid viscosifier in enhanced oil recovery applications and, for that reason, a considerable amount of experimental data has been published on the flow of xanthan solutions in various porous media.     

Three-dimensional crystal alignment using a time-dependent elliptic magnetic field

A theoretical study has been presented to show that it is possible to align three different diamagnetic susceptibility axes (chi(3) < chi(2) < chi(1) < 0) of a crystallite with respect to the laboratory coordinate system (x, y, z). The time-dependent magnetic field that periodically changes in direction and intensity on the xy plane in an elliptic manner (the intensity stronger in the x direction) at a rate quicker than the intrinsic rate of magnetic response causes the three-dimensional alignment, that is, chi(1) parallel x, chi(2) parallel y, and chi(3) parallel z. The fluctuation of the three susceptibility axes around the corresponding laboratory coordinates is estimated in terms of the fluctuation around the minimum of the time-averaged magnetic potential. This technique enables the three-dimensional alignment of the crystallographic axes.     

Two-fluid model for the simultaneous flow of colloids and fluids in porous media

To describe the velocities of particles such as ions, protein molecules and colloids dispersed or dissolved in a fluid, it is important to also describe the forces acting on the fluid, including pressure gradients and friction of the fluid with the particles and with the porous media through which the fluid flows. To account for this problem, the use of a two-fluid model is described, familiar in the field of fluid mechanics, extended to include osmotic effects. We show how familiar relationships follow in various situations and give examples of combined fluid/particle transport in neutral and charged membranes driven by a combination of electrostatic, diffusional and pressure forces. The analysis shows how the same modeling framework can be generally used both for multidimensional electrokinetic flow through macroscopic channels and around macroscopic objects, as well as for mean-field modeling of transport through porous media such as gels and membranes.     

Nuclear magnetic resonance diffusion with surface relaxation in porous media

Nuclear magnetic resonance (NMR) diffusion simulations with surface relaxation were performed numerically in unconsolidated and consolidated porous media by a random walk technique. Two uniform and nonuniform models of surface relaxation were proposed and compared. The apparent diffusion coefficient and extinction function were determined and studied in the fast, slow and intermediate diffusion regimes of relaxation. According to theoretical predictions, it was observed that the extinction function does not depend on surface relaxivity parameter rho 2 in the slow diffusion regime. The apparent diffusion coefficients are independent of rho 2 in the fast diffusion regime and tend to be superposed onto a single curve in the slow one. The evolution of the apparent diffusion coefficients is gathered by a reduced representation in the fast diffusion regime.     

Vapor-liquid transitions of dipolar fluids in disordered porous media: performance of angle-averaged potentials

Using replica integral equations in the reference hypernetted-chain (RHNC) approximation we calculate vapor-liquid spinodals, chemical potentials, and compressibilities of fluids with angle-averaged dipolar interactions adsorbed to various disordered porous media. Comparison with previous RHNC results for systems with true angle-dependent Stockmayer (dipolar plus Lennard-Jones) interactions indicate that, for a dilute hard sphere matrix, the angle-averaged fluid-fluid (ff) potential is a reasonable alternative for reduced fluid dipole moments m( *2)=mu(2)/(epsilon(0)sigma(3))< or =2.0. This range is comparable to that estimated in bulk fluids, for which RHNC results are presented as well. Finally, results for weakly polar matrices suggest that angle-averaged fluid-matrix (fm) interactions can reproduce main features observed for true dipolar (fm) interactions such as the shift of the vapor-liquid spinodals towards lower temperatures and higher densities. However, the effective attraction induced by dipolar (fm) interaction is underestimated rather than overestimated as in the case of angle-averaged ff interactions.     

Thermal performance assessment of a flat-plate solar collector considering porous media,hybrid nanofluid and magnetic field effects

Journal of Thermal Analysis and Calorimetry - In this work, a numerical study of thermal performance enhancement of a flat plate solar collector has been presented. The collector channel is...     

The effect of magnetic field on conductive films

Conductive paints consisting of nickel powder (conductive material), solvent, and binder polymer were treated in a magnetic field. The efficiency of magnetic treatments on conductivity of coating films was evaluated. The effect of the following factors on conductivity of composite films by magnetic treatment was studied: metal powder concentration, magnetic flow density, time difference between film preparation and magnetic treatment, drying time of paint films, and effect of distance between terminals. Results showed that the volume resistivity of paint films treated magnetically was lower than that for untreated films at each nickel content. Magnetic treatment provided high conductivity even at low magnetic flow density, and conductivity increased with magnetic flow density.     

Measurement of gas diffusion coefficient in liquid-saturated porous media using magnetic resonance imaging

In this study, the dual-chamber pressure decay method and magnetic resonance imaging (MRI) were used to dynamically visualize the gas diffusion process in liquid-saturated porous media, and the relationship of concentration-distance for gas diffusing into liquid-saturated porous media at different times were obtained by MR images quantitative analysis. A non-iterative finite volume method was successfully applied to calculate the local gas diffusion coefficient in liquid-saturated porous media. The results agreed very well with the conventional pressure decay method, thus it demonstrates that the method was feasible of determining the local diffusion coefficient of gas in liquid-saturated porous media at different times during diffusion process.     

Time-resolved, optically detected NMR of fluids at high magnetic field

We report on the use of optical Faraday rotation to monitor the nuclear-spin signal in a set of model (19)F- and (1)H-rich fluids. Our approach integrates optical detection with high-field, pulsed NMR so as to record the time-resolved evolution of nuclear-spins after rf excitation. Comparison of chemical-shift-resolved resonances allows us to set order-of-magnitude constrains on the relative amplitudes of hyperfine coupling constants for different bonding geometries. When evaluated against coil induction, the present detection modality suffers from poorer sensitivity, but improvement could be attained via multipass schemes. Because illumination is off-resonant i.e., the medium is optically transparent, this methodology could find extensions in a broad class of fluids and soft condensed matter systems.     

Low-frequency magnetic field effect on cytoskeleton and chromatin

The effect of magnetic fields on the living systems is studied in vivo or in vitro in very broad spectrum of organisms, cells and tissues. The mechanism of their acting is not known until now. We studied low-frequency magnetic field effect on cytoskeleton and on the structure of chromatin in human cells. We used cell line of small lung carcinoma (A549) and the effects of magnetic field on cytoskeleton and higher-order chromatin structure were analyzed 96 h of magnetic field exposure. Magnetic field generated by the cylindrical soil was homogenous and the cells were cultivated at 37 degrees C in humidified atmosphere containing 5% CO(2). Magnetic field induction was B(m)=2 mT and the net frequency f=50 Hz. In such affected and control cells the F-actin was estimated using FITC-conjugated Phalloidin and mitochondria were studied using MitoTracker (Molecular Probes). Images of cytoskeleton and genetic loci were acquired using confocal microscopy and analysis was performed by FISH 2.0 software. Slight morphological changes of F-actin filaments and mitochondria were observed in affected cells and nuclear condensation was found. These effects could be related to the process of cell death apoptosis probably induced by magnetic field. The studies aimed at centromeric heterochromatin (9cen) did not show statistically significant changes. Therefore, we suggest that magnetic field has no influence on higher order chromatin structure but certain changes could be observed on the level of cytoskeleton. However, these statements need a thorough verification. Our preliminary experiments will be extended and the effect of magnetic field on another structures of cytoskeleton and cell nuclei will be further studied.     

Mean field kinetic theory for a lattice gas model of fluids confined in porous materials

We consider the mean field kinetic equations describing the relaxation dynamics of a lattice model of a fluid confined in a porous material. The dynamical theory embodied in these equations can be viewed as a mean field approximation to a Kawasaki dynamics Monte Carlo simulation of the system, as a theory of diffusion, or as a dynamical density functional theory. The solutions of the kinetic equations for long times coincide with the solutions of the static mean field equations for the inhomogeneous lattice gas. The approach is applied to a lattice gas model of a fluid confined in a finite length slit pore open at both ends and is in contact with the bulk fluid at a temperature where capillary condensation and hysteresis occur. The states emerging dynamically during irreversible changes in the chemical potential are compared with those obtained from the static mean field equations for states associated with a quasistatic progression up and down the adsorption/desorption isotherm. In the capillary transition region, the dynamics involves the appearance of undulates (adsorption) and liquid bridges (adsorption and desorption) which are unstable in the static mean field theory in the grand ensemble for the open pore but which are stable in the static mean field theory in the canonical ensemble for an infinite pore.     

Emulsion particle size in porous media and its effect on the displacement efficiency

The emulsification process in reservoirs was simulated using core displacement experiments. Emulsions with different particle sizes were prepared using different permeability cores, and the different emulsion particle sizes formed using different permeability cores were studied. The emulsion particle size was graded against the core throat diameter, and the displacement efficiency of the different particle size emulsions was studied. The displacement mechanism of the emulsion was analyzed. The results indicated that the emulsion is mostly pore-throat scale formed in the porous media, with a particle size distribution similar to that of the core throat diameter, and the emulsion particle size increases with the increase of core permeability. The recovery percentage of emulsion flooding is greater when the matching ability of the emulsion was favorable, it is 17.07% when the matching factor is 1.08. The pore-throat scale emulsion can block high permeability zones and expand sweep volume. Moreover, due to deformation of the emulsion, the elastic stress can make the residual oil migrate forward and improve the efficiency of oil displacement.     

Simulation of nonwetting phase entrapment within porous media using magnetic resonance imaging

Models representing the pore structures of amorphous, mesoporous silica pellets have been constructed using magnetic resonance images of the materials. Using magnetic resonance imaging (MRI), maps of the macroscopic (approximately 0.01-1 mm) spatial distribution of porosity and pore size were obtained. The nature and key parameters of the physical mechanism for mercury retraction, during porosimetry experiments on the silica materials, were determined using integrated gas sorption experiments. Subsequent simulations of mercury porosimetry within the structural models derived from MRI have been used to successfully predict, a priori, the point of the onset of structural hysteresis and the final levels of mercury entrapment for the silicas. Hence, a firm understanding of the physical processes of mercury retraction and entrapment in these amorphous silica materials has been established.     

Relativistic hydrodynamics of magnetic and dielectric fluids in interaction with the electromagnetic field

The semirelativistic hydrodynamic equations of motion of de Groot and Mazur [Non-Equilibrium Thermodynamics (North-Holland, Amsterdam, 1962)] for a fluid with polarization and magnetization are derived from the relativistic energy-momentum conservation equation, mass conservation, and Maxwell's equations on the basis of a systematic expansion in inverse powers of the velocity of light. A modification of the de Groot-Suttorp field energy-momentum tensor [de Groot and Suttorp, Foundations of Electrodynamics (North-Holland, Amsterdam, 1972)] based on a shift of terms to the internal energy density of the fluid yields relatively simple expressions for energy density, energy current density, momentum density, and stress tensor.     

Nanosecond time-resolved magnetic field effect on radical recombination in solution

The time dependence of the magnetic field effect on radical recombination in solution has been analyzed experimentally and theoretically. For the geminate recombination of anthracene anions and dimethylaniline cations in a polar solvent, the effect originates from a magnetic field dependent production of triplet states in an initially singlet phased radical pair, induced by hyperfine interaction of the unpaired electrons with the nuclei. The magnetic field dependence of the triplet yield shows a lifetime broadening of the energy levels of the radical pair if a short delay-time between radical production and triplet observation is chosen. The agreement of this delay-time dependent broadening effect with the theoretical results proves directly the coherence of the spin motion in the radical pairs.     

The effect of magnetic field on the band structure of polymers Part 1. Static magnetic field

The Hartree-Fock equations for a periodic polymer chain in the presence of a static magnetic field are formulated. The cases of homogeneous and inhomogeneous static field are treated separately. In the case where the magnetic field strength is dependent on the coordinate z (the direction of the main axis of the polymer), the periodic symmetry of the polymer breaks down. For the case of weak z dependence of the magnetic field the chain can be divided into segments, each one characterized by an averge field, and the interface of these segments can be treated with the help of two Dyson equations (one for each segment at the interface).     

Separation of different physical forms of plasmid DNA using a combination of low electric field strength and flow in porous media: effect of different field gradients and porosity of the media

The retention of different physical forms of DNA by an electric field in a chromatography system was studied. We were able to effectively separate the supercoiled and the open circular forms of plasmid DNA using this type of electrochromatography system. Chromatography columns were packed with porous beads, and an axial electric field was applied so that convective buffer flow opposed the direction of electrophoresis of the DNA. A model system composed of approximately equal amounts of the super-coiled and open circular forms of the plasmid pBR 322 (4322 base pairs) was used to test the separation. Chromatography beads (agarose-based) with different porosities were used to determine the effect of the stationary phase on the separation. The porous media did not have a major effect on the separation, but the best separations were obtained using porous chromatography media made with the highest agarose concentration (10% agarose). Selective elution of plasmid DNA with different forms was obtained by either increasing the flow rates or decreasing the electric field strength (by steps or a gradient). In all the separations, the more compact supercoiled form of the plasmid was retained less strongly than either the open circular form (nicked) or the linear form. High molecular weight host genomic DNA was more strongly retained than the plasmid DNA. Increasing the ionic strength of the buffer improved resolution and capacity. The capacity of the separation was determined by injecting increasing amounts of plasmid DNA. Satisfactory separation was obtained at sample loading of up to 360 microg of total DNA on a column with dimensions of 2.5 by 11 cm (bed volume of 54 mL). The retention of DNA depends upon a counter-current flow of electrophoresis and convective flow and could be regarded as a type of field flow fractionation. The retention of the DNA by the electric field and flow is discussed in relation to the diffusion coefficients of the DNA.