Diffusion deposition of aerosol nanoparticles in model granular filters

The diffusion deposition of point aerosol particles from a flow in model granular (grained) filters, i.e., separate layers composed of parallel chains of spherical granules, has been studied at small Reynolds numbers. Numerical solution of the Stokes and convective diffusion equations has been employed to determine the drag forces and granule collection efficiencies as depending on the Peclet diffusion number in a range Pe = 0.02–2 × 10⁴ and the ratio between the granule diameter and the distance between chain axes. Layers of closed chains with square and hexagonal packings have been considered. Approximation formulas have been derived for calculation of nanoparticle penetration in model granular filters.     

Ultra-low background measurements of decayed aerosol filters

Aerosol samples collected on filter media were analyzed using HPGe detectors employing varying background-reduction techniques in order to experimentally evaluate the opportunity to apply ultra-low background measurement methods to samples collected, for instance, by the Comprehensive Test Ban Treaty International Monitoring System (IMS). In this way, realistic estimates of the impact of low-background methodology on the sensitivity obtained in systems such as the IMS were assessed. The current detectability requirement of stations in the IMS is 30 μBq/m³ of air for ¹⁴⁰Ba, which would imply ~10⁶ fissions per daily sample. Importantly, this is for a fresh aerosol filter. One week of decay reduces the intrinsic background from radon daughters in the sample allowing much higher sensitivity measurement of relevant isotopes, including ¹³¹I. An experiment was conducted in which decayed filter samples were measured at a variety of underground locations using Ultra-Low Background (ULB) gamma spectroscopy technology. The impacts of the decay and ULB are discussed.     

Transport processes through track-etch membrane filters in a reagent delivery cell

A reagent delivery cell with a track-etch membrane filter for on-line dilution of concentrated salt solutions is described. The influence of several system parameters such as concentration of the stock solution, temperature, transmembrane pressure and the dependence on the diffusion coefficients of several salt components on the dilution was evaluated. As an application example, the use of the reagent delivery cell for on-line calibration of an atomic absorption spectrometer was studied. Fluxes through the membrane filter of 10 to 50 nL mm^–2 min^–1 with relative standard deviations of 0.8% within a day and 1.9% from day to day were achieved. The permeation experiments with the track-etch membrane filter for the dilution of aqueous solutions of several chlorides and sodium salts confirm a diffusion process. Flux rates can be estimated mathematically using Fick’s first law with an agreement between measured and calculated dilution factors within 86 to 113%.     

Transport processes through track-etch membrane filters in a reagent delivery cell

A reagent delivery cell with a track-etch membrane filter for on-line dilution of concentrated salt solutions is described. The influence of several system parameters such as concentration of the stock solution, temperature. transmembrane pressure and the dependence on the diffusion coefficients of several salt components on the dilution was evaluated. As an application example, the use of the reagent delivery cell for on-line calibration of an atomic absorption spectrometer was studied. Fluxes through the membrane filter of 10 to 50 nL mm(-2) min(-1) with relative standard deviations of 0.8% within a day and 1.9% from day to day were achieved. The permeation experiments with the track-etch membrane filter for the dilution of aqueous solutions of several chlorides and sodium salts confirm a diffusion process. Flux rates can be estimated mathematically using Fick's first law with an agreement between measured and calculated dilution factors within 86 to 113%.     

Modeling of diffusive transport of benzoic acid through a liquid membrane

A model of diffusive transport of benzoic acid through a liquid membrane (LM) separating two aqueous solutions, based on diffusion layers and the assumption of a steady state, has been developed and tested using experimental results. It has been found that a model with the apparent partition coefficient dependent on the concentration is able to describe the time dependence of acid concentration in LM with and without a maximum on that dependence. The quality of the model fit with the single apparent diffusion coefficient of benzoic acid is the same as the one which takes into account the diffusion of benzoic acid in different forms (undissociated and dissociated form in aqueous phase, monomer and dimer in organic phase); however, in the second case, the model becomes overparameterized. Assuming that the partition and diffusion coefficients are constant, the diffusion layer model corresponds to the model of reversible consecutive reactions. Analytical solution for such case is given. Apart from the partition equilibrium, also kinetics of partitioning was considered. It was shown that in some basic situations both cases yield identical results.     

Membrane-Potential-induced electroporation as a physical mechanism for metal nanoparticle penetration through a cell membrane

A mechanism of the penetration of nanosized metal particles from an ambient solution into the cytoplasm of a living cell, has been proposed. The driving force of this new mechanism is membrane potential, i.e., the potential difference between the cellular cytoplasm and the ambient solution. The essence of the mechanism consists in the fact that a metal particle occurring at a cell membrane shunts the potential drop in the diffuse part of the electrical double layer of the membrane. As a result, almost the entire membrane potential, which, at a normal state of the cell, is distributed between its electrical double layer and the lipid bilayer of the cell membrane, appears to be completely applied to the latter. As a consequence, the field strength in the lipid bilayer rises, thereby increasing the probability of the formation of a pore in it, through which a metal particle with a diameter lying in a certain range (in the case under consideration, from two to three tens of nanometers) can penetrate into the cytoplasm without inflicting any damage on the cell.     

Inertial deposition of aerosol particles from laminar flows in fibrous filters

The deposition of aerosol particles onto filter fibers under the effect of inertial forces is studied in a wide range of Stokes numbers (St) at Reynolds numbers close to unity (Re ∼ 1). Coefficients η of the capture of inertial particles with finite sizes in model filters composed of parallel rows of identical parallel fibers located normal to the direction of a flow are determined based on the numerical solution of the Navier-Stokes and particle motion equations. It is shown that, at Re < 1 and a constant particle-to-fiber radius ratio, R = r _p/a, number St uniquely characterizes capture coefficients η for particles with different densities, while, at Re ≥ 1, the capture coefficient depends on both St and Re. At constant R and St values, the larger Re the higher the capture coefficient. The influence of the structure of the model filter on pressure drop Δp and η is investigated. A nonuniform arrangement of fibers in rows is shown to increase the Δp/U ratio at lower Re values and to make the η -St dependence more pronounced than that for systems of uniformly ordered fibers. The results of calculations agree with the experimental data.     

Modeling of the dynamic adsorption of an anionic dye through ion-exchange membrane adsorber

The present study examines the dynamic adsorption through ion-exchange membrane adsorbers. The model used in the study includes convection, axial dispersion with simultaneous adsorption and desorption of the solute in the membrane. Adsorption and desorption processes give the Langmuir isotherm for the equilibrium. The mathematical model makes use of dimensionless parameters in terms of characteristic times for the different mechanisms that take place during the process (convection, dispersion, adsorption and desorption characteristic times). The model has five independent dimensionless parameters. Three of these parameters are related to the equilibrium isotherm and the other two are related to the dynamic process. Equilibrium and dynamic experiments were carried out in order to fit their respective parameters. In order to examine the suitability of the model to describe real processes, the adsorption of an anionic dye (Orange-G) through the ion-exchange membrane adsorber was investigated as a function of dye and KCl concentration, obtaining strong correlation between fitted and experimental breakthrough curves. The results show the relative importance of axial dispersion, adsorption and desorption as a function of operational variables.     

Deposition of aerosol nanoparticles on filters coated with layer of carbon nanotubes

Problems concerning intensification of fine gas purification from suspended particles by coating filter fibers with permeable layers of carbon nanotubes, as well as problems relevant to the rebound of Brownian nanoparticles from nanofibers, are discussed. When passing nanoaerosols of sodium chloride through different filtering partitions, the internal surface of which is coated with a thin layer of carbon nanotubes, no peculiarities are observed for deposition of nanoparticles with diameters of larger than 3.5 nm.     

Deposition of aerosol nanoparticles in filters composed of fibers with porous shells

The diffusion deposition of submicron aerosol particles in model filters consisting of fibers covered with permeable porous shells is studied. An ordered system of parallel cylinders arranged perpendicular to the flow is used as a model filter. The results of calculations are given for the dependences of the capture coefficient on the shell radius, the shell permeability, the packing density of the filters, the particle radius, and the flow velocity. Calculations are performed within a wide range of Peclet numbers. It is shown that the capture coefficient and the quality criterion γ of a filter increase with the diffusion mobility of particles and shell permeability, as well as that the dependence of the quality criterion on the radius of permeable shells has a maximum. It is also shown that the capture coefficients for fibers with porous shells, calculated using the cell model and the isolated row of fibers, almost coincide with one another.     

Modeling of thermal conduction in granular silica aerogels

Monolithic silica aerogels of large sizes are difficult to synthesize and manipulate. Granular form is the easiest way of conditioning them. One of the most promising applications is probably thermal superinsulation applied to the building sector. To understand and quantify thermal conduction in granular silica aerogels, numerical simulations are necessary. Our method is based on two steps: determining properties of monoliths at the nanopore scale and applying them to macroscopic grain packings. The two-dimensional heat diffusion equation is applied to periodic fractal patterns representing an ideal nanoporous medium made of two phases (silica and air) in order to infer macroscopic effective properties of monolithic silica aerogels. The same equation is then applied to granular aerogels. Grains are represented by regular cubic or hexagonal packings of spheres. The thermal contact resistance between grains is taken into account in an original analytical way.     

Microwave-assisted extraction of metal elements from glass fibrous filters for aerosol sampling

Atmospheric aerosols are generally collected on filters according to the International Monitoring System (IMS) designed in the Comprehensive Nuclear-Test-Ban Treaty (CTBT). More information could be revealed when the filter sample is pretreated rather than measured directly by g-ray spectrometer. Microwave-assisted extraction (MAE) is a suitable method that gives higher recoveries of elements from glass fibrous filters under different conditions. The results indicate that the MAE is a highly efficient and robust method for the treatment of glass fibrous filter samples. The recoveries of potential fission products from glass fibrous filter samples by microwave-assisted extraction meet the efficiency of the extraction by both aqua regia and 2% HCl.     

Filtration of nanoaerosols through a granular layer

Diffusion deposition of nanoparticles in model granular filters with different structures has been considered at low Reynolds numbers. On the basis of a three-dimensional flow field calculated for layers of granules, nanoparticle-collection efficiences have been determined for the granules in a wide range of the Peclet diffusion numbers. The interference of the layers has been studied, and approximation equations have been derived for calculating the pressure drop and the nanoparticle-collection efficiency.     

Stokes flow and deposition of aerosol nanoparticles in model filters composed of elliptic fibers

The calculation is implemented for the fiber collection efficiencies due to diffusion of nanoparticles in model filters, i.e., separate rows of fibers with an elliptic cross section located normal to the flow at different orientations of the ellipse axes with respect to the flow. The Stokes flow field in the system of the fibers is found by the method of fundamental solutions. The concentration field of Brownian particles and the efficiency of their deposition onto the fibers are determined from the numerical solution of the equation for the convective diffusion. The dependence of the capture coefficient on the Peclet number for elliptic fibers is shown to have the form η = APe^−m, where exponent m changes from 2/3 to 3/4 at the parallel and normal orientation of the major axes of the ellipses with respect to the flow, respectively. It is shown that, from the viewpoint of aerosol nanoparticle capture, the best filters are those in which the fibers have a maximum midsection at the same cross-sectional area.     

Measurement of filtering efficiency of artificial radioactive aerosol filter membrane

Journal of Radioanalytical and Nuclear Chemistry - When monitoring radioactive aerosol in the atmosphere, choosing a filter membrane with better surface collection characteristics and filtering...     

Modeling of spontaneous penetration of viscoelastic fluids and biofluids into capillaries

A theoretical model was developed to describe the dynamics of spontaneous penetration of viscoelastic fluids into capillaries. The model agrees quantitatively with recent experiments on absorption of droplets of polymer solutions by glass capillaries [A.V. Bazilevsky, K.G. Kornev, A.N. Rozhkov, A.V. Neimark, J. Colloid Interface Sci. (2003)]. The rate of penetration progressively reduces with the increase in fluid elasticity. Analysis revealed two main contributions to the viscoelastic drag of the liquid column: (i) viscous resistance, which is independent of fluid elasticity, and (ii) viscoelastic resistance, known as the Weissenberg effect. We analytically derived an augmented Bosanquet equation for the maximal velocity of penetration by balancing capillary, inertia, and viscoelastic forces. For slow creep of a liquid column, the Lucas-Washburn equation was modified by accounting for the Weissenberg effect. A series of numerical calculations were performed to demonstrate characteristic features of absorption of fluids at different conditions. This article also discusses some problems specific to absorption of biofluids. We show that deformations of cell membranes in the external converging flow may cause their rupture at the pore entrance.     

Electron transport through human epithelial amniotic membrane in vitro: Comparative study with artificial membranes (bilayer and milipore filters)

The transfer of solute and water across the amniotic, in vico, is controlled by hydraulic, osmotic and electrochemical factors. In this study the electrochemical component is specified in vitro, and the mechanism of ion transport across the amnion is characterized by comparison with artificial membranes(bilayers and Teflon milipore filters). The electrochemical study shows that the amniotic conductance depends on time and salt concentration; the cationic transference number is comprised between 0.75 and 0.85; the electro-osmotic potentials are negative; the I(V) curves are linear; the activation conductance energy ranges between 4 and 8 kcal degree⁻¹ mole⁻¹ and the cation selectivity sequence is: Rb=Cs=K>Na>Li. The pharmacological agents (oubain, amiloride, dinitrophenol) decrease gNa. These results suggest: (1) that the amnion has fixed neutral sites in external epithelial cell membranes and on intercellular channels with, under cetain conditions, negative cites in greater number than the positive sites; (2) that the amnion is comparable to negative artificial membranes; (3) that the paracellular pathway is more important that the transcellular pathway; (4) that there are specific sites on the external membranes and in intercellular channels.     

Modeling the pharmacodynamics of passive membrane permeability

Small molecule permeability through cellular membranes is critical to a better understanding of pharmacodynamics and the drug discovery endeavor. Such permeability may be estimated as a function of the free energy change of barrier crossing by invoking the barrier domain model, which posits that permeation is limited by passage through a single “barrier domain” and assumes diffusivity differences among compounds of similar structure are negligible. Inspired by the work of Rezai and co-workers (JACS 128:14073–14080, 2006), we estimate this free energy change as the difference in implicit solvation free energies in chloroform and water, but extend their model to include solute conformational affects. Using a set of eleven structurally diverse FDA approved compounds and a set of thirteen congeneric molecules, we show that the solvation free energies are dominated by the global minima, which allows solute conformational distributions to be effectively neglected. For the set of tested compounds, the best correlation with experiment is obtained when the implicit chloroform global minimum is used to evaluate the solvation free energy difference.     

Modeling the induction of lipid membrane electropermeabilization

Experiments show significant effects of an electric field on lipid membrane, leading to a pore formation when a high intensity field is applied. The phenomenon of electroporation is preceded by the induction and expansion of defects, responsible for the pre-pore excitation. We examine the mechanism of the induction of the field-driven defects by Monte Carlo simulations. The study is based on the improved Pink's model, which includes explicit interactions between the polar heads and energy of interactions between the heads and the field. No anomalous deformation of the molecules is considered. The study, provided for bilayer dipalmitoyl-phosphatidylcholine (DPPC) membrane in the gel (300 K) and fluid (330 K) phases, shows dependence of the membrane conformational and energetical state on the value of the electric field. We observe that the electric field affects the number of molecules in the gel and in the fluid states. In the layer at the negative potential, when the transmembrane voltage is above U(c) approximately 280 mV, lipid heads abruptly reorient and the number of local spots with fluid conformation increases. The other layer slightly tends to tighten its structure, producing additional mechanical stress between layers. Lipids showed complete insensitivity to the electric field within physiological limits, U<70 mV.