Function Of Packing Density Research Articles

Influence of multimodal fiber distribution on the permeability of fibrous media

This article deals with the impact of fiber size distribution on the permeability of nonwoven fibrous media, a major issue in the general field of filtration. The conventional method to determine the permeability of nonwoven fibrous media involves equating complex fibrous media with structures composed of a single equivalent fiber diameter. Based on numerical simulations of flow through tridimensional structures with polymodal and polydisperse fiber size distributions, we proposed a new equivalent diameter. In conjunction with a packing density function, the approach is compared to experimental data and simulations available in the scientific literature and shows better permeability predictions than other existing models for a wide range of solid volume fractions (1 % to 20 %) and fiber diameters (1.5 µm to 30 µm). This predictive model of permeability is an essential step in developing a decision-making tool for the design of fibrous media.

Modeling pressure drop values across ultra-thin nanofiber filters with various ranges of filtration parameters under an aerodynamic slip effect

Computational fluid dynamics simulations of fibrous filters with 56 combinations of different fiber sizes, packing densities, face velocities, and thicknesses were conducted for developing models that predict pressure drops across nanofiber filters. The accuracy of the simulation method was confirmed by comparing the numerical pressure drops to the experimental data obtained for polyacrylonitrile electrospun nanofiber filters. In the simulations, an aerodynamic slip effect around the surface of the small nanofibers was considered. The results showed that, unlike in the case of conventional filtration theory, pressure drops across the thin layers of electrospun nanofiber filters are not proportional to the thickness. This might be a critical factor for obtaining precise pressure drops across the electrospun nanofiber filters with extremely thin layers. Finally, we derived the product of drag coefficient and Reynolds number as a function of packing density, Knudsen number, and ratio of thickness to fiber diameter to get the correlation equation for pressure drop prediction. The obtained equation predicted the pressure drops across the nanofiber filters with the maximum relative difference of less than 15%.

Local and Global Order in Dense Packings of Semi-Flexible Polymers of Hard Spheres.

The local and global order in dense packings of linear, semi-flexible polymers of tangent hard spheres are studied by employing extensive Monte Carlo simulations at increasing volume fractions. The chain stiffness is controlled by a tunable harmonic potential for the bending angle, whose intensity dictates the rigidity of the polymer backbone as a function of the bending constant and equilibrium angle. The studied angles range between acute and obtuse ones, reaching the limit of rod-like polymers. We analyze how the packing density and chain stiffness affect the chains' ability to self-organize at the local and global levels. The former corresponds to crystallinity, as quantified by the Characteristic Crystallographic Element (CCE) norm descriptor, while the latter is computed through the scalar orientational order parameter. In all cases, we identify the critical volume fraction for the phase transition and gauge the established crystal morphologies, developing a complete phase diagram as a function of packing density and equilibrium bending angle. A plethora of structures are obtained, ranging between random hexagonal closed packed morphologies of mixed character and almost perfect face centered cubic (FCC) and hexagonal close-packed (HCP) crystals at the level of monomers, and nematic mesophases, with prolate and oblate mesogens at the level of chains. For rod-like chains, a delay is observed between the establishment of the long-range nematic order and crystallization as a function of the packing density, while for right-angle chains, both transitions are synchronized. A comparison is also provided against the analogous packings of monomeric and fully flexible chains of hard spheres.

Drying Shrinkage and Cracking Tendency of Concrete Pavement Mixtures with Variable Packing Densities of Aggregates and Paste Contents

Excessive drying shrinkage, and associated cracking, can lead to serious durability problem in concrete pavements and bridges. In the course of this study, the magnitude of drying shrinkage and cracking potential was evaluated for several concrete pavement mixtures as a function of packing density of the aggregate and paste contents. The results indicated that both, the shrinkage and the cracking potential depend on the volume of voids between aggregate particles (packing density), paste content of concrete mixture, and the paste-aggregate void saturation ratio.

Influence of packing density and stress on the dynamic response of granular materials

Laboratory geophysics tests including bender elements and acoustic emission measure the speed of propagation of stress or sound waves in granular materials to derive elastic stiffness parameters. This contribution builds on earlier studies to assess whether the received signal characteristics can provide additional information about either the material’s behaviour or the nature of the material itself. Specifically it considers the maximum frequency that the material can transmit; it also assesses whether there is a simple link between the spectrum of the received signal and the natural frequencies of the sample. Discrete element method (DEM) simulations of planar compression wave propagation were performed to generate the data for the study. Restricting consideration to uniform (monodisperse) spheres, the material fabric was varied by considering face-centred cubic lattice packings as well as random configurations with different packing densities. Supplemental analyses, in addition to the DEM simulations, were used to develop a more comprehensive understanding of the system dynamics. The assembly stiffness and mass matrices were extracted from the DEM model and these data were used in an eigenmode analysis that provided significant insight into the observed overall dynamic response. The close agreement of the wave velocities estimated using eigenmode analysis with the DEM results confirms that DEM wave propagation simulations can reliably be used to extract material stiffness data. The data show that increasing either stress or density allows higher frequencies to propagate through the media, but the low-pass wavelength is a function of packing density rather than stress level. Prior research which had hypothesised that there is a simple link between the spectrum of the received signal and the natural sample frequencies was not substantiated.

Creating Aptamers and Their Use in Resisitive Pulse Sensors

Resistive pulse sensors, RPS, are allowing the transport mechanism of molecules, proteins and even nanoparticles to be characterized as they traverse small channels. Here we present our recent advancement of the technique identifying key experimental designs for potential POC assays. The first assay utilized superparamagnetic beads, if the surfaces of the beads are modified with an aptamer, the frequency of beads (translocations/minute) through the pore can be related to the concentration of specific proteins in the solution. Herein, we have demonstrated the successful use of TRPS to observe the binding of two proteins, to their specific aptamers simultaneously. We then adapt the measurement strategy and demonstrate that the translocation times of particles can be used to infer the zeta potential to measure the change in zeta potential of DNA modified particles. By measuring the translocation times of DNA modified nanoparticles as a function of packing density, length, structure, and hybridisation time, we observe a clear difference in zeta potential using both mean values, and population distributions as a function of the DNA structure. Finally we present the first comparison between assays that use resistive pulses or rectification ratios on a tunable pore platform.

Dolos Hydraulic-Stability Formula Update and Optimum Packing Density

This paper effectively summarizes dolos stability results for a slope of 1:1.5 that were obtained from the same flume at the Council for Scientific and Industrial Research (CSIR) laboratory in Stellenbosch, South Africa, over a period of more than 10 years. The initial CSIR investigation into dolos stability was conducted with regular waves and included the effect of packing density, concrete density, storm duration, and waist-to-height ratio. Subsequent investigations into the effect of wave period, armor slope, and waist-to-height ratio were conducted with irregular waves yielding 316 data points. These data formed the basis of the present dolos stability formulas. For establishing the effect of packing density, 286 regular wave results were used. The analysis included the establishment of a relationship between packing density and ultimate displacement just prior to failure. An optimum packing density can be estimated by using this approach together with the change in displacements as a function of packing density. The effect of concrete density on stability as previously established in the same flume was included in the formulas, as was the effect of storm duration. Final stability formulas were compared with those of other researchers for dolosse, tetrapods, cubes, and Accropode (SOGREAH, Echirolles, France).

Particle-by-Particle Charge Analysis of DNA-Modified Nanoparticles Using Tunable Resistive Pulse Sensing.

Resistive pulse sensors, RPS, are allowing the transport mechanism of molecules, proteins and even nanoparticles to be characterized as they traverse pores. Previous work using RPS has shown that the size, concentration and zeta potential of the analyte can be measured. Here we use tunable resistive pulse sensing (TRPS) which utilizes a tunable pore to monitor the translocation times of nanoparticles with DNA modified surfaces. We start by demonstrating that the translocation times of particles can be used to infer the zeta potential of known standards and then apply the method to measure the change in zeta potential of DNA modified particles. By measuring the translocation times of DNA modified nanoparticles as a function of packing density, length, structure, and hybridization time, we observe a clear difference in zeta potential using both mean values and population distributions as a function of the DNA structure. We demonstrate the ability to resolve the signals for ssDNA, dsDNA, small changes in base length for nucleotides between 15 and 40 bases long, and even the discrimination between partial and fully complementary target sequences. Such a method has potential and applications in sensors for the monitoring of nanoparticles in both medical and environmental samples.

How to predict the ideal glass transition density in polydisperse hard-sphere packings.

The formula for the entropy s of the accessible volume of the phase space for frictionless hard spheres is combined with the Boublík-Mansoori-Carnahan-Starling-Leland (BMCSL) equation of state for polydisperse three-dimensional packings to obtain an analytical expression for s as a function of packing density φ. Polydisperse hard-sphere packings with log-normal, Gaussian, and Pareto particle diameter distributions are generated to estimate their ideal glass transition densities φg. The accessible entropy s at φg is almost the same for all investigated particle diameter distributions. We denote this entropy as sg and can predict φg for an arbitrary particle diameter distribution through an equation s(φ) = sg. If the BMCSL equation of state is used for s(φ), then φg is found to depend only on the first three moments of a particle diameter distribution.

Monte Carlo simulations of densely-packed athermal polymers in the bulk and under confinement

We review the main results from extensive Monte Carlo (MC) simulations on athermal polymer packings in the bulk and under confinement. By employing the simplest possible model of excluded volume, macromolecules are represented as freely-jointed chains of hard spheres of uniform size. Simulations are carried out in a wide concentration range: from very dilute up to very high volume fractions, reaching the maximally random jammed (MRJ) state. We study how factors like chain length, volume fraction and flexibility of bond lengths affect the structure, shape and size of polymers, their packing efficiency and their phase behaviour (disorder–order transition). In addition, we observe how these properties are affected by confinement realized by flat, impenetrable walls in one dimension. Finally, by mapping the parent polymer chains to primitive paths through direct geometrical algorithms, we analyse the characteristics of the entanglement network as a function of packing density.

Contact nonlinearities and linear response in jammed particulate packings.

Packings of frictionless athermal particles that interact only when they overlap experience a jamming transition as a function of packing density. Such packings provide the foundation for the theory of jamming. This theory rests on the observation that, despite the multitude of disordered configurations, the mechanical response to linear order depends only on the distance to the transition. We investigate the validity and utility of such measurements that invoke the harmonic approximation and show that, despite particles coming in and out of contact, there is a well-defined linear regime in the thermodynamic limit.

Evolution of fivefold local symmetry during crystal nucleation and growth in dense hard-sphere packings

Crystal nucleation and growth of monodisperse hard-spheres as a function of packing density is studied by collision-driven molecular dynamics simulations. Short-range order in the form of fivefold local symmetry is identified and its dynamical and structural evolution is tracked as the originally amorphous assembly transits to the stable ordered phase. A cluster-based approach shows that hard-sphere configurations having initially a similar average fraction of fivefold and ordered sites can crystallize in completely different patterns both in terms of dynamics and morphology. It is found that at high volume fractions crystallization is significantly delayed in assemblies where sites with fivefold symmetry are abundant. Eventually, once the crystal phase is reached, fivefold symmetry either diminishes or arranges in specific geometric patterns. Such defects are spatially strongly correlated with twinning planes at crystalline boundaries. A detailed analysis is provided on the structural characteristics of the established crystal morphologies.

Structure of a monolayer of poly(ethylene glycol) end-capped with a fluoroalkyl group and its relationship with protein adsorption at the aqueous interface

Poly(ethylene glycol) (PEG, 6 kg mol−1) terminated at both ends by a hydrophobic fluoroalkyl segment [-(CH2)2C10F21] (Rf-PEG) has been shown to self-assemble into an essentially water insoluble hydrogel. Here, we characterize the monolayer, formed by Rf-PEG and the subsequent adsorption of a protein, as a function of packing density at the aqueous interface by using thermodynamic methods, neutron reflectivity, and fluorescence microscopy. The π–A isotherms showed two transitions. The first transition was attributed to the start of a conformational change of PEG chains from a pancake-like shape to a stretched brush-like shape due to intermicellar packing. The second transition was assigned to a collapse of the Rf-PEG monolayer, submerging some of the micellar Rf-PEG underneath the monolayer and forming a multilayered state. BSA (bovine serum albumin), a model protein, adsorption onto the Rf-PEG monolayer from the subphase was also correlated with the transition state of the monolayer. At a lower packing density below the first transition, the Rf-PEG did not suppress BSA adsorption at all. Conversely, an almost complete prevention of BSA adsorption by the Rf-PEG was observed once the Rf-PEG had formed a fully-covered monolayer state.

Activity of Membrane Proteins Immobilized on Surfaces as a Function of Packing Density

A systematic study of the influence of the packing density of proteins on their activity is performed with cytochrome c oxidase (CcO) from R. sphaeroides as an example. The protein was incorporated into a protein-tethered bilayer lipid membrane and CcO was genetically engineered with a histidine-tag, attached to Subunit II, and then tethered by an interaction with functionalized thiol compounds bound to a gold electrode. The packing density was varied by diluting the functionalized thiol with a nonfunctionalized thiol that does not bind to the enzyme. After attaching the CcO to the gold surface, a lipid bilayer was formed to incorporate the tethered proteins. The reconstituted protein-lipid bilayer was characterized by surface enhanced infrared reflection absorption spectroscopy (SEIRAS), electrochemical impedance spectroscopy, surface plasmon resonance, and atomic force microscopy. The activity of the proteins within the reconstituted bilayer was probed by direct electrochemical electron injection and was shown to be very sensitive to the packing density of protein molecules. At low surface density of CcO, the bilayer did not effectively form, and protein aggregates were observed, whereas at very high surface density, very little lipid is able to intrude between the closely packed proteins. In both of these cases, redox activity, measured by the efficiency to accept electrons, is low. Redox activity of the enzyme is preserved in the biomimetic structure but only at a moderate surface coverage in which a continuous lipid bilayer is present and the proteins are not forced to aggregate. Electrostatic and other interaction forces between protein molecules are held responsible for these effects.

Shell-side mass transfer of hollow fibre modules in osmotic distillation process

The effect of packing density of hollow fibre modules on mass transfer in the shell side of osmotic distillation process was studied. The osmotic distillation experiments were carried out with several modules of the packing densities ranging from 30.6 to 61.2%. It was found that the Reynolds number was a function of packing density and packing density affected mass transfer performance. The shell-side mass transfer coefficient increased with the brine velocity. The membrane permeability can be predicted from the experimental flux at the maximum brine velocity. The mass transfer correlation was proposed in order to determine the shell-side mass transfer coefficient in the randomly packed modules for osmotic distillation process. The empirical correlation proposed was fitted to the experimental results and it was found that the mass transfer coefficients calculated from the proposed correlation were in good agreement with those from the experimental data. Comparison of the results obtained from the proposed correlation with other correlations in the literature was discussed.

Pore structure of the packing of fine particles

This paper presents a numerical study of the pore structure of fine particles. By means of granular dynamics simulation, packings of mono-sized particles ranging from 1 to 1000 μm are constructed. Our results show that packing density varies with particle size due to the effect of the cohesive van der Waals force. Pores and their connectivity are then analysed in terms of Delaunay tessellation. The geometries of the pores are represented by the size and shape of Delaunay cells and quantified as a function of packing density or particle size. It shows that the cell size decreases and the cell shape becomes more spherical with increasing packing density. A general correlation exists between the size and shape of cells: the larger the cell size relative to particle size, the more spherical the cell shape. This correlation, however, becomes weaker as packing density decreases. The connectivity between pores is represented by throat size and channel length. With decreasing packing density, the throat size increases and the channel length decreases. The pore scale information would be useful to understand and model the transport and mechanical properties of porous media.

Multiscaling at PointJ: Jamming is a Critical Phenomenon

We analyze the jamming transition that occurs as a function of increasing packing density in a disordered two-dimensional assembly of disks at zero temperature for "Point J" of the recently proposed jamming phase diagram. We measure the total number of moving disks and the transverse length of the moving region, and find a power law divergence as the packing density increases toward a critical jamming density. This provides evidence that the T=0 jamming transition as a function of packing density is a second order phase transition. Additionally, we find evidence for multiscaling, indicating the importance of long tails in the velocity fluctuations.

High-density recording on magnetic thin film disk media

Experimental curves of output as a function of packing density are presented for a thin film medium and two types of head. These curves are then fitted to theoretical predictions in such a way that useful data is determined about the factors which limit the packing density observed. From this secure base, predictions are made about the ultimate attainable packing densities if improvements are made to the limiting parameters. It is shown that packing densities of half a million bits per inch and more along with area densities of 50 Gb/in 2 are possible but that substantial improvements to the magnetic and mechanical aspects of disk systems as well as to the signal processing methods are necessary especially if densities of one million bits per inch and 100 Gb/in 2 are to be achieved.

Local interaction field in periodic arrangement of rectangular prisms

Exact analytical expressions for the local interaction field produced by an infinite array of ferromagnetic rectangular prisms are derived. For tablet-like prisms, the local interaction is positive for certain packing density; while for needle-like prisms, the local interaction is always negative. Moreover, local interaction is not a linear function of packing density.