Showing papers on "Particle-size distribution published in 2015"

Size distribution of particles in Saturn's rings from aggregation and fragmentation

Abstract: Saturn's rings consist of a huge number of water ice particles, with a tiny addition of rocky material. They form a flat disk, as the result of an interplay of angular momentum conservation and the steady loss of energy in dissipative interparticle collisions. For particles in the size range from a few centimeters to a few meters, a power-law distribution of radii, ~r(-q) with q ≈ 3, has been inferred; for larger sizes, the distribution has a steep cutoff. It has been suggested that this size distribution may arise from a balance between aggregation and fragmentation of ring particles, yet neither the power-law dependence nor the upper size cutoff have been established on theoretical grounds. Here we propose a model for the particle size distribution that quantitatively explains the observations. In accordance with data, our model predicts the exponent q to be constrained to the interval 2.75 ≤ q ≤ 3.5. Also an exponential cutoff for larger particle sizes establishes naturally with the cutoff radius being set by the relative frequency of aggregating and disruptive collisions. This cutoff is much smaller than the typical scale of microstructures seen in Saturn's rings.

Splash erosion of clay–sand mixtures and its relationship with soil physical properties: The effects of particle size distribution on soil structure

Abstract: Large quantities of disturbed soils and residues derived from engineering constructions have provided material resources for soil erosion and geological disasters, which is becoming a serious problem in China To date, studies on the erosion mechanism of these soils in a wide particle size distribution (PSD) are limited, and the relationships between soil PSD and physical characteristics are not explicit Here, we analyzed the effects of PSD on structural and mechanical parameters and attempted to quantify their relationships by blending silty clay soil and engineering sand as clay–sand mixtures with various sand mass proportions ranging from 0 to 90% at a 10% increment in a wet and a dry mixing type (MT) In addition, the effect of PSD on splash erosion was measured under simulated rainfall Changes of bulk density and porosity of soil mixtures (for clay contents more than 13%) with clay content displayed apparent differences separately in the wet and dry mixing types mainly due to the deformation and reorganization of soil grains Internal friction angle φ and cohesion c of saturated soils showed a parabolic trend and an exponential increase with clay content respectively, while cohesion c was better quantified by particle fractal dimension D than clay content Shear strength indexes in the wet mixing type were generally larger than those in the dry mixing type when clay content was larger than 13% Splash erosion temporal variation was significantly affected by PSD, MT, rainfall duration (RD) and their interaction effects (PSD × MT, PSD × RD), among which MT was the most prominent source of variation (F = 9997, p

On polydispersity and the hard sphere glass transition

Abstract: We investigate the dynamics of polydisperse hard spheres at high packing fractions ϕ. We use extensive numerical simulations based on an experimentally-realistic particle size distribution (PSD) and compare to commonly-used PSDs such as Gaussian or top hat distribution. We find that the mode of kinetic arrest depends on the PSD's shape and not only on its variance. For the experimentally-realistic PSD we find ageing dynamics even though the density correlators decay fully to zero for ϕ ≥ 0.59. We observe substantial decoupling of the dynamics of the smallest and largest particles. While the smallest particles remain diffusive in all our simulations, a power-law describes the largest-particle diffusion, suggesting an ideal arrest at ϕc ∼ 0.588. The latter is however averted just before ϕc, due to the presence of the mobile smallest particles. In addition, we identify that a partial aging mechanism is at work, whose effects are most pronounced for the largest particles. By comparing our results with recent experimental observations of ergodic behavior up to ϕ ∼ 0.6 in a hard-sphere system, we argue that this is an effect of polydispersity, which smears out the glass transition.

Flow Synthesis of Biocompatible Fe3O4 Nanoparticles: Insight into the Effects of Residence Time, Fluid Velocity, and Tube Reactor Dimension on Particle Size Distribution

Abstract: PEGylated Fe3O4 nanoparticles were prepared through flow synthesis upon the pyrolysis of ferric acetylacetonate (Fe(acac)(3)) in anisole at 250 degrees C under pressure of 33 bar, in the presence of a,?-dicarboxyl-terminated polyethylene glycol (HOOC-PEG-COOH) and oleylamine. In combination with theoretical analysis, the effects of linear velocity, residence time, and reactor dimension on particle size distribution were systematically investigated. In addition, the impact of Ostwald ripening on particle size distribution was also revealed. In particular, the impacts of monomer concentration distributions along both axial and radial directions of the tube reactor on the particle size distribution were carefully investigated. Under optimized conditions, PEGylated Fe3O4 nanoparticles with the relative standard deviation of particle size down to 10.6% were thus obtained. The resulting 4.6 nm particles exhibited excellent colloidal stability and high longitudinal relaxivity (r(1)) up to 11.1 mM(-1) center dot s(-1), which manifested the reliability of flow synthesis in preparing PEGylated Fe3O4 nanoparticles as contrast agents for magnetic resonance imaging applications.

Assessing Optical Properties and Refractive Index of Combustion Aerosol Particles Through Combined Experimental and Modeling Studies

Abstract: The variability of optical properties of combustion particles generated from a propane diffusion flame under varying fuel-to-air (C/O) ratios was studied with a three-wavelength nephelometer, a particle soot absorption photometer, and an integrating sphere photometer. Information on particle size distribution, morphology, and elemental carbon to total carbon (EC/TC) ratios were obtained from scanning mobility particle sizer measurements, transmission electron microscopy analyses, and thermal-optical analyses. Particles generated under a low C/O ratio (0.22) showed high elemental carbon fraction (EC/TC = 0.77) and low brown carbon to equivalent black carbon (BrC/EBC) ratio (0.01), and were aggregates composed of small primary particles. Rayleigh–Debye–Gans theory reproduced experimental single-scattering albedo, ω, absorption, and scattering Angstrom exponents within 56, 3, and 18%, respectively. In contrast, particles produced under a high C/O ratio (0.60) showed low elemental carbon fraction (EC/TC = 0.0...

Effect of Particle Size on Shear Stress of Magnetorheological Fluids

Abstract: Magnetorheological fluids (MRF), known for their variable shear stress contain magnetisable micrometer-sized particles (few micrometer to 200 micrometers) in a nonmagnetic carrier liquid To avoid settling of particles, smaller sized (3-10 micrometers) particles are preferred, while larger sized particles can be used in MR brakes, MR clutches, etc as mechanical stirring action in those mechanisms does not allow particles to settle down Ideally larger sized particles provide higher shear stress compared to smaller sized particles However there is need to explore the effect of particle sizes on the shear stress In the current paper, a comparison of different particle sizes on MR effect has been presented Particle size distributions of iron particles were measured using HORIBA Laser Scattering Particle Size Distribution Analyser The particle size distribution, mean sizes and standard deviations have been presented The nature of particle shapes has been observed using scanning electron microscopy To explore the effect of particle sizes, nine MR fluids containing small, large and mixed sized carbonyl iron particles have been synthesized Three concentrations (9%, 18% and 36% by volume) for each size of particles have been used The shear stresses of those MRF samples have been measured using ANTON PAAR MCR-102 Rheometer With increase in volume fraction of iron particles, the MR fluids synthesized using “mixed sized particles” show better shear stress compared to the MR fluids containing “smaller sized spherical shaped particles” and “larger sized flaked shaped particles” at higher shear rate

The effect of oleic acid on the synthesis of Fe3−xO4 nanoparticles over a wide size range

Abstract: This work reports on the effect of the oleic acid concentration on the magnetic and structural properties of Fe3−xO4 nanoparticles synthesized by thermal decomposition of Fe(acac)3 in benzyl-ether. This method allows the synthesis of highly monodisperse particles ranging from 7 to 100 nm in size by only varying the concentration of oleic acid in the reaction mixture. The structural and magnetic characterization reveal homogeneous particles in composition, with narrow particle size distribution, which are single-phase magnetite with almost bulk-like values of the saturation magnetization of about 90–99 emu g−1 at low temperatures and show the characteristic anomaly in the zero field-cooling magnetization curves associated with the Verwey transition for nanoparticles bigger than about 7 nm. In addition, the analyses of aliquots of the reaction mixtures by Fourier transform infrared spectroscopy at various stages shed light on the nucleation and growth processes of the particles.

Synthesis of ultrafine cubic tungsten carbide in a discharge plasma jet

Abstract: Hexagonal tungsten carbide (WC) is widely-used for the production of metal-working tools, but there is a great interest to the cubic modification of WC. The possibility of obtaining the ultrafine cubic tungsten carbide in an electrodischarge plasma jet generated by a high-current pulsed coaxial magnetoplasma accelerator is shown in this report. According to X-ray diffraction and high resolution transmission electron microscopy the product predominantly consists of a cubic tungsten carbide phase WC0.86 (95% mass). Lattice constant of obtained tungsten carbide is a = 4.2536 A. This constant differs from the lattice constant (a = 4.2355 A) for ICDD card no. 00-020-1316 (cubic WC1 − x) nonetheless both of them are in the possible range for cubic tungsten carbide structures. The high cooling rate, realized in the system based on coaxial magnetoplasma accelerator, provides the formation of the cubic WC lattice and the narrow range of particle size distribution (10–40 nm).

Analysis of strength development in non-traditional liquid additive-stabilized laterite soil from macro- and micro-structural considerations

Abstract: The stabilization of soils with additives is a chemically modified method that can be used to improve soils with weak engineering properties. It has been well established that the size, shape, and arrangement of soil particles will affect the treatment process of natural soil with stabilizers. Also, the degree of enhancement is dependent on the morphology of the new formed products that bond the soil particles together. In this paper, unconfined compressive strength (UCS) test was performed as an index of soil improvement on liquid-stabilized (TX-85) mix designs. The time-dependent change in shear properties and compressibility behavior of treated soil was also studied using standard direct shear and consolidation tests. To better understand the structure and surface morphology of treated particles, FESEM, N2-BET and particle size distribution analysis were performed on soil-stabilizer matrix. From engineering point of view, the UCS results indicated that the degree of improvement for TX-85-stabilized laterite soil was approximately four times greater than the natural soil in a 7-day curing time period. Also, increased compressibility resistance of treated samples with curing time was evident. Based on the results, it was found that the stabilization process modifies the porous network of laterite soil. In addition, new white layers of reaction products were formed on the surface of clay particles.

Further details on particle inception and growth in premixed flames

Abstract: Soot inception in flames remains mostly unknown. What are, if any, the differences between inception particles formed in non-sooting and sooting flames, and to what extent soot formation is governed primarily by coagulation or by a different composition/aromatization of inception particles are open questions. In this work, the initial particle growth occurring in premixed flames of ethylene has been investigated. On line size-selected photoionization efficiency of inception particles produced in various flames has been measured. Structural/chemical differences of both nucleating and growing particles have been additionally investigated by off-line cyclic voltammetry, Raman spectroscopy, and light absorption. Results show that the size of aromatic domains within particles slightly increases when moving from flames in which the particle size distribution remains nearly constant and mono-modal in the nanometer range along the flame height, to flames in which particle coagulation gives rise to the formation of a second mode constituted by larger but still nanometric particles. Only in particles of this latter mode, d ≈ 10 nm, the first appearance of stacking of polyaromatic units is observed. On this basis, it has been hypothesized that two graphitization processes occur during the first stages of soot inception: a slight increase of in plane aromatic islands in primary particles and the formation of aromatic plane stacks in coagulated primary particles. This second type of coagulating-graphitization process explains the closure of the band gap observed in the grown particles respect to the primary ones, and hence the change of optical properties, towards the typical values observed in soot.

Sensitivity of dispersion model forecasts of volcanic ash clouds to the physical characteristics of the particles.

Abstract: This study examines the sensitivity of atmospheric dispersion model forecasts of volcanic ash clouds to the physical characteristics assigned to the particles. We show that the particle size distribution (PSD) used to initialise a dispersion model has a significant impact on the forecast of the mass loading of the ash particles in the atmosphere. This is because the modeled fall velocity of the particles is sensitive to the particle diameter. Forecasts of the long-range transport of the ash cloud consider particles with diameters between 0.1 μm and 100 μm. The fall velocity of particles with diameter 100 μm is over 5 orders of magnitude greater than a particle with diameter 0.1 μm, and 30 μm particles fall 88% slower and travel up to 5× further than a 100 μm particle. Identifying the PSD of the ash cloud at the source, which is required to initialise a model, is difficult. Further, aggregation processes are currently not explicitly modeled in operational dispersion models due to the high computational costs associated with aggregation schemes. We show that using a modified total grain size distribution (TGSD) that effectively accounts for aggregation processes improves the modeled PSD of the ash cloud and deposits from the eruption of Eyjafjallajokull in 2010. Knowledge of the TGSD of an eruption is therefore critical for reducing uncertainty in quantitative forecasts of ash cloud dispersion. The density and shape assigned to the model particles have a lesser but still significant impact on the calculated fall velocity. Accounting for the density distribution and sphericity of ash from the eruption of Eyjafjallajokull in 2010, modeled particles can travel up to 84% further than particles with default particle characteristics that assume the particles are spherical and have a fixed density.

Poly (lactic-co-glycolic acid) particles prepared by microfluidics and conventional methods. Modulated particle size and rheology

Abstract: Hypothesis Microfluidic techniques are expected to provide narrower particle size distribution than conventional methods for the preparation of poly (lactic-co-glycolic acid) (PLGA) microparticles. Besides, it is hypothesized that the particle size distribution of poly (lactic-co-glycolic acid) microparticles influences the settling behavior and rheological properties of its aqueous dispersions. Experiments For the preparation of PLGA particles, two different methods, microfluidic and conventional oil-in-water emulsification methods were employed. The particle size and particle size distribution of PLGA particles prepared by microfluidics were studied as a function of the flow rate of the organic phase while particles prepared by conventional methods were studied as a function of stirring rate. In order to study the stability and structural organization of colloidal dispersions, settling experiments and oscillatory rheological measurements were carried out on aqueous dispersions of PLGA particles with different particle size distributions. Findings Microfluidics technique allowed the control of size and size distribution of the droplets formed in the process of emulsification. This resulted in a narrower particle size distribution for samples prepared by MF with respect to samples prepared by conventional methods. Polydisperse samples showed a larger tendency to aggregate, thus confirming the advantages of microfluidics over conventional methods, especially if biomedical applications are envisaged.

PLUME-MoM 1.0: A new integral model of volcanic plumes based on the method of moments

Abstract: . In this paper a new integral mathematical model for volcanic plumes, named PLUME-MoM, is presented. The model describes the steady-state dynamics of a plume in a 3-D coordinate system, accounting for continuous variability in particle size distribution of the pyroclastic mixture ejected at the vent. Volcanic plumes are composed of pyroclastic particles of many different sizes ranging from a few microns up to several centimeters and more. A proper description of such a multi-particle nature is crucial when quantifying changes in grain-size distribution along the plume and, therefore, for better characterization of source conditions of ash dispersal models. The new model is based on the method of moments, which allows for a description of the pyroclastic mixture dynamics not only in the spatial domain but also in the space of parameters of the continuous size distribution of the particles. This is achieved by formulation of fundamental transport equations for the multi-particle mixture with respect to the different moments of the grain-size distribution. Different formulations, in terms of the distribution of the particle number, as well as of the mass distribution expressed in terms of the Krumbein log scale, are also derived. Comparison between the new moments-based formulation and the classical approach, based on the discretization of the mixture in N discrete phases, shows that the new model allows for the same results to be obtained with a significantly lower computational cost (particularly when a large number of discrete phases is adopted). Application of the new model, coupled with uncertainty quantification and global sensitivity analyses, enables the investigation of the response of four key output variables (mean and standard deviation of the grain-size distribution at the top of the plume, plume height and amount of mass lost by the plume during the ascent) to changes in the main input parameters (mean and standard deviation) characterizing the pyroclastic mixture at the base of the plume. Results show that, for the range of parameters investigated and without considering interparticle processes such as aggregation or comminution, the grain-size distribution at the top of the plume is remarkably similar to that at the base and that the plume height is only weakly affected by the parameters of the grain distribution. The adopted approach can be potentially extended to the consideration of key particle–particle effects occurring in the plume including particle aggregation and fragmentation.

Vertical Profiles and Chemical Properties of Aerosol Particles upon Ny-Ålesund (Svalbard Islands)

Abstract: Size-segregated particle samples were collected in the Arctic (Ny-Alesund, Svalbard) in April 2011 both at ground level and in the free atmosphere exploiting a tethered balloon equipped also with an optical particle counter (OPC) and meteorological sensors. Individual particle properties were investigated by scanning electron microscopy coupled with energy dispersive microanalysis (SEM-EDS). Results of the SEM-EDS were integrated with particle size and optical measurements of the aerosols properties at ground level and along the vertical profiles. Detailed analysis of two case studies reveals significant differences in composition despite the similar structure (layering) and the comparable texture (grain size distribution) of particles in the air column. Differences in the mineral chemistry of samples point at both local (plutonic/metamorphic complexes in Svalbard) and remote (basic/ultrabasic magmatic complexes in Greenland and/or Iceland) geological source regions for dust. Differences in the particle size and shape are put into relationship with the mechanism of particle formation, that is, primary (well sorted, small) or secondary (idiomorphic, fine to coarse grained) origin for chloride and sulfate crystals and transport/settling for soil (silicate, carbonate and metal oxide) particles. The influence of size, shape, and mixing state of particles on ice nucleation and radiative properties is also discussed.

Spatio-temporal evolution of the dust particle size distribution in dusty argon rf plasmas

Abstract: An imaging Mie scattering technique has been developed to measure the spatially resolved size distribution of dust particles in extended dust clouds. For large dust clouds of micrometre-sized plastic particles confined in an radio frequency (rf) discharge, a segmentation of the dust cloud into populations of different sizes is observed, even though the size differences are very small. The dust size dispersion inside a population is much smaller than the difference between the populations. Furthermore, the dust size is found to be constantly decreasing over time while the particles are confined in an inert argon plasma. The processes responsible for the shrinking of the dust in the plasma have been addressed by mass spectrometry, ex situ microscopy of the dust size, dust resonance measurements, in situ determination of the dust surface temperature and Fourier transform infrared absorption (FT-IR). It is concluded that both a reduction of dust size and its mass density due to outgassing of water and other volatile constituents as well as chemical etching by oxygen impurities are responsible for the observations.

Fractal features of soil particle size distribution in newly formed wetlands in the Yellow River Delta

Abstract: The characteristic of particle size distribution (PSD) in the newly formed wetlands in coast has seldom been studied. We applied fractal-scaling theory in assessing soil particle size distribution (PSD) features of newly formed wetlands in the Yellow River Delta (YRD), China. The singular fractal dimensions (D) values ranged from 1.82 to 1.90, the capacity dimension (D-0) values ranged from 0.84 to 0.93, and the entropy dimension (D-1) values ranged from 0.66 to 0.84. Constrained corresponding analysis revealed that 43.5% of the variance in soil PSD can be explained by environmental factors, including 14.7% by seasonal variation, 8.6% by soil depth, and 8.0% by vegetation type. The fractal dimensions D and D-1 were sensitive with fine particles with size ranging less than 126 mu m, and D-0 was sensitive with coarse particles with size ranging between 126 mu m to 2000 mu m. Fractal analysis makes full use of soil PSD information, and offers a useful approach to quantify and assess the soil physical attributes in the newly formed wetland.