Maximum Particle Concentration Research Articles

UV laser irradiation of IR laser generated particles ablated from nitrobenzyl alcohol

Particles generated by 2.94 μm pulsed IR laser ablation of liquid 3-nitrobenzyl alcohol were irradiated with a 351 nm UV laser 3.5 mm above and parallel to the sample target. The size and concentration of the ablated particles were measured with a light scattering particle sizer. The application of the UV laser resulted in a reduction in the average particle size by one-half and an increase in the total particle concentration by a factor of nine. The optimum delay between the IR and UV lasers was between 16 and 26 μs and was dependent on the fluence of the IR laser: higher fluence led to a more rapid appearance of particulate. The ejection velocity of the particle plume, as determined by the delay time corresponding to the maximum two-laser particle concentration signal, was 130 m/s at 1600 J/m 2 IR laser fluence and increased to 220 m/s at 2700 J/m 2. The emission of particles extended for several ms. The observations are consistent with a rapid phase change and emission of particulate, followed by an extended emission of particles ablated from the target surface.

Particle Aggregation

A basic problem in marine biogeochemistry is understanding material and elemental distributions and fluxes in the oceans, and a key part of this problem is understanding the processes that affect particulate material in the ocean. Aggregation of particulate material is a primary process because it alters the transport properties of particulate material and provides a mechanism for transferring material from the dissolved into the particulate pools. Aggregation theory not only provides a framework for understanding these processes, but it also provides a means for making predictions and has been successfully used to predict maximum particle concentrations in the oceans and the fate of diatom blooms (including those from iron fertilization), the size spectra of particles in the oceans, and the size distributions of trace metals. Here we review the basic theory involved, summarize recent developments, and explore unresolved issues.

Effect of mixed layer depth on phytoplankton removal by coagulation and on the critical depth concept

Coagulation theory predicts that there is a maximum particle concentration that is sustainable in an algal bloom; the critical depth theory of Sverdrup [Sverdrup, H.U., 1953. On conditions for the vernal blooming of phytoplankton. Journal du Conseil/Conseil International pour l’ Exploration de la Mer 18, 287–295.] predicts a different maximum particle concentration based on light-limitation of phytoplankton growth. This paper uses a model incorporating both coagulation and light-limited algal growth to predict the effect of mixed layer depth and wind velocity on maximum phytoplankton populations. As part of this process, the critical concentration parameter is compared to the results of full coagulation simulations. Including phytoplankton aggregation decreases the maximum phytoplankton concentration but increases the rate of particle export through sedimentation. The results help explain field observations made in the Antarctic Ocean. They also suggest that the accuracy of the critical concentration parameter depends on the average shear rate.

Ultrafine particles produced by ozone/limonene reactions in indoor air under low/closed ventilation conditions

Formation of ultrafine particles, dp<100nm, from gas-phase reaction of limonene with O3 was studied. The concentration of reactants was chosen as close to realistic indoor conditions as possible. Two reaction chambers (1 and 14m3) were used. Particle number concentrations were measured using a CPC and size distributions by using a scanning mobility particle sizer (SMPS) system. Rapid formation of new particles was observed at low concentrations of reactants and close to zero ventilation rates. The maximum number of particles was correlated with the initial rate of formation of reaction products. An excess of O3 tends to give higher maximum particle concentrations. Modeling work lead to the conclusion that significant nucleation starts when the mixing ratio of “product” from the reaction Limonene+O3→productox exceeds 0.5–1ppb. The secondary particles formed by atmospheric chemistry in indoor air contribute to the total particulate matter indoors and should be considered in terms of low-dose long-term exposure.

Maximum phytoplankton concentrations in the sea

A simplification of plankton dynamics using coagulation theory provides predictions of the maximum algal concentration sustainable in aquatic systems. These predictions have previously been tested successfully against results from iron fertilization experiments. We extend the test to data collected in the North Atlantic as part of the Bermuda Atlantic Time Series program as well as data collected off Southern California as part of the Southern California Bight Study program. The observed maximum particulate organic carbon and volumetric particle concentrations are consistent with the predictions. The results imply that physical processes control maximum particle concentrations in planktonic systems.

Null‐Scattering Hybrid Particles Using Controlled Radical Polymerization

The particular properties of nanometer-sized inorganic materials are of central importance to the design of modern composite materials such as polymer composites, coatings, or cosmetic products where particle additives are used to improve mechanical, thermal, transport, or optical properties. [1–7] However, in many instances the improvement of some performance characteristic is compromised by a loss in transparency that results from the scattering of visible light by the embedded particle inclusions – a consequence of the significantly different refractive index n of most inorganic materials and the organic embedding medium. For applications that capitalize on optical transparency, the scattering of particle inclusions presents severe limitations to the maximum concentration of filler particles as well as the design possibilities of the organic-matrix composites. For optically isotropic particles with linear dimensions significantly less than the wavelength of light, the particle scattering cross-section is given by Csca ∼ Vp 2 (Da) 2 (with Vp denoting the particle volume and Da the polarizability of the particle within the embedding medium). [8,9] Since Da ∼ (ep – em)/(ep +2 em) >> 0 (with ei = ni 2 denoting the dielectric constant of medium ‘i’; ‘p’, and ‘m’ represent the particle and embedding medium, respectively) for most inorganic/organic material combinations, significant scattering can arise even for small particle sizes. In this Communication we demonstrate that the scattering of inorganic particles can efficiently be suppressed by grafting of polymers of appropriate composition, molecular weight, and grafting density from the particles’ surface such as to match the effective dielectric constant of the resulting coreshell particle to the dielectric constant of the embedding medium. Key to the presented approach is the observation that for core-shell particles with a size less than the wavelength of light the optical properties are approximately equal to those of a homogeneous particle with an effective dielectric constant that depends on the optical properties and volume fractions of the respective constituents. [10] In particular, for a core-shell particle at wavelengths larger than the particle dimension the particles’ effective dielectric constant is given by Maxwell–Garnett theory as eeff eshell 1 3 x 1 x 1

Effect of particle collisions on the expulsion of heavy particles from a vortex core

A computational study has been performed on the effect of particle collisions as heavy particles are expelled from a vortex core. In the absence of collisions, particles collect in a band surrounding the vortex core, where the maximum particle concentration within this band increases indefinitely as the band of particles propagates radially outward. Particle collisions act to regulate the increase in concentration within this band by the enhanced spreading of the particles that occurs through the action of shear-induced migration. The presence of axial flow within the vortex core introduces a drift of the particles within the core toward the low-shear region at the core center, thus resisting the outward particle drift caused by centrifugal force. With increasing axial flow rates, the particles are retained in the vortex core for longer time periods. The role of various parameters on the particle expulsion is evaluated, including particle diameter, restitution coefficient, and initial concentration. The study also examines the effect of collisions on cases with two distinct particle sizes, in which the different outward expulsion rates act to separate particles of different size.

Parametric Effects on Particle Deposition in Abrasive Waterjet Surface Treatments

The abrasive waterjet (AWJ) has primarily been used for net-shape sectioning of engineering materials. In this study an AWJ was adopted for the surface treatment of commercially pure titanium (cpTi) and the contribution of treatment parameters to material removal and the deposition of particles within the substrate were examined. The surface texture and material removal rate were analyzed using conventional techniques whereas the quantity of abrasive particles embedded within the cpTi surfaces was determined using energy dispersive X-ray analysis (EDXA). Parametric effects related to particle hardness were distinguished from a comparison of surfaces treated with Aluminum oxide, garnet, and crushed glass abrasives. According to an analysis of variance (ANOVA), treatment responses were found to be primarily dependent on the abrasive size, abrasive hardness, angle of incidence, and jet pressure. The minimum and maximum concentration of particles embedded in the AWJ treated cpTi (in percent surface area covered) was 2.5 and 21.6%, respectively. Particle concentration and mean particle size in the cpTi increased with abrasive size, angle of incidence and jet pressure; the particle concentration reached saturation at an angle of incidence near 80°. Although abrasive hardness was important to the treatment responses, an increase in hardness beyond 800 on the Rosiwal scale resulted in minimal changes in material removal or other features of the process.

Measurement of Ultrafine Particles: A Comparison of Two Handheld Condensation Particle Counters

The objective of this study was to compare two real-time condensation particle counters for measurement of number concentrations of ultrafine particles (UFPs). The comparison is based on the data from side-by-side measurements conducted in several locations, both indoors and outdoors. CPC 3007 and P-Trak™ 8525 manufactured by TSI (instruments A and B, respectively) were used simultaneously. They measure particles in sizes from 0.01 to greater than 1 μ m and 0.02 to greater than 1 μ m, respectively. The results reveal a good correlation between the two instruments. The ratios of measured aerosol concentrations varied from 0.81 to 1.17, which implies that in all data sets the difference between the two instruments was less than ± 20%. About 63% of the results were in the range of ± 10%, and about 44% showed differences less than ± 5%. The maximum particle concentration detected by instrument A was approximately 105,000 particles cm − 3 and the minimum was about 230 particles cm − 3 . Because of the lower pa...

Parametric Effects on Particle Deposition in Abrasive Waterjet Surface Treatments

Abstract The abrasive waterjet (AWJ) has primarily been used for net-shape sectioning of engineering materials. In this study an AWJ was adopted for the surface treatment of commercially pure titanium (cpTi) and the contribution of treatment parameters to material removal and the deposition of particles within the substrate were examined. The surface texture and material removal rate were analyzed using conventional techniques whereas the quantity of abrasive particles embedded within the cpTi surfaces was determined using energy dispersive X-ray analysis (EDXA). Parametric effects related to particle hardness were distinguished from a comparison of surfaces treated with Aluminum oxide, garnet, and crushed glass abrasives. According to an analysis of variance (ANOVA), treatment responses were found to be primarily dependent on the abrasive size, abrasive hardness, angle of incidence, and jet pressure. The minimum and maximum concentration of particles embedded in the AWJ treated cpTi (in percent surface area covered) was 2.5 and 21.6%, respectively. Particle concentration and mean particle size in the cpTi increased with abrasive size, angle of incidence and jet pressure; the particle concentration reached saturation at an angle of incidence near 80°. Although abrasive hardness was important to the treatment responses, an increase in hardness beyond 800 on the Rosiwal scale resulted in minimal changes in material removal or other features of the process.

Exponential formula for computing effective viscosity

An exponential model is proposed for evaluating the effective viscosity of a particle–fluid mixture. First, theoretical consideration is restricted to the dilute condition without effects of dynamic particle interactions and fluid turbulence. This leads to a power series expressed in terms of particle concentration, which can be viewed as an extension of the Einstein's formula. The derivation is then extended using an exponential model for the condition of high particle concentrations. The exponential formula obtained, which is not subjected to the maximum particle concentration, is found comparable to various empirical formulas available in the literature.

Simultaneous charging and Brownian coagulation of nanometre aerosol particles

The electric charging process, by bipolar ions, of nanometre-sized aerosol particles undergoing simultaneous Brownian coagulation is examined. Two approximate analytical models, valid for symmetrical charging and size-independent coagulation rate constant, are derived and compared with the numerical solution of the rigorous population balance equations. For each particle size, there exists an optimum mean aerosol residence time in the bipolar charging device for which the output concentration of charged particles is a maximum. The optimum residence time for the smallest particles is about one order of magnitude lower than the time required to attain the charge equilibrium state. In practical situations, the maximum attainable number concentration of charged particles is accordingly much lower than in equilibrium conditions.

Applicability limits of Beer’s law for dispersion media with a high concentration of particles

This study analyzes the values of volume concentrations of scatterers at which radiation extinction in dispersion media obeys Beer's law. The dependence of the maximum particle concentration at which Beer's law holds on the properties of the dispersion medium is investigated. It is shown that the maximum concentration is strongly dependent on the scatterers' parameters and varies over a wide range, from tenths to tens of percent.

Transient bipolar charging of a coagulating nanometer aerosol

The transient charging process of a nanometer aerosol undergoing Brownian coagulation is studied theoretically and experimentally. The numerical solution of the population balance equations, which include charging and coagulation terms but not diffusion losses, is in reasonable agreement with the experimental results. Due to coagulation, for each particle size there exists an optimum residence time at which the output concentration of charged particles is a maximum. The optimum residence time decreases as the particle size decreases, an obvious consequence of Brownian coagulation. Furthermore, the experimentally measured optimum residence times for the smallest particles are about one order of magnitude lower than the time required to attain the charging equilibrium state, so that the maximum attainable concentration of charged nanometer particles is considerably smaller than theoretically expected. These results are important to the design of equipment aimed to generate or measure nanometer particles in concentrations as high as possible.

Rheological study of magnetic particle suspensions

Viscometry was used to investigate the microstructural properties of various magnetic particle suspensions. Our study was focused primarily on the shape (acicular versus plate-like effects) of magnetic particles, magnetic particle and/or floc concentration, as well as the effects of shear rate. Mean field theory and the Mooney equation were adopted to estimate a maximum packing concentration of magnetic particles ( ∝) as well as the intrinsic viscosity ([ η]). The values of ∝ and [ η] for different suspensions were used to interpret the microstructure of magnetic particle suspensions. Further, the Casson equation was employed to investigate the dependence of shear rate on the viscosity.

A Model for Predicting Airborne Dust Concentrations within a Ventilated Airspace

A random flight model is used to predict mean concentrations of 1μm diameter particles within mechanically ventilated airspace, an experimental section of a typical UK intensive livestock building. The model is used in conjunction with the statistical properties of air flow predicted by thek-ϵmodel, a standard computational fluid dynamics (CFD) model. The patterns of predicted mean particle concentrations are shown to be in accord wtih experimental findings. Particularly well predicted are the locations of maximum particle concentration and the shape of contours of constant particle concentration. The root mean square differences between predicted and measured normalized mean particle concentrations are 0·38 and 0·49, for two different locations of the particle source. These differences are shown to be 16% and 23% smaller than those obtained using a simple random flight model which is widely available within CFD packages.

Kinetics of localized adsorption of colloid particles

Methods of analyzing localized adsorption of colloid particles at solid/liquid interfaces were extensively reviewed. First, the initial adsorption fluxes calculated using the Levich-Smoluchowski approximation were discussed. The uniformly, and nonuniformly accessible interfaces were distinguished and the superiority of the former in experimental studies was pointed out. A criterion was introduced for estimating the relative significance of the bulk transfer and surface adsorption steps. It was shown that for the majority of experimental and practical situations the surface mass balance equation can be decoupled from the bulk continuity equation. Thus, in due course attention was focused on theoretical and experimental methods of determining the surface blocking parameter B. It was shown that for low and moderate surface concentration range the statistical mechanic approach can be effectively used for predicting B. By introducing the equivalent hard sphere radius r ∗ it became possible to analyze quantitatively blocking effects of interacting as well as nonspherical particles. The analytical solutions were compared with numerical simulation methods valid for the entire range of surface concentrations. The Monte-Carlo algorithm based on the random sequential adsorption (RSA) concept was compared with the sequential Brownian-Dynamics (SBD) method. Theoretical results obtained using these approaches were extensively discussed especially the role of repulsive electrostatic interaction among adsorbing particles. It was shown that these interactions diminish profoundly both the particle adsorption rate and the maximum surface concentration of particles forming “random” monolayers. When the electrostatic forces were operating (lower ionic strength) two distinctive adsorption regimes were predicted (i) fast Langmuir-type adsorption for short times and then (ii) very slow RSA-type approach to the maximum surface concentrations. As discussed such long lasting transient adsorption states could erroneously be interpreted as equilibrium adsorption isotherms. Then, the indirect and direct experimental methods aimed at a quantitative determination of particle adsorption kinetics were described. Illustrative experimental results performed for model latex suspensions were evoked. A satisfactory agreement with theoretical predictions was found for a variety of important physicochemical parameters studied. The RSA approach was found useful for describing particle adsorption kinetics for low and moderate surface concentrations in the case when the flow induced effects could be neglected. On the other hand, the SBD method was found of general validity especially in describing the hydrodynamic scattering effect observed experimentally for higher shear rates. Finally, the theoretical and experimental results concerning structure formation in adsorption processes were presented. The experimentally measured two-dimensional (2D) pair correlation function g12 of adsorbed particles suggested a liquid-like short range ordering occurring for larger surface concentrations. The extent of the 2D ordering was influenced by the adsorption mechanisms of particles, especially the presence of external field of forces.

Particle velocity characteristics of dilute to moderately dense suspension flows in stirred reactors

Measurements of particle mean and r.m.s. velocity were obtained by laser-Doppler velocimetry in solid-liquid turbulent flows in fully baffled stirred reactors driven by Rushton-type impellers of different sizes at rotational speeds of 150, 300 and 313 rpm. The effects of particle size, density and volumetric concentration were investigated. The maximum particle concentration at which the solid-phase velocity measurements could be made was improved from 0.02 to 2.5% when the refractive index of the continuous-phase was matched to that of the dispersed particles. The results showed a steep particle concentration gradient in the vertical direction below the impeller and a mild one above the impeller, that the particles lagged or led the bulk fluid when the flow direction was upwards and downwards, respectively, and that the particle turbulence levels were in general lower than those of the single-phase flow levels, especially in the impeller stream and wall jet regions. Particle velocities decreased with an increase in particle concentration, while the particle turbulence levels remained the same. The apparent relative velocity of glass particles was higher than that of Diakon by up to 2.5 times and the effect of the particle size, at least for the sizes used in the experiment, was negligible.

Ice particle concentrations and precipitation development in small polar maritime cumuliform clouds

AbstractIncreases in the concentrations of ice particles, as well as precipitation development, can proceed very rapidly in even quite small maritime cumuliform clouds. In the upper portions of these clouds, ice particle concentrations can increase from &lt;1 per litre to &gt;100 per litre in ∼10 minutes, even when cloud tops are warmer than −12°C. Just prior to the onset of the high ice particle concentrations in the upper regions of these clouds, a few liquid and frozen drizzle drops appear; these are followed almost immediately by high concentrations of largely vapour‐grown ice crystals. Subsequently, the ice particle concentrations gradually decline as the ice particles grow, aggregate and fall out. Crystal habits and sizes indicate that nearly all of the ice crystals encountered in the upper regions of the clouds form close to those regions.Prior to the formation of high ice particle concentrations, the clouds generally contained, near cloud top, droplets &gt;30μm diameter in concentrations &gt;1cm−3, and often drizzle drops (100–400μm diameter) in concentrations &gt;1 per litre. For maritime and continental cumuliform clouds with widths D ≥ 3km, the breadth of the droplet spectrum near the cloud top is a better predictor of the maximum ice particle concentrations that will develop in this region than is cloud‐top temperature.Exceptions to the general picture described above are the very narrow (D&lt;3km), ‘chimney‐type’, maritime cumuliform clouds with tops that quickly subside after reaching their maximum altitude. For cloud‐top temperatures between −7 and −13°C these clouds produce fewer ice particles (∼1–20 per litre), even though droplets with diameters &gt;30μm are present in concentrations &gt;1cm−3 and drizzle drops are often present in concentrations &gt;1 per litre.Several proposed mechanisms for the formation of high ice particle concentrations in clouds are discussed in the light of these field observations. In a detailed case‐study, ice particle concentrations produced by the ejection of ice splinters during riming were calculated to be about an order of magnitude less than the ice particle concentrations measured in the upper regions of the wider maritime clouds. The freezing of evaporating drops by contact nucleation could be responsible for the frozen drops that precede the high concentrations of vapour‐grown ice crystals. The latter could be produced in localized regions of unexpectedly high supersaturation that may be produced when drops begin to grow by collisions. This explanation is consistent with several of our findings concerning the conditions under which high concentrations of vapour‐grown crystals appear almost spontaneously in maritime clouds. Similarities between these natural crystals, and the conditions under which they appear, and those produced artificially by dry‐ice seeding and by aircraft flying through supercooled clouds are discussed.

A numerical study of steady plane granular chute flows using the Jenkins–Savage model and its extension

AbstractThe granular flow model proposed by Jenkins and Savage and extended by us is used here to construct numerical solutions of steady chute flows thought to be typical of granular flow behaviour.We present the governing differential equations and discuss the boundary conditions for two flow cases: (i) a fully fluidized layer of granules moving steadily under rapid shear and (ii) a fluidized bottom‐near bed covered by a rigid slab of gravel in steady motion under its own weight. The boundary value problem is transformed into a dimensionless form and the emerging system of non‐linear ordinary differential equations is numerically integrated. Singularities at the free surface and (in one case) also at an unknown point inside the solution interval make the problem unusual. Since the non‐dimensionalization is performed with the maximum particle concentration and the maximum velocity, which are both unknown, these two parameters also enter the formulation of the problem through algebraic equations. The two‐point boundary value problem is solved with the aid of the shooting method by satisfying the boundary conditions at the end of the soluton interval and these normalizing conditions by means of a minimization procedure. We outline the numerical scheme and report selective numerical results. The computations are the first performed with the exact equations of the Jenkins–Savage model; they permit delineation of the conditions of applicability of the model and thus prove to be a useful tool for the granular flow modeller.