Distinguishing Between Spherical and Nonspherical Particles by Measuring the Variability in Azimuthal Light Scattering

Azimuthal variabilities in scattering of monochromatic, circularly polarized light by individual spherical and nonspherical particles were measured using the DAWN-A (Wyatt et al. Appl. Opt. 27:2405–2421, 1988) differential light scattering detector. Measured aerosols included polystyrene latex spheres (PSL), quartz, and sodium chloride particles of 0.576, 0.741, 0.966, and 1.250 μm diameter. Signals from eight detectors at different azimuthal angles at a polar angle of 55° showed that variabilities for nonspherical particles significantly exceeded values for the spherical PSL. The probability that a quartz or sodium chloride particle would be incorrectly identified as a sphere are less than about 5% for all sizes investigated.     

Online Nanoparticle Mass Measurement by Combined Aerosol Particle Mass Analyzer and Differential Mobility Analyzer: Comparison of Theory and Measurements

A combination of a differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM) is used to measure the mass of NIST Standard Reference Materials (SRM®) PSL spheres with 60 and 100 nm nominal diameter, and NIST traceable 300 nm PSL spheres. The calibration PSL spheres were previously characterized by modal diameter and spread in particle size. We used the DMA to separate the particles with modal diameter in a narrow mobility diameter range. The mass of the separated particles is measured using the APM. The measured mass is converted to diameter using a specific density of 1.05. We found that there was good agreement between our measurements and calibration modal diameter. The measured average modal diameters are 59.23 and 101.2 nm for nominal diameters of 60 and 100 nm (calibration modal diameter: 60.39 and 100.7 nm) PSL spheres, respectively. The repeatability uncertainty of these measurements is reported. For 300 nm, the measured diameter was 305.5 nm, which is an agreement with calibration diameter within 1.8%.

The effect of spread in particle size on the APM transfer function is investigated. Two sources of the spread in “mono-dispersed” particle size distributions are discussed: (a) spread due to the triangular DMA transfer function, and (b) spread in the calibration particle size. The APM response function is calculated numerically with parabolic flow through the APM and diffusion broadening. As expected from theory, the calculated APM response function and measured data followed a similar trend with respect to APM voltage. However, the theoretical APM transfer function is narrower than the measured APM response.     

Evaluation of a Method to Measure Black Carbon Particles Suspended in Rainwater and Snow Samples

We conducted a detailed evaluation of a method for measuring the mass concentrations and size distributions of black carbon (BC) particles in rainwater and snow. The method uses an ultrasonic nebulizer (USN) and a single particle soot photometer (SP2). The USN disperses sample water into micron-size droplets at a constant rate and then dries them to release BC particles into the air. The masses of individual BC particles are measured by the SP2, using the laser-induced incandescence technique. The loss of BC particles during the extraction from liquid water to air depends on their sizes. We determined the size-dependent extraction efficiency using polystyrene latex (PSL) spheres with 12 different diameters between 107 and 1025 nm. The PSL concentrations in water were measured by the light extinction at 532 nm. The extraction efficiency of the USN showed a broad maximum of about 10% in the diameter range 200–500 nm and decreased substantially at larger sizes. The accuracy and reproducibility of the measured mass concentration of BC in sample water after long-term storage were about ±25% and ±35%, respectively. We tested the method by analyzing rainwater and surface snow samples collected in Okinawa and Sapporo, respectively. The measured number size distributions of BC in these samples showed negligible contributions of BC particles larger than 300 nm to the total number of BC particles. A dominant fraction of BC mass in these samples was observed in the diameter range 100–500 nm.

Copyright 2013 American Association for Aerosol Research     

Characterization of nanosized silica size standards

Nanosized silica size standards produced with a sol–gel synthesis process were evaluated for particle size, effective density, and refractive index in this study. Particle size and effective density measurements were conducted following protocol from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. Particle sizes were measured via electrical mobility analysis using a differential mobility analyzer (DMA) at sheath flow rates (Q_sh) of 3.0 and 6.0 L/min and a constant aerosol flow rate (Q_a) of 0.3 L/min. The measured mean and mode diameters agreed well with the labeled sizes in the size range 40–200 nm, with differences ranging from 0.03% to 0.8%, well within the labeled expanded uncertainties (95% confidence intervals) of 1.8%–2.2%. The coefficient of variation (CV) of the size distribution was 0.012–0.027 for 40–200 nm. Particle sizes measured for 20 nm and 30 nm standards showed size differences with respect to the certified sizes of 1.7% and 8.3% at Q_sh = 6.0 L/min, but the size distributions were narrow, with CV = 0.047–0.064. The average effective density for the range 40–200 nm measured with an aerosol particle mass analyzer (APM) was 1.9 g/cm³. The real component of the refractive index measured with an optical particle counter (OPC) was 1.41 at a wavelength of 633 nm. All properties (size, effective density, and refractive index) were stable and could be measured with good repeatability. From these evaluations, it was found that the nanosized silica size standards have good characteristics for use as size standards and constitute a feasible alternative to PSL particles.

© 2017 American Association for Aerosol Research     

Electrostatic Enhancement of the Collection Efficiency of Stainless Steel Fiber Filters

This study investigated the effect of using a stainless steel fibrous filter as the ground electrode of a point-to-plate electrostatic precipitator on particle penetration. The effect of the electrical field on particle penetration was investigated at 4 different filter face velocities (25, 50, 100, and 125 cm/s) for monodisperse PSL and silica particles (size range 0.05-1 m) as well as polydis perse ammonium sulfate, ammonium nitrate, and ultrafine indoor air particles. Particle penetration was greatly reduced by the application of the electrical field. By comparison, the performance of electrically active fibrous filters has been shown to rapidly degrade as particle loading exceeds 2-3 g/m². The effect of particle loading on particle penetration was also investigated at a filter face velocity of 50 cm/s. Particle penetration seemed to slightly decrease with particle loading and was independent of particle size. These results indicated that the accumulation of nonconductive particles up to 15 cm³/ m² does not create ''back corona,'' which would substantially decrease the collection efficiency of the grounded filter. In conclusion, our experiments suggest that using metal filters as the collector electrodes may be an attractive alternative design for electrostatic precipitators.     

Effect of particle characteristics on particle pickup velocity

Particle entrainment is investigated by measuring the velocity required to pick up particles from rest, also known as pickup velocity. Pickup velocity is a function of individual particle characteristics and interparticle forces. Although 5-200 μm particles are investigated, the work presented here focuses on the pickup of particles in a pile in the size range of 5-35 μm. These smaller particle sizes are more typical for pharmaceutical and biomedical applications, such as dry powder inhalers (DPIs). Pickup velocities varied from 3.9 to 16.9 m/s for the range of particle sizes investigated.There is a strong correlation between particle size and the dominating forces that determine the magnitude of the pickup velocity. Preliminary data investigating pickup velocity as a function of particle size indicate the existence of a minimum pickup velocity. For larger particle sizes, the mass of the particle demands a greater fluid velocity for entrainment, and for smaller particle sizes, greater fluid velocities are required to overcome particle-particle interactions. Pickup velocity remains relatively constant at very small particle diameters, specifically, less than 10 μm for glass spheres and 20 μm for nonspherical alumina powder. This can be attributed to the negligible changes in London-van der Waals forces due to a hypothesized decrease in interparticle spacing. At intermediate particle diameters, electrostatic forces are dominant.     

二氧化硅纳米颗粒对冰浆中冰晶粒径分布及存储演化特性的影响

分别以乙二醇水溶液和氯化钠水溶液为基液配制不同浓度的二氧化硅纳米流体并以此制备冰浆，通过显微装置获得冰晶图像，将实验得到的粒径分布与正态分布、对数正态分布、Gamma分布和Weibull分布进行对比，探讨纳米二氧化硅对冰晶平均粒径与分布特性的影响，同时观测储存过程中冰晶粒径演化规律。结果表明：加入纳米二氧化硅前后冰晶粒径分布均可用Gamma分布描述；纳米二氧化硅可起到细化晶粒的作用，而且添加浓度越高冰晶颗粒越小；当基液为乙二醇水溶液时，加入纳米二氧化硅可较好地抑制储存过程中的冰晶粒径增长，但基液为氯化钠水溶液时，纳米二氧化硅浓度需达到0.75%，才可抑制冰晶增大。研究结果证明，一定浓度的纳米二氧化硅流体可作为制冰溶液，起到减小冰晶粒径并控制冰晶生长的作用，这对冰浆流动和传热性能的改善具有重要的应用价值。     

Synthesis of hard mesoporous macro-spheres with silicate and aluminosilicate compositions

This study reports the synthesis mechanism and the influence of different variables in the preparation of mesoporous macro-spheres having silicate and aluminosilicate compositions. The spheres possess sizes in the range 200–1000 μm with a narrow particle size distribution and a significant mechanical resistance (hard macro-spheres). In addition, the presence of a highly regular mesoporosity (around 3 nm) and of a high surface area (normally about 1000 m²/g) make these materials as very interesting self-supported adsorbents or catalysts that could be directly applied without no binder, which is a great advantage compared to materials in powder form. The synthesis of the hard mesoporous spheres takes place in a biphasic system, generated by the use of tetrabutoxysilane as silica source, which is reacted with water and NaOH leading to the formation of small primary particles which initially do not present any mesoscopic ordering. However, after about 5 h of synthesis, the presence of cetyltrimethylammonium chloride, as surfactant, gives rise to the detection of a mesostructured silica material. Subsequently, after 9 h of synthesis the small mesostructured particles, with sizes in the micrometer range, acquire a spherical shape. Finally, the reorganization and fusion of these particles cause the formation of the silica macrospheres, with a particle size of several hundreds of micrometers. When the synthesis is carried in the presence of an aluminium source, the aluminosilicate materials so obtained exhibit smaller surface area and pore volume and greater particle size than the pure silica ones.     

Deposition of silica agglomerates in a cast of human lung airways: Enhancement relative to spheres of equal mobility and aerodynamic diameter

This paper reports on an experimental study of the deposition of well-characterized silica agglomerates in a cast of a section of a human lung. Deposition of the agglomerates is compared with the deposition of oleic acid spheres and sodium chloride particles for a range of mobility sizes, agglomerate properties (primary particle size and mass–mobility exponent) and inspiratory flow rates. In most cases, agglomerate deposition was significantly greater than that of the oleic acid and sodium chloride particles. Deposition of agglomerates with a more open structure was greater than that of relatively more compact (but still non-spherical) agglomerates. Deposition also increased with the flow rate. Because of the large physical size of the agglomerates, as well as the crenulated flow path through the model and the flow rate dependence, it is likely that interception is responsible for the enhanced deposition of the agglomerates.     

Assembling Colloidal Silica into Porous Hollow Microspheres

A non-surfactant-based synthesis approach to mesoporous hollow spheres through the use of colloidal silica is presented. Based on nanoparticle assembly chemistry developed previously for silica/polymer hybrid microcapsules, the room-temperature preparation follows a two-step sequence: (1) the electrostatic reaction of cationic polymer with an anionic salt solution, resulting in a suspension of salt-bridged polymer aggregates; and (2) the electrostatic reaction between this suspension and an aqueous suspension of nanoparticles (NPs). As a specific example, 13-nm silica particles, combined with polyallylamine and sodium citrate, gave silica/polymer hollow spheres with a mean diameter of 2.1 μm and a BET surface area of 4 m²/g. After calcination at 600 °C, the resulting silica-only microcapsules had a BET surface area of 259 m²/g, a modal pore size of 4.0 nm, and a pore volume of 0.38 cc/g, values that exceeded those of calcined silica NPs. This colloidal silica-based material is an example of the simultaneous control of pore size (at the nanometer scale) and particle morphology (at the micrometer scale) that is possible through charge-driven NP assembly.     

低成本制备纳米氧化锌工艺条件研究

以氯化锌和碳酸钠为原料,通过均匀沉淀法制备纳米氧化锌。借助激光粒度分析仪及透射电镜等分析手段,探索制备工艺条件对纳米氧化锌粒径及形貌的影响规律。最佳工艺条件：锌离子（Zn2+）初始浓度为0.74 mol/L,碳酸根与锌离子浓度比［c（CO32-）/c（Zn2+）］为1.1,反应温度为90 ℃,反应时间为40 min,煅烧温度为600 ℃,煅烧时间为1 h。在此条件下制备纳米氧化锌颗粒形貌为球形,粒度均匀,粒径约为25 nm,二次粒径（D50）为549.9 nm,分散性好。该工艺条件为低成本工业化制备纳米氧化锌提供了基础数据。     

表面改性球形二氧化硅的制备与表征

在醇水混合溶剂中,以氨作催化剂,通过正硅酸乙酯(TEOS)水解制备SiO2球形颗粒,并以十八醇作为改性剂对SiO2进行表面修饰。研究了TEOS浓度对SiO2球形颗粒粒径的影响,并用TEM、XPS、IR、TG-DTG对所得产品进行了表征。结果表明:在TEOS∶NH3∶H2O(物质的量比)为1∶4.4∶12.7时,可得到粒径约为450nm的球形SiO2颗粒;在其它条件不变的情况下,SiO2颗粒粒径随TEOS浓度的增大而增大;用十八醇作为改性剂得到的二氧化硅能在环己烷中很好地分散。     

Sizing of Aerosol in Gases Other Than Air Using a Differential Mobility Analyzer

The differential mobility analyzer (DMA) is a device that sizes aerosol particles based on their electrical mobility. The relationship between particle size and mobility depends, among other factors, on three gas specific parameters, namely, dynamic viscosity, mean free path, and Cunningham slip correction factor C c . Provided these parameters are known, DMA theory is expected to be valid independent of gas type. The present study demonstrates the sizing accuracy of DMAs for gases other than air using monodisperse polystyrene latex (PSL) spheres with nominal diameters of 60 nm, 149 nm, and 343 nm in He, Ar, H 2 , CO 2 , and N 2 O. Eliminating possible systematic errors due to uncertainties in DMA geometry and nominal PSL diameter by normalizing the measured PSL diameters to their respective diameters measured in air, the sheath flow rate Q sh and C c are expected to be the main sources for measurement errors. Since C c data are lacking for PSL spheres in gases other than air, an expression given by Allen and Raabe (1985b) was used to approximate C c . The experimental results of the present study are consistent with a 2% accuracy of this expression for C c , which is considerably better than the 5% accuracy estimated by Rader (1990) for a similar expression for oil drops. Finally, we discuss other aspects of operating a DMA with gases other than air, namely, flow meter calibration and dependence of electrical breakdown voltage on gas type. In the present study a thermal mass flow meter (MFM) was used to measure Q sh . Calibration of this MFM revealed that the gas specific MFM correction factors ( K factors) provided by the technical literature can be highly inaccurate (here between -12% and +31%). More accurate K factors are presented.     

A Wet Electrostatic Precipitator (WESP) for Soft Nanoparticle Collection

Many nanoparticle collection devices have limitations related to retention of particle integrity from bounce, shattering, or aggregation. Suspensions of soft nanoparticles (e.g., proteins, lipids) are required for drug delivery and therapy. To enable direct collection of soft nanoparticles into liquid media, a wet electrostatic precipitator (WESP) was designed and evaluated in this work. Different sections were used for ion generation and particle charging, for minimal contact between the corona wire and particles, which were charged using positive nitrogen ions. WESP dimensions and operating parameters were optimized using charge distribution modeling. The prototype WESP was designed for operation with a continuous flow of liquid over the collection plate, to allow continuous particle collection from the exit stream of an aerosol reactor. The collection efficiency of the WESP, in dry and wet modes, was measured using aerosols of monodisperse polystyrene latex (PSL), polydisperse sucrose, and stearic acid (soft lipid) particles, through SMPS measurements, corrected for diffusional losses, at the entry and exit of the device. Measured collection efficiency was 70%–90% for particles of sizes 80–600 nm diameter in reasonable agreement with theoretical estimates. However, for small particles (20–80 nm diameter) measured collection efficiency ranged 40%–70%, significantly lower than theoretical estimates, possibly from incomplete neutralization of negative charges attained during air-jet atomization. Transmission electron microscopy (TEM) images and dynamic light scattering (DLS) measurements confirm that wet collection produces a suspension of free, unaggregated nanoparticles with sizes similar to their measured mean mobility diameter.

Copyright 2012 American Association for Aerosol Research     

Dense CO2 antisolvent precipitation of levothyroxine sodium: A comparative study of GAS and ARISE techniques based on morphology and particle size distributions

Morphology and particle size distribution of levothyroxine sodium are experimentally investigated by comparing gas antisolvent (GAS) and atomized rapid injection for solvent extraction (ARISE) techniques using dense CO₂. Precipitation of levothyroxine sodium from ethanol was carried out at 25, 40 and 50 °C, with pressure in the 90–120 bar range and different concentrations of the organic solution. Particles produced by the GAS process are nanospheres whereas ARISE processed particles are either spherical or rod-like micro and nanoparticles. Particle size and size distributions of GAS processed levothyroxine sodium are in the 370–500 nm range, while the ARISE process produced particles in the 360–1200 nm range. In most cases, both techniques produced bimodal size distributions, due to particle agglomeration. The different morphological characteristics and particle size distributions of levothyroxine sodium obtained using GAS and ARISE at different operating conditions can be useful depending on the type of drug formulation chosen, as well as the route of drug administration and delivery system.     

Layer-by-Layer Self-Assembly of Ceramic Particles for Coating Complex Shape Substrates

Key parameters for controlling the layer-by-layer self-assembly of ceramic particles were studied using a model system, which consisted of an Si substrate, 100 and 500 nm silica spheres, and a polycation/polyanion combination that provided binding between the substrate and silica particles, as well as between the silica particle layers. The overall quality of the particle assemblies was mainly dictated by (1) the degree of electrostatic attraction between the negatively charged silica particles and the outermost positively charged electrolyte layer of a polyelectrolyte multilayer and (2) electrostatically repulsive interactions between the silica particles. The surface coverage of the silica particles improved with an increase of NaCl used in deposition of the polyelectrolyte layers and silica particles, and with an increased number of the polyelectrolyte/particle layers deposited. We found that the effects of particle size, polydispersity, and electrolyte concentration in the particle suspension on the surface coverage and morphology of the first silica particle layer deposited on the polyelectrolyte layer surface were highly coupled, and resolution of these effects was important in generating a rational basis for infiltrating a uniform coating of multilayer silica particle assemblies into a cellular structure.     

Aerosol measurement by laser doppler spectroscopy—I. Theory and experimental results for aerosols homogeneous

The basic theory, experimental techniques and results are presented describing a technique for sizing aerosol particles in situ using laser Doppler spectroscopy. Unlike conventional light scattering procedures which use average intensity information, this technique utilizes the Doppler shifted frequency of the scattered light produced by the Brownian motion of the aerosol particles to determine particle diffusion coefficients and size. Experiments were carried out using monodisperse dibutylpthalate aerosols and monodisperse polystyrene latex spheres, in concentrations ranging from 10³ to 10⁶ particles per cubic centimeter. Measured particle sizes were within 10 per cent of the size predicted by conventional light scattering methods for the DBP particles and the reported sizes of the PSL particles. Based on these results it is concluded that laser Doppler spectroscopy can be utilized to accurately measure aerosol particle size in situ.     

A High Resolution Study of the Effect of Morphology On the Mass Spectra of Single PSL Particles with Na-containing Layers and Nodules

The interpretation and quantification of measurements of particle composition by laser ablation based single particle mass spectrometry is complex. Among the most difficult systems to quantify are internally mixed particles containing alkali metals and organics. The alkali atoms in such particles tend to suppress the formation of other ions sometimes to below the detection limit. Here we present a study of the behavior of single particle mass spectral peak intensities as a function of the amount of the sodium containing compounds deposited on the surface of 240 nm polystyrene latex (PSL) spheres. We generate three morphologically distinct and well defined coating types: uniform layers, cubic nodules and rounded nodules, and measure the individual particle mass spectra as a function of the vacuum aerodynamic diameter with nanometer resolution. The data show that the probability of detecting the PSL spheres depends on the amount of the alkali metal on the PSL sphere surface, its morphological distribution and the ablation laser power. The data suggest that PSL spheres with localized Na-containing nodules are easier to detect than those which are completely encapsulated. We show, for example, that at low laser power, PSL particles that are completely encapsulated with Na-containing compounds, whose weight fraction is close to 50%, cannot be detected, while 35% of PSL spheres with same amount of coating can be detected if coating is localized in nodules on a fraction of the particle surface.     

Hydrocarbon Nanoparticles Formed in Flames and Diesel Engines

The formation of nanoparticles in laboratory hydrocarbon flames is reviewed in terms of particle morphology, chemical composition, and health hazards. The nascent nanoparticles in the nucleation mode have been widely reported in diverse laboratory flames, and are distinguished by their occurrence as singlet particles that form translucent images in transmission electron microscopy (TEM). Their sizes range from about 10 nm or more down to 2 to 3 nm, the limit of resolution of the TEM, and they possess a liquid-like quality. These particles are widely considered to be the precursor stage to the more readily observed carbonaceous aggregates consisting of chained primary particles that are opaque to the electron beam of the TEM. Nanoparticles sampled from the inverse diffusion flame and the particle effluent from diesel engines show a strong resemblance by GCMS analysis, and they contain many of the stabilomer PAHs and their isomers in the 200 to 302 atomic mass range. Many of these chemical species have high relative mutagenicities. Distinctive bimodal particle size distributions can be observed in both flame and engine samples. Recent TEM micrographs of diesel particulates show images of precursor-like nanoparticles, of as yet unknown chemical composition, that are formed in a diesel engine at many operating conditions.     

A Cylindrical Thermal Precipitator with a Particle Size-Selective Inlet

We designed a thermal precipitator in a cylindrical configuration with a size-selective inlet, and investigated its performance in experiments using differential mobility analyzer (DMA)-classified particles of sodium chloride (NaCl) and polystyrene latex (PSL). Our investigation was performed in two parts: (1) using the size-selective inlet to determine the best inlet-to-wall distance for optimal impaction of 1 μm particles; (2) using a simple inlet tube to measure particle collection via thermophoresis over a size range from 40 nm to 1000 nm. The results showed that the inlet had a particle cut-off curve, with a 50% particle cut-off Stokes number of 0.238, resulting in removing particles with sizes larger than 1 μm at an aerosol flow rate of 1.5 lpm. The thermophoretic particle collection efficiency in the prototype was measured without the size-selective inlet installed. The size dependence of the collection efficiency was negligible for particles with diameters ≤300 nm and became noticeable for those with diameters >300 nm. An analytical model was further developed to estimate the particle collection efficiency due to thermophoresis of the prototype under various aerosol flow rates and temperature gradients. For particles with diameters less than 400 nm, reasonable agreement was obtained between the measured data and the collection efficiency calculated from the developed analytical model. It was further concluded that the derived formula for the calculation of thermophoretic particle collection efficiency could serve as the backbone for future design of thermal precipitators in any configuration, when combined with the proper formula for the dimensionless thermophoretic particle velocity.

Copyright 2012 American Association for Aerosol Research