Comparative Bounce Properties of Particle Materials

A test impactor with a 10-μm 50% cutoff diameter at a flow rate of 7 liters/min was designed to measure particle bounce by the observed penetration to an optical counter. The effects of surface loading and relative humidity on particle bounce were determined, and a set of test criteria was developed to avoid these effects. The particle bouncing experiments were performed for ten common aerosols: ammonium fluorescein, sodium chloride, ammonium sulfate, potassium biphthalate, polystyrene latex, lycopodium spores, paper mulberry pollen, short ragweed pollen, glass beads, and Pinole soil. The differences in the fraction of bounce for different particle materials are discussed. Systematic measurements for ammonium fluorescein revealed a correlation between the fraction of particles bouncing and the particle kinetic energy and aerodynamic diameter. The EPA bounce criterion for PM ₁₀ samplers is criticized in terms of this correlation.     

Evaluation of Particle Bounce in Various Collection Substrates to be Used as Vaporizer in Aerosol Mass Spectrometer

The determination of the collection efficiency (CE) of particles during transport, vaporization, and ionization in the aerosol mass spectrometer (AMS), which uses vaporizer to evaporate non-refractory particles with subsequent ionization, is important for accurately quantifying the concentrations of chemical constituents. Particle bounce in the vaporizer can be considered as one of the most important parameters influencing the CE of particles. Substrates with various shapes (flat, cylindrical, reverse-conical, cup, trapezoidal, and reverse-T), materials (stainless steel, copper, tungsten, and molybdenum), pores with average sizes of 0.2, 1, 5, 20, and 100 μm, and mesh with a size of 79 μm, which can be a possible candidate for the vaporizer in the AMS, were constructed. Bounce fractions of sub-micrometer particles (polystyrene latex, oleic acid, and dioctyl phthalate) were determined using the differential mobility analyzer (DMA)-impactor technique under a constant impact velocity. For the porous substrate, the particle bounce fraction significantly decreased with increasing pore size and porosity, but there was an upper limit for the pore size above which the particle bounce fraction no longer decreased significantly (i.e., the rebounded particles successfully escaped from the pores). The mesh substrate also had a lower particle bounce fraction than the flat substrate. Among the tested materials, the copper substrate having the lowest hardness and elasticity had the lowest particle bounce fraction. In addition, the reverse-T shape substrate having more available surfaces for particle entrapment led to the reduction of particle bounce fraction. In terms of phase, the liquid particles had lower particle bounce fractions than the solid particles. Our results suggest that the vaporizer in the AMS should provide traps for multiple collisions of the rebounding particles with an appropriate porosity or mesh and should be made of low-hardness materials to minimize particle bounce.

Copyright 2015 American Association for Aerosol Research     

Reducing Particle Bounce and Loading Effect for a Multi-Hole Impactor

Aerosol sampling is used to evaluate the health hazards associated with particles deposited in the human breathing system. Impactors, which are extensively employed as aerosol samplers, have low collection efficiency because of particle bounce. The impaction plate is typically coated with oil or grease to prevent particle bounce. However, such coating materials cannot sustain long-term heavy particle loading.

In this study, the impaction plate was recessed, forming a cavity filled with Trypticase Soy Agar (TSA) to reduce particle bounce and re-entrainment. An ultrasonic atomizing nozzle was employed to generate challenge aerosols. An Aerodynamic Particle Sizer (APS) was utilized to measure the number concentrations and the size distributions upstream and downstream of the size-selective devices. A multi-hole impactor and Personal Environmental Monitor PM _2.5 (PEM–PM _2.5 ) were used to evaluate particle bounce and heavy particle loading. Liquid type-Dioctyl phthalate (DOP), soluble solid type-potassium sodium tartrate tetrahydrate (PST) and insoluble solid type-polymethyl methacrylate (PMMA) were investigated, as were different impaction surfaces/surface combinations. The multi-hole impactor coated with silicone oil was compared with a TSA-filled plate. Laboratory results demonstrate that the solid PST particles bounced off the TSA-filled plate less than off the silicone-coated aluminum plate. This study also used a 700-μm-thick layer of silicone oil to prevent TSA dehydration. The experimental results revealed that the silicone-TSA double layer minimized PST particle bounce during the two-hour heavy sampling (mass concentration was around 7.22 mg/m ³ ). Moreover, the PEM-PM _2.5 impactor yielded consistent results when the silicone-TSA double layer method was used. These results are useful for designing bounce-free impaction substrates during heavy load sampling.     

Prediction of Dimensionless Cutsize for Size‐Fractionated Measurements of Particles that Impact on a Sintered Stainless‐Steel Filter

Abstract

This investigation experimentally studied the penetration curve of particles that impact on a sintered stainless‐steel filter with various pore sizes, sampling flow rates and jet diameters. The penetration curves were compared to those with an aluminum foil substrate. Test data reveal that when the sintered stainless‐steel filter has larger pore sizes (100 µm or 40 µm), the particle penetration, P(%), is lower and the curve is less steep than that obtained from the aluminum foil substrate. The penetration curve of the sintered stainless‐steel filter with smaller pore size (5 µm) is close to that of the aluminum foil substrate. The dimensionless cutsize‐shift (the ratio of the dimensionless cutsize of sintered stainless‐steel filter to that of aluminum foil) falls as the pore sizes and the Reynolds number increase. Experimental data were then compared with theoretical results, and theory over‐predicted the dimensionless cutsize‐shift. Hence, a regression equation for the dimensionless cutsize‐shift is proposed by fitting the experimental data. The discrepancy between the experimental data and the regression prediction is within 4%. The regression equation can be used to predict the dimensionless cutsize for the size‐fractionated measurements of particles that impact on a sintered stainless‐steel filter with various sized pores and Reynolds numbers.     

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.     

Calculation of Leakage and Particle Loss in Filter Cassettes

Experimental evidence of aerosol bypass leakage around the filter in plastic filter cassettes prompted an investigation using computational fluid dynamics to explain particle penetration through the leak. Axi-symmetric models of a cassette with several leak dimensions were constructed. The models predicted that submicrometer particles penetrated the leak, but that larger particles impacted on the filter surface. Experimental data from another study clearly indicated that larger solid particles were being lost from the surface of the filter during sampling. When particle bounce was invoked as an explanation for this loss of sampled solid particles, the theoretical loss from the filter in cassettes with large leaks exhibited characteristics similar to the experimental data. For small leaks, the mass loss behavior appeared to be more complex.     

Particle Bounce During Filtration of Particles on Wet and Dry Filters

This paper experimentally examines the bounce and immediate re-entrainment of liquid and solid monodisperse aerosols under a stable filtration regime (precake formation) by wet and dry fibrous filters. PSL and DEHS were the solid and liquid aerosols, respectively, used in four monodisperse sizes of 0.52, 0.83, 1.50, and 3.00 w m. Three different fibrous filters were used to filter the aerosol streams, and the efficiency of the filtration process for each aerosol type under dry and wet regimes was measured. It was found that the solid particles generally exhibited a lower fractional filtration efficiency than liquid particles, although this difference decreased in the smaller size fractions. The difference between solid and liquid efficiencies was found to be greatest in the 1.5 w m size range. As particle sizes of liquid/solid aerosols and filtration parameters were similar, this difference is most likely to be due to the effect of particle bounce and or immediate re-entrainment occurring inside the filter, with the greater efficiency of filtration of the liquid particles being due to their greater capacity to plastically/elastically deform in order to absorb the impact forces. However, for the wet filtration regime (each fibre of the filter was coated by a film of water), no significant difference in filtration efficiency was detectable between solid and liquid aerosols. Therefore, the conclusion can be drawn that the either the bounce effect of the particles is inhibited by the liquid film, or the filtration conditions in the wet filter are so different that the aerosol properties are less significant with respect to capture.     

Filtration of nanosized particles with different shape on oil coated fibres

In our previous work it has been shown that perfectly spherical polystyrene latex (PSL) particles have higher filtration efficiency compared to cubic magnesium oxide (MgO) particles of the same electrical mobility as PSL particles. This disparity was ascribed to the different nature of motion of the spherical and cubic particles along the fibre surface, following the initial collision. After touching the fibre surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could slide or tumble. During tumbling, the area of contact between the particle and the fibre changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of particle trajectory. In this project, the polypropylene filter was coated with a thin layer of mineral oil that was used to absorb the energy and, respectively, to minimize particle motion along the fibre after collision. The filtration efficiency of spherical PSL, and cubic MgO particles was measured in the size range of 50–300 nm, for filtration velocity of 10 and 20 cm/s. It was found that, regardless of shape, both particle types have very similar filtration efficiency. The theoretical predictions are in good agreement with our experimental results. Therefore, the conclusion can be drawn that the oil coating minimizes the amount of particle motion along the fibre after initial collision, making results for all particle shapes similar.     

Calibration of the QCM Impactor for Stratospheric Sampling

The performance of the QCM impactor (Model 3000, California Measurements, Sierra Madre, CA) has been evaluated at inlet pressures of 59 and 88 Torr, corresponding to the conditions of stratospheric sampling aboard the U-2 aircraft. Impactor mass flow rates and stage operating pressures were measured as a function of inlet pressure. The collection efficiencies of monodisperse liquid oleic acid aerosols were measured for individual impactor stages by counting particle penetration with an optical particle counter. Modification of the optical counter for this low pressure application is described. Response of the assembled QCM to near-mono-disperse Apiezon grease aerosols was measured using the signal from the quartz crystal impaction substrates. Aerodynamic cutoff diameters as a function of inlet pressure agree with impaction theory for most stages. Good comparison is found between the response of the QCM microbalance impaction plates and the optical particle counter data. For solid particles the QCM collection is poor due to rebound of the particles off the uncoated surface.     

Deposition of Charged Aerosol Particles on a Substrate by Collimating Through an Electric Field Assisted Coaxial Flow Nozzle

Electroaerodynamic (EAD) jet printing, where aerodynamic force is coupled with electrostatic force in order to obtain a wide range of controlled pattern sizes, is introduced. Charged and sheathed aerosol particles yield a high deposition rate even at low velocity owing to the force of their electrostatic attraction to the substrate. In this study, two coaxial nozzles (inner diameters of 6 mm and 100 μm) were designed and tested theoretically and experimentally in order to observe the effects of electrostatic force, particle size, and air flow rate on particle trajectory and dot pattern size. A higher sheath air flow rate (higher Stokes number) caused the aerosol jet stream to be focused. For Stokes numbers higher than 1, the effect of applied voltage on pattern size was less than that of the sheath air flow rate. However, for Stokes number lower than 1, the pattern size was affected by both the applied voltage and the sheath air flow rate. After incorporating all data, the diameter of the particle deposition area (W_p) was expressed as a function of nozzle diameter (W), sheath air flow rate (Q_sheath), aerosol flow rate (Q_aerosol), Stokes number (Stk), and Electrostatic number (Es). Three different equations were obtained for Stk < 1, for 1 ≤ Stk < 5, and for Stk ≥ 5, respectively. These equations would be used to predict pattern width for given conditions of aerosol and sheath flow rates, particle size, electric field, and nozzle size.

Copyright 2013 American Association for Aerosol Research     

Influence of Particle Shape on Filtration Processes

The influence of particle shape on filtration processes was investigated. Two types of particles, including spherical polystyrene latex (PSL) and iron oxide, and perfect cubes of magnesium oxide, were examined. It was found that the removal efficiency of spherical particles on fibrous filters is very similar for corresponding sizes within the range of 50–300 nm, regardless of the fact that the densities of PSL and iron oxide differ by a factor of five. On the other hand, the removal efficiency of magnesium oxide cubic particles was measured, and found to be much lower than the removal efficiency for the aerodynamically similar spheres. Such disparity was ascribed to the different nature of the motion of the spherical and cubic particles along the fiber surface, following the initial collision. After touching the fiber surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could either slide or tumble. During tumbling, the area of contact between the particle and the fiber changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of the particle trajectory. The extra probability of particle bounce by the cubes was derived from the experimental data. The particle kinetic energy was proposed to be responsible for the difference in removal efficiency of particles with alternative shapes, if all other process parameters remain the same. The increase in kinetic energy is shown to favor the increase of the bounce probability.     

Structural Property Effect of Nanoparticle Agglomerates on Particle Penetration through Fibrous Filter

Most filtration studies have been conducted with spherical particles; however, many aerosol particles are agglomerates of small primary spheres. Filtration efficiency tests were conducted with silver NP agglomerates, with the agglomerate structure controlled by altering the temperature of a sintering furnace. The mobility diameter and mass of the silver NP agglomerates were measured using a differential mobility analyzer together with an aerosol particle mass analyzer. From these measurements, it was found that the fractal-like dimension, D _fm, varied from 2.07 to 2.95 as the sintering temperatures was increased from ambient to 600°C. The agglomerates were essentially fully coalesced at 600°C allowing direct comparison of the filtration behavior of the agglomerate to that of a sphere with the same mobility diameter. Other agglomerate properties measured include the primary diameter, the agglomerate length and aspect ratio, and the dynamic shape factor.

Agglomerate filtration modeling with no adjustable parameters has been investigated in terms of diffusion, impaction, and interception. The model results agree qualitatively with the experimental results in the particle size range of 50 to 300 nm. The results indicated that the larger interception length of agglomerates is responsible for the smaller penetration through a fibrous filter in comparison to spherical particles with the same mobility diameters.     

The deposition of haematite particles from flowing water

Deposition of 0.2 μ haematite (α – Fe₂O₃) particles from suspension in turbulently flowing water, Reynolds Number = 11000, onto metal tube surfaces has been studied with particular reference to the effect of pH. Measurements on stainless steel showed trivial deposition outside the pH range 5 to 8. Within this range deposition rose sharply to a peak at pH = 6.2. This is similar to the findings of Kuo and Matijevic (1980) who studied haematite particle deposition onto a packed bed of stainless steel beads under laminar flow conditions. This shows that particle/surface interactions govern deposition in this system and that hydrodynamic and geometric factors are secondary.     

Particle Count Statistics Applied to the Penetration of a Filter Challenged with Nanoparticles

Statistical confidence in a single measure of filter penetration (P) is dependent on the low number of particle counts made downstream of the filter. This article discusses methods for determining an upper confidence limit (UCL) for a single measure of penetration. The magnitude of the UCL was then compared to the P value, UCL ≤ 2P, as a penetration acceptance criterion (PAC). This statistical method was applied to penetration trials involving an N95 filtering facepiece respirator challenged with sodium chloride and four engineered nanoparticles: titanium dioxide, iron oxide, silicon dioxide, and single-walled carbon nanotubes. Ten trials were performed for each particle type with the aim of determining the most penetrating particle size (MPPS) and the maximum penetration, P _max. The PAC was applied to the size channel containing the MPPS. With those P values that met the PAC for a given set of trials, an average P _max and MPPS was computed together with corresponding standard deviations. Because the size distribution of the silicon dioxide aerosol was shifted toward larger particles relative to the MPPS, none of the ten trials satisfied the PAC for that aerosol. The remaining four particle types resulted in at least four trials meeting the criterion. MPPS values ranged from 35 to 53 nm with average P _max values varying from 4.0% for titanium dioxide to 7.0% for iron oxide. The use of the PAC is suggested for determining the reliability of penetration measurements obtained to determine filter P _max and MPPS.

© 2013 American Association for Aerosol Research     

耐腐蚀超疏水超亲油不锈钢网的制备及其在油水分离过程中的应用

以不锈钢网为基底,通过化学刻蚀法制备微米级粗糙表面,通过一步浸泡法将st9ber法制得的疏水亲油纳米Si O2颗粒沉积到粗糙的不锈钢网表面,制备了具有微纳二级粗糙结构的超疏水超亲油不锈钢网。利用扫描电子显微镜(SEM)、傅里叶变换红外光谱仪(FT-IR)和接触角测量仪(CA)表征了超疏水超亲油不锈钢网的表面形貌、化学组成和润湿性能,并将其用于油水分离过程中。结果表明,疏水亲油纳米Si O2颗粒成功的沉积到不锈钢网表面;水滴在超疏水超亲油不锈钢网上的接触角最大为151°,煤油的接触角为0°;制备的超疏水超亲油不锈钢网不仅能高效的分离不同种类油和水的混合物,还能高效的分离油和腐蚀性液体(强酸或强碱水溶液)的混合物,其耐腐蚀特性可满足复杂环境下的油水分离要求。     

Penetration of Particles into Buildings and Associated Physical Factors. Part I: Model Development and Computer Simulations

The PM_2.5 standard proposed by the U.S. Environmental Protection Agency (EPA) has stimulated research on the relationships between particulate matter concentrations and the exposures and subsequent health responses of sensitive subpopulations, such as the elderly. Since individuals in these subpopulations may spend more than 90% of their time indoors, understanding the relationship between outdoor particle concentrations and those found in indoor microenvironments is critical. This research resulted in a time-dependent indoor air quality model incorporating all potential particle sources and loss mechanisms. Monte Carlo simulations of the model identified the mechanisms, such as particle loss during penetration through the building envelope, that modify the outdoor particle size distribution during transport into the interior of a building, calculated indoor-to-outdoor (I/O) concentration ratios, and estimated penetration factors as a function of particle size. Indoor particle generation and transport of outdoor particles through the HVAC system were the most important contributors to the indoor concentration in residential and commercial buildings, respectively. The most significant removal mechanisms included ventilation through and particle removal by the HVAC filter if an HVAC system was present, or particle deposition on indoor surfaces if an HVAC system was not present. The modeled I/O concentration ratios varied between 0.05 and 0.5, depending on particle size and type of ventilation system, and agreed well with published experimental results. Penetration factors less than unity were calculated for particles with aerodynamic diameters larger than 0.2 θ m if the air exchange rate and steady-state I/O concentration ratio were correlated during the simulations. An additional correlation between the air exchange rate and particle deposition velocity is required if penetration factors less than unity are to be modeled for particles with aerodynamic diameters smaller than 0.2 θ m. These results     

Nanoparticle Detection by Aerosol Mass Spectrometry

An aerodynamic lens system with efficient transmission of particles in the 10-300 nm size range is used to study the efficiency of nanoparticle detection by laser ablation mass spectrometry with 193 nm and 266 nm radiation. Ideally, all particles in the beam path when the laser fires should be detected. However, the probability of particle detection is much less than 1 and dependent upon the particle type, defined by particle size and chemical composition, and the ablation conditions, defined by the laser wavelength and irradiance. Particles above 100 nm can be ablated and detected with near unit efficiency. Below 100 nm, the detection probability decreases with decreasing particle size and salt particles (sodium chloride, potassium chloride) are detected with higher efficiency than organic particles (oleic acid, 3-nitrobenzyl alcohol). The results are discussed in relation to the mechanism of laser ablation and the instrumental requirements for particle detection.     

A Mathematical Study of Particle Rebound for Thin-Walled Samplers

The aspiration of spherical particles which are suspended without sedimentation in an ideal fluid that flows past a thin-walled sampler is investigated. A recent experimental study showed that secondary aspiration caused by particles bouncing off the walls of the nozzle and subsequently entering the nozzle inlet is of importance in determining the aspiration efficiency of samplers in many situations. Therefore the extent of oversampling caused by particle rebound is investigated using a numerical method. Three situations are considered, namely, when the particles (1) adhere to the sampler surface on impact, (2) bounce off the surface and all those that hit within a certain area, known as the stagnation area, are eventually sampled, and (3) bounce off the surface with no loss of energy. Results for the three cases are compared and they are found to display the same trends as found in the experimental investigations.     

A Partial Theory of Atomization in Internal Mix Swirl Nozzles

The aspiration of spherical particles which are suspended without sedimentation in an ideal fluid that flows past a thin-walled sampler is investigated. A recent experimental study showed that secondary aspiration caused by particles bouncing off the walls of the nozzle and subsequently entering the nozzle inlet is of importance in determining the aspiration efficiency of samplers in many situations. Therefore the extent of oversampling caused by particle rebound is investigated using a numerical method. Three situations are considered, namely, when the particles (1) adhere to the sampler surface on impact, (2) bounce off the surface and all those that hit within a certain area, known as the stagnation area, are eventually sampled, and (3) bounce off the surface with no loss of energy. Results for the three cases are compared and they are found to display the same trends as found in the experimental investigations.     

Sampling efficiency of low-volume PM10 inlets with different impaction substrates

The louvered 16.7?L min^?1 PM₁₀ inlet is commonly used in PM₁₀ and PM_2.5 FRM samplers or FEM monitors. Its sampling efficiency is influenced by particle bounce, re-entrainment, and overloading since the PM₁₀ inlet contains a PM₁₀ impactor with an uncoated impaction surface. In this study, a modified PM₁₀ (M-PM₁₀) inlet with an oil-soaked glass fiber filter substrate supported by an oil-soaked porous metal disc was developed to eliminate the particle bounce and overloading effects. The oiled M-PM₁₀ inlet and the traditional PM₁₀ inlets with and without grease coating were collocated at the field for long-term comparison tests. The results show that the traditional uncoated PM₁₀ inlet which is cleaned initially but not cleaned daily afterwards oversamples PM₁₀ concentration after one 24-h sampling day and has the high positive average sampling bias during 14 sampling days due to particle bounce and re-entrainment. The grease-coated PM₁₀ inlet without daily cleaning shows a better performance with a smaller sampling bias, but it still oversamples PM₁₀ concentrations after the first three 24-h sampling days and then undersamples after 10 sampling days due to particle bounce and overloading effects, respectively. In comparison, the M-PM₁₀ inlet shows a good performance with a small average sampling bias during 35 sampling days since vacuum oil wicks up through the deposit to eliminate particle bounce and overloading. It is suggested that the oiled M-PM₁₀ inlet can be used to replace the traditional EPA PM₁₀ inlet and for long-term sampling of over 1 month without the frequent maintenance need.

Copyright © 2019 American Association for Aerosol Research