Particle Electrical Mobility Research Articles

Particle charge distributions in the effluent of a flow-through atmospheric pressure low temperature plasma

Atmospheric pressure low-temperature plasmas are often utilized to perform particle synthesis, treatment, and removal. It is well-known that dust particles are highly negatively charged in these plasmas; however, little is known about dust particle charging behavior as particles leave the plasma volume and pass through the spatial afterglow region. In this work, monodisperse particles of various sizes and work functions were introduced into an atmospheric pressure radiofrequency capacitively coupled flow-through plasma. Dust particle electrical mobility distributions downstream of the flow-through plasma were measured utilizing a differential mobility analyzer in conjunction with a condensation particle counter at various gas flow velocities. Charge distributions were determined from the measured electrical mobility distributions. Experiments confirm that particles become less negatively charged, and even net-positively charged after leaving the plasma volume, with a distribution that follows a shifted Boltzmann charge distribution. Additionally, particle charge in the effluent of the flow-through plasma is negligibly dependent on work function but highly size and flow velocity dependent. Larger particles were shown to have a higher magnitude of charge under all studied conditions; however, particle polarity was switchable by varying gas flow velocity. The charging dynamics were simulated utilizing a constant number Monte Carlo model that accounts for electron temperature decay and the transition from ambipolar to free diffusion of electrons and ions in the spatial afterglow. Simulation results also suggest that, at the same flow velocity, larger particles obtain a greater magnitude of charge, negative or positive. The decrease in electron mobility and the difference between ion and electron convective loss rates create an ion-rich region in the plasma effluent that promotes ion–particle collisions and drives particle charge removal and even reversal of polarity. Larger particles more favorably collide with energetic species in these environments, which results in higher charge states.

A Build-In Data Inversion Method to Retrieve Aerosol Size Distributions for a Portable Ultrafine Particle Sizer (PUPS)

A build-in data inversion method to retrieve aerosol size distributions based on the principle of particle electrical mobility has been introduced, whose required computational effort is low for a portable ultrafine particle sizer (PUPS). The PUPS is cost-effective for the measurements of the fine and ultrafine particles in polluted environments near-source (e.g., cities with high traffic density, freeways, airports or stationary combustion sources). The particle sizer is mainly composed of a unipolar charger, a plate differential mobility analyzer (PDMA), and a Faraday cup electrometer (FCE). The classification efficiencies of the PDMA are strongly dependent on the charging distribution of the unipolar charger, in which multiple charging is more significant than that for a bipolar charger. To reduce the excessive overlap in the kernel function caused by multiple charging, a guidance method for selecting the classification voltages and operating parameters of the PDMA is proposed with the help of MATLAB. Subsequently, a combination of the nonnegative least squares (NNLS) algorithm and post facto smoothing method has been employed to derive the discretized solution of the Fredholm integral equation of the first kind. In the end, the accuracy and stability of the proposed approach are tested under a range of particle size distribution scenarios. The synthesized data results show that for the unimodal aerosol distribution, almost all of the relative errors are less than 20 %, regardless of nonideal operating conditions. The inversion algorithm can be run on Cortex-M3, an ARM embedded chip for low-cost and low-power consumption applications. When a reasonable error range can be permitted, the inversion algorithm can meet the requirements of the PUPS.

Numerical investigation of the separation mechanism in an electrostatic aerosol-to-hydrosol separator by glow corona discharge: a quantitative comparison of the effects of ionic wind and Coulomb force

Electrostatic separation of particles from air onto a liquid surface has attracted increasing attention for various applications such as wet electrostatic precipitators and bioaerosol samplers. In this study, the electrostatic separation of particles onto the liquid surface is numerically modeled in a simplified manner. The simulated separation efficiency is validated by experimental values reported in literature. The effects of ionic wind and Coulombic force on particle separation for various electrical mobilities, applied voltages, and air flow rates are studied by decomposing the total separation efficiency into two components: Coulombic and ionic-wind separation efficiencies. Both Coulombic and ionic-wind separation efficiencies show the same behaviors as the total separation efficiency with changes in the operating conditions: they increase with increasing applied voltage and particle electrical mobility, and with decreasing air flow rate. Overall, the ionic-wind separation efficiency is higher than the Coulombic separation efficiency, mainly because of the short residence time. However, when the applied voltage and particle electrical mobility are high, and the air flow rate is low, the Coulombic force outperforms the ionic wind. Time-scale analysis well explains the relations between the relative importance of each separation mechanism and the operating parameters.

A fast integrated mobility spectrometer for rapid measurement of sub-micrometer aerosol size distribution, Part I: Design and model evaluation

A fast integrated mobility spectrometer (FIMS) was previously developed to characterize submicron aerosol size distributions at a frequency of 1Hz and with high size resolution and counting statistics (Kulkarni & Wang, 2006a, 2006b; Olfert, Kulkarni, & Wang, 2008). However, the dynamic size range of the FIMS was limited to one decade in particle electrical mobility. It was proposed that the FIMS dynamic size range can be greatly increased by using a spatially varying electric field (Wang, 2009). This electric field creates regions with drastically different field strengths in the separator, such that particles of a wide diameter range can be simultaneously classified and subsequently measured. A FIMS incorporating this spatially varying electric field is developed. This paper describes the theoretical frame work and numerical simulations of the FIMS with extended dynamic size range, including the spatially varying electric field, particle trajectories, activation of separated particles in the condenser, and the transfer function, transmission efficiency, and mobility resolution. The influences of the particle Brownian motion on FIMS transfer function and mobility resolution are examined. The simulation results indicate that the FIMS incorporating the spatially varying electric field is capable of measuring aerosol size distribution from 8 to 600nm with high time resolution. The experimental characterization of the FIMS is presented in an accompanying paper.

Aerosol effective density measurement using scanning mobility particle sizer and quartz crystal microbalance with the estimation of involved uncertainty

Abstract. In this work, we have used a scanning mobility particle sizer (SMPS) and a quartz crystal microbalance (QCM) to estimate the effective density of aerosol particles. This approach is tested for aerosolized particles generated from the solution of standard materials of known density, i.e. ammonium sulfate (AS), ammonium nitrate (AN) and sodium chloride (SC), and also applied for ambient measurement in New Delhi. We also discuss uncertainty involved in the measurement. In this method, dried particles are introduced in to a differential mobility analyser (DMA), where size segregation is done based on particle electrical mobility. Downstream of the DMA, the aerosol stream is subdivided into two parts. One is sent to a condensation particle counter (CPC) to measure particle number concentration, whereas the other one is sent to the QCM to measure the particle mass concentration simultaneously. Based on particle volume derived from size distribution data of the SMPS and mass concentration data obtained from the QCM, the mean effective density (ρeff) with uncertainty of inorganic salt particles (for particle count mean diameter (CMD) over a size range 10–478 nm), i.e. AS, SC and AN, is estimated to be 1.76 ± 0.24, 2.08 ± 0.19 and 1.69 ± 0.28 g cm−3, values which are comparable with the material density (ρ) values, 1.77, 2.17 and 1.72 g cm−3, respectively. Using this technique, the percentage contribution of error in the measurement of effective density is calculated to be in the range of 9–17 %. Among the individual uncertainty components, repeatability of particle mass obtained by the QCM, the QCM crystal frequency, CPC counting efficiency, and the equivalence of CPC- and QCM-derived volume are the major contributors to the expanded uncertainty (at k = 2) in comparison to other components, e.g. diffusion correction, charge correction, etc. Effective density for ambient particles at the beginning of the winter period in New Delhi was measured to be 1.28 ± 0.12 g cm−3. It was found that in general, mid-day effective density of ambient aerosols increases with increase in CMD of particle size measurement but particle photochemistry is an important factor to govern this trend. It is further observed that the CMD has good correlation with O3, SO2 and ambient RH, suggesting that possibly sulfate secondary materials have a substantial contribution in particle effective density. This approach can be useful for real-time measurement of effective density of both laboratory-generated and ambient aerosol particles, which is very important for studying the physico-chemical properties of particles.

Aerosol electrical mobility spectrum analyzer

An aerosol electrical mobility spectrum analyzer (AEMSA) that consists of saturator, mobility analyzer, condenser, and position detector is newly devised. This device can detect charged particle electrical mobility spectrum regardless of its polarity without electrical scanning procedure in nanometer size range. The performance of AEMSA is tested with equilibrium charging state particles using soft X-ray neutralizer and Am 241 radioactive source and compared with the approximated Fuchs model ( Wiedensohler, 1988) and Boltzmann prediction. The result shows the approximated Fuchs model predicts better for neutralized nano-particles.

Electrical Mobility Separation of Airborne Particles Using Integrated Microfabricated Corona Ionizer and Separator Electrodes

Airborne particles are separated according to their electrical mobilities using a microfabricated corona ionizer and separator electrodes. Oleic acid particles with sizes ranging from 30 to 300 nm are used to characterize the device. They are generated using a TSI 3075 constant output atomizer. These particles are electrically charged by a microfabricated corona ionizer, and the resultant particle electrical mobility is a function of the size of the particle. A varying DC potential difference of 0-2 kV across the separator electrodes selects charged particles of various electrical mobilities. These separated particles are subsequently counted using a TSI 3025A condensation particle counter. The device demonstrated its ability to separate particles between 50 and 130 nm into five distinct size bins. The operational flow rate is 0.5 L/min, and the micropin-between-planes corona ionizer operates at 1.3 kV with 7 muA. The theoretical and experimental electrical mobilities of the particles are compared.

Charged Droplet Dynamics in the Submicrometer Size Range

The highly charged droplets produced by electrospray are of significant importance in analytical chemistry, particularly in mass spectrometry. In spite of this, little is known about the mechanisms of charge loss for highly charged droplets in the submicrometer size range. In this study, electropsrayed droplet charge loss dynamics in the submicrometer size range were examined for the first time using tandem differential mobility analysis (TDMA) of aerosol particles originally enclosed in methanol-water droplets. After complete droplet evaporation, the remaining aerosol particles carried the nonejected charges from droplets; thus, particle electrical mobility spectra are reflective of the electrical mobility spectra of droplets of similar size. Bayesian data inversion was used to compare measured electrical mobility spectra to models for droplet losing charge by Coulombic fission and by single ion emission. Data inversion showed that, at a size of approximately 40 nm, a transition of the mechanism of charge loss occurred. Methanol-water droplets larger than this size lost charge through Coulombic fissions at the Rayleigh limit with an effective surface tension of 0.050 N m(-1), while droplets in the 10-40 nm size range lost charge through ion emission, maintaining a constant electric field of 1.1 V nm(-1) on their surface during ion emission. This transition was observed experimentally for the first time in this study and is in good agreement with theoretical predictions from prior study of ion emitting nanodroplets. The data presented here provide the necessary link between studies of highly charged micrometer sized droplet and those of highly charged liquid clusters.

Use of an electrical aerosol detector (EAD) for nanoparticle size distribution measurement

Recently, Nanoparticle Surface Area Monitor (NSAM, TSI model 3550) and EAD (EAD, TSI Model 3070A) have been commercially available to measure the integral parameters (i.e., total particle surface area and total particle length) of nanoparticles. By comparison, the configuration of the EAD or NSAM is similar to that of electrical mobility analyzer of the early generation for particle size distribution measurement. It is therefore possible to use the EAD or NSAM as a particle sizer. To realize the objective of using the EAD as a sizer, we characterized the average electrical charges of monodisperse particles passing through the EAD particle charger and ion trap set at voltages ranging from 20 to 2500 V. The average charge data collected at different ion-trap voltages were then summarized by the empirical correlation using the parameter of Zp*V, where Zp is the particle electrical mobility and V is the ion-trap voltage. A data-reduction scheme was further proposed to retrieve the size distribution of sampled particles from the EAD readout at different ion-trap voltages. In the scheme, the functional format of each mode in a number size distribution of particles was assumed as log-normal, but the number of modes in an entire size distribution is not limited. A criterion was used to best fit the simulated EAD readouts with experimental ones by varying the count median diameter (CMD), geometric standard deviation (σg), and total particle number (Nt) of each mode in a particle size distribution. Experiments were performed to verify the proposed scheme.

Performance study of a unipolar aerosol mini-charger for a personal nanoparticle sizer

We designed and constructed a corona-discharge-based unipolar mini-charger and experimentally evaluated its performance. The simple design and compact size make the prototype mini-charger well suited for use with portable aerosol sizing instruments based on particle electrical mobility techniques. In this work, we optimized the extrinsic charging efficiency of the prototype mini-charger for two different aerosol flow rates (i.e., 0.3 and 1.5 lpm). The optimal settings for the prototype operation are: (i) a corona current of 1 μ A and an ion driving voltage of 40 V for the 0.3 lpm flow rate, and (ii) 2 μ A and 120 V for the 1.5 lpm flowrate. Both intrinsic and extrinsic charging efficiencies of the prototype mini-charger at the optimal operational conditions were then evaluated for particles in diameters ranging from 10 to 200 nm. The intrinsic charging efficiency of the prototype reaches 100% at 20 nm for the 0.3 lpm flow rate, and 45 nm for the 1.5 lpm flow rate. The higher intrinsic charging efficiency at the low flowrate is due to the longer residence time of particles in the device. The extrinsic charging efficiency, however, is higher for the 1.5 lpm flow rate than for the 0.3 lpm flow rate due to charged particle loss in the prototype. Charge distributions of test monodisperse particles of different sizes were also measured by the Tandem-DMA technique.

Development of an LDMA-FCE System for the Measurement of Submicron Aerosol Particles

An LDMA-FCE (long differential mobility analyzer–faraday cup electrometer) system capable of measuring particle sizes up to larger than 1 μm was developed, and a data reduction process for converting an electrical mobility distribution measured by the LDMA-FCE system into a particle size distribution was derived by the modification of a process previously used for a DMA-CNC (differential mobility analyzer–condensation nucleus counter) system. The performance of the LDMA was evaluated by a tandem DMA system, electrical classification of the PSL standard particles, and field emission-scanning electron microscopy observations. The accuracy of the modified data reduction process was confirmed by comparing particle size distributions measured by the LDMA-FCE system with those determined using an LDMA-CNC system. Particle electrical mobility distributions, as measured by the tandem DMA system, were in good agreement with the theoretical distribution. Good agreement was also found between particle size distributions measured with the LDMA-FCE and the LDMA-CNC systems. Thus, the LDMA-FCE system performed well and represents a useful device for measuring the size of submicron aerosol particles over a wide range.

Comparison of ambient particle surface area measurement by epiphaniometer and SMPS/APS

During measurement campaigns at an urban background and a rural site, simultaneous measurements of particle size distributions using a scanning mobility particle sizer (SMPS)/aerodynamic particle sizer (APS) combination and Fuchs surface using an epiphaniometer have been made. The epiphaniometer was calibrated using sub-100 nm monodisperse aerosol and it was found that a calibration based upon particle electrical mobility diameters measured with a SMPS was consistent irrespective of the use of singlet particles of sodium chloride and ammonium sulphate or clusters of carbon. The field intercomparison of surface areas derived directly from the epiphaniometer and calculated from the size distributions determined by the SMPS/APS combination showed a good agreement of Fuchs surface estimates at both measurement sites. However, attempts to estimate a “geometric” surface area from the epiphaniometer data led to significant divergence from the estimates of the SMPS/APS combination when there was a significant fraction of coarser (>700 nm) particles contributing to the aerosol surface area.

Measurement of particle aerodynamic diameter and electrical mobility using laser doppler anemometry

Measurement of particle aerodynamic diameter and electrical mobility using laser doppler anemometry

Technique for aerosol flow check in differential mobility analyzer and its imfluence on classification performance.

The flow pattern of aerosol and sheath air in a cylindrical annulus of a Differential Mobility Analyzer (DMA) is critical to determine the classification performance of the DMA. In the present work, a new technique to check aerosol flow inside the DMA is proposed and the influences of aerosol flow distortion and broadening on the classification performance are investigated by means of the Tandem DMA method. As a result, it is found that (1) the annular flow inside the studied DMA is a fully developed laminar flow with a negligible mixing between aerosol and sheath air when the aerosol flow rate is less than 3 l/min and the sheath air flow rate is 17.5 l/min, (2) a larger width of aerosol entrance slit causes uneven distribution of aerosol flow in the circumferential direction, and (3) both the uneven distribution and broadening of the aerosol flow lead to a broader band of classified particle electrical mobility, as well as a shift of peak electrical mobility to a smaller range. These results suggest that aerosol flow inside a short-type DMA is more sensitive to the flow rates and its structure compared to the conventional TSI type DMA.

Technique for Assessing the Electrical Charge Levels of Aerosols

A relatively low-cost and easy-to-use method for estimating the charge level on aerosols has been developed. It uses the properties of an electrostatically enhanced (electret) filter combined with an optical particle counter to obtain size-dependent charge levels of workplace aerosols. The optical particle counter is calibrated to give a “filtration equivalent” particle size. For the size range of the present measurements, this is similar to geometric size. The aerosol concentration is measured before and after neutralization to determine a penetration ratio that can be approximately correlated with particle electrical mobility. For a particle of a given size, the penetration ratio increases with increasing particle charge level. The method was calibrated with monodisperse methylene blue particles charged to a known level. A polydisperse CaCO3 aerosol, characterized as to size-dependent charge levels, also was measured. Finally, the charge level on a copy-machine toner dust was measured to simulate a highly charged workplace aerosol. The method is limited to the size range of 0.1 µm to 0.7 µm by the characteristics of the electret filter. Electrical mobilities ranging from 0.01 to 1 cm2/statV sec can be measured. The charge level on one particle size in a size distribution is proportional to the charge on other sizes, and can thus be an indicator of charge level of the overall aerosol. Although of limited size-range capability, the method can serve as an indicator of the importance of aerosol charge for sampling or for health effects.

Technique for Assessing the Electrical Charge Levels of Aerosols

A relatively low-cost and easy-to-use method for estimating the charge level on aerosols has been developed. It uses the properties of an electrostatically enhanced (electret) filter combined with an optical particle counter to obtain size-dependent charge levels of workplace aerosols. The optical particle counter is calibrated to give a “filtration equivalent” particle size. For the size range of the present measurements, this is similar to geometric size. The aerosol concentration is measured before and after neutralization to determine a penetration ratio that can be approximately correlated with particle electrical mobility. For a particle of a given size, the penetration ratio increases with increasing particle charge level. The method was calibrated with monodisperse methylene blue particles charged to a known level. A polydisperse CaCO3 aerosol, characterized as to size-dependent charge levels, also was measured. Finally, the charge level on a copy-machine toner dust was measured to simulate a high...

Electrostatic phenomena during powder handling

During powder handling operations, particles make frequent contact with surfaces often metallic, and become electrically charged due to the process of contact electrification. It is often more appropriate to describes such a contact electrification process as triboelectrification as sliding/frictional contact is invariably involved. Triboelectrification is a complex process as charge exchange involving insulating surfaces takes place. The concept of work function may be applied to some insulator materials, especially those that charge negatively as many polymeric materials do. Surface charging depends upon surface condition and some materials are sensitive to the presence of oxidising agents in the atmosphere and also to the presence of moisture. To quantify the triboelectrification process, the dynamics of particle surface contact must be fully defined and contact area accurately assessed. The dynamic behaviour of charged particles may be significantly dependent upon charge. Space charge fields which exist in charged powder clouds act upon individual particles whose motion depends upon particle electrical mobility. Even in the absence of space charge fields, charged particles experience attractive forces towards nearby neutral surfaces. Upon contact with surfaces, charged particles may adhere strongly by a combination of electrostatic and van der Waals forces. Particle/surface adhesion is important in powder coating applications and in electrostatic precipitators.

Electrical neutralization of charged aerosol particles by bipolar ions.

The electrical neutralization of charged aerosol particles by bipolar ions in a flow-type chamber was studied theoretically and experimentally. The time-dependent changes in electrical charge of aerosol particles were evaluated by numerically solving the birth-death equation under various conditions. The calculated results were arranged using dimensionless parameters, and the necessary conditions for charged particles to attain equilibrium charge distributions were clarified. The graphs presented in this paper enable one to estimate the minimum concentration of bipolar ions required to neutralize the charged particles under various conditions. Some of these results are found to agree with experimental results obtained by means of the visual technique in the measurement of particle size and electrical mobility.