An Improved Method for Charging Submicron and Nano Particles with Uniform Charging Performance

An improved method for charging submicron and nano silver particles with uniform charging performance was developed. Monodisperse silver particles were grown into microdroplets through condensation. The aerodynamic diameter and GSD of the condensed droplets were the same regardless of their original diameter. The diameter of the droplets increased from 1.7μm to 2.5 μm as the temperature of the saturator increased from 45°C to 55°C. They were charged by an indirect corona-based charger, in which the ion-generation zone is followed by a particle-charging zone through which the condensed droplets pass. The charges of the droplets were controlled by varying the droplet size, ion concentration, and strength of electric field in the charger. The solvent of the charged droplets was evaporated in an evaporator. The size distribution of the evaporated particles was measured by SMPS spectrometer and compared with their original size distribution. The particles after evaporation were slightly larger than their original particles, due to recondensation. The total charge and number concentration of the particles were measured by aerosol electrometer and CPC, to calculate the average charge. Their electrical mobility distribution was measured by SMPS spectrometer without a neutralizer, to calculate the charge distribution and average charge of the evaporated particles. The results showed the average charges of the particles were similar, regardless of initial diameter and manner of calculation. The charge distributions of the evaporated particles were identical, except for 16.9 nm particles. Ion evaporation phenomenon of particles smaller than 40 nm in diameter was not detected.     

A Device for Generating Singly Charged Particles in the 0.1–1.0-μm Diameter Range

In this paper an aerosol charger that largely avoids the production of multiply charged particles in the 0.1–1.0 μm diameter range is described. The input aerosol is first passed through an electrostatic condenser to remove all charged particles and ions. The remaining neutral aerosol then flows into a 23-cm-long, 2.1-cm inner diameter cylindrical tube; the inner surface of this tube is uniformly coated with 0.09 μCi⁶³ Ni, a 0.067 MeV β-emitter with a half-life of 92 years. At typical airflow rates of 0.2–1.0 lpm, this low-activity source of ionizing radiation produces bipolar ion concentrations ranging from 1 × 10⁴ to 9 × 10⁴ ion/cm³, which is much lower than levels required to bring the aerosol to Boltzmann charge equilibrium. At a flow rate of 1.0 lpm, particles smaller than about 1.0 μm typically interact with no more than one ion en route through the charger. Therefore, particles at the charger exit are mostly either neutral or singly charged. Charge distributions of initially-neutral mono-disperse polystyrene latex particles were measured at the exit from the charger for particle diameters ranging in size from 0.09 to 1.09 μm. It was found that, at an airflow rate of 1.0 lpm and particle size 1.09 μm, the ratios of singly, doubly, and triply charged to total positively charged concentrations were 0.75, 0.19, and 0.06 respectively; particles with more than three charges were not detected. In contrast, the analogous charge ratio at Boltzmann equilibrium is 0.28 (+ 1), 0.24 (+ 2), 0.19 (+ 3), 0.13 (+ 4), 0.08 (+ 5), 0.05 (+ 6), and 0.7 (+ 02).     

Practical limitations of aerosol separation by a tandem differential mobility analyzer–aerosol particle mass analyzer

A cavity ring-down spectrometer and condensation particle counter were used to investigate the limitations in the separation of singly and multiply charged aerosol particles by a tandem differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM). The impact of particle polydispersity and morphology was investigated using three materials: nearly monodisperse polystyrene latex nanospheres (PSL); polydisperse, nearly spherical ammonium sulfate (AS), and polydisperse lacey fractal soot agglomerates. PSL and AS particles were easily resolved as a function of charge. For soot, the presence of multiply charged particles severely affects the isolation of the singly charged particles. In cases where the DMA–APM was unable to fully resolve the singly charged particles of interest, the peak mass deviated by up to 13% leading to errors in the mass specific extinction cross section of over 100%. For measurements of nonspherical particles, nonsymmetrical distributions of concentration as a function of mass were a sign of the presence of multiply charged particles. Under these conditions, the effects of multiply charged particles can be reduced by using a second charge neutralizer after the DMA and prior to the APM. Dilution of the aerosol stream serves to decrease the total number concentration of particles and does not remove the contributions of multiply charged particles.     

Towards traceable particle number concentration standard: Single charged aerosol reference (SCAR)

A concept of realizing a standard for aerosol particle number concentration was tested, based on generating singly charged aerosol particles in the size range from 10 up to 500 nm. To this end, a device named single-charged aerosol reference (SCAR) was designed, built, and tested. The device is based on electrical charging of nanoparticles and subsequent growth of the particles. With an accurate measurement of volume flow and electrical current from the singly charged particles, the number concentration can be accurately, and in the end, traceably determined. Laboratory tests have shown that the device can be used to generate a narrow (GSD<1.3) particle size distribution of singly charged particles. The device can be used for traceable calibration of instruments measuring the number concentration of the particles.     

Electrically Produced Standard Aerosols in a Wide Size Range

Non-uniformly charged particles cause a major problem in electrical methods for producing standard aerosols, imposing strict limitations on the usability of the obtained standard aerosols. This article gives a quick overview of this problem, examining the ways how differently charged particles affect the generation of standard aerosols, and presenting a new method for producing standard aerosols and avoiding the effects of multiply charged particles, with the key idea that the probability for small particles to have more than one charge is very low. A two-stage aerosol generator is used. Small silver particles (d< 40 nm) generated by homogeneous nucleation of silver vapors obtain in bipolar charger not more than one elementary charge. These condensation nuclei are enlarged by a condensational growth device. Big particles are now also not more than singly charged, and narrow size distribution can be obtained by electrical separation. Properties of standard aerosols produced experimentally by both the conventional and the new method, are compared and the validity of the new idea behind the new method is confirmed.     

The Effect of Particle Pre-Existing Charge on Unipolar Charging and Its Implication on Electrical Aerosol Measurements

We investigated the effect of particle pre-existing charges on unipolar charging. Particles carrying a defined number and polarity of pre-existing charges were used to study the unipolar charging process in a unipolar diffusion charger with positive ions. It was found that the particles initially carrying negative charges have almost the same amount of positive charges as the initially uncharged particles after passing the test charger; and the particles initially carrying more positive charges have more final charges. An analytical solution of a model for particle charge distribution of initially charged particles was provided for unipolar charging based on Fuchs' theory and the birth-and-death theory. The N _ion t value used in this model was obtained by fitting the experimental data of average charge on particles for initially uncharged particles. The results from the analytical solution show very good agreements with experimental data regarding the relationship between the pre-existing charge and the final charge on particles (50–200 nm in this study). Experimental tests of the response of Nanoparticle Surface Area Monitor (NSAM) against initially charged particles demonstrated that NSAM could have a large response deviation (more than 20% in the tested charge level) depending on the particle size and the amount of pre-existing positive charges on particles. Modeling of NSAM response showed similar deviation and predicted that when pre-existing charge is high enough, the NSAM response can be as large as 5 and 9 times of the uncharged particle response for alveolar and tracheobronchial surface area concentration, respectively.     

Molecular dynamics study of TiO2/poly(acrylic acid‐co‐methyl methacrylate) and Fe3O4/polystyrene composite latex particles prepared by heterocoagulation

All‐atom molecular dynamics simulations were used to study the morphology of polymer/inorganic composite particles prepared by heterocoagulation. The results were also compared to those of our previous study of the preparation of TiO₂/poly(acrylic acid‐co‐methyl methacrylate) and Fe₃O₄/polystyrene composite particles. In the simulation system, polymer or inorganic particles were simulated by surface‐charge‐modified C₆₀ or Na atoms. Through a combination of analysis of the radial distribution functions of charged atoms and snapshots of the equilibrated structure, three kinds of particle distributions were observed under different conditions. When the polymer and inorganic particles had opposite surface charges and their sizes were very different, the composite morphology showed a core–shell structure with small particles adsorbed onto the surfaces of large particles. Furthermore, when the polymer and inorganic particles had opposite surface charges but comparable sizes, the polymer and inorganic particles aggregated domain by domain. Finally, when the polymer and inorganic particles were endowed with the same surface charge, the distribution of these two types of particles was homogeneous, regardless of their size difference. The simulation results were in agreement with the experimental results. The electrostatic interaction and the size of the particles dominated the final morphology of the composite particles when the heterocoagulation method was used. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

On Measuring the Critical Diameter of Cloud Condensation Nuclei Using Mobility Selected Aerosol

Cloud condensation nuclei (CCN) instruments determine the so-called “critical diameter” for activation of particles into cloud droplets at a fixed water supersaturation. A differential mobility analyzer is often used to size-select particles for purposes of scanning for the critical diameter. Usually the diameter where 50% of the particles have activated to cloud droplets is assumed to be equal to the critical diameter. We introduce a model that describes the transfer of polydisperse charge-equilibrated particles through an ideal differential mobility analyzer followed by transit through an ideal CCN instrument. We show that if the mode diameter of the polydisperse size distribution exceeds the critical diameter of the particles, multiply-charged particles may lead to nonmonotonic CCN counter response curves (plots of CCN-active fraction vs. mobility diameter) that exhibit multiple peaks, rather than a simple sigmoidally-shaped curve. Hence, determination of the 50% activation diameter is ambiguous. Multiply-charged particles significantly skew the CCNc response curves when sampling particles with critical diameters exceeding 0.1 μ m from particle size distributions with mode diameters also larger than the critical diameter. We present a method for inversion of CCN counter data that takes multiple-charging effects into account, and demonstrate its application to laboratory data. Our calculated CCN counter response curves are in good agreement with observations, and can be used to infer the critical activation diameter for a specified supersaturation.     

Maximizing the singly charged fraction of sub-micrometer particles using a unipolar charger

Particle charging via the mixing of aerosols with unipolar ions typically results in multiple charges on particles. Particle classification and sizing, based on the electrical mobility, ideally requires all the particles being singly charged to the performance enhancement. In this study, we explored the feasibility of maximizing the singly charged fraction of particles via the control of the N_it product in a unipolar charger. The feasibility was first investigated by modeling unipolar diffusion charging. It was found that the singly charged fraction of monodisperse particles could be maximized by the control of the N_it product. A corona-based unipolar charger was also constructed to study the maximization of the singly charged fraction of monodisperse particles. It was found that a wider range of ion concentration in the charging zone could be obtained by the variation of ion-driving voltage compared to that by changing the corona-discharge current. The maximum singly charged fraction of monodisperse particles in various sizes was characterized when the charger was operated at the flow rates of 1.5 and 3.0 lpm. It was evidenced that the current charger could be conditioned to achieve a higher singly charged fraction of particles than that by bipolar chargers in the particle size range of 20–200?nm, particularly in the ultrafine particle size range. The control of N_it product in the charging zone of a unipolar charger offers a simple and effective means to enhance the singly charged fraction of particles in a given size range.

Copyright © 2019 American Association for Aerosol Research     

Volatility characterization of nanoparticles from single and dual-fuel low temperature combustion in compression ignition engines

This work explores the volatility of particles produced from two diesel low temperature combustion (LTC) modes proposed for high-efficiency compression ignition engines. It also explores mechanisms of particulate formation and growth upon dilution in the near-tailpipe environment. The number distribution of exhaust particles from low- and mid-load dual-fuel reactivity controlled compression ignition (RCCI) and single-fuel premixed charge compression ignition (PPCI) modes were experimentally studied over a gradient of dilution temperature. Particle volatility of select particle diameters was investigated using volatility tandem differential mobility analysis (V-TDMA). Evaporation rates for exhaust particles were compared with V-TDMA results for candidate pure n-alkanes to identify species with similar volatility characteristics. The results show that LTC particles are mostly comprised of material with volatility similar to engine oil alkanes. V-TDMA results were used as inputs to an aerosol condensation and evaporation model to support the finding that smaller particles in the distribution are comprised of lower volatility material than large particles under primary dilution conditions. Although our results show that saturation levels are high enough to drive condensation of alkanes onto existing particles under the dilution conditions investigated, they are not high enough to allow homogeneous nucleation of these same compounds in the primary exhaust plume. Therefore, we conclude that observed particles from LTC operation must grow from low concentrations of highly nonvolatile compounds present in the exhaust.

Copyright © 2016 American Association for Aerosol Research     

Bench-scale tests to determine mechanisms of charge generation due to particle-particle and particle-wall contact in binary systems of fine and coarse particles

Bench-scale methods were utilized to determine changes in electrostatic charges and their mechanisms for various coarse and fine particles as they came into contact with each other and/or their containing vessel walls. Techniques included shaking tests and particle-copper plate contacting experiments. Electrostatic behaviour of coarse particles (glass beads and polyethylene) and fine particles (Larostat 519, glass beads and silver-coated glass beads) were investigated. Shaking tests resulted in charge separation in which the fine particles acquired significant positive charges, opposite to those carried by the large particles. In copper-plate contacting tests, charge transfer occurred between the fines and the copper plate with fines carrying away almost all of the initial charges on the plate followed by further charge separation. Charge separation was found to be the dominant charging mechanism between the coarse particle and copper plate, with the particles becoming negatively charged.     

Charge distributions of aerosol dioctyl sebacate particles charged in a dielectric barrier discharger

The collection efficiencies of submicron aerosol particles using a two-stage, dielectric barrier discharge (DBD) type electrostatic precipitator have been reported previously [Byeon et al. (2006). Collection of submicron particles by an electrostatic precipitator using a dielectric barrier discharge. Journal of Aerosol Science, 37, 1618–1628]. In this paper, the charge distributions of aerosol dioctyl sebacate (DOS) particles, which had a mobility equivalent diameter of 118, 175, and 241 nm and were charged in a DBD charger, were examined using a tandem differential mobility analyzer (TDMA) system at applied voltages of 9–11 kV and frequencies of 60–120 Hz. The mean number of elementary charges for positively or negatively charged particles increased slightly with increasing applied voltage or frequency. However, the number of elementary charges increased significantly with increasing particle size. At any applied voltage and frequency, the charge distributions of these particles of these sizes indicated asymmetric bipolar charging. The positive-to-negative charge ratios were 10.4, 4.7, and 3.0 for particle sizes of 118, 175, and 241 nm, respectively, at a DBD voltage and frequency was 9 kV and 60 Hz, respectively. Fluorometric analysis showed that average positive-to-negative charge ratios were 11.5, 4.9, and 3.7 for particle sizes of 118, 175, and 241 nm, which agrees well with the TDMA results. Further fluorometric analyses with larger particles (514 and 710 nm) and higher frequencies (1 and 2 kHz) showed that the positive-to-negative charge ratio reached almost unity with increasing particle size or frequency.     

Analysis of particle contamination in plasma reactor by 2-sized particle growth model

Rapid particle growth in the silane plasma reactor by coagulation between 2-sized particles was analyzed for various process conditions. The particle coagulation rate was calculated considering the effects of particle charge distribution based on the Gaussian distribution function. The large size particles are charged more negatively than the small size particles. Some fractions of small size particles are in neutral state or charged positively, depending on the plasma conditions. The small size particle concentration increases at first and decreases later and reaches the steady state by the balance of generation rate and coagulation rate. The large size particles grow with discharge time by coagulation with small size particles and their size reaches the steady state, while the large size particle concentration increases with discharge time by faster generation rate and reaches the steady state by the balance of generation and disappearance rates. As the diameter of small size particles decreases, the diameter of large size particles increases more quickly by the faster coagulation with small size particles of higher concentration. As the residence time increases, the concentration and size of large size particles increase more quickly and the average charges per small size and large size particle decrease.     

Automated Spore Measurements Using Microscopy,Image Analysis,and Peak Recognition of Near-Monodisperse Aerosols

Rapid detection of airborne fungal and bacterial spores would enable public agencies to respond quickly and appropriately to intentional releases of hazardous aerosols. Automated analysis of microscope images and automated detection of near-monodisperse peaks in aerosol size distribution data offer complementary approaches to traditional methods for the identification and counting of fungal and bacterial spores. First, spores of the fungus Scopulariopsis brevicaulis were aerosolized in a chamber and then collected with a slit impactor; later, digital microscope images were analyzed manually to determine spore cluster distributions. The images also were analyzed with ImageJ, a program that automatically outlined objects and measured Feret's diameter, area, perimeter, and circularity. These characteristics were used to identify spore clusters automatically using two data analysis methods. Second, a computer program was developed to discriminate near-monodisperse bioaerosol peaks from those for polydisperse ambient particulate matter (PM) and was successfully tested using simulated and real aerosol mixtures. The observed agreement between manual and automated spore counts and the ability to detect spore peaks suggest that it may be possible to develop a system to recognize intentional releases rapidly through examination of particle morphology and size distributions. The peak detection procedure is potentially the fastest technique when used with real-time instrument data, but assumes that intentional releases would consist of large numbers of uniformly sized particles in the respirable size range.

Copyright 2012 American Association for Aerosol Research     

Electrostatic effects on inertial particle transport in bifurcated tubes

Most aerosols found naturally in the ambient environment or those dispersed from artificial devices such as dry powder inhalers, are electrically charged. It is known that a strong electrostatic charge on aerosols can result in transport behavior dramatically different from that of uncharged aerosols, even in the absence of an external electric field. In the present work, we study pneumatic transport of corona‐charged particles in bifurcated tubes. This is accomplished by tracking the motion of discrete particles numerically under the influence of drag, gravitational, and electrostatic forces. The model aerosol is fly ash powder, whose size and charge distributions have been determined experimentally. The electrical mobility of the charged particle cloud is modeled through coulombic interactions between discrete point charges. For the case of polydispersed particles electrically charged across a distribution, the deposition efficiency was found to be greater than what is indicated by the mean charge and size. In particular, use of negatively charged fly ash powder of mean size of 2 μm and mean charge of ?1.5 C/kg led to significant increase in deposition efficiency (～29%) compared with uncharged fly ash powder of the same size distribution (～8%). Analysis of particle residence times suggests significant interaction between electrical and drag forces. These findings could have implications for pneumatic powder conveying or pulmonary drug delivery applications. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Particle Charging and Transmission Efficiencies of Aerosol Charge Neutralizes

ABSTRACT

Diffusion losses and charging efficiency were measured for three types of charge neutralizers commonly used in aerosol research: two with ⁸⁵Kr and one with ²¹⁰Po as radiation sources. The diffusion losses were characterized at flows of 0.5 -6 1 min^?1 typically used in atmospheric aerosol physics measurements. All of the neutralizers tested exhibited high transmission efficiencies, with losses up to 25% at the smallest tested size of 3 nm, varying with size and flow in general agreement with diffusion loss theory. Charging efficiency was measured for a singly charged, monodisperse aerosol at the same flows and at concentrations of 10³-10⁴ particles cm^?3. Neither of the ⁸⁵Kr chargers brought the charge distribution close to equilibrium at 2 1 min^?1, except at concentrations ≤ 10³ cm^?3. The ²¹⁰Po charger produced the theoretically expected fraction of singly charged particles within the uncertainty of the experiment.     

Mathematical Description of Experimentally Determined Charge Distributions of a Unipolar Diffusion Charger

The charge distributions of an improved opposed flow unipolar diffusion charger were measured using a tandem differential mobility analyzer (DMA) set up in a size range of approximately 20–400 nm. The charger is intended to be used in a portable aerosol sizer to measure particle size distributions. The determined charge distributions were represented by lognormal distributions, and a set of equations and coefficients was developed to calculate the charge distributions. These equations can be easily implemented in software for size distribution measurements. The agreement between the mathematically derived and measured charge distributions is very good, with regression coefficients R ² > 0.96. The investigations showed that approximately 55% of 20-nm particles remain uncharged, while up to 25 elementary charges need to be considered for multiple charge correction of 400-nm particles. Comparison with the Fuchs theory delivered satisfying agreement with the measured average charge levels, but charge distributions cannot be described by the Fuchs theory, likely caused by the charger geometry.

Copyright 2012 American Association for Aerosol Research     

The effect of the generation and handling in the acquired electrostatic charge in airborne particles

The measurement of the charge distribution in laboratory generated aerosols particles was carried out. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases, the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. In the other two cases, a corona charger and an impact charger were utilized as supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas stream: the vibrating orifice generator TSI model 3450 and the rotating plate generator TSI model 3433. In the vibrating orifice generator, a solution of methylene blue was used and the generated particles were mono-dispersed. Different mono-aerosols were generated with particle diameters varying from 6.0 × 10^− 6 m to 1.4 × 10^− 5 m. In the rotating plate generator, a poly-dispersed phosphate rock concentrate with Stokes mean diameter of 1.30 × 10^− 6 m and size range between 1.5 × 10^− 7 m and 8.0 × 10^− 6 m was utilized as powder material in all tests. In the tests performed with the mono-dispersed particles, the median charges of the particles varied between − 3.0 × 10^− 16 C and − 5.0 × 10^− 18 °C and a weak dependence between particle size and charge was observed. The particles were predominantly negatively charged. In the tests with the poly-dispersed particles the median charges varied fairly linearly with the particle diameter and were negative. The order of magnitude of the results obtained is in accordance with data reported in the literature. The charge distribution, in this case, was wider, so that an appreciable amount of particles were positively charged. The relative spread of the distribution varied with the charging method. It was also noticed that the corona charger acted very effectively in charging the particles.     

Granular size and shape effect on electrostatics in pneumatic conveying systems

In solid processing systems, electrostatic problems are commonly observed for granules composed of various sizes and shapes. However, complete understanding about the functional dependence of electrostatic charge generation and transfer on the particle shape and size distributions has yet to be established. This observation has motivated the present study where novel methods are proposed to examine the effect of particle size and shape distributions on electrostatics. In this work, polyvinyl chloride (PVC) granule (original diameter 3.35-4.1 mm, in the shape of cylinders) was first discharged to remove any residual charges and subsequently its electrostatic charging was studied. Granular size and shape were varied by mechanical attrition conducted in a rotary valve jointly with a pneumatic conveying system. Characterised by induced current, particle charge density and equivalent current of the charged granular flow, granular electrostatics was found to increase with the extent of granular attrition in a continuous recycled pneumatic conveying process. In a separate setup, single particles (collected from the attrited granules formed in the rotary valve) were examined by correlating the extent of charge variation with size/shape. It was found that a dimensionless group, defined by the ratio of charge variation to size variation, is useful in describing the particle attrition process as this parameter increased with decreasing granule size. Smaller granules were found to be the main contributors in the enhancement of electrostatics charge density in bulk particles. By a separate shape analysis, it was uncovered that face shape requiring more shearing actions for its formation tended to give rise to a higher charge variation and so did column geometry. In this fashion, charge variation evaluated for whole attrited granules exhibited good agreement with the temporal variation of attrition weight; this applied for all air flow rates used in the conveying system. Furthermore, there is a reasonably good matching between results obtained by shape and size analyses.By the correlations presented above for single particle electrostatics either by size or shape analysis, charge variation of granular flow matched very well with that measured in the conveying system as well as the attrition process in the rotary valve. As such, the joint granule size and shape analysis has proven to be useful for characterisation of electrostatics in conveying systems where granules are made up of complex combinations of different particle sizes and shapes.     

Induction Charging and Electrostatic Classification of Micrometer-Size Particles for Investigating the Electrobiological Properties of Airborne Microorganisms

Our earlier studies have shown that the electrostatic collection technique, a potentially "gentle" bioaerosol collection method, allows for efficient collection of airborne bacteria, but sensitive bacteria such as Pseudomonas fluorescens ( P. fluorescens ) lose their culturability during collection. We hypothesized that excessive stress was imposed on the sensitive bacteria by the sampler's conventional corona charging mechanism. In this research, we developed and built an experimental setup that allows us to analyze electrobiological properties of airborne microorganisms. In this experimental system, we imparted electric charges on airborne biological and nonbiological particles by aerosolizing them in the presence of an electric field. The charged P. fluorescens test bacteria and NaCl test particles were then channeled into a parallel plate mobility analyzer, which we have designed so that bacteria and inert particles carrying specific charge ranges can be extracted and made available for further analysis. When testing the experimental system, we related the extracted particle concentrations to the total particle concentration and obtained the charge distributions of these particles at different charging conditions. Our results have shown that even without charging, aerosolized P. fluorescens bacteria have a net negative charge and can carry up to 13,000 elementary charges per bacterium. In contrast, the NaCl particles were found to carry very few electric charges. We concluded that the electric charge carried by a bacterium consists of 2 components: its own natural charge, which can be high, and the charge imposed on it by the dispersion process. Our experiments have shown that the charge distributions on biological and nonbiological particles can be effectively manipulated by varying the external electric field during their aerosolization. Since airborne microorganisms may carry high internal electric charges, their collection by electrical field forces may be possible without first electrically charging them.