Charged Monodisperse Research Articles

Microscale Templating of Functional Particles Using Self-Limiting Electrospray Deposition.

Electrospray deposition (ESD) uses strong electric fields applied to solutions and dispersions exiting a capillary to produce charged monodisperse droplets driven toward grounded targets. Self-limiting electrospray deposition (SLED) is a phenomenon in which highly directed, uniform, and even 3D coatings can be achieved by trapping charge in the deposited film, redirecting the field lines to uncoated regions of the target. However, when inorganic particles are added to SLED sprays, the buildup of charge required to repel incoming material is disrupted as particle loading increases. Due to its fibril gelling behavior, methylcellulose (MC) SLED can form nanowire morphologies. These wires, when used as a binder, can separate particles and prevent percolation. In this work, a variety of conductive and insulating particles are explored using patterned and un-patterned substrates. This exploration allows us to maximally load particles for high-concentration and highly controlled self-limiting functional sprays. This is demonstrated using Ti3C2Tx MXene to functionalize an interdigitated electrode for use as a supercapacitor.

Molecular dynamics simulation of the cone-jet electrospray: Role of surfactants

Electrospray in the cone-jet regime could produce highly charged and fine monodisperse drops, and further disintegrates into nanoscale drops, even ions due to synergism of Rayleigh instability and evaporation. The jet and its breakup instability into drops are significantly affected by liquid properties including surface tension, conductivity, density and others. Surfactants may play important roles on gas-liquid interface and are usually added in solution to vary liquid property and may further affect electrospray behaviors. In present work, the ethanol containing surfactants is atomized in the cone-jet to discuss the synergistic effect of surfactant types, concentrations and surfactant-salt on the jet evolution behaviors under direct current electric fields using molecular dynamics (MD) method. It is found that the jet length of ethanol solution with Span-80 is the longest (Span-80, SDS, CTAB and TX-100), while it has excellent dispersion. The addition of CTAB could make ethanol solution transform from continuous jet into dripping. SDS interacts most violently with ethanol due to the presence of sodium ions. As surfactant concentration increasing, the number of solution molecules sprayed remarkably increases, while the interaction generated from ethanol decreases. The number of molecular clusters (composed of three or more ethanol molecules) in the cone-jet electrospray increases with increasing Span-80 concentration. It is also found that the addition of Span-80 slightly changes the average bond angle of ethanol and bond angle distribution. The ethanol atomization morphology transforms to spindle becasue of the synergistic effect of surfactant and salt ions. The interaction generated between ethanol molecules plays a dominating role and decreases due to the generation of spindle-shaped drops. The hydrogen bond generated from ethanol decreases with increasing conductivity and surface tension. The conclusions of this paper will be latent value for the optimization of physical parameters in electrospray.

Collective effects in a dusty plasma with a magnetic field irradiated by hard radiation

The transfer coefficients of a rarefied low-temperature plasma (hereinafter referred to as LTP) with a condensed dispersed phase (hereinafter referred to as CDF) were calculated for characteristic temperatures T = 50 – 100 K in weak external magnetic fields (B = 10–6 – 1 Gauss). These effects of irradiation of LTP CDF plasma with X-ray and γ-ray fluxes, highenergy charged particles (electrons, positrons, protons and α-particles) are shown. For systems of positively charged monodisperse nanoparticles, new frequencies of collective MHD oscillations are determined. Significant physical differences between such interacting systems and their influence on the redistribution between the kinetic energy of particle flows and (also the energy of incident quanta of hard radiation) to magnetic and vice versa are shown. In this regard, the degree of dissipative scattering of energy entering the system is relevant for the interpretation of experimental spectroscopic data on scattering. The effects of such scattering are considered on specific examples. By its nature, the first type of scattering of the energy entering the physical systems under consideration consists in local energy losses due to pair interactions with atoms, molecules, and CDF of the medium. The second type of interactions considered is the scattering of charged particle flows and turbulent (like cyclonic vortices) LTP structures with CDF located in an external magnetic field. Comparison of these two mechanisms of dissipative losses shows their comparability.For the physical systems considered above, the transfer coefficients we are received. The relaxation times–τand gyrofrequencies– ω for a two-component electron gas during its interaction with CDF and ions of atoms and molecules in an external magnetic field are shown.For this multicomponent plasma, based on the Braginsky criteria (ωτ≫1, or ωτ≪1), for applying the previously obtained relations for the transfer coefficients between various plasma components are shown. It was found that the thermal, low-energy electronic component of the LTP is responsible for the recombination of atomic ions and the discharge of CDFs. The high-energy electrons Auger component leads to large temperature gradients and, as a consequence, significant heat fluxes. In turn, it has been shown that large temperature gradients cause significant currents of the charged component of NTP (electrons, atomic and molecular ions, CDF). It is shown that, in turn, the plasma generates local magnetic fields. The magnetic Reynolds number ea ≫ 1who was calculated in this work are shown that the plasma becomes turbulent. The main conclusion of the work is the proof that a charged monodisperse ultra-low density CDF in the field of hard radiation and an external magnetic field is almost always turbulent.

Highly charged, magnetically sensitive magnetite/polystyrene colloids: synthesis and tunable optical properties

Highly charged monodisperse magnetite/polystyrene colloidal particles are successfully synthesized by using a miniemulsion polymerization method. The particles, with high saturation magnetization (Ms ≈ 38.6 emu g−1), display high sensitivity to the magnetic field and can self-assemble into periodically ordered colloidal structure in aqueous solution, thus diffract light strongly, under magnetic induction. By changing the strength of magnetic field, the position of reflection peaks can be reversibly tuned in the range of 420–720 nm, covering almost the entire visible light range. After adding 1.5 wt.% of acid black 210 into the colloidal suspension, the color saturation and brightness of the colloids are significantly improved. As-prepared magnetically responsive colloidal particles have great potential to be used in color display, biosensors, anti-counterfeiting, camouflage, pigments and dyes.

Modeling simple-jet mode electrohydrodynamic-atomization droplets' trajectories and spray pattern for a single nozzle system

Electrohydrodynamic atomization (EHDA), or simply Electrospraying is the process of influencing the breakup of a liquid into droplets by using a strong electric field. There can be different modes of Electrospraying depending, basically, on the created electric field strength and the liquid flowrate, for a specified liquid. Among these modes, the so-called cone-jet mode is the most explored one. This is due to its ability to produce highly charged monodisperse droplets in the nano- to micro-meter size range. Another mode of interest, which can also produce monodisperse droplets is the simple-jet mode. This mode is less explored when compared to the former. Within the papers that were explored by the authors, Agostinho et al. (2012) were the first authors to carefully investigate and characterize this mode. In their work, the authors reported about the influence of the electric field and the liquid flowrate on the droplets' size and spray dispersion. They also pointed out that the charge on these droplets can be expressed as a certain percentage of their Rayleigh limit.So far, there has been no model proposed to describe the droplets' trajectories in the simple-jet mode. This paper describes the design and the implementation of a physical model for determining the droplet trajectories in this mode. The model is done, specifically, for a single nozzle/ring-up configuration. It is a two-dimensional model, which solves the force balance equation for each droplet breaking up from the jet. It takes into consideration; the initial droplet velocity, the force of gravity, the electric field force, the inter-droplet coulombic force and the drag force. The droplets' deformation and reorientation were hypothesized, from observations, to play a major role in initiating the droplets' dispersion. They were simulated by implementing periodic displacements on the droplets' center of charge from its center of mass. The calculated droplets' trajectories' envelope angle was fitted to the experimental envelope angle by adjusting the droplet charge around the values that were reported by Agostinho et al. (2012). The model was validated by comparing the shapes of the theoretical and experimental sprays.The model offers new possibilities of modeling the droplets' trajectories in complex geometries, and of introducing additional forces to manipulate their trajectories in the simple-jet mode.

BSA Adsorption and Immobilization onto Charged Monodisperse Polymer Nanoparticles

Controlling the behavior of adsorbed and immobilized proteins is essential for protein purification and a wide range of applications, including biosensors, biocatalysis and biomedical devices. In this study, monodisperse polymer particles were synthesized by soap free emulsion polymerization and the surface functional groups were directly introduced as a monomer or chemically modified with epoxy groups to enable protein immobilization through covalent bonding. Three kinds of surface charged particles having cationic, anionic, or both, groups were synthesized and characterized. Bovine serum albumin (BSA) was selected as a model protein to study the effect of pH of a buffer solution containing protein on the adsorption and immobilization amount. Protein adsorption was found to be strongly affected by pH and matching of the global protein and polymer particle charges, respectively. When pH was below the pI of either protein (pH 3.8), negatively charged polymer particles were found to adsorb a high amount of proteins. At maximum surface coverage of BSA on negatively charged polymer particles at pH 3.8 and after subsequent rinsing of the BSA-polymer particle complex with phosphate buffer (pH 7), ~50% of BSA was desorbed. Therefore, 50% of BSA was adsorbed by ionic interaction and the remaining fraction was immobilized covalently at pH 3.8. The remaining immobilized fraction was sufficient to completely shield the anionic charge of the polymer particles. Fluorescence spectroscopy suggests that at maximum immobilized amounts, the conformation of immobilized BSA appears to be the same as in aqueous solution.

Cross-linked poly(methyl methacrylate) particles with surface amino groups

Positively charged monodisperse particles have been synthesized via emulsifier-free emulsion copolymerization of methyl methacrylate with vinylformamide and ethylene glycol dimethacrylate. The effect of the cross-linking agent on the conversion of the monomers, as well as on the shape, size distribution, and surface characteristics of the obtained particles, has been determined. Addition of the cross-linking agent (5 wt %) to the reaction system has been shown to result in the formation of particles, which, after hydrolysis, have positive ζ potentials at pH 2–10.

Effect of Electrostatic Charge on Deposition of Uniformly Charged Monodisperse Particles in the Nasal Extrathoracic Airways of an Infant

The fraction of inhaled particles depositing in the nasal extrathoracic airways determines the amount of particles delivered to the lungs of infants. Electrostatic charge on particles can affect this deposition, and for this reason, deposition of charged aerosol particles in the Alberta Idealized Infant nasal geometry is examined. Charged aerosol particles were generated via Plateau-Rayleigh jet breakup atomization with induction charging. Nasal deposition was measured by collecting particles on a filter membrane at the inlet and outlet of the airway and measuring their mass with an ultramicrobalance. The experiments were carried out using monodisperse, uniformly charged particles with aerodynamic diameters of 3-6 μm at two flow rates of 7.5 and 15 L/min, for a charge range of 0-10,000 e per particle. Electrostatic charge effects are largest for the lowest flow rate, smallest particle size, and highest charge level, with deposition in this case being approximately three times that for neutral particles. Higher flow rates and larger particle size result in much weaker electrostatic effects, with even the highest charge levels giving only a few percent higher deposition for the largest particle size and flow rate considered in this study. A dimensionless empirical relation based on the experimental data was developed for predicting deposition of charged particles in the idealized infant airway. Electrostatic charge on inhaled aerosol particles has only a minor effect on deposition for large particles at higher flow rates, because in this case inertial impaction dominates deposition. However, for particles with low inertia, for example, small particles or low flow rates, large values of electrostatic charge strongly increase nasal deposition in the present infant extrathoracic airway.

Effect of electrostatic charge on oral-extrathoracic deposition for uniformly charged monodisperse aerosols

An in vitro study was conducted with the aim of investigating the effect of particle charge on deposition in the oral-extrathoracic airways of human adults. To generate monodisperse droplets with uniform charge, an atomizer was designed and built. The atomizer generates monodisperse droplets using controlled Rayleigh jet breakup and charges the droplets uniformly via electrostatic induction. Experiments were conducted at various aerodynamic particle diameters (3.3, 4.6 and 5.9μm) and volume flow rates (15 and 30L/min) for a wide range of charges per particle (0–25,000 elementary charges). The results show substantial increases in deposition due to induced charge electrostatics, especially for smaller particle sizes. A dimensionless empirical correlation was developed for predicting deposition in the mouth–throat region of human adults including induced charge electrostatic effects.

Silver Deposition on a Polymer Substrate Catalyzed by Singly Charged Monodisperse Copper Nanoparticles

Aerosol deposition of singly charged monodisperse copper nanoparticles was used to catalytically activate a polymer substrate for electroless silver deposition. An ambient spark discharge was used to produce aerosol copper nanoparticles, and the particles were electrostatically classified at an equivalent mobility diameter of 10 nm, using a nanodifferential mobility analyzer. Deposition of the copper particles onto the surface of the substrate was enhanced by thermophoresis. The copper-deposited substrate was then immersed in a Ag(I) solution, resulting in the electroless deposition of silver (∼17 μm line width) on the previously deposited copper (∼12 μm line width, using a shadow mask with a 100 μm in width patterned stripe). The arithmetic mean roughness and electrical resistivity of the silver pattern were 44.7 nm and 7.9 μΩ cm, respectively, which showed an enhancement compared to those from the nonclassified copper particles (roughness = 162.2 nm, resistivity = 13.3 μΩ cm), because of a more-uniform copper deposition.

Evaluation of the Filtration Performance of NIOSH-Approved N95 Filtering Facepiece Respirators by Photometric and Number-Based Test Methods

N95 particulate filtering facepiece respirators are certified by measuring penetration levels photometrically with a presumed severe case test method using charge neutralized NaCl aerosols at 85 L/min. However, penetration values obtained by photometric methods have not been compared with count-based methods using contemporary respirators composed of electrostatic filter media and challenged with both generated and ambient aerosols. To better understand the effects of key test parameters (e.g., particle charge, detection method), initial penetration levels for five N95 model filtering facepiece respirators were measured using NaCl aerosols with the aerosol challenge and test equipment employed in the NIOSH respirator certification method (photometric) and compared with an ultrafine condensation particle counter method (count based) for the same NaCl aerosols as well as for ambient room air particles. Penetrations using the NIOSH test method were several-fold less than the penetrations obtained by the ultrafine condensation particle counter for NaCl aerosols as well as for room particles indicating that penetration measurement based on particle counting offers a more difficult challenge than the photometric method, which lacks sensitivity for particles < 100 nm. All five N95 models showed the most penetrating particle size around 50 nm for room air particles with or without charge neutralization, and at 200 nm for singly charged NaCl monodisperse particles. Room air with fewer charged particles and an overwhelming number of neutral particles contributed to the most penetrating particle size in the 50 nm range, indicating that the charge state for the majority of test particles determines the MPPS. Data suggest that the NIOSH respirator certification protocol employing the photometric method may not be a more challenging aerosol test method. Filter penetrations can vary among workplaces with different particle size distributions, which suggests the need for the development of new or revised “more challenging” aerosol test methods for NIOSH certification of respirators.

Enhanced stem cell tracking via electrostatically assembled fluorescent SPION–peptidecomplexes

For cellular MRI there is a need to label cells with superparamagnetic iron oxidenanoparticles (SPION) that have multiple imaging moieties that are nontoxic and haveincreased NMR relaxation properties to improve the detection and tracking of therapeuticcells. Although increases in the relaxation properties of SPION have been accomplished,detection of tagged cells is limited by either poor cell labeling efficiency or low intracellulariron content. A strategy via a complex formation with transfection agents to overcomethese obstacles has been reported. In this paper, we report a complex formation betweennegatively charged fluorescent monodisperse SPION and positively charged peptides anduse the complex formation to improve the MR properties of labeled stem cells. Asa result, labeled stem cells exhibited a strong fluorescent signal and enhancedT 2*-weighted MR imaging in vitro and in vivo in a flank tumor model.

Self-assembly of monodisperse nanoparticles of styrene copolymers with N-vinylformamide into periodic colloidal structures

532 Studying the self-assembly of nanoparticles is important for nanotechnology, since this process is promising for the fabrication of twoand three-dimensional ordered matrices of optical elements, detectors, and sensors [1]. For example, submicron monodisperse particles of silicon dioxide or polystyrene are applied as structural elements of photonic crystals [1, 2]. The formation of periodic colloidal structures (PCS) with negatively charged polymer nanoparticles has been revealed even in dispersions at low ionic force and a volume ratio of 0.001–0.04 [3]. The action of proportional attraction and repulsion forces between particles is a necessary condition for the process. An enhancement of the ability of nanoparticles to self-assemble, provided that they have a hydrophilic surface layer, has been noted [4]. Due to the extension of chains of the layer into the aqueous phase the particles “sense” each other at distances on the order of 50 nm, which is longer than follows from the DLVO theory. These chains also facilitate a high capillary adhesion of the particles. To extend the range of nanoparticles capable of selfassembling, a series of positively charged monodisperse nanoparticles with hydrophilic surface layer and 55–110 nm in diameter with its standard deviation less than 2% was obtained via the emulsion copolymerization of styrene (St) with N -vinylformamide (VFA) (3– 7 wt %) initiated by 0.2–3.5 wt % of 2,2'-azobis-[2-(2imidazoline-2-yl)propane] dihydrochloride (AIP) in 0.9–1.5 × 10 –3 mol/l cetyltrimethylammonium bromide (CTAB) solutions with pH 3.5–6.5 at 90°e (Fig. 1). The particle size decreased in a weakly acidic medium with an increase in the VFA and AIP concentrations as a result of an increase in the amount of oligomer chains enriched in VFA units. Such chains are formed in the initial stage of the synthesis and participate in the nucleation process along with CTAB. At the initial pH below 5.8, the hydrolysis of the AIP residues yielding carboxylic groups at the ends of polymer chains caused a decrease in the surface charge and an increase in the particle size. The holding of the nanoparticles in a 1 M PHOTONIC CRYSTALS

Collective Particle Interactions in the Sedimentation of Charged Colloidal Suspensions

The sedimentation of a similarly charged monodisperse colloidal suspension was studied to investigate the role of collective particle interactions affecting the rate of sedimentation and thereby the stability. The particle concentration profiles, microstructure formation, and the sedimentation rates were determined using a nondestructive back-light scattering technique. Stochastic particle dynamics simulations using the Beresford-Smith and Derjaguin−Landau−Verwey−Overbeek repulsive pair potentials were carried out to predict the sedimentation rates. In addition, Monte Carlo simulations were performed to reveal the particle microstructure formation. The collective particle interactions result in an oscillatory effective potential of interaction between the identically charged particles, even though the pair potential is repulsive, and lead to a more ordered microstructure formation and reduction in the sedimentation rate. The theoretical predictions, which were in good agreement with the experimental measu...

Polymerized PolyHEMA Photonic Crystals: pH and Ethanol Sensor Materials

The surface of monodisperse silica particles synthesized using the Stober process were coated with a thin layer of polystyrene. Surface charge groups were attached by a grafting polymerization of styrene sulfonate. The resulting highly charged monodisperse silica particles self-assemble into crystalline colloidal arrays (CCA) in deionized water. We polymerized hydroxyethyl methacrylate (HEMA) around the CCA to form a HEMA-polymerized crystalline colloidal array (PCCA). Hydrofluoric acid was utilized to etch out the silica particles to produce a three-dimensional periodic array of voids in the HEMA PCCA. The diffraction from the embedded CCA sensitively monitors the concentration of ethanol in water because the HEMA PCCA shows a large volume dependence on ethanol due to a decreased Flory-Huggins mixing parameter. Between pure water and 40% ethanol the diffraction shifts across the entire visible spectral region. We accurately modeled the dependence of the diffraction wavelength on ethanol concentration using Flory theory. We also fabricated a PCCA (which responds to pH changes in both low and high ionic strength solutions) by utilizing a second polymerization to incorporate carboxyl groups into the HEMA PCCA. We were also able to model the pH dependence of diffraction of the HEMA PCCA by using Flory theory. An unusual feature of the pH response is a hysteresis in response to titration to higher and lower pH. This hysteresis results from the formation of a Donnan potential at high pH which shifts the ionic equilibrium. The kinetics of equilibration is very slow due to the ultralow diffusion constant of protons in the carboxylated PCCA as predicted earlier by the Tanaka group.

Dense and square lattice-free colloid crystals of highly charged monodisperse latex particles on 3-aminopropyl trimethoxysilane-modified glass substrate

Two- and three-dimensional colloid arrays are fabricated using highly charged, monodisperse poly(styrene/sodium p-styrene sulfonate) particles and 3-aminopropyl trimethoxysilane (APTMS)-modified glass substrates at 20 °C. The colloidal array patterns were investigated by SEM, AFM, and UV–visible analyses, and the pattern on the APTMS-modified glass substrate shows a denser packing and square lattice-free pattern without any crevices, as compared with that of cleaned, bare glass substrates. The adhesion force curves obtained from AFM analysis proved a negligible attractive force between APTMS and the poly(St/NaSS) particles. The APTMS layer guaranteed the free-slipping condition to prevent scattered pinnings of drawing particles into the nuclei. Consequently, the free-slipping led to a denser hexagonal close packing and particle deformation by a stronger capillary force arising from the reduced interstices among the particles. As a result, a dense (packing density ∼ 0.80) fcc (or hcp) packing and narrower stop bands were obtained.

Parallel patterning of nanoparticles via electrodynamic focusing of charged aerosols

The development of nanodevices that exploit the unique properties of nanoparticles will require high-speed methods for patterning surfaces with nanoparticles over large areas and with high resolution. Moreover, the technique will need to work with both conducting and non-conducting surfaces. Here we report an ion-induced parallel-focusing approach that satisfies all requirements. Charged monodisperse aerosol nanoparticles are deposited onto a surface patterned with a photoresist while ions of the same polarity are introduced into the deposition chamber in the presence of an applied electric field. The ions accumulate on the photoresist, modifying the applied field to produce nanoscopic electrostatic lenses that focus the nanoparticles onto the exposed parts of the surface. We have demonstrated that the technique could produce high-resolution patterns at high speed on both conducting (p-type silicon) and non-conducting (silica) surfaces. Moreover, the feature sizes in the nanoparticle patterns were significantly smaller than those in the original photoresist pattern.

Theoretical Analysis of Factors Affecting the Formation and Stability of Multilayered Colloidal Dispersions

A mathematical analysis of the major factors influencing the formation and stability of colloidal dispersions containing spherical particles surrounded by multilayered polymeric interfacial membranes formed by the layer-by-layer electrostatic deposition technique is carried out. The mathematical model assumes that (i) the colloidal dispersion initially consists of a mixture of electrically charged monodisperse spherical particles and oppositely charged polymer molecules, (ii) the adsorption of polymer molecules to the particle surfaces is diffusion-limited, and (iii) the dominant particle-particle collision mechanism is Brownian motion. This approach was used to produce stability maps that highlight conditions under which bridging flocculation, multilayer formation, or depletion flocculation occurs. The stability maps are derived from calculations of the critical polymer concentrations required to (i) saturate the particle surfaces (C(Sat)), (ii) ensure that polymer adsorption is faster than particle collisions (C(Ads)), and (iii) promote depletion flocculation (C(Dep)). In addition, the influence of interfacial properties on the stability of multilayer colloidal dispersions was assessed by calculating the colloidal interactions between the coated particles (i.e., van der Waals, electrostatic, steric, and depletion). These calculations indicated that the major factors are the interfacial charge and composition rather than the interfacial thickness. This article provides useful insights into the factors affecting the formation of stable multilayer colloidal dispersions.

Highly charging of nanoparticles through electrospray of nanoparticle suspension

Patterned deposition of nanoparticles is a prerequisite for the application of unique properties of nanoparticles in future nanodevices. Recent development of nanoxerography requires highly charged aerosol nanoparticles to avoid noise deposition due to random Brownian motion. However, it has been known that it is difficult to charge aerosol nanoparticles with more than two elementary charges. The goal of this work is to develop a simple technique for obtaining highly charged monodisperse aerosol nanoparticles by means of electrospray of colloidal suspension. Highly charged aerosol nanoparticles were produced by electrospraying (ES) and drying colloidal suspensions of monodisperse gold nanoparticles. Size and charge distributions of the resultant particles were measured. We demonstrate that this method successfully charges monodisperse nanoparticles very highly, e.g., 122 elementary charges for 25.0 nm, 23.5 for 10.5 nm, and 4.6 for 4.2 nm. The method described here constitutes a convenient, reliable, and continuous tool for preparing highly charged aerosol nanoparticles from suspensions of nanoparticles produced by either wet chemistry or gas-phase methods.

Characteristics of aerosol charge neutralizers for highly charged particles

Aerosol charge neutralizers containing various radioactive sources have been used to apply an equilibrium charge distribution to aerosols of unknown charge distribution. However, the performance of aerosol charge neutralizers is not well known especially for highly charged particles. In this paper, we discuss some issues related to the neutralizers’ low performance in the measurement of highly charged aerosols, especially with high flow rates. Characteristics of two aerosol charge neutralizers for charging singly charged monodisperse particles and highly-charged polydisperse particles were investigated. One neutralizer had a radioactive source of 85 Kr (beta source, 2 mCi) and the other 210 Po (alpha source, 0.5 mCi). The air flow rate of each aerosol charge neutralizer was varied from 0.2 to 3.0 l min −1 . The results for the 85 Kr aerosol charge neutralizer at an air flow rate of 0.3 l min −1 agree well with the empirical curve (Journal of Aerosol Science 19 (1988) 387), but deviate from the empirical curve at air flow rates of 0.6, 1.0, and 1.5 l min −1 . Unlike the 85 Kr aerosol charge neutralizer, the effect of air flow rate was insignificant for the 210 Po aerosol charge neutralizer. For highly charged polydisperse particles penetrating the 85 Kr aerosol charge neutralizer, the non-equilibrium character also appeared as the air flow rate of the aerosol neutralizer increased. The shift of size distribution was observed most severely at 1.0 l min −1 for both negatively and positively charged particles. Measurements by a scanning mobility particle sizer (SMPS) system showed that the low neutralization efficiency of the 85 Kr aerosol charge neutralizer could distort the particle size and number concentration in measurements of highly charged particles. However, the efficiency was insensitive to the air flow rate for the 210 Po aerosol charge neutralizer.