Bipolar Charge Distribution Research Articles

Aerosol charge neutralization and its impact on particle filtration efficiency of common face mask materials

Despite the significant increase in research on mask filtration testing since the COVID-19 pandemic, there remains considerable ambiguity regarding which parameters affect particle filtration efficiency (PFE) and how differences in standard testing protocols can lead to divergent PFE values. We evaluated the PFE (and differential pressure) of several common face masks and community face mask materials including woven cotton, spunbond polypropylene, and meltblown polypropylene, testing in accordance with ASTM F2100/2299 standards for medical masks, using polystyrene latex (PSL) aerosol, as well as NIOSH standards for respirators, using NaCl aerosol. In both cases, PFE was measured with and without aerosol charge neutralization, which is used to bring the particle population to a known, equilibrium bipolar charge distribution. Aerosols of either composition that were untreated (not neutralized) led to significant increases in measured PFE, especially in the case of PSL. In contrast, effective neutralization led to lower PFE measurements that also showed little to no dependence on aerosol composition across most materials. To investigate further, the bipolar charge distributions of PSL and NaCl aerosols, both neutralized and untreated, were characterized using an aerodynamic aerosol classifier operated in tandem with a scanning mobility particle sizer (AAC-SMPS). This technique illustrated the differences in the distribution of particle charge states between PSL and NaCl aerosols of the same size, and between PSL particles of different sizes, revealing the presence of highly charged particles in many cases. Most importantly, the equilibrium charge distribution after neutralization is shown to be independent of particle composition or initial charge distribution, highlighting the crucial role of aerosol charge neutralizers in preventing overestimates of mask performance (due to electrostatic effects) and promoting consistency in standard testing procedures.

Tuning interactions in nanopore arrays with charges on the pore walls

Physiological processes at the cellular level are regulated by nanoconfined ionic transport. Protein channels regulate the passage of specific ions into and out of the cell, and biological systems rely on the concerted function of many channels embedded in their membranes. For instance, the propagation of an action potential down an axon is moderated by the coordinated transport of sodium and potassium ions across neuronal membranes. Beyond its ubiquity in biological systems, transport across nanoporous devices has industrial applications such as single-molecule sensing, energy storage, and water desalination. While single nanopore systems have been thoroughly studied in both biological and industrial contexts, systems containing an array of multiple nanopores are of increasing interest. Due to the close proximity of channels in an array, local environmental changes at the interface of one channel may influence the transport properties of neighboring channels. In this work, we fabricated nanopore arrays with tunable geometries and surface chemistries to probe this cross-talk between nanopores. We employed electron beam sculpting of silicon nitride membranes to fabricate arrays containing two and three nanopores. This method allowed us to engineer devices of varying interpore distances to probe neighboring interactions as a function of interpore distance. Using electrochemical methods, we characterized the current-voltage response of each array at varying sizes of the electric double layer to probe the nature of interpore interactions. Furthermore, we explored surface modifications to create nanopores with bipolar surface charge distributions, which effectively act as ionic diodes. Our results are also complemented by computational modeling. An exploration of the electrokinetic phenomena of these arrays will deepen our understanding of the interplay between neighboring channels, elucidating the mechanisms behind ionic sieving on the scale of an entire network of nanopores.

Nanofluidic diodes based on asymmetric bio-inspired surface coatings in straight glass nanochannels.

In this study, we present nanofluidic diodes fabricated from straight glass nanochannels and functionalized using bio-inspired polydopamine (PDA) and poly-L-lysine (PLL) coatings. The resulting PDA coatings are shown to be asymmetric due to a combination of transport considerations which can be leveraged to provide a measure of control over the effective channel geometry. By subsequently introducing a layer of amine-bearing PLL chains covalently bound to the PDA, we enhance heterogeneities in the charge and ion distributions within the channel and enable significant current rectification between forward-bias and reverse-bias modes; our PDA-PLL-coated channels yielded a rectification ratio greater than 1000 in a 100 nm channel filled with 0.01× phosphate-buffered saline solution (PBS). We further demonstrated that at higher ionic strength conditions, reducing the solution pH increased the number of protonated amines within the PLL layer, amplifying the charge disparities along the channel and leading to greater rectification. As nanofluidic diodes with bipolar surface charge distributions tend to provide superior performance compared to those with a single wall charge polarity, we imposed a more bipolar charge distribution in our devices by partially coating our PDA-PLL-coated channels with negatively charged polyacrylic acid (PAA). These enhanced bipolar channels exhibited greater current rectification than the PDA-PLL-coated channels, reaching rectification ratios in excess of 100 even in more physiologically-relevant 1× PBS solutions. Our fabrication approach and the results herein provide a promising platform from which the scientific community can build upon in the relentless endeavor for improved sensitivity in biosensors and other analytical devices.

A pH gradient induced rectification inversion in asymmetric nanochannels leads to remarkably improved osmotic power

Biological channel membranes regulate direction and degree of ion transport in response to external stimuli, yet it is a challenge to develop artificial nanofluidics with such functions without any chemical modification of the channel walls. Herein, we report the asymmetric branch-type alumina nanochannel (BAN) membrane, that comprises large-pore stem channels and small-pore branched channels and exhibits reversed and significantly enhanced ionic rectification when subjected to a pH gradient. The anomalous transport properties are supported by our rigorous model that takes into account the equilibrium surface reactions of alumina hydroxyl groups on the nanochannel walls with surface protons. The simulation results indicated that the pH gradient-induced bipolar surface charge distributions on the BAN walls lead to the reversal and enhancement of ion transport properties. This work highlights the link between surface chemistry, concentration polarization and ion current rectification, significantly extending the knowledge of how asymmetric pH environments influence transport properties of asymmetric nanofluidics. The application of remarkably amplified osmotic power is also demonstrated, highlighting that the BAN membrane achieves a jump of power density output from ∼2.1 to ∼5.9 W/m2 in the presence of a pH gradient, thus exceeding previously reported values.

Particle beam deflection imaging for simultaneous characterization of aerosol particle size and charge

We present a novel method referred to as Particle Beam Deflection Imaging (PBDI) for the simultaneous characterization of the size and electric charge distribution of submicrometer (currently ∼200–800 nm) aerosol particles in real-time (currently ≤30 s). It combines aerodynamic and electrostatic particle deflection, providing two-dimensional information on size and charge in a single two-dimensional light scattering image. The image allows one to extract information on charge-averaged and charged-resolved particle size distributions, and on size-dependent bipolar charge distributions. The current prototype has a size resolution of <50 nm and can resolve individual charge states. Compared to Scanning Mobility Particles Sizers (SMPS), the PBDI offers the advantage of providing simultaneous size and charge information at higher size resolution in a significantly shorter measurement time, hence allowing real-time characterization of volatile aerosol particles.

Simulation of PMMA powder flow electrification using a new charging model based on single-particle experiments

Thus far, simulations have failed to predict accurately electrostatic powder charging during pneumatic transport. We advanced the modeling of powder flow charging by a three-part study: first, we shot individual particles on a metal target and measured the exchanged charge. Second, based on these results, we formulated an empirical model and implemented it in our CFD tool. Using this tool, we performed large-eddy simulations of the powder flow through a square duct with a cylindrical obstacle inside. Finally, we compared the simulations to measurements in our pneumatic conveying test rig. The simulations successfully predicted the charging of powder consisting of monodisperse particles of a size of 200 μ m. Contrary to the usual procedure for this type of simulation, the tool requires no tuning of any parameters. According to our simulations, the powder mostly charged when hitting the cylindrical obstacle. The contacts led to bipolar charge distributions.

Microscale Bipolar Charge Distributions on Surfaces after Liquid Wetting and Evaporation in an Electric Field.

Studies have shown that when insulator surfaces become electrostatically charged, complex spatial distributions of charge are produced, which are made up of micrometer-scale regions of both charge polarities. The origin of these charge patterns, often called "charge mosaics", is not understood. Here, we carried out controlled Kelvin force microscopy experiments on microfabricated interdigitated electrode systems to show that the process of wetting a surface by a liquid followed by evaporation of the liquid in an electric field can lead to neighboring micrometer-scale regions of positive and negative charge, which remain stable long after the electric field is removed. We thus suggest that local electric fields, perhaps due to the existing charge on the surface, can act in concert with liquid evaporation to contribute to the creation of charge mosaics.

Generating an aerosol of homogeneous, non-spherical particles and measuring their bipolar charge distribution

The Aerodynamic Aerosol Classifier (AAC) and Differential Mobility Analyzer (DMA) are aerosol classifiers commonly used to generate a monodispersed aerosol by selecting particles within a narrow range of relaxation times or electrical mobilities, respectively. However, generating an aerosol of homogeneous particles, which has narrow ranges of particle mass, mobility and relaxation time simultaneously, with either of these methods is challenging. Particles classified by the DMA are often not homogeneous (or monodispersed) due to multiply-charged particles. While the AAC overcomes this challenge for spherical particles, homogeneity is not achieved with non-spherical particles due to their effective density varying with particle size. This study demonstrates using an AAC and DMA in tandem to generate an aerosol of homogeneous, non-spherical particles. This approach is validated using scanning electron microscope (SEM) images and electrical mobility measurements of the tandem-classified particles to highlight their homogeneity. To limit the effects of multiple charging during DMA classification, only a subset of DMA and AAC setpoints are permitted. While this subset is not representative of “average” non-spherical particles from the same aerosol source, this subset of low-density particles deviates the most from spherical morphology, and thus, provides insights into the upper bound of other particle properties, such as charging. Using this approach to select homogeneous particles, the bipolar charge distribution of low-density soot aggregates is then measured using another DMA. This AAC-DMA-DMA approach is demonstrated to measure up to 17 individual charge states (i.e. −8 to +8) after neutralization (with 85Kr) of size-resolved, soot aggregates with mobility diameters between 80 and 433 nm. The low-density soot aggregates obtain higher charges than predicted by theory, which overestimates the uncharged fraction (by 0.042–0.069) and, to a lesser extent, the single charge fractions (by up to 0.037) of the low-density soot aggregates, while underestimating their proportion of multiple charging (by up to 0.135 cumulatively at one particle size or up to 0.039 at one multiple charge state and size). These charging discrepancies represent an upper bound of the bipolar charging of average aggregates from the same source of flame soot.

An experimentally validated model of diffusion charging of arbitrary shaped aerosol particles

Particle shape strongly influences the diffusion charging of aerosol particles exposed to bipolar/unipolar ions and accurate modeling is needed to predict the charge distribution of non-spherical particles. A prior particle-ion collision kernel βi model including Coulombic and image potential interactions for spherical particles is generalized for arbitrary shapes following a scaling approach that uses a continuum and free molecular particle length scale and Langevin dynamics simulations of non-spherical particle-ion collisions for attractive Coulomb-image potential interactions. This extended βi model for collisions between unlike charged particle-ion (bipolar charging) and like charged particle-ion (unipolar charging) is validated by comparing against published experimental data of bipolar charge distributions for diverse shapes. Comparison to the bipolar charging data for spherical particles shows good agreement in air, argon, and nitrogen, while also demonstrating high accuracy in predicting charge states up to ±6. Comparisons to the data for fractal aggregates reveal that the LD-based βi model predicts within overall ±30% without any systematic bias. The mean charge on linear chain aggregates and charge fractions on cylindrical particles is found to be in good agreement with the measurements (~±20% overall). The comparison with experimental results supports the use of LD-based diffusion charging models to predict the bipolar and unipolar charge distribution of arbitrary shaped aerosol particles for a wide range of particle size, and gas temperature, pressure. The presented βi model is valid for perfectly conducting particles and in the absence of external electric fields; these simplifications need to be addressed in future work on particle charging.

Crystal structure of bacteriophage T4 Spackle as determined by native SAD phasing.

The crystal structure of a bacteriophage T4 early gene product, Spackle, was determined by native sulfur single-wavelength anomalous diffraction (SAD) phasing using synchrotron radiation and was refined to 1.52 Å resolution. The structure shows that Spackle consists of a bundle of five α-helices, forming a relatively flat disc-like overall shape. Although Spackle forms a dimer in the crystal, size-exclusion chromatography with multi-angle light scattering shows that it is monomeric in solution. Mass spectrometry confirms that purified mature Spackle lacks the amino-terminal signal peptide and contains an intramolecular disulfide bond, consistent with its proposed role in the periplasm of T4 phage-infected Escherichia coli cells. The surface electrostatic potential of Spackle shows a strikingly bipolar charge distribution, suggesting a possible mode of membrane association and inhibition of the tail lysozyme activity in T4 bacteriophage superinfection exclusion.

Triboelectric charging of polyethylene powders: Comparison of same-material and different-material contributions to the charge build-up

The understanding of triboelectric charging of industrially important powders can help in avoiding or controlling the particle agglomeration and wall sheeting. We developed a new experimental approach to study both same-material particle-particle (PP) and different-material particle-wall (PW) charging of nascent polyethylene (PE) particles born by catalytic polymerization. We constructed a simple tribocharger consisting of a metal box divided inside into several compartments filled with the PE sample and attached to the vibration shaker. The particle charge distribution was characterized by a self-constructed electrostatic separator. This way, we investigated the charging dynamics of PE particles colliding with other PE particles as well as with metal walls of the box compartments. Both PP and PW charging were measured either separately, or in sequence, and in combination of simultaneous PP and PW. The different-material PW charging produced unipolarly charged particles and was much faster than the same-material PP charging leading to bipolar particle charge distribution.

Triboelectric Charging and Separation of Fine Powder Mixtures

AbstractFor increasingly finer powders, the material‐specific separation at high loadings is a challenging task, for instance in recycling processes. Here, a combination of triboelectric charging and electrostatic separation was investigated for powder mixtures of talcum and calcite. The dependencies of the triboelectric charge on the mass loading, the gas velocity, and the mixture ratio were investigated. While higher charge levels were achieved with increasing gas velocity, the mass loading had an opposite effect on the net charge. Although bipolar charge distributions were observed within pure materials and mixtures, electrical neutralization did not occur in the mixtures. Therefore, already in a non‐optimized setup, a decent degree of material enrichment (of up to 53 %) was found on the separating electrodes.

Measuring the bipolar charge distribution of nanoparticles: Review of methodologies and development using the Aerodynamic Aerosol Classifier

A review of methodologies to measure the bipolar charge distribution of nanoparticles is completed, including their advantages/disadvantages and sequential development. This summary also provides context for a new development, which uses an Aerodynamic Aerosol Classifier (AAC) and Differential Mobility Analyzer (DMA) in tandem for a similar purpose. It is demonstrated that the tandem AAC-DMA system overcomes some significant limitations of the previous methodologies, such as multiply-charged particle artefacts and low measurement signals. The tandem AAC-DMA methodology also has the sensitivity to detect other charging phenomena, such as the effects of different sample flow rates through the charger, free-ions downstream of the charger, the inlet insert on the 85Kr charger and different particle chargers (x-ray, old 85Kr and new 85Kr).The charge fractions of the particles at low-flow (0.6 L/min) through the new 85Kr charger agreed well (average absolute difference of 0.007) with widely-used charging theory. However, significant deviations from theory (up to a 0.044 difference in charge fractions) were found with a higher sample flow rate (1.2 L/min), with different exposure times to free-ions downstream of the charger, or with the inlet insert on the new 85Kr charger. It was found that regardless of flow rate, a soft x-ray charger resulted in charge fractions which deviated significantly from theory (up to a 0.084 difference in charge fractions), producing higher fractions of positively charged particles and lower fractions of negatively charged particles relative to theory. All of these deviations are likely due to the simplifying assumptions made by the charging theory. Therefore, rigorous measurement of particle charge distributions are necessary for accurate aerosol characterization, such as standard SMPS measurements.

Growth of nanoparticles in dynamic plasma.

Coagulation growth kinetics of nanoparticles in plasma is affected by interparticle electrostatic forces due to the charging phenomenon. In stationary plasmas, unipolar charging of particles results in retardation of particle growth and may result in a limitation on particle size. We demonstrate the opposite effect of enhanced particle growth in atmospheric pressure nonstationary arc discharge. Modeling of the nanoparticle growth kinetics reveals the formation of a bipolar charge distribution. As a result, reversed (attractive) Coulomb forces promote the formation of micrometer-size particles in a millisecond timescale as observed in experiment.

Holographic deflection imaging measurement of electric charge on aerosol particles

We propose a simple noninvasive technique called holographic deflection imaging (HDI) to measure electric charge on individual aerosol particles. Particles in many flows such as atmospheric turbulence can carry electric charges, which add forces that compete with turbulence and affect particle dynamics. To measure particle charges, we devised a novel technique for polydispersed particles with bipolar charge distributions. The technique uses common optical hardware and a novel particle sizing algorithm for polydispersed size distributions. A charged particle sample is introduced into a vertical channel air flow and a horizontal electric field. For each particle, In-line digital holography is used to simultaneously track particle positions over successive holograms and decode size information from hologram interference fringes. This enables measurement of the electric-force-induced horizontal component of terminal velocity and the particle size. From velocity and size, particle charge is calculated using a force balance, assuming Stokes drag on spherical particles. Two experiments were conducted, one to validate HDI measurements of a bipolarly charged particle sample against an electrometer, and the other to demonstrate particle size effect on triboelectric charging of aerosols in a fan-driven isotropic turbulence chamber. The HDI setup was configured for each experiment, and measurement uncertainty was estimated. Experiment 1 found good agreement (< 5%) in mean charge between measurements by HDI and the electrometer. Experiment 2 found nearly symmetric bipolar charge distributions, which broadened with increasing particle surface area, as expected for triboelectric charging by the symmetrically installed fans.

Determination of nanoparticle collision cross section distribution functions in low pressure plasma synthesis reactors via ion mobility spectrometry

Low pressure (<20 Torr) non-thermal plasma flow tube reactors are commonly used for the synthesis of high purity nanoparticles (NPs) via nucleation and subsequent growth from vapor phase precursors. In spite of their utility, process monitoring (i.e. output NP size characterization) in such reactors is difficult, as there is a dearth of techniques available for online NP size distribution function measurement. In this study, we developed and applied an ion mobility spectrometry (IMS) system consisting of a low pressure differential mobility analyzer (LPDMA) and electrical detector to determine the collision cross section distribution functions of Si NPs synthesized in a radio frequency non-thermal SiH4–Ar plasma operated at ∼2 Torr (266 Pa). The collision cross section, roughly proportional to projected area, is a parameter quantifying the size and structure of nanometer scale species in the vapor phase. We introduce the collision cross section distribution function as a metric for potential use in online process monitoring in NP synthesis. Proper inversion of the collision cross section distribution function requires a priori knowledge of the LPDMA transfer function. We utilized a tandem differential mobility analyzer approach, coupled with a Twomey–Markwoski based scheme to determine LPDMA transfer functions. Subsequent application of these transfer functions in collision cross section distribution function determination showed that at the outlet of the non-thermal plasma flow tube reactor, both negatively and positively charged Si NPs persisted with nearly identical collision cross section distribution functions. NPs were found to have mode ‘mobility equivalent’ diameters near 10 nm, and via TEM analysis were found to persist at the reactor outlet as small aggregates composed of ∼5 nm diameter primary particles. Size distribution functions inferred from collision cross section measurements were compared to size distribution functions inferred from TEM images; excellent agreements were found between IMS and TEM for both mean mobility equivalent diameter and distribution function width. In total, this study shows that (1) IMS is a viable approach for process monitoring in non-thermal plasma NP flow tube synthesis systems, (2) although NPs are modestly aggregated at low-pressure plasma reactor outlets, the extent of aggregation is considerably less than observed in most atmospheric pressure and equilibrium NP synthesis systems (e.g. flames), and (3) after exiting the plasma reactor, the decharging of NPs from highly negative charge levels to a bipolar charge distribution likely drives aggregation on the plasma boundary.

Quantification of the soluble Receptor of Advanced Glycation End-Products (sRAGE) by LC-MS after enrichment by strong cation exchange (SCX) solid-phase extraction (SPE) at the protein level

The study of low abundant proteins contributes to increasing our knowledge about (patho)physiological processes and may lead to the identification and clinical application of disease markers. However, studying these proteins is challenging as high-abundant proteins complicate their analysis. Antibodies are often used to enrich proteins from biological matrices prior to their analysis, though antibody-free approaches have been described for some proteins as well. Here we report an antibody-free workflow on the basis of strong cation exchange (SCX) enrichment and liquid chromatography-mass spectrometry (LC-MS) for quantification of the soluble Receptor of Advanced Glycation End-products (sRAGE), a promising biomarker in chronic obstructive pulmonary disease (COPD). sRAGE was quantified in serum at clinically relevant low to sub ng mL−1 levels. The method was validated according to U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA) guidelines and was compared to an antibody-based LC-MS sRAGE method. The SCX-based method builds upon the bipolar charge distribution of sRAGE, which has a highly basic N-terminal part and an acidic C-terminal part resulting in an overall neutral isoelectric point (pI). The highly basic N-terminal part (pIcalculated = 10.3) allowed for sRAGE to be enriched by SCX at pH 10, a pH at which most serum proteins do not bind. This study shows that ion exchange-based enrichment is a viable approach for the LC-MS analysis of several low abundant proteins following a thorough analysis of their physical-chemical properties.

Numerical and experimental study of submicron aerosol deposition in electret microfiber nonwovens

Highly efficient microfiber electret nonwovens, in which the fibers are electrostatically charged to complement the mechanical deposition mechanisms by electrostatic effects, are nowadays widely used in numerous applications for example to improve the indoor air quality or to reduce emissions. Simulations of the filtration efficiency of electret filters are typically based on a single fiber approach and thus imply strong simplifications. Here, a novel simulation method based on direct numerical solution of the Navier-Stokes equations and the Euler-Lagrange approach for particle tracking taking into account the real three-dimensional microscopic structure of the electret media is presented.Besides the conventional mechanical deposition mechanisms, the model also considers the Coulomb effect as well as dielectrophoresis. It furthermore allows to allocate different surface charge amounts and distributions of both polarities to the microfibers and thus to investigate their influence on the filtration efficiency.To validate the simulations, the results were compared to an empirical approach and measurements for an exemplary electret medium. In the investigated case, the results from the simulations agree well with the measurements for a bipolar charge distribution on the microfibers. The simulation results also point out that by the used empirical approach, for a given surface charge amount, the filtration efficiency is overestimated.

Saharan dust electrification perceived by a triangle of atmospheric electricity stations in Southern Portugal

Atmospheric Electric Potential Gradient (PG) measurements were carried out in three sites forming a triangular array in Southern Portugal. The campaign was performed during the summer, characterized by Saharan dust outbreaks; 16th–17th July 2014 dust event is considered. Short time-scale oscillations of the PG at two of the stations and a mid time-scale suppression of the PG in the three stations are found. Results are interpreted as evidencing long-range dust electrification; attributed to the air-Earth electrical current creating a bipolar charge distribution inside of the dust layer. The relevance of using arrays of sensors, instead of single sited, is highlighted.

Methodology to quantify the ratio of multiple-to single-charged fractions acquired in aerosol neutralizers

ABSTRACTA methodology for the quantification of the ratio of multiple- to single-charged fractions acquired in aerosol neutralizers is presented. These quantities are necessary for an accurate monodisperse calibration of aerosol instrumentation. A tandem Differential Mobility Analyzer (DMA) setup is required, with the second DMA scanning the electrical mobility spectra classified in the first DMA. In contrast to previous studies on the quantification of bipolar charge distribution utilizing tandem DMA schemes, the methodology targets at the direct determination of the multiple- to single-charge fractions and does so through the analysis of the raw signal instead of the inverted size distributions, thus circumventing errors associated with the assumptions in the DMA data inversion. The proposed methodology is employed for the characterization of different types of aerosols commonly employed for instrument calibration. Spherical liquid particles (emery oil and dioctyl sebacate) were found to acquire lower m...