Showing papers on "Particle published in 2020"

The acidity of atmospheric particles and clouds

Abstract: . Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semivolatile gases such as HNO3 , NH3 , HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine-particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicate acidity may be relatively constant due to the semivolatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

Pickering emulsions: Versatility of colloidal particles and recent applications.

Abstract: The versatility of colloidal particles endows the particle stabilized or Pickering emulsions with unique features and can potentially enable the fabrication of a wide variety of derived materials. We review the evolution and breakthroughs in the research on the use of colloidal particles for the stabilization of Pickering emulsions in recent years for the particle categories of inorganic particles, polymer-based particles, and food-grade particles. Moreover, based on the latest works, several emulsions stabilized by the featured particles and their derived functional materials, including enzyme immobilized emulsifiers for interfacial catalysis, 2D colloidal materials stabilized emulsions as templates for porous materials, and Pickering emulsions as adjuvant formulations, are also summarized. Finally, we point out the gaps in the current research on the applications of Pickering emulsions and suggest future directions for the design of particulate stabilizers and preparation methods for Pickering emulsions and their derived materials.

Sustainable food-grade Pickering emulsions stabilized by plant-based particles

Abstract: This review summarizes the major advances that have occurred over the last 5 years in the use of plant-based colloidal particles for the stabilization of oil-in-water and water-in-oil emulsions. We consider the characteristics of polysaccharide-based particles, protein-based particles and organic crystals (flavonoids) with respect to their particle size, degree of aggregation, anisotropy, hydrophobicity and electrical charge. Specific effects of processing on particle functionality are identified. Special emphasis is directed towards the issue of correctly defining the stabilization mechanism to distinguish those cases where the particles are acting as genuine Pickering stabilizers, through direct monolayer adsorption at the liquid–liquid interface, from those cases where the particles are predominantly behaving as ‘structuring agents’ between droplets without necessarily adsorbing at the interface, for example, in many so-called high internal phase Pickering emulsions. Finally, we consider the outlook for future research activity in the field of Pickering emulsions for food applications.

Size and surface charge characterization of nanoparticles with a salt gradient.

Abstract: Exosomes are nanometer-sized lipid vesicles present in liquid biopsies and used as biomarkers for several diseases including cancer, Alzheimer's, and central nervous system diseases. Purification and subsequent size and surface characterization are essential to exosome-based diagnostics. Sample purification is, however, time consuming and potentially damaging, and no current method gives the size and zeta potential from a single measurement. Here, we concentrate exosomes from a dilute solution and measure their size and zeta potential in a one-step measurement with a salt gradient in a capillary channel. The salt gradient causes oppositely directed particle and fluid transport that trap particles. Within minutes, the particle concentration increases more than two orders of magnitude. A fit to the spatial distribution of a single or an ensemble of exosomes returns both their size and surface charge. Our method is applicable for other types of nanoparticles. The capillary is fabricated in a low-cost polymer device.

Machine-learning-revealed statistics of the particle-carbon/binder detachment in lithium-ion battery cathodes

Abstract: The microstructure of a composite electrode determines how individual battery particles are charged and discharged in a lithium-ion battery. It is a frontier challenge to experimentally visualize and, subsequently, to understand the electrochemical consequences of battery particles' evolving (de)attachment with the conductive matrix. Herein, we tackle this issue with a unique combination of multiscale experimental approaches, machine-learning-assisted statistical analysis, and experiment-informed mathematical modeling. Our results suggest that the degree of particle detachment is positively correlated with the charging rate and that smaller particles exhibit a higher degree of uncertainty in their detachment from the carbon/binder matrix. We further explore the feasibility and limitation of utilizing the reconstructed electron density as a proxy for the state-of-charge. Our findings highlight the importance of precisely quantifying the evolving nature of the battery electrode's microstructure with statistical confidence, which is a key to maximize the utility of active particles towards higher battery capacity.

Numerical investigation of aerosol transport in a classroom with relevance to COVID-19

Abstract: The present study investigates aerosol transport and surface deposition in a realistic classroom environment using computational fluid-particle dynamics simulations. Effects of particle size, aerosol source location, glass barriers, and windows are explored. While aerosol transport in air exhibits some stochasticity, it is found that a significant fraction (24%-50%) of particles smaller than 15 µm exit the system within 15 min through the air conditioning system. Particles larger than 20 µm almost entirely deposit on the ground, desks, and nearby surfaces in the room. Source location strongly influences the trajectory and deposition distribution of the exhaled aerosol particles and affects the effectiveness of mitigation measures such as glass barriers. Glass barriers are found to reduce the aerosol transmission of 1 µm particles from the source individual to others separated by at least 2.4 m by ∼92%. By opening windows, the particle exit fraction can be increased by ∼38% compared to the case with closed windows and reduces aerosol deposition on people in the room. On average, ∼69% of 1 µm particles exit the system when the windows are open.

Remarkable nucleation and growth of ultrafine particles from vehicular exhaust

Abstract: High levels of ultrafine particles (UFPs; diameter of less than 50 nm) are frequently produced from new particle formation under urban conditions, with profound implications on human health, weather, and climate. However, the fundamental mechanisms of new particle formation remain elusive, and few experimental studies have realistically replicated the relevant atmospheric conditions. Previous experimental studies simulated oxidation of one compound or a mixture of a few compounds, and extrapolation of the laboratory results to chemically complex air was uncertain. Here, we show striking formation of UFPs in urban air from combining ambient and chamber measurements. By capturing the ambient conditions (i.e., temperature, relative humidity, sunlight, and the types and abundances of chemical species), we elucidate the roles of existing particles, photochemistry, and synergy of multipollutants in new particle formation. Aerosol nucleation in urban air is limited by existing particles but negligibly by nitrogen oxides. Photooxidation of vehicular exhaust yields abundant precursors, and organics, rather than sulfuric acid or base species, dominate formation of UFPs under urban conditions. Recognition of this source of UFPs is essential to assessing their impacts and developing mitigation policies. Our results imply that reduction of primary particles or removal of existing particles without simultaneously limiting organics from automobile emissions is ineffective and can even exacerbate this problem.

Poly- N-isopropylacrylamide Nanogels and Microgels at Fluid Interfaces

Abstract: The confinement of colloidal particles at liquid interfaces offers many opportunities for materials design. Adsorption is driven by a reduction of the total free energy as the contact area between the two liquids is partially replaced by the particle. From an application point of view, particle-stabilized interfaces form emulsions and foams with superior stability. Liquid interfaces also effectively confine colloidal particles in two dimensions and therefore provide ideal model systems to fundamentally study particle interactions, dynamics, and self-assembly. With progress in the synthesis of nanomaterials, more and more complex and functional particles are available for such studies. In this Account, we focus on poly(N-isopropylacrylamide) nanogels and microgels. These are cross-linked polymeric particles that swell and soften by uptaking large amounts of water. The incorporated water can be partially expelled, causing a volume phase transition into a collapsed state when the temperature is increased above approximately 32 °C. Soft microgels adsorbed to liquid interfaces significantly deform under the influence of interfacial tension and assume cross sections exceeding their bulk dimensions. In particular, a pronounced corona forms around the microgel core, consisting of dangling chains at the microgel periphery. These polymer chains expand at the interface and strongly affect the interparticle interactions. The particle deformability therefore leads to a significantly more complex interfacial phase behavior that provides a rich playground to explore structure formation processes. We first discuss the characteristic "fried-egg" or core-corona morphology of individual microgels adsorbed to a liquid interface and comment on the dependence of this interfacial morphology on their physicochemical properties. We introduce different theoretical models to describe their interfacial morphology. In a second part, we introduce how ensembles of microgels interact and self-assemble at liquid interfaces. The core-corona morphology and the possibility to force these elements into overlap upon compression results in a complex phase behavior with a phase transition between microgels with extended and collapsed coronae. We discuss the influence of the internal particle architecture, also including core-shell microgels with rigid cores, on the phase behavior. Finally, we present new routes for the realization of more complex structures, resulting from multiple deposition protocols and from engineering the interaction potential of the individual particles.

Single-Iron Site Catalysts with Self-Assembled Dual-size Architecture and Hierarchical Porosity for Proton-Exchange Membrane Fuel Cells

Abstract: Atomically dispersed and nitrogen coordinated single iron site (i.e., FeN4) catalysts (Fe-N-C) are the most promising platinum group metal (PGM)-free cathode for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, current Fe-N-C catalysts are limited by the inferior exposure of active FeN4 sites due to the inevitable agglomeration of particles in cathodes. Herein, we report a self-assembled strategy to synthesize the atomically dispersed FeN4 site catalysts with a hierarchically porous matrix derived from dual-size Fe-doped ZIF-8 crystal precursors by using large particles to support small particles. The tailored structure is effective in mitigating the particle migration, agglomeration, and spatial overlap, thereby exposing increased accessible active sites and facilitating mass transport. The best performing catalyst composed of 100 nm “nucleated seed” assembled by 30 nm “satellite” demonstrates exceptional ORR activity in acidic electrolyte and membrane electrode assembly. This work provides new concepts for designing hierarchically porous catalysts with single metal atom dispersion via self-assembly of ZIF-8 crystal precursors with tunable particle sizes and nanostructures.

Study on the surface particle distribution characteristics of silt with different moisture content

Abstract: Soil structure and particle distribution determine the action form of water and the distribution and movement form of water in the soil, which may cause many engineering and environmental problems, so it is of great significance to study the distribution form of soil particles under different moisture content. Taking the silt as the research object, the surface micro features of soil samples with the same dry density and different moisture content were collected, and the binarization analysis and particle statistics were carried out to study the action mode between water and particles. The results show that (1) moisture content has a certain impact on the micro features. With the increase of moisture content, the pores between soil particles gradually decrease, and the fractal dimension of surface particles also decreases. (2) In the case of the difference of specific surface area, the smaller the particle size, the more obvious the response to water. For this silt sample, the particles with a size of 0.03~0.05 mm are relatively stable. With the increase of moisture content, small particles flocculate to form large particles under a series of forces. (3) When the moisture content was 8.92% and 13.78%, the soil features changed significantly, which may be the critical moisture contents affecting the structural quality of the sample. (4) Advantage orientation of silt particles is 90°~105°, which may cause the soil mechanics characteristics of anisotropy. (5) The interaction model between water and soil particles is as follows: dissolution, the change of stress on particles, and the change of electrical double layer on the surface of soil particles.

Analysis of particle deposition in a new‐type rectifying plate system during shale gas extraction

Abstract: Much less attention has been focused on the particle deposition in rectifying plate though it is a common problem in shale gas pipe systems. The effects of particle parameter and flow field on the deposition and distribution of particle in a new‐type rectifying plate system are investigated. Both the computational fluid dynamics (CFD) method and the experiment method are used in order to analyze the particle deposition under various conditions. The accuracy of simulation model is verified with measurements in the experiment and from analyzing, and it is found that the Boltzmann equation can well describe the relationship between gas Reynolds number and particle deposition in the rectifying plate system. It is also found from investigation that the particle deposition is greatly affected by the particle parameter. Deposition rate rises with the increase of the particle diameter; however, it reduces gradually with the decrease of particle shape factor. Moreover, the particle mass concentration is an essential dimension that can give a prediction of where the particle may deposit.

Evaluation of PM2.5 measured in an urban setting using a low-cost optical particle counter and a Federal Equivalent Method Beta Attenuation Monitor

Abstract: We present the results of a multi-season field evaluation of a low-cost optical particle counting sensor (Purple Air PA-II) that reports mass concentration of particulate matter with diamet...

Incompressible smoothed particle hydrodynamics simulation of natural convection in a nanofluid-filled complex wavy porous cavity with inner solid particles

Abstract: This study focus on the simulation of the natural convection of a nanofluid in a wavy cavity saturated with a partially layered non-Darcy porous medium. The motion of the embedded solid particles, which carry two different isothermal conditions inside a wavy cavity, was considered. The meshfree nature of incompressible smoothed particle hydrodynamics (ISPH) method helped us to simulate the motion of solid particles inside a wavy cavity. The dummy wall boundary particles with enough layers were used to prevent the particle penetrations during simulation of natural convection. The wavy cavity is filled with a nanofluid and a non-Darcy porous medium is embedded in the upper half of the wavy cavity. The results from the current investigation showed that, the motion of the inserted solid particles affects strongly on the strength of the fluid flows and heat transfer inside a wavy cavity. The position and isothermal condition of the inner solid particles try to change the distributions of temperature and fluid flow inside a wavy cavity. Average Nusselt number has higher values in the case of cool solid particles compare to hot solid particles. At the current model, an addition of nanoparticles has slight effects on enhancement heat transfer inside a wavy cavity.

Crystallization by particle attachment is a colloidal assembly process.

Abstract: The nucleation of crystals has long been thought to occur through the stochastic association of ions, atoms or molecules to form critical nuclei, which will later grow out to crystals1. Only in the past decade has the awareness grown that crystallization can also proceed through the assembly of different types of building blocks2,3, including amorphous precursors4, primary particles5, prenucleation species6,7, dense liquid droplets8,9 or nanocrystals10. However, the forces that control these alternative pathways are still poorly understood. Here, we investigate the crystallization of magnetite (Fe3O4) through the formation and aggregation of primary particles and show that both the thermodynamics and the kinetics of the process can be described in terms of colloidal assembly. This model allows predicting the average crystal size at a given initial Fe concentration, thereby opening the way to the design of crystals with predefined sizes and properties.

Minimum sizes of respiratory particles carrying SARS-CoV-2 and the possibility of aerosol generation

Abstract: This study calculates and elucidates the minimum size of respiratory particles that are potential carriers of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); furthermore, it evaluates the aerosol generation potential of SARS-CoV-2. The calculations are based on experimental results and theoretical models. In the case of maximum viral-loading derived from experimental data of COVID-19 patients, 7.18 × 10−4% of a respiratory fluid particle from a COVID-19 patient is occupied by SARS-CoV-2. Hence, the minimum size of a respiratory particle that can contain SARS-CoV-2 is calculated to be approximately 4.7 μm. The minimum size of the particles can decrease due to the evaporation of water on the particle surfaces. There are limitations to this analysis: (a) assumption that the viruses are homogeneously distributed in respiratory fluid particles and (b) considering a gene copy as a single virion in unit conversions. However, the study shows that high viral loads can decrease the minimum size of respiratory particles containing SARS-CoV-2, thereby increasing the probability of aerosol generation of the viruses. The aerosol generation theory created in this study for COVID-19 has the potential to be applied to other contagious diseases that are caused by respiratory infectious microorganisms.

Spatial quantification of dynamic inter and intra particle crystallographic heterogeneities within lithium ion electrodes

Abstract: The performance of lithium ion electrodes is hindered by unfavorable chemical heterogeneities that pre-exist or develop during operation. Time-resolved spatial descriptions are needed to understand the link between such heterogeneities and a cell’s performance. Here, operando high-resolution X-ray diffraction-computed tomography is used to spatially and temporally quantify crystallographic heterogeneities within and between particles throughout both fresh and degraded LixMn2O4 electrodes. This imaging technique facilitates identification of stoichiometric differences between particles and stoichiometric gradients and phase heterogeneities within particles. Through radial quantification of phase fractions, the response of distinct particles to lithiation is found to vary; most particles contain localized regions that transition to rock salt LiMnO2 within the first cycle. Other particles contain monoclinic Li2MnO3 near the surface and almost pure spinel LixMn2O4 near the core. Following 150 cycles, concentrations of LiMnO2 and Li2MnO3 significantly increase and widely vary between particles. Dynamic chemical and structural heterogeneities within electrodes are known to lead to battery degradation and failure. Here, the authors show that X-ray diffraction computed tomography can be used to spatially quantify the dynamic crystallographic states of electrodes as they operate and degrade.

Infiltration Behavior of Microplastic Particles with Different Densities, Sizes, and Shapes-From Glass Spheres to Natural Sediments.

Abstract: In this study, the infiltration behavior of 21 microplastic particles with different densities, diameters, and shapes was investigated using columns of glass spheres with different grain diameters. The glass spheres were considered as an analogy for natural sediment and the results were afterward transferred to natural sediment and compared to fine sediment infiltration. The infiltration depth of the microplastic particles increased with decreasing diameter of the microplastic particles (dMP) and with increasing diameter of the glass spheres (dGS). At ratios of dMP/dGS > 0.32, hardly any infiltration could be observed. In regard to the particle shape, the data shows that spherical particles infiltrate deeper than fragments and fibers. In case of fibers, the fiber diameter, in particular, influences the depth of infiltration, with thinner fiber diameters leading to deeper infiltration depths. Fragments, such as tire abrasion, infiltrated less deeply than spherical particles, probably due to the entanglement of the angular particles in the pores. Finally, based on the experiments, this study provides initial indications of reasonable sampling depths in dependence of the grain sizes of the bottom sediment and the microplastic particles. According to this, microplastics in detectable particle sizes (>100 μm) are only found on the surface of sediment consisting of coarse silt and fine sand, while the particles might infiltrate up to 13 cm into sediment consisting of coarse sand, fine gravel, and medium gravel. A statement of depth-variable microplastic concentrations is therefore mainly useful for these sediment types. Accordingly, in future sediment samples, the grain size distribution of the sediment should always be indicated to better evaluate the detected microplastic concentrations.

Completely Dark Photons from Gravitational Particle Production During Inflation

Abstract: Starting with the de Broglie--Proca Lagrangian for a massive vector field, we calculate the number density of particles resulting from gravitational particle production (GPP) during inflation, with detailed consideration to the evolution of the number density through the reheating. We find plausible scenarios for the production of dark-photon dark matter of mass in a wide range, as low as a micro-electron volt to $10^{14}$ GeV. Gravitational particle production does not depend on any coupling of the dark photon to standard-model particles.

Switchable Full-Color Reflective Photonic Ellipsoidal Particles.

Abstract: Full-color reflective photonic ellipsoidal polymer particles, capable of a dynamic color change, are created from dendronized brush block copolymers (den-BBCPs) self-assembled by solvent-evaporation from an emulsion. Surfactants composed of dendritic monomer units allow for the precise modulation of the interfacial properties of den-BBCP particles to transition in shape from spheres to striped ellipsoids. Strong steric repulsions between wedge-type monomers promote rapid self-assembly of polymers into large domains (i.e., 153 nm ≤ D ≤ 298 nm). Of note, highly ordered axially stacked lamellae (i.e., number of layers >100) within an ellipsoid give rise to a near-perfect photonic multilayer. The reflecting color is readily tunable across the entire visible spectrum by alteration of the molecular weight from 477 to 1144 kDa. Finally, the photonic ellipsoids are functionalized with magnetic nanoparticles organized into bands on the particle surface to produce real-time on/off coloration by magnetic field-assisted activation. In total, the reported photonic ellipsoidal particles represent a new class of switchable photonic materials.

Assessing the accuracy of low-cost optical particle sensors using a physics-based approach.

Abstract: . Low-cost sensors for measuring particulate matter (PM) offer the ability to understand human exposure to air pollution at spatiotemporal scales that have previously been impractical. However, such low-cost PM sensors tend to be poorly characterized, and their measurements of mass concentration can be subject to considerable error. Recent studies have investigated how individual factors can contribute to this error, but these studies are largely based on empirical comparisons and generally do not examine the role of multiple factors simultaneously. Here, we present a new physics-based framework and open-source software package (opcsim) for evaluating the ability of low-cost optical particle sensors (optical particle counters and nephelometers) to accurately characterize the size distribution and/or mass loading of aerosol particles. This framework, which uses Mie theory to calculate the response of a given sensor to a given particle population, is used to estimate the fractional error in mass loading for different sensor types given variations in relative humidity, aerosol optical properties, and the underlying particle size distribution. Results indicate that such error, which can be substantial, is dependent on the sensor technology (nephelometer vs. optical particle counter), the specific parameters of the individual sensor, and differences between the aerosol used to calibrate the sensor and the aerosol being measured. We conclude with a summary of likely sources of error for different sensor types, environmental conditions, and particle classes and offer general recommendations for the choice of calibrant under different measurement scenarios.