Control of the radial distribution of chemical components in spray-dried crystalline microparticles

The process of particle formation from evaporating droplets containing more than one solute was studied. Two-component microparticles were produced using a piezoceramic dispenser with an inner diameter of 30 µm. Initial droplets had a diameter in the range of 70–85 µm and contained sodium nitrate and potassium nitrate in different molar ratios of 30:70, 50:50, and 70:30, corresponding to weight ratios of 26.5:73.5, 45.7:54.3, and 66.2:33.8, in the form of aqueous solutions with initial concentrations of 1 or 10 mg/ml. The monodisperse droplets were dried in a dry laminar gas flow with temperatures of 50°C or 100°C. Different initial conditions affected the particle formation process and the particle morphology. The diameter of the final dried microparticles ranged from 4 to 10 µm. Their density varied from 1250 to 1950 mg/ml. The formulation and process conditions determined the distribution of chemical components in the dried microparticles, especially their surface composition as determined by energy-dispersive X-ray spectroscopy. The distribution of the chemical components was theoretically explained using characteristic times for the crystallization kinetics of the drying process. It was shown that the solute that reached supersaturation first formed most of the outer shell of the microparticles.

© 2016 American Association for Aerosol Research     

A comparison of spherical and cylindrical microparticles composed of nanoparticles for pulmonary application

Nano-embedded microparticles represent promising carrier systems to tackle the challenges of nanoparticle delivery into the lungs by inhalation. While spray drying is widely used for the incorporation of nanoparticles into microparticles, the template-assisted technique is a novel method to prepare aspherical, cylindrical microparticles composed of nanoparticles. In this work, both techniques were applied to produce both spherical and cylindrical nano-embedded microparticles. For both geometries particles consisting of gelatin nanoparticles, mannitol and leucine were prepared in three different sizes each. Cylindrical microparticles could be prepared with defined dimensions and narrow size distributions, allowing to target a wide range of aerodynamic diameters. The size of spherical microparticles was influenced by the spraying feed concentration, yielding only small differences in geometric and aerodynamic diameters and broad particle size distributions. Regarding the redispersibility of the nano-embedded microparticles, spherical particles showed better disintegration behavior and higher nanoparticle release in comparison to cylindrical particles upon contact with water. The template-assisted technique yielded higher nanoparticle content in contrast to spray drying. In summary, cylindrical particles represent a promising drug delivery system with high potential for later application. However, further improvements in the preparation method are required to enable higher yields and a possible later scale-up.

Copyright © 2018 American Association for Aerosol Research     

Water-based single-flow mixing condensation particle counter

A novel water-based condensation particle counter has been developed using a patented, single-flow mixing (SFM) condenser that permits a conventional thermal approach of using a hot saturator followed by a cold condenser to activate and grow particles for counting with an optical detector. A computational fluid dynamics (CFD) model of the internal flow, temperature, and vapor profiles was used to predict the effectiveness of the SFM condenser. Using the results from the CFD model, the counting efficiency was numerically calculated for pure water droplets, and the CPC cut-point (i.e., 50% counting efficiency) was predicted to be 8.3 nm. The experimental performance of the new CPC was measured with differential mobility analyzer-classified, monodisperse particles. The measured cut-points were 8.2 nm for Ag particles and 3.9 nm for NaCl particles. The reduction in the cut-point for NaCl is the result of a compound effect: water uptake by NaCl particles, which increases their size before entering into the growth section (condenser), and the reduction of the equilibrium vapor pressure of water over NaCl-water droplets, resulting in a decrease of the activation diameter.

Copyright © 2016 American Association for Aerosol Research     

Generation of monodisperse aerosols by combining aerodynamic flow-focusing and mechanical perturbation

A novel instrument has been developed for generating highly monodisperse aerosol particles with a geometrical standard deviation of 1.05 or less. This aerosol generator applies a periodic mechanical excitation to a micro-liquid jet obtained by aerodynamic flow-focusing. The jet diameter and its fastest growth wavelength have been optimized as a function of the flow-focusing pressure drop and the liquid flow rate. The monodisperse aerosol generated by this instrument is also charge neutralized with bipolar ions produced by a non-radioactive, corona discharge device. Monodisperse droplet generation in the 15- to 72-μm diameter range from a single 100-micron nozzle has been demonstrated. Both liquid and solid monodisperse particles can be generated from 0.7- to 15-μm diameter by varying solution concentration, liquid flow rate, and excitation frequency. The calculated monodisperse particle diameter agrees well with independent measurements. The operation of this new monodisperse aerosol generator is stable and reliable without nozzle clogging, typical of other aerosol generators at the lower end of the operating particle size ranges.

Copyright © 2016 American Association for Aerosol Research     

Effect of Induced Charge on Deposition of Uniformly Charged Particles in a Pediatric Oral-Extrathoracic Airway

An in vitro study was conducted in the Alberta idealized child mouth-throat, which mimics average deposition in a set of nine 6–14-year-old subjects, to examine the enhancement of deposition of monodisperse uniformly charged particles as a result of induced electrostatic forces. A purpose-based atomizer was designed and built for generating monodisperse, uniformly charged particles. The atomizer generates droplets by jet break up under the action of capillary waves and charges them via electrostatic induction. The experiments cover different particle aerodynamic diameters (d _a = 3.6, 4.4, and 5.9 μm), at two flow rates (Q = 10 and 20 L/min), over a wide range of elementary charges per particle (0–10,000 e). The results show substantial increases in particle deposition in the present idealized pediatric mouth throat compared to neutral aerosols. Two empirical equations, as a function of Reynolds number, Stokes number, and induced charge number are introduced for the prediction of mouth-throat deposition in children, based on two different characteristic diameters of the airway.

Copyright 2014 American Association for Aerosol Research     

A practical CFD modeling approach to estimate outlet boundary conditions of industrial multistage spray dryers: Inert particle flow field investigation

Industrial multistage spray drying systems often have limited in situ process measurements to provide sufficient information for computational fluid dynamics (CFD) simulations of the primary drying chamber. In this case study on the spray dryer at Davis Dairy Plant (South Dakota State University), uncertainties were encountered in specifying the outlet boundary conditions of the spray drying chamber with two outlets: the side outlet and the bottom outlet leading to the second stage external vibrating bed. Using the available data on the vacuum pressure of the chamber, a numerical framework was introduced to approximate suitable outlet boundary conditions for the drying chamber. The procedure involved analyzing the ratio of the airflow rate between the two outlets and using a pseudo-tracer inert particle injection analysis. The goal of this approach was to determine a suitable range of outlet vacuum pressure that will lead to realistic particle movement behaviors during the actual plant operation. The protocol developed here will be a useful tool for CFD modeling of large scale multistage spray drying systems.

Abbreviations: ARC: Australian Research Council; CFD: Computational Fluid Dynamics; FFT: Fast Fourier Transform; MCC: Micellar Casein Concentrate; PRESTO: Pressure Staggering Option; SDSU: South Dakota State University; SIMPLE: Semi???Impilicit Method for Pressure Linked Equations; WPC: Whey Protein Concentrate     

Modeling the thermodynamics and kinetics of sulfuric acid-dimethylamine-water nanoparticle growth in the CLOUD chamber

Dimethylamine (DMA) has a stabilizing effect on sulfuric acid (SA) clusters, and the SA and DMA molecules and clusters likely play important roles in both aerosol particle formation and growth in the atmosphere. We use the monodisperse particle growth model for acid-base chemistry in nanoparticle growth (MABNAG) together with direct and indirect observations from the CLOUD4 and CLOUD7 experiments in the cosmics leaving outdoor droplets (CLOUD) chamber at CERN to investigate the size and composition evolution of freshly formed particles consisting of SA, DMA, and water as they grow to 20 nm in dry diameter. Hygroscopic growth factors are measured using a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA), which combined with simulations of particle water uptake using the thermodynamic extended-aerosol inorganics model (E-AIM) constrain the chemical composition. MABNAG predicts a particle-phase ratio between DMA and SA molecules of 1.1–1.3 for a 2 nm particle and DMA gas-phase mixing ratios between 3.5 and 80 pptv. These ratios agree well with observations by an atmospheric-pressure interface time-of-flight (APi-TOF) mass spectrometer. Simulations with MABNAG, direct observations of the composition of clusters <2 nm, and indirect observations of the particle composition indicate that the acidity of the nucleated particles decreases as they grow from ～1 to 20 nm. However, MABNAG predicts less acidic particles than suggested by the indirect estimates at 10 nm diameter using the nano-HTDMA measurements, and less acidic particles than observed by a thermal desorption chemical ionization mass spectrometer (TDCIMS) at 10–30 nm. Possible explanations for these discrepancies are discussed.

Copyright © 2016 American Association for Aerosol Research     

Cu-Sn binary metal particle generation by spray pyrolysis

Cu-Sn binary particles were generated via spray pyrolysis from metal salt precursors with ethylene glycol as the co-solvent and reducing agent. The morphology, crystallinity, and elemental distribution of particles were tunable by changing the reaction temperature, residence time, and quench gas flow rate. Hollow porous particles were fabricated with a higher Sn concentration on the particle surface when the furnace set point was 500°C, while solid particles with a lower surface Sn concentration were generated when the furnace set point was 1000°C. Particles with spherical morphologies were obtained at long residence time conditions (4.5 s). Cu-Sn binary particles with irregular structures (e.g., pores on the particle surface, fragmented spherical particles, and lamellar fragments) were formed at short residence time conditions (0.92 s). A possible spray pyrolysis mechanism was proposed that incorporates chemical reaction steps and structural progression. By this mechanism, the metal salts are believed to sequentially undergo hydrolysis to metal hydroxides, decomposition to metal oxides, reduction to metals, and finally diffusion of Sn into the Cu matrix to generate the Cu-Sn solid solution.

Copyright © 2017 American Association for Aerosol Research     

Mass,charge, and radius of droplets in a linear quadrupole electrodynamic balance

Single particle levitation is a key tool in the analysis of the physicochemical properties of aerosol particles. Central to these techniques is the ability to determine the size of the confined particle or droplet, usually achieved via optical methods. While some of these methods are extremely accurate, they are not suitable for all applications and sample types, such as solid or optically absorbing particles. In this work, measurements of the radius, mass, and charge of droplets in a linear quadrupole electrodynamic balance (LQ-EDB) are reported. Using the elastic light scattering pattern produced by laser illumination, a method to determine the radius is described, with an accuracy of as good as ±60?nm and a sensitivity to changes on the order of 10?nm. The effect of refractive index on these measurements is explored by application of the technique to simulated data using Mie theory. In addition to radius, the relative and absolute mass and charge of droplets in the trap is measured from the voltage required to stabilize their vertical position. These measurements are facilitated by stacking multiple droplets in the LQ-EDB and solving the force balance equations to yield both parameters. These approaches are demonstrated through measurements of the evaporation of pure ethylene glycol and pure water droplets, the change in density of an aqueous glycerol solution as water evaporates, and the mass and charge of pure glycerol droplets.

Copyright © 2019 American Association for Aerosol Research     

Influence of flame-generated ions on the simultaneous charging and coagulation of nanoparticles during combustion

Flames generate a large amount of chemically and thermally ionized species, which are involved in the growth dynamics of particles formed in flames. However, existing models predicting particle formation and growth do not consider particle charging, which may lead to bias in the calculated size distribution of particles. In this study, Fuchs' charging theory was coupled with a monodisperse particle growth model to study the simultaneous charging and coagulation of nanoparticles during combustion. In order to quantify the charging characteristics of nanoparticles, a high-resolution DMA was used to measure the mobilities of ions generated from a premixed flat flame operated at various conditions. The effect of temperature on ion–particle and particle–particle combination coefficients was further examined. The proposed model showed that the influence of charging on particle growth dynamics was more prominent when the ion concentration was comparable to or higher than the particle concentrations, a condition that may be encountered in flame synthesis and solid fuel-burning. Simulated results also showed that unipolar ion environments strongly suppressed the coagulation of particles. In the end, a simplified analysis of the relative importance of particle charging and coagulation was proposed by comparing the characteristic time scales of these two mechanisms.

© 2017 American Association for Aerosol Research     

Influence of surfactants on growth of individual aqueous coarse mode aerosol particles

Understanding the links between aerosol and cloud and radiative properties remains a large uncertainty in predicting Earth's changing energy budget. Surfactants are observed in ambient atmospheric aerosol particles, and their effect on cloud droplet growth is a mechanism that was, until recently, neglected in model calculations of particle activation and droplet growth. In this study, coarse mode aqueous aerosol particles were created containing the surfactant Igepal CA-630 and NaCl. The evaporation and condensation of these individual aqueous particles were investigated using an aerosol optical trap combined with Raman spectroscopy. For a relative humidity (RH) change from 70% to 80%, droplets containing both Igepal and NaCl at atmospheric concentrations exhibited on average more than 4% larger changes in droplet radii, compared to droplets containing NaCl only. This indicates enhanced water uptake in the presence of surfactants, but this result is unexpected based on the standard calculation of the effect of surfactants, using surface tension reduction and/or hygroscopicity changes, for particles of this size. One implication of these results is that in periods with increasing RH, surfactant-containing aqueous particles may grow larger than similarly sized aqueous NaCl particles without surfactants, thus shifting atmospheric particle size distributions, influencing particle growth, and affecting aerosol loading, visibility, and radiative forcing.

Copyright © 2018 American Association for Aerosol Research     

Particle formation in spray drying

Theoretical and experimental investigations of the particle formation process during spray drying are presented. A novel experimental method allows observation of individual, free flowing droplets during drying in a laminar gas flow and subsequent analysis of the resulting monodisperse, monomorphic dry particles. A second method combines a vibrating orifice generator and a bench top spray drier, which allows production and sampling of monodisperse particles at different drying stages. The experimental results are compared to a full numerical model and a simplified analytical model. Two dimensionless parameters are identified that influence particle formation: the Peclet number, which is the ratio of the diffusion coefficient of the solute and the evaporation rate, and the initial saturation of the excipients. In an application example, particle design is shown to improve the aerosol properties of powders intended for pulmonary drug delivery.     

Modeling Spray Drying of Redispersible Polyacrylate Powder

Spray drying was used to produce redispersible polyacrylate powder (RPP) from polyacrylate latex particles with a soft-core/hard-shell configuration. The optimal pretreatment parameters were determined by investigating the effects of the contents of silica sol and the protective colloid, polyvinyl alcohol (PVA), the viscosity of the latex, and solid content of the latex on the redispersibility, moisture content, and residual ratio of RPP particles. The optimal spray-drying parameters were established by investigating the effects of the drying temperature and atomizing speed on the moisture content, residual ratio, redispersibility, and film-forming ability of RPP particles. The dynamic mass- and energy-control equations were combined to establish a model of spray-drying RPP particles. The changes in the temperature and moisture content of the RPP droplets were predicted by coupling the two equations and running a code developed in MATLAB.

[Supplementary materials are available for this article. Go to the publisher's online edition of Drying Technology for the following free supplemental resource(s): MATLAB code.]     

Influence of Particle Size on the Drying Kinetics of Single Droplets Containing Mixtures of Nanoparticles and Microparticles: Modeling and Pilot-Scale Validation

The particle size of the primary particles is an important parameter influencing the drying behavior of droplets. In this work, the influence of particle size on the drying kinetics and grain properties was analyzed for droplets containing silica nanoparticles, microparticles, and mixtures of the two. The presence of microparticles was found to increase the drying rate and shrinkage of the droplet. The drying curves were modeled using a reaction engineering approach (REA) model. Finally, different suspensions were dried in a pilot-scale spray dryer in order to prove the influence of the particle size obtained in the levitator tests.     

Modification of the TSI 3081 differential mobility analyzer to include three monodisperse outlets: Comparison between experimental and theoretical performance

Differential mobility analyzers (DMAs) are widely used to determine the size of aerosol particles, and to probe their size-dependent physicochemical properties when two are employed in tandem. A limitation of tandem DMA (TDMA) systems is their long measuring cycle when the properties of more than one monodisperse population of particles need to be probed. In this work, we propose a simple modification of the classical cylindrical DMA by including three monodisperse-particle outlets in its central electrode (namely, the 3MO-DMA), with the objective of using it as the first DMA in TDMA systems for reducing their measuring cycle. The performance of the 3MO-DMA at different flow conditions was evaluated using laboratory-generated aerosol particles, and compared with theoretical predictions. The theory predicted accurately (i.e., within 3%) the geometric mean diameters of the three distinct populations, as well as the resolutions of the first and the third outlet, under all experimental conditions. For the second outlet, the resolution was 10% to 74% lower than that predicted theoretically depending on the sheath-to-aerosol flow ratio. Nevertheless, the geometric standard deviation of the monodisperse aerosol from all the outlets was less than 1.09, which is sufficient for using the 3MO-DMA designed and tested in this work as a first DMA to produce a monodisperse aerosol flow containing three distinct particle populations in TDMA systems.

Copyright © 2016 American Association for Aerosol Research     

Spatial aerosol flow maldistribution: A design flaw confounding the proper calibration and data interpretation of stages “0” and “1” of the Andersen eight-stage nonviable cascade impactor

We introduced monodisperse calibrant particles into an eight-stage non-viable Andersen cascade impactor (ACI) operated at 28.3 L/min and separately quantified the particle mass captured under each of the four concentric rings of nozzles on stages 0 and 1, the entry and succeeding stages of this impactor. On both stages, we found that each ring of nozzles has a particle capture efficiency behavior that differs from the others, and the fraction of calibrant particles deposited under each of the individual rings of nozzles depended on the particle size. We believe this behavior derives primarily from a radial flow velocity non-uniformity associated with recirculation zones introduced by the 110° expansion angle of the inlet cone. Because of these recirculation zones, the inertia of particles larger than about 5 µm aerodynamic diameter will cause their point-wise local concentration to differ from the concentration at the inlet entry. This concentration maldistribution continues to stage 1 primarily because of the annular collection plate at stage 0. The influence of the inlet cone aerodynamics on the performance of both stages means that the size of particles deposited on these plates will be uncertain unless the aerosol transport entering the impactor associated with calibration using monodisperse particles exactly simulates the in-use aerosol flow conditions. The degree of realism necessary in the calibration method has heretofore not been discussed in published calibrations of the ACI, introducing uncertainty in the size interpretation of the particle mass collected on stages 0 and 1 in practical applications of this impactor.

Copyright © 2017 American Association for Aerosol Research     

Synthesis of Porous Particles of SOFC Anode and Cathode Materials by Citric Acid-Addition Ultrasonic Spray Pyrolysis (CA-USP)

Ultrasonic spray pyrolysis method to synthesize porous submicron particles of solid oxide fuel cell (SOFC) electrode materials was developed in which citric acid is added in a precursor solution. This citric acid-addition ultrasonic spray pyrolysis (CA-USP) method was then used to synthesize NiO-Gd_0.1Ce_0.9O_1.95 (GDC) particles for SOFC anodes and La_0.8Sr_0.2Co₃ (LSC) and La_0.8Ca_0.2MnO₃ (LCM) particles for SOFC cathodes. The synthesized particles were submicron-sized, porous, and spherical, and had a sponge-like structure. Energy dispersive X-ray spectrometry images of the synthesized NiO-GDC particles revealed that Ni, Ce, Gd, and O were uniformly distributed within individual particles. NiO-GDC and LCM particles with sponge-like structure were not crushed by a 2-h ball mill grinding test. The formation process of the sponge-like structure was clarified by synthesizing GDC particles at various furnace temperatures between 473 and 1273 K.

Copyright 2014 American Association for Aerosol Research     

Synthesis of Porous Submicron Gd0.1Ce0.9O1.95 Particles by Carbon Nanoparticle-Addition Ultrasonic Spray Pyrolysis (CNA-USP) and Nanoparticle Preparation by Ball Milling

A new ultrasonic spray pyrolysis method, called carbon nanoparticle-addition ultrasonic spray pyrolysis (CNA-USP), is developed to synthesize nanoparticles of electrolyte material for solid oxide fuel cell applications. In CNA-USP, carbon nanoparticles are added in a precursor solution. First, Gd_0.1Ce_0.9O_1.95 (GDC) particles were synthesized from an aqueous solution of Ce(NO₃)₃ 6H₂O and Gd(NO₃)₃ 6H₂O by using the CNA-USP method. The resulting synthesized GDC particles were agglomerated, porous, primary particles on the order of 10 nm in diameter. EDX images revealed uniform distributions of Ce, Gd, and O in these porous particles. Then, these agglomerated, porous submicron GDC particles were ground into primary nanoparticles by ball milling for 24 h. The average diameter of the ground GDC nanoparticles was about double of their average crystallite size.

Copyright 2014 American Association for Aerosol Research     

Detachment of droplets by air jet impingement

Surface cleaning using air jets is an appropriate method to remove particles from surfaces especially when cleaning by mechanical methods is not suitable. The detachment behavior of droplets using an air jet is not necessarily the same as solid particles and there is a lack of studies regarding this behavior. In this article, the detachment of droplets on a plastic substrate by air jet impingement was investigated experimentally. Droplets of two different size ranges were impinged by an air jet with different impinging angles. For micrometer-sized droplets, a smaller horizontal velocity was required to detach large droplets. Moreover, the horizontal velocity required to detach 50% number fraction of droplets decreased when the air jet impinging angle increased. Millimeter-sized droplets split into many portions. Most portions remained on the substrate and only a few were resuspended. The remaining portions were distributed in a fan shape, with larger droplets traveling further on the substrate. A linear lower bound of traveled distance was observed. Due to the splitting and the small fraction of resuspension, it should not be expected that air jet cleaning of droplets is the same as that for solid particles.

Copyright © 2017 American Association for Aerosol Research     

On the Feasibility of a Number Concentration Calibration Using a Wafer Surface Scanner

A new primary standard method for calibrating optical particle counters (OPC) has been developed based on quantitative gravitational deposition on a silicon wafer and accurate counting of the particles by a wafer surface scanner (WSS). The test aerosol consists of 3-μm diameter monodisperse polystyrene latex (PSL) spheres at concentrations in the range of 0.1 cm^?3 to 1 cm^?3. A key element to the calibration is the ability to generate monodisperse PSL spheres without residue particles by use of a virtual impactor and differential mobility analyzer. The use of these devices reduced the percentage of residue particles from more than 99.98% to about 5%. The expanded relative uncertainty (95% confidence level) in the number concentration determined with a WSS for a deposition of 200 particles is 17.8%. The major uncertainty component arises from the Poisson fluctuations in the aerosol concentration because of the low concentration. This methodology has advantages of a fast scanning time by the WSS of minutes compared to hours or days by microscopy and of counting every particle deposited compared to often only a small fraction via microscopy.

The WSS was used in the calibration of an OPC based on 12 depositions with concentrations ranging from 0.1 cm^?3 to 1 cm^?3 for each deposition. Make-up air was added to the aerosol entering the OPC so that the lowest achievable concentration for the OPC measurement is about 0.01 cm^?3 in this study. The detection efficiency of the OPC was measured to be 0.984 with an expanded uncertainty of 13.4%.

Copyright 2014 American Association for Aerosol Research