Detachment of Droplets in a Fully Developed Turbulent Channel Flow

Liquid aerosols deform and detach from solid surfaces under an external force. It is a familiar phenomenon in many engineering applications. This article experimentally investigates the deformation and detachment of liquid droplets on three different solid surfaces in a fully developed turbulent channel flow. It is shown that the droplets either are compressed or elongate under the turbulent flow. The elongation of the droplet due to the turbulent flow is measured and presented. When the friction velocity of the flow exceeds a critical value, the droplets slide along the surface. The critical friction velocity is found empirically to be inversely proportional to the square root of the contact diameter. The sliding velocity after detachment is also reported. It has been observed by many researchers that, when the external force is gravity or a simple shear flow, the retention force of the droplet is proportional to the difference between the cosines of the receding and advancing contact angles. As the shape of a deformed droplet is much more complex under a turbulent flow, this article discusses the applicability of the same relation to the turbulent channel flow.

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     

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     

Detachment of adhered particles from a cloth surface subjected to a rod strike

A mechanical impulse can cause adhered particles to detach from a surface. For various purposes, particle detachment may need to be enhanced or restricted. Unlike rigid solids, cloth material can be deformed or bent by a mechanical impulse. However, neither the cloth deformation nor the induced turbulent airflow has been well studied. This investigation experimentally measured the detachment of Arizona test dust (ATD) from cloth segments. The vertical margins of each cloth segment were fastened to a frame, and the cloth surface with the ATD adhered to the reverse side was struck with a rod. The cloth motion, induced airflow, and particle detachment were recorded by a high-speed camera. In addition, the displacement and acceleration of the cloth were monitored with a laser distance sensor. The mass percentages of detached particles from the cloth and the particle residual were weighed. Several factors that affected particle detachment were compared. The results revealed that the particle detachment was caused by a combination of the vibrating motion of the cloth surface, the hydrodynamic action of the induced turbulent airflows, and the particle agglomeration when the cloth was bent. A strike could even leave fewer residual particles when a much higher surface dust load had initially adhered to the cloth.

Copyright © 2019 American Association for Aerosol Research     

Aerosol Generation by a Spray-Drying Technique Under Coulomb Explosion and Rapid Evaporation for the Preparation of Aerosol Particles for Inhalation Tests

In this study, nanosized (<100 nm) aerosol particles with high mass concentrations for inhalation tests were generated by a spray-drying technique with combining Coulomb explosion and rapid evaporation of the droplets. Under typical spray-drying conditions, aerosol particles with average diameter of 50–150 nm were prepared from a suspension of NiO nanoparticles with a primary diameter of 15–30 nm. Under the Coulomb explosion method, the sprayed droplets were charged by being mixed with unipolar ions to break up the droplets, which resulted in the generation of smaller aerosol particles with diameters of 15–30 nm and high number concentrations. Under the rapid evaporation method, the droplets were heated immediately after being sprayed to avoid inertial impaction on the flow path due to shrinkage of the droplet, which increased the mass concentration of the aerosol particles. The combination of the Coulomb explosion and rapid evaporation of droplets resulted in the generation of aerosol particles with sizes less than 100 nm and mass concentrations greater than 1 mg/m³; these values are often necessary for inhalation tests. The aerosols generated under the combined method exhibited good long-term stability for inhalation tests. The techniques developed in this study were also applied to other metal oxide nanoparticle materials and to fibrous multiwalled carbon nanotubes.

Copyright 2014 American Association for Aerosol Research     

Advanced aerosol optical tweezers chamber design to facilitate phase-separation and equilibration timescale experiments on complex droplets

The phase-separation of mixed aerosol particles and the resulting morphology plays an important role in determining the interactions of liquid aerosols with their gas-phase environment. We present the application of a new aerosol optical tweezers chamber for delivering a uniformly mixed aerosol flow to the trapped droplet's position for performing experiments that determine the phase-separation and resulting properties of complex mixed droplets. This facilitates stable trapping when adding additional phases through aerosol coagulation, and reproducible measurements of the droplet's equilibration timescale. We demonstrate the trapping of pure organic carbon droplets, which allows us to study the morphology of droplets containing pure hydrocarbon phases to which a second phase is added by coagulation. A series of experiments using simple compounds are presented to establish our ability to use the cavity enhanced Raman spectra to distinguish between homogeneous single-phase, and phase-separated core–shell or partially engulfed morphologies. The core–shell morphology is distinguished by the pattern of the whispering gallery modes (WGMs) in the Raman spectra where the WGMs are influenced by refraction through both phases. A core–shell optimization algorithm was developed to provide a more accurate and detailed analysis of the WGMs than is possible using the homogeneous Mie scattering solution. The unique analytical capabilities of the aerosol optical tweezers provide a new approach for advancing our understanding of the chemical and physical evolution of complex atmospheric particulate matter, and the important environmental impacts of aerosols on atmospheric chemistry, air quality, human health, and climate change.

Copyright © 2016 American Association for Aerosol Research     

Droplet-based microfluidics detector for bioaerosol detection

Detection of bioaerosols is important in fields ranging from environmental health monitoring to biosurveillance, and current detector weaknesses have motivated the development of new technologies. In this work, a detector was built, which applies the principles of droplet microfluidics to bioaerosol detection. Droplet microfluidics is a subfield of microfluidics based on the creation of monodisperse microdroplets with compartmentalized reagents and supports enhanced assays and fluidic manipulations. The bioaerosol detector operates by aerodynamically focusing aerosols directly into these droplets to harness the benefits of the microreactor environment. A breadboard detector system, which consisted of an aerodynamic focusing lens, aerosol-focusing capillary, microfluidic droplet chip, and optical microscope, was constructed. Computational fluid dynamic simulations and Lagrangian particle tracking modeling were conducted to identify the optimal conditions for focusing. Preliminary experiments, where aerosols were deposited onto a solid substrate, demonstrated sub 200-µm spot diameters for aerodynamic diameters of 2–5 µm. Test aerosols were then generated, and collected into the microfluidic liquid interface on the chip as verified by microscopy. Recovery efficiency of the aerosols was dependent on aerosol size and ranged from about 27% to nearly 100%. Finally, to prove bioaerosol collection and detection, a droplet propidium iodide (PI) assay was performed: the system distinguished between E. coli and non-biological aerosols within 20 s. Overall, this work established the technique of direct collection of bioaerosols into a convenient droplet microfluidic platform for detection.

Copyright © 2017 The Johns Hopkins University Applied Physics Laboratory     

HEAT TRANSFER IN PACKED BED REACTORS WITH ONE PHASE FLOW

An extensive array of literature data on the heat transfer from a reactor wall to a fluid flowing through a packed bed and those obtained from some experimental runs were interpreted with a model containing two parameters: k_e, (effective radial thermal conductivity within the bed) and h_w (heat transfer coefficient at the wall).

Both parameters were considered in terms of a stagnant contribution (due to the heat conduction through the solid particles and the fluid in the void space) and a radial mixing contribution (due to the heat convection by turbulent mechanism.

The stagnant contribution was interpreted with a model similar to that proposed by Kunii and Smith (1966) for heat transfer in a packed bed with motionless fluid.

General correlating equations for calculating the stagnant and the turbulent contributions of both k_e, and h_w are proposed.     

Laboratory measurements of light scattering properties of kaolinite dust at 532 nm

Light scattering by kaolinite dust samples at 532 nm is studied using a newly developed laboratory apparatus. During the experiments, dust samples are suspended in water, aerosolized by a nebulizer, and then injected into the scattering zone, with or without going through a diffusion drier, to generate either dried dust particles or water droplets with dust inclusions. The light source is a dual wavelength (532 and 1064 nm) diode-pumped solid state laser. Light scattered by an ensemble of particles is collected by a charge-coupled device (CCD) camera, which is mounted on the rotating arm of a stepper motor. The stepper motor rotates the CCD to cover the scattering angle range from 3° to 177°. Polarized scattering light is measured for the horizontally and vertically polarized incident light. The apparatus is calibrated, using pure water droplets as the scattering media. The response function with respect to the scattering angle is obtained by comparing the measurements with Lorenz–Mie calculations and then used in the later data analysis. Measurements show that the backward scattering features of the water droplets are smoothened due to their dust inclusions. Numerical simulations and measurements are extensively compared and discussed. It is found that the Lorenz–Mie theory is inadequate to reproduce the scattering phase functions of either dust particles or water droplets with dust inclusions. A nonspherical aggregate model is applied to simulate the scattering phase functions. The simulation is able to reproduce the overall scattering features; however, substantial discrepancies still exist due to uncertainties in particle shape and refractive index.

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     

Transient aerodynamic atomization model to predict aerosol droplet size of pressurized metered dose inhalers (pMDI)

Pressurized metered dose inhalers (pMDI) produce large numbers of droplets with smaller sizes than 5 μm to treat asthma and other pulmonary diseases. The mechanism responsible for droplet generation from bulk propellant liquid is poorly understood, mainly because the small length scales and short time scales make it difficult to characterize transient spray formation events. This article describes the development and findings of a numerical atomization model to predict droplet size of pharmaceutical propellants from first principles. In this model, the velocity difference between propellant vapor and liquid phase inside spray orifice leads to formation of wave-like instabilities on the liquid surface. Two variants of the aerodynamic atomization model are presented based on assumed liquid precursor geometry: (1) cylindrical jet-shaped liquid ligaments surrounded by vapor annulus; (2) annular liquid film with vapor flow in the core. The growth of instabilities on the liquid precursor surfaces and the size of the subsequently formed droplets are predicted by numerical solutions of dispersion equations. The droplet size predictions were compared with phase doppler anemometry (PDA) data and the predictions were in good agreement with the number mean diameter D₁₀, which is representative of the respirable droplets. The temporal behavior of droplet size production was captured consistently well during the period of the first 95% of the propellant mass emission. The outcome of our modeling activities also suggests that, in addition to saturated vapor pressure of the propellant, its viscosity and surface tension are also key properties that govern pMDI droplet size.

© 2017 American Association for Aerosol Research     

HEAT TRANSFER IN TRICKLE-BED REACTORS

The mechanism of radial heat transfer in two-phase flow through packed beds is examined. A model with 2 parameters: an effective radial thermal conductivity in the bed, k_e , and a heat transfer coefficient, h_w , at the wall, give a satisfactory interpretation of the radial temperature profile.

k_e was expressed in terms of a stagnant contribution, due to the heat conduction through the solid and the fluid in the void space, and a radial mixing contribution of the gas and liquid phases, due to the radial component of the velocity of both fluids. The radial mixing contribution of the liquid ( k_e ) _L was compared with radial mass dispersion data, and a satisfactory agreement was obtained.

Moreover, ( k_e )was much higher than the gas mixing and the stagnant contributions.

Correlations for hw and k_e ) _L have been proposed in accordance with the hydrodynamic regimes of the two-phase flow.     

Collection of soot particles into aqueous suspension using a particle-into-liquid sampler

Steam collection devices collecting aerosol particles into liquid samples are frequently used to analyze water-soluble particulate material. The fate of water-insoluble components is often neglected. In this work, we show that fresh soot particles can be suspended into pure water using a steam collection device, the particle-into-liquid sampler (PILS, Weber et?al. 2001). The overall collection efficiency of freshly generated soot particles was found to be on the order of 20%. This shows that, depending on the analytic technique employed, the presence of insoluble, and/or hydrophobic particles in liquid samples from steam collection cannot be neglected.

Copyright © 2018 The Author(s). Published with license by Taylor & Francis Group, LLC     

Thermophoresis of a particle in a concentric cavity with thermal stress slip

The thermophoretic motion of a spherical particle situated at the center of a spherical cavity filled with a gaseous medium under a prescribed temperature gradient is studied analytically. The Knudsen number is small for the gas motion in the slip-flow regime, and the temperature jump, thermal creep, frictional slip, and particularly, thermal stress slip are allowed on the solid surfaces. After solving the equations of heat conduction and fluid motion, an explicit formula for the migration velocity of the confined particle is obtained for different temperature conditions of the cavity with arbitrary values of the particle-to-cavity radius ratio and other parameters. Contributions from the thermoosmotic flow along the cavity wall and from the wall-corrected thermophoretic force to the particle velocity are equivalently important and can be linearly superimposed. With either or both of these contributions, the particle velocity in general is a decreasing function of the particle-to-cavity radius ratio and vanishes in the limit. The effects of the thermal stress slip at the solid surfaces to the migration velocity of the confined particle can be significant and interesting, dependent on the thermal and interfacial properties of the particle and surrounding gas. The wall effect on the thermophoretic migration of the particle in a cavity is qualitatively different from that on the motion of the particle in a circular tube.

Copyright © 2018 American Association for Aerosol Research     

Silver-incorporated poly vinylidene fluoride nanofibers for bacterial filtration

An anti-bacterial filter was developed using poly vinylidene fluoride (PVDF) nanofibers using electrospinning method blended with silver nanoparticles (AgNO₃) of varying weight percentages of filler. Polypropylene (PP) non-woven substrate was used as base material for collecting the nanofibers. It also acted as a barrier to protect the fibers. UV-visible spectroscopy and fourier transform infra red spectroscopy confirmed the uniform dispersion of silver nanoparticles throughout the nanofibers. The experiment was designed using Box–Behnken statistical tool through three different variables namely, PP non-woven sheets (GSM), electrospinning time (hours), concentration of silver (wt%) in 15 runs. Surface morphology was analyzed using scanning electron microscopy and atomic force microscopy. Thermogravimetric analysis was performed for the analyses of mass decomposition of the material. Bacterial filtration efficiency and anti-bacterial activity studies were tested against Staphylococcus aureus and Escherichia coli for both PVDF + 0?wt% Ag fibers and PVDF-Ag nanofibers. This research shows the bacterial filtration efficiency for the prepared PVDF-Ag nanofibers as 99.86%. The prepared nanofilter was shown providing greater possibilities towards the application for clean air management.

© Copyright 2019 American Association for Aerosol Research     

Impactor Apparatus for the Study of Particle Rebound: Relative Humidity and Capillary Forces

The effect of relative humidity (RH) on the adhesion of particles colliding with a hard surface was studied for submicron particles of liquid oleic acid, solid ammonium sulfate, and solid polystyrene latex (PSL). For this purpose, a three-arm impactor was designed and constructed. The three arms consisted of one impactor having an uncoated impaction plate (i.e., a rebound arm), one impactor having a viscous-liquid-coated impaction plate (i.e., a capture arm), and one impactor having no impaction plate (i.e., a null arm). The particle number concentrations downstream of each arm were measured by condensation particle counters (CPCs). Data were analyzed to obtain the particle rebound fraction. Use of ambient upstage pressure allowed measurements from 5 to 95% RH at the impaction plate. Particle rebound depended strongly on RH, even for non-hygroscopic PSL particles. The rebound fraction for PSL particles dropped monotonically from nearly unity at 50% RH to 0.4 at 95% RH. For ammonium sulfate, the rebound fraction dropped from nearly unity at 25% RH to 0.5 at 70% RH. The decreased rebound at higher RH was explained by the formation of a water meniscus. The resulting capillary forces inhibited particle detachment. A model, taking into account the impact kinetic energy compared to the contact adhesion energy arising from van der Waals and capillary forces, captured the observations well. The reduced rebound arising from increased adhesion at high RH, independent of particle water content, potentially confounds a recent assumption that non-rebounding atmospheric particles are liquid.

Copyright 2014 American Association for Aerosol Research     

MEASUREMENT OF INTERFACIAL AREA AND MASS TRANSFER COEFFICIENTS BY CHEMICAL METHOD OF C02 ABSORPTION INTO AQUEOUS SOLUTIONS OF NaOH IN A MODEL COLUMN WITH EXPANDED METAL SHEET PACKING

The theory of gas absorption accompanied by fast pseudo-fast order reaction which considered dependences of diffusivity, kinetic constant and Henry's law constant on absolute temperature and ionic strength was used to obtain values of effective interfacial areas and mass transfer coefficients in gas and liquid phase.

Experimental measurement of carbon dioxide absorption from mixture with air was performed in a pilot-plant column with expanded metal sheet packing irrigated with sodium hydroxide solution.

Resulting liquid and gas-side mass transfer coefficients are compared with values obtained from physical Absorption measurement of carbon dioxide into water and with measurement of gas-side mass transfer coefficient for sulphur dioxide in the same column.

The differences between determined values are discussed.     

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