Inertial impaction of aerosol particles on single and multiple spherical targets

Inertial deposition of aerosol droplets (diameter: 1–14 μm) on steel spheres (diameter: 3–9 mm) was investigated. Air velocity was varied between 7 and 28 m/s (corresponding sphere Reynolds numbers: 1400–17000). The impaction on single spheres as well as that on linear arrays of eight spheres was measured. Theoretical results, based on potential flow investigations were verified by single sphere experiments. Of special interest was the range of lower Stokes numbers, where the theoretically predicted limit of deposition cannot be verified. The experiments on sphere arrays were for the first time performed in the low Stokes number range. Deposition on the leading sphere, relative to that on the shielded spheres, exhibits a maximum in the high Stokes number range, but this changes drastically in the low Stokes number range. Here, maximum deposition can be found on the shielded spheres while the leading sphere shows a markedly lower deposition.     

Eddy motions between fluid spheres in a stokes flow

In the axisymmetric Stokes streaming flow past two fluid spheres it is shown that, in contrast to when the spheres are solid, no separation from the surfaces of the spheres occurs. However, for oil droplets in water, free toroidal eddies are predicted for a certain range of proximities and for a ratio of radii up to about 7. The limit in which one sphere becomes a half space is discussed.     

Multilayer Construction with Various Ceramic Films for Electronic Devices Fabricated by Aerosol Deposition

Aerosol deposition (AD) is applicable as a fabrication technology for microstructures comprising different materials. We used this method for electronic devices that consist of ceramic films and metal electrodes. Various ceramic thick films (5–50 μm thickness), for example, Al₂O₃, 2MgO·SiO₂, and BaTiO₃, were deposited on substrates using room-temperature aerosol deposition. The dielectric constant of BaTiO₃ was 78 at 1 MHz. Multilayer constructions with ceramic films and copper electrodes were obtained using aerosol deposition and sputtering. During deposition, photoresist film masks were applied to produce patterns of ceramic films and connections between upper and lower electrodes through the ceramic films.     

Deposition of Charged Aerosol Particles on a Substrate by Collimating Through an Electric Field Assisted Coaxial Flow Nozzle

Electroaerodynamic (EAD) jet printing, where aerodynamic force is coupled with electrostatic force in order to obtain a wide range of controlled pattern sizes, is introduced. Charged and sheathed aerosol particles yield a high deposition rate even at low velocity owing to the force of their electrostatic attraction to the substrate. In this study, two coaxial nozzles (inner diameters of 6 mm and 100 μm) were designed and tested theoretically and experimentally in order to observe the effects of electrostatic force, particle size, and air flow rate on particle trajectory and dot pattern size. A higher sheath air flow rate (higher Stokes number) caused the aerosol jet stream to be focused. For Stokes numbers higher than 1, the effect of applied voltage on pattern size was less than that of the sheath air flow rate. However, for Stokes number lower than 1, the pattern size was affected by both the applied voltage and the sheath air flow rate. After incorporating all data, the diameter of the particle deposition area (W_p) was expressed as a function of nozzle diameter (W), sheath air flow rate (Q_sheath), aerosol flow rate (Q_aerosol), Stokes number (Stk), and Electrostatic number (Es). Three different equations were obtained for Stk < 1, for 1 ≤ Stk < 5, and for Stk ≥ 5, respectively. These equations would be used to predict pattern width for given conditions of aerosol and sheath flow rates, particle size, electric field, and nozzle size.

Copyright 2013 American Association for Aerosol Research     

Particle deposition in the wake of charged spheres

Earlier work in this laboratory, wherein aerosols were collected from gas streams moving around charged water droplets, indicated the likelihood of high rates of deposition on the aft side of the droplet where flow conditions are not adequately represented by potential flow theory. Further tests were made with metal spheres and the aerosol deposition pattern observed with scanning electron microscopy. Results from these test confirm that the developed wake has a definite influence on the collection pattern as well as the overall collection efficiency. Generally, minima were observed aft of the separation point at an angle at which the vortices flow tangentially to the sphere. Maxima appeared forward of the separation and advanced toward the forward stagnation point as the Reynolds number increased and overall collection efficiency declined. At higher levels of field intensity the deposition profile appeared flat as one scanned around the sphere, with a peak at the rear stagnation point. Very little information is available concerning deposition in the wake of moving droplets. This seems particularly important for scrubbers and spray towers where particles are collected by droplets moving at high Reynolds numbers where wakes and eddies are quite common.     

Anisokinetic sampling of aerosols at a slot intake

The flow at intake to some aerosol samplers may be regarded as two-dimensional flow into a thin-walled slot. A theoretical analysis is given of the efficiency with which such a slot collects particles from an approaching gas flow for the case where the approach velocity exceeds the mean velocity in the intake. In the theory, which takes regions of separated flow into account, the gas flow is treated as a potential flow and Stokes law is assumed to govern the interaction between particles and gas.

Curves of calculated collection efficiency are given over a wide range of Stokes number for ratios of approach velocity to mean intake velocity up to 20. Where Stokes law is not applicable the collection efficiency obtained from the curves is an upper limiting value.     

Inertial Deposition of Aerosol Particles in a Periodic Row of Porous Cylinders

The aerosol flow through a periodic row of parallel porous cylinders is investigated. The air flow field outside the cylinders is described by the Navier–Stokes equations of viscous incompressible fluid. The extended Darcy–Brinkman equations are used to calculate the flow velocity inside a porous cylinder. The dependence of the efficiency of the deposition of aerosol particles by inertial impaction and interception on the Stokes number for various values of the Darcy number is studied. Comparison of the results obtained from the numerical model and an approximate analytical model is given. The combined approximate formula for the deposition efficiency of a cylindrical fiber in a parallel array proposed by Müller et al. (2014) is extended for the porous cylindrical fiber. The aerosol flow through the porous body composed by a random array of cylinders is calculated to estimate the interior deposition.

Copyright 2015 American Association for Aerosol Research     

Rear surface deposition of fine particles on spheres — eddy deposition or electrostatic effects?

The aim of this investigation is to show the demarcation of two possible mechanisms for surface deposition of fine particles on the rear surface of single spheres. By means of single particle trajectory computation, based on numerically determined flow fields (Re_max = 10³), it is shown that the mere existence of a wake is not in itself sufficient to produce eddy deposition. In addition, the particle's motion must undergo a lateral transfer promoted by fluid trubulence, in order to effect eddy deposition commencing at a Reynolds number of about 100. On the other hand, rear deposition, influenced by electrostatic forces, especially by the Coulomb force, is possible at any Reynolds number. Consequently, for Reynolds numbers of less than 100, only electrostatic effects can produce rear surface deposition. In the range of high Reynolds numbers, the coexistence of both mechanisms is possible. Very high Reynolds numbers (Re > 10³) and low Stokes numbers indicated the predominance of the electrostatic effect over eddy deposition, whereas at very high Reynolds numbers and medium to high Stokes numbers the electrostatic effect is only predominant in presence of high electrostatic charges.     

Synthesis of nano-crystalline Gd0.1Ce0.9O2−x for IT-SOFC by aerosol flame deposition

Nano-sized gadolinia-doped ceria (GDC) can be used as an IT-SOFC electrolyte, oxygen gas sensor or abrasives. In this study, nano-sized GDC powders with bimodal particle distribution of about 10 nm and 200 nm particle size were successfully synthesized by aerosol flame deposition (AFD). The resulting effects of sintering temperature on microstructure and electrical properties were investigated in the sintering temperature range 1100–1400 °C. The pellet had a completely dense microstructure after sintering at 1400 °C for 10 h. Raman measurement showed an increase of oxygen vacancy due to shift between reduced and oxidized states (Ce³⁺ ↔ Ce⁴⁺) with increasing sintering temperature. The formation of oxygen vacancies noticeably increased the ionic conductivity above 1300 °C.     

Analytical expression for the friction coefficient of DLCA aggregates based on extended Kirkwood–Riseman theory

We use a self-consistent field method, which we have previously validated, to calculate the translational friction coefficient of fractal aerosol particles formed by diffusion-limited cluster aggregation (DLCA). Our method involves solving the Bhatnagar–Gross–Krook model for the velocity around a sphere in the transition flow regime. The velocity and drag results are then used in an extension of Kirkwood–Riseman theory to obtain the drag on the aggregate. Our results span a range of primary sphere Knudsen numbers from 0.01 to 100 for clusters with up to N = 2000 primary spheres. Calculated friction coefficients are in good agreement with experimental data and approach the correct continuum and free molecule limits for small and large Knudsen numbers, respectively. Results show that particles exhibit more continuum-like behavior as the number of primary spheres increase, even when the primary particle is in the free molecule regime; as an illustrative example, the friction coefficient for aggregates with primary sphere Kn = 1 is approximately equal to the continuum friction coefficient for N > 500. We estimate that our calculations are within 10% of the true values of the friction coefficients for the range of Kn and N presented here. Finally, we use our results to develop an analytical expression (Equation (38)) for the friction coefficient over a wide range of aggregate and primary particle sizes.

Copyright © 2017 American Association for Aerosol Research     

The effect of the contact between platinum and soot particles on the catalytic oxidation of soot deposits on a diesel particle filter

Experiments were performed with two model soot aerosols brought into different forms of contact with Pt aerosol particles, to investigate the effectiveness of this contact in lowering the catalytic soot oxidation temperature. The contact was either generated between individual particles in the aerosol state (Pt-doped soot to simulate a fuel borne catalyst), or by sequential or simultaneous deposition of separately generated soot and Pt aerosols onto a sintered metal filter. (Formation of a soot cake on previously deposited Pt aerosol would simulate a catalyst coated diesel particle filter.) The catalytic activity was determined in all cases from temperature ramped oxidation in air of the filtered particles, and defined as the 50% conversion temperature.

It was found that Pt-doped soot and simultaneously filtered aerosols were both equally effective in reducing the oxidation temperature by up to 140–250 °C for the spark discharge soot (with 3–47 wt% Pt concentration in the soot cake), and by up to 140 °C for the pyrolysis soot (3 wt% Pt). Conversely, the deposition of a thin soot layer of 5–10 μm thickness onto Pt, or vice versa, produced only a slight temperature reduction on the order of about 13–42 °C. These results suggest that the distance between soot and Pt particles plays a key role in promoting an effective oxidation on the filter, which is consistent with the role of Pt particles as local generators of activated oxygen.     

SETTLING VELOCITIES OF TWO EQUAL-SIZED SPHERES FOLLOWING EACH OTHER IN VISCOUS FLUIDS CONTAINED IN CYLINDERS

Experimental data are presented for two equal-sized spheres falling along the axis of a cylinder. The two spheres settle with the same velocity as that of a single falling sphere as long as their separation distance is larger than a critical value. When the distance is smaller than the critical value, the two spheres fall faster than a single sphere. The drag on the two spheres is less affected when two spheres fall in a cylinder in comparison to the situation when two spheres fall in an unbounded medium. The data are correlated and shown to agree with numerical calculations.     

Aerosol deposition of dense lead zirconate titanate thin films at room temperature

Lead zirconate titanate (Pb(Zr,Ti)O₃, PZT) films were grown on silicon 1 0 0 substrate by aerosol deposition, using solid-state reacted powder containing donor oxide Nb₂O₅, while the substrate was maintained at room temperature. The PZT films were simultaneously sintered upon deposition on a highly densified ceramic layer. Crystalline phases of the deposited films have been determined by X-ray diffractometry (XRD), and microstructures analysed by transmission electron microscopy (TEM). The cross-section microstructure consisted of several thin layers, including the PZT film and the platinum electrode and titanium-buffered layers on the substrate. High-resolution images revealed that the PZT layer contained a mixture of randomly oriented grains of nanometre size, which were embedded in an amorphous matrix. In contrast to the conventional liquid-phase sintering mechanism, sintering of the PZT films involved amorphised phases generated by pressure-induced amorphisation (PIA) from plastic deformation when the initial powder particles collided amongst one another upon reaching the silicon substrate during aerosol deposition. An analogy may be drawn to the impact of extraterrestrial meteorites in which diaplectic glass, i.e., amorphised phase, was formed and retained metastably at room temperature. The individual PZT grains were joined with the amorphised phase(s) and sintered to become a dense, thin film on the silicon substrate.     

气溶胶在毛细管扩散池组中谱带展宽的实验测定

气溶胶粒子在毛细管中的输运过程是一个谱带展宽的过程,影响谱带展宽的因素大体上可分为柱内和柱外效应两大部分.在Reynolds数0.07～0.26范围内,采用亚微米聚苯乙烯球形硬气溶胶粒子脉冲,测定了气溶胶粒子在毛细管扩散池组中的谱带展宽,由此获得了气溶胶在毛细管中的平均保留时间和穿透率.研究了平均保留时间、穿透率与流体运动速度和气溶胶粒子大小之间的关系.实验发现：气溶胶粒子在毛细管扩散池组中的平均停留时间滞后于流体平均停留时间;气溶胶粒子在毛细管扩散池组中的穿透率随流体运动速度和气溶胶粒子粒径的减小而显著减小.研究结果在气溶胶采样、监测和泄漏评估方面具有应用价值.     

INERTIAL COUPLING IN DISPERSED POTENTIAL FLOW—USE OF THE METHOD OF IMAGES

Net first and second dipole reflections are computed for several finite and infinite arrays of cylinders or spheres held stationary in a uniform potential flow. The polarizability and resistivity, from which the macroscopic properties of a two-phase flow with corresponding microstructure can be derived, are deduced for the infinite arrays. Finite arrays resemble infinite ones apart from edge and end effects, which are small for some geometries. The added mass coefficient for a regular monolayer of spheres is obtained and shown to be very close to Smythe's (1964) solution for a sphere in a circular tube. Similar agreement is obtained between the solutions for a cubical array of spheres and for a row of spheres in a circular tube.     

Experimental measurements and computational modeling of aerosol deposition in the Carleton-Civic standardized human nasal cavity

Aerosol deposition in the novel, “Carleton-Civic” standardized geometry of the human nasal cavity was studied both numerically and experimentally. Inhalation flow rates varied from 30 to 90 L/min in the experiments, and aerosol droplets had diameters ranging from 1.71 to 9.14 μm (impaction parameters ranging from 123.3 to 2527.6 μm L/min). For the numerical simulations, both the RANS/EIM (Reynolds averaged Navier–Stokes equations for the gas phase and eddy-interaction random walk models for the particulate phase) and large eddy simulations were used. The mechanism of aerosol deposition in the standardized nasal cavity was dominated by inertial impaction. Deposition data from the standardized nasal cavity transected cited in vitro data based on individual subjects. The data also correlated very well with cited in vivo measurements but generally showed less aerosol deposition for a given value of the impaction parameter. Regional deposition characteristics within the nasal passages were also investigated both experimentally and numerically and new trends of regional deposition versus impaction parameter are discussed. These trends provide new insight into the general deposition behaviour of various sized aerosols within the human nasal cavity.     

Influence of purge gas on the characteristics of lead–zirconium–titanate thin films prepared by metalorganic chemical vapor deposition

In order to optimize the metalorganic chemical vapor deposition process for PbZr_xTi_1−xO₃ (PZT) thin films, the effect of purge gas species was investigated. Two steps of gas input process for stabilizing reaction chamber pressure, the gas flow prior to PbTiO₃ (PTO) seed layer deposition and PZT thin film deposition, were varied and their effect on structural and electrical properties were examined with regard to the memory device application. PZT film properties exhibited remarkable dependency on the gas species before PTO seed deposition, and insignificant dependency on the gas species before PZT film deposition. With the optimized pre-deposition gas flow, PZT thin film showed excellent properties such as high (1 1 1)-orientation (92.2%), high remnant polarization value of 71 μC/cm² at 3 V. Retention property also showed a heavy dependency on the pre-deposition gas flow that 91.1% of initial charge could be maintained after 100 h of baking at 150 °C.     

Low-Reynolds-number motion of a heavy sphere between two parallel plane walls

A novel boundary-integral algorithm [Staben, M.E., Zinchenko, A.Z., Davis, R.H., 2003. Motion of a particle between two parallel plane walls in low-Reynolds-number Poiseuille flow. Physics of Fluids 15, 1711-1733; Erratum: Phys. Fluids 16, 4206] is used to obtain O(1)-nonsingular terms that are combined with two-wall lubrication asymptotic terms to give resistance coefficients for near-contact or contact motion of a heavy sphere translating and rotating between two parallel plane walls in a Poiseuille flow. These resistance coefficients are used to describe the sphere's motion for two cases: a heavy sphere driven by a Poiseuille flow in a horizontal channel and a heavy sphere settling due to gravity through a quiescent fluid in an inclined channel. When the heavy sphere contacts a wall in either system, which occurs when the gap between the sphere and the wall becomes equal to the surface roughness of the sphere (or plane), a contact-force model using the two-wall resistance coefficients is employed. For a heavy sphere in a Poiseuille flow, experiments were performed using polystyrene particles with diameters 10%-60% of the channel depth, driven through a glass microchannel using a syringe pump. The measured translational velocities for these particles show good agreement with theoretical results. The predicted translational velocity increases for increasing particle diameter, as the spheres extend further into the Poiseuille flow, except for particles that are so large (diameters of 80%-85% of the channel depth) that the upper wall has a dominant influence on the particle velocity. For a heavy sphere settling in a quiescent fluid in an inclined channel, the transition from the no-slip regime to slipping motion occurs for a larger inclination angle of the channel with respect to the horizontal for an increase in particle diameter, since the larger particles are more slowed by the second wall. Limited experiments were performed for Teflon spheres with diameters 64%-95% of the channel depth settling in a very viscous fluid along the lower wall of an inclined acrylic channel. The measured translational velocities, which are only about 15%-25% of the tangential component of the undisturbed Stokes settling velocity, are in close agreement with theory using physical parameters obtained from similar experiments with a single wall [Galvin, K.P., Zhao, Y., Davis, R.H., 2001. Time-averaged hydrodynamic roughness of a noncolloidal sphere in low Reynolds number motion down an inclined plane. Physics of Fluids 13, 3108-3119].     

System design and test method for measuring respirator filter efficiency using mycobacterium aerosols

Recently, the protection of health care workers from tuberculosis-containing aerosols has been the subject of considerable debate. An experimental apparatus and test protocol were developed to measure the collection efficiency of surgical mask and respirator filter media using a microbial aerosol challenge. Mycobacterium chelonae (M. chelonae), used as a surrogate for Mycobacterium tuberculosis, was generated from liquid suspension using a Collison nebulizer. Upstream and downstream concentrations of viable aerosol particles were measured using Andersen cascade impactors, while total particle concentrations were measured with an aerodynamic particle sizer (APS). A monodisperse polystyrene latex (PSL) sphere aerosol (0.804 μm) was used in separate experiments to measure filter efficiency; concentrations were determined with the APS. The mycobacterial aerosol ranged in size from 0.65 to 2.2 μm when measured with the cascade impactor. A similar size range was found with the APS, yielding a count median diameter of about 0.8 μm. Samples of the mycobacterial aerosol were collected on glass slides, stained M. chelonae, as determined by environmental scanning electron microscope, were found to be rod shaped with an average length of 2 μm and average width of 0.3 μm. To evaluate the apparatus over a range of filter efficiencies (10–100%), different layers of fiberglass filter paper were tested for penetration using a 0.12 μm dioctyl phthalate (DOP) aerosol measured with a light scattering photometer, in addition to the mycobacterial and PSL aerosols. For the range of efficiencies tested it was shown that filter collection of DOP was linearly related to that of both mycobacterial and PSL sphere aerosols (r² = 0.99), demonstrating that an inert aerosol may be used to predict the collection of biological aerosols by such filter media.