Boundary effects on thermophoresis of aerosol cylinders

An analytical study is presented for the thermophoretic motion of a circular cylindrical particle in a gaseous medium with a transversely imposed temperature gradient near a large plane wall parallel to its axis in the quasisteady limit of negligible Peclet and Reynolds numbers. The Knudsen number is assumed to be small so that the fluid flow is described by a continuum model with a temperature jump, a thermal slip, and a frictional slip at the particle surface. The presence of the confining wall causes two basic effects on the particle velocity: first, the local temperature gradient on the particle surface is altered by the wall, thereby speeding up or slowing down the particle; secondly, the wall enhance the viscous retardation of the moving particle. Through the use of cylindrical bipolar coordinates, the transport equations governing this problem are solved and the wall effects on the thermophoresis of the aerosol cylinder are computed for various cases. The presence of the plane wall can reduce or enhance the particle velocity, depending upon the relative thermal conductivity and surface properties of the particle, the relative particle-wall separation distance, and the direction of the applied temperature gradient. The direction of the thermophoretic motion of a cylindrical particle near a plane wall is different from that of the prescribed thermal gradient, except when it is oriented parallel or perpendicular to the wall. The effects of the plane wall on the thermophoresis of a cylinder are found to be much more significant than those for a sphere at the same separation.     

Bioactive glass coating using aerosol deposition

This work demonstrates the successful deposition of bioactive glass (BG) 45S5 coatings on various metallic and ceramic substrates at room temperature under low vacuum condition by using aerosol deposition (AD). This room temperature and particle impact consolidation-based deposition method enabled us to deposit well-adhered and dense BG coatings directly on metallic and ceramic substrates. In vitro tests with human osteoblast-like cells on substrates with a 45S5 BG coating demonstrated high cell activity on the surfaces. All tested materials exhibited high in vitro biocompatibility as no inhibition in cell proliferation could be observed. The utilization of AD process for achieving non-crystalline BG coatings is promising for practical bio-medical applications, e.g., bioactive coatings on bioinert metallic and ceramic substrates.     

Eulerian modelling of lung deposition with sectional representation of aerosol dynamics

A dynamical model of respiratory deposition is developed based on an Eulerian approach. The model simulates detailed lung deposition along all generations of the respiratory tract by solving numerically the aerosol general dynamics equation (GDE). All deposition mechanisms are described mechanistically, without using any empirical correlations. The GDE is solved in a one-dimensional form using a sectional method to describe the aerosol size distribution. To describe lung geometry the classical Weibel's morphometric model is used, employing a time-varying alveolar geometry to accommodate inhalation dynamics. A computationally efficient methodology is implemented based on a time-step splitting and subcycling approach, combined with a moving grid method for the growth process. The model is validated by comparing extensively with experimental and numerical results. The simulation results show that aerosol dynamics, in particular condensational growth, significantly influence respiratory deposition of fine hygroscopic particles. Instead, the effect of coagulation was found to be negligible. Particle deposition in terms of number, surface, or mass is addressed, which is of interest to current inhalation toxicology studies.     

Modeling and measurement of aerosol deposition on a heated substrate

Formation of an inorganic film by chemical aerosol deposition has been investigated experimentally and theoretically. Carrier gas flow rate, nozzle-to-substrate distance and substrate temperature were chosen as major process variables. The experimental work has been carried out to find their effect on the deposition efficiency, film thickness and its distribution. Both the deposition efficiency and film thickness increased with the carrier gas flow rate and substrate temperature but decreased with the nozzle-to-substrate distance. Especially at higher deposition rates, the central part of the film has a concave surface like a bowl. Flow and temperature fields of the fluid phase in the region between the nozzle and substrate were calculated numerically. Particle trajectories and particle evaporation were simulated numerically. As a result, the evaporation of the aerosol particles occurred so abruptly that the aerosol-existing region has a clear boundary. The extent of the region was found to be a determining factor in the film deposition, which characterizes the process of the chemical aerosol deposition.     

Mechanisms of aerosol deposition in a nasal model

Total and regional aerosol deposition were investigated in a model of a normal human nasal airway. Contributions of fluid turbulence and particle inertia were evaluated using monodisperse aerosols. At fixed turbulent flow conditions, deposition percentage increased with particle size greater than 1 μm, suggesting that turbulent inertial deposition is a primary mechanism.

With same size aerosol, deposition increased with increasing fluid turbulence but its contribution was less with larger size aerosol. Turbulent diffusion was the dominant transport mechanism for particles less than 1 μm, where deposition decreased with particle size. Two major deposition sites were visualized with radio-aerosol in the anterior region of the nasal airway. One is close to the ostium internum where turbulent eddies are well developed, and the other is the anterior region of the middle turbinate where direction of airflow changes from upward to horizontal.     

Exact analysis of aerosol deposition during steady breathing

Using a trumpet lung model, a transport equation is derived for an aerosol breathed into and out of the human airways. The partial differential equation is solved exactly using the method of characteristics. From this solution, the deposition of particles along the airways is obtained for steady breathing with and without pause. The total and regional depositions for particle sizes ranging from 0.01 μm to 10 μm aerodynamic diameter are calculated for Weibel's lung model, and they compare satisfactorily with the existing experimental data. The effects of the rest lung volume and breathing pattern on the total and regional depositions are also determined in order to explain the observed intersubject variability. Comparison between the present results with that recommended by the Task Group on Lung Dynamics shows that the present pulmonary deposition is considerably lower. while the tracheobronchial deposition differs only slightly.     

Thermophoretic deposition of aerosol particles in laminar tube flow with mixed convection

A model is developed to investigate the thermophoretic deposition of aerosol particles on the wall of a relatively cool cylindrical tube. A key aspect of the flow is the presence of mixed convection. Continuity, momentum, and energy equations are solved to determine the velocity and temperature profiles of the system. Additionally, an aerosol population balance that incorporates thermophoresis and Brownian diffusion is solved. The results indicate that more particles deposit at shorter axial distances in downward flow through a vertical pipe than in upward flow. This behavior is traced to the larger residence time associated with downward flow, despite the presence of a reduced radial temperature gradient relative to the upward configuration. Model predictions also indicate that a bulk Richardson number (Ri) of at least unity is necessary, but not sufficient for free convection effects to be important. The additional evaluation of a local Ri provides a more reliable indicator.     

Plasma resistance of quartz with a glass coating layer by aerosol deposition

ABSTRACT

To improve the plasma resistance behaviour, glass frits of SiO₂–Al₂O₃–Y₂O₃ with various powder sizes were coated onto quartz substrates by the aerosol deposition (AD) method. The thickness and microstructure of the coating layers were observed using a surface profiler and scanning electron microscopy. Plasma resistance was measured via the quartz substrate, after exposure to an inductively coupled plasma etcher. The coating layers were densely formed on the quartz substrates without additional heat treatment, and the layer thickness changed for the glass frit size distribution and AD process conditions. The SiO₂–Al₂O₃–Y₂O₃ glass coating layer showed a higher plasma resistance than quartz. Furthermore, the AD coating layer was evenly etched after plasma exposure. This study improves the lifetime of plasma chamber components in the semiconductor industry.     

Electrical image deposition of charges from laminar flow in cylinders

The electrical force on a point charge in a conducting cylindrical channel is calculated exactly and compared with approximations used in the literature. The exact form is shown to lead to larger deposition rates for charges in a laminar flow in a viscous medium. Several numerical applications are presented.     

Silver metallization for microwave device using aerosol deposition

We examined the possibility of using aerosol deposition (AD) as a simple, environmentally friendly and dry metallization process capable of acting as an alternative to the electroless and electroplating methods. Silver thick films were fabricated, their characteristics evaluated, and the factors influencing their growth investigated. As a result, silver thick films were successfully fabricated by AD with high deposition rates (10 μm/min) at room temperature. The resistivity of the as-deposited silver thick films was 8–10 times larger than that of the bulk silver. Post-annealing increased the resistivity of the silver films by approximately 2–3 times compared to that of the bulk silver. Microstructural observations revealed an increase in the connectivity between the silver particles after the heat treatments, which reduced the resistivity of the as-deposited silver films.     

A new theory of aerosol deposition from turbulent fluids

The author's previously published theory[2] of mass (or heat) transfer to very rough surfaces predicts a power dependence on Sc of ?$\text{1}\text{2}$ (compared with ?$\text{2}\text{3}$ for smooth surfaces). This expected change of $\text{1}\text{6}$ in the power has previously proved difficult to test precisely, but it is shown here that available data on the deposition of small aerosol particles (Sc ～ 10⁶) confirm that the deposition is indeed 10 times faster on very rough surfaces.For larger aerosol particles, however, deposition rates depend strongly on roughness characteristics, as expected from the author's interception and impaction theory[13]. This assumes an x-directional deposition on to the roughness elements, the latter being very important to the process. Comparison of published data with the new theory, and with typical surface roughnesses, supports the proposed theory (eqn 4).     

Fabrication of porous thick films using room-temperature aerosol deposition

A novel technique for the rapid room-temperature deposition of porous ceramic, glass, or metal thick films using the aerosol deposition (AD) method is presented. The process is based on the co-deposition of the desired film material and a second water-soluble constituent, resulting in a ceramic-ceramic composite. Following the subsequent removal of water-soluble end member, a network of pores is retained. To demonstrate the process, porous BaTiO₃ thick films were fabricated through co-deposition with NaCl. Microstructural images show the clear development of a porous structure, which was found to enhance the dielectric properties over dense thick films, possibly related to the lower extent of internal residual stress. This simple but highly effective porous structure fabrication can be applied to any film and substrate material stable in water and is promising for the application of AD-processed films in gas sensors, solid oxide fuel cells, and humidity sensors.     

Computational study on the deposition of ultrafine particles from Diesel exhaust aerosol

Short-term in vitro assays are arising in order to determine the toxicity of native Diesel aerosol particles, due to its association with adverse health effects. Estimation of the real quantity and characteristics of the particles deposition on the cells cultures in these assays becomes necessary to establish correlations between the particle deposition conditions and the biological indicators of toxicity of the exposed cells.

On that basis, it is the aim of this paper to analyze the deposition of sub-micron particles in an air/liquid interface cells exposure system by means of a CFD (computational fluid dynamics) code. Some comparisons with experimental and modelled results are shown to probe the validity of the method and a detailed analysis on the deposition of particles and efficiency of exposure-cells systems, with perpendicular contact, is carried out.

Furthermore, a parametric study on the effect of particle diameter, particle density and flow inlet velocity revealed the importance of such parameters in the efficiency and pattern of particle deposition.     

A note on the diffusional deposition of aerosol particles in packed beds

Aerosol penetration through packed beds of spheres under diffusional conditions is given by the empirical equation

$\text{C}{}_{\mathrm{L}}\text{C}{}_{\mathrm{L}}\text{=}\text{exp}\text{?6·39}\text{D}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{ν}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{R}{}^{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}\text{L}$

recently proposed by Gebhart et al. (1973). The present note shows that this equation may be derived independently from Wilson and Geankoplis' (1966) correlation for mass transfer coefficients. It therefore follows that standard expressions for such coefficients may be used to predict aerosol collection under diffusional conditions.