An approach for evaluating aerosol particle deposition from a natural convection flow onto a vertical flat plate.

An approach through numerical integration for evaluating aerosol particle deposition onto a vertical flat plate is proposed. The airflow was based on the assumption of a two-dimensional, incompressible and steady state laminar flow driven by a buoyancy force. The mechanisms of particle deposition were coupled from natural convection, Brownian diffusion, thermophoresis and electrophoresis due to constant electric strength. This approach demonstrated an easier method of prediction and produced a very good agreement with the thermophoresis exact solution. Results described the role of thermophoretic and electrophoretic forces on particle deposition. The thermophoresis effect was predicted to be particularly important for particles of d(p)>/=0.1 microm moving toward a cold surface or away from a hot surface at a given temperature gradient. The electrophoresis effect dominates the deposition of submicron particles.     

Modelling and analyses do not support the hypothesis that charging by power-line corona increases lung deposition of airborne particles

The National Radiological Protection Board's advisory Group on Non-ionising Radiation has recommended further study on the effects of electric charge on the deposition of 0.005-1 microm particles in the lung. Estimates have been made regarding the integrated ion exposure within the corona plume generated by a power line and by ionisers in an intensive care unit. Changes in the charge state of particles with sizes in the range 0.02-13 mum have been calculated for these exposures. The corona plume increases the charge per particle of 0.02 and 0.1 microm particles by the order of 0.1. The ionisers in the intensive care unit produced negative ions-as do power lines under most conditions. Bacteria can carry in the order of 1000 charges (of either sign) and it is shown that the repulsion between such a negatively charged bacterium and negative ions prevents further ion deposition by diffusion charging. Positively charged bacteria can, however, be discharged by the ions which are attracted to them. The data provide no support for the hypothesis that ion exposure, at the levels considered, can increase deposition in the lung.     

Effect of electrostatic charge of particles on hydrodynamics of gas-solid fluidized beds

The aim of this work was to investigate effect of electrostatic charge of particles on the fluidization hydrodynamics. Behavior of bubbles in beds of polyethylene particles was studied through analysis of pressure fluctuations in the frequency domain. Fluidized beds of uncharged, pre-charged and bed-charged particles were used in the experiments. Results revealed that in the bed of pre-charged particles, compared to uncharged experiments, particle-particle repulsive force increases the bed voidage and reduces equilibrium bubble size while the transition velocity to turbulent fluidization is decreased. In the case of bed-charged particles, at low gas velocities bubble fraction is greater compare to the other cases due to faster bubble coalescence in the presence of particle-wall attractive electrostatic force. Electrostatic charge of bulk increases by increasing the gas velocity. At high gas velocities, the repulsion force between highly charged particles overcomes the particle-wall effect on bubble formation and reduces the bubble size to less than in uncharged experiments. Accumulation of particles near the wall in the bed od bed-charged particles affects the hydrodynamics in two ways: first it accelerates bubble growth via bubble coalescence at low gas velocities, second it limits the bubble growth and reduces the transition velocity to turbulent regime to a value less than for pre-charged particles.     

Thermophoretic measurements in presence of thermal stress convection in aerosols in microgravity conditions of drop tower

The aim of the work is getting reference data on thermophoretic motion eliminating gravity-induced perturbation, developing new instrumentation and procedures. A series of experiments on measuring phoretic velocities was performed in the Bremen drop tower providing 4.7 s of high quality microgravity conditions, which allowed making negligible particle sedimentation and buoyancy driven convection. Motion of aerosol particles was observed simultaneously at low resolution to control nongravity convective motion in the cell and at high resolution by the digital holographic velocimeter in order to register particle three-dimensional trajectories. By choosing appropriate cell size and experimental procedures the heat and mass transfer relaxation processes were reduced to less than 0.3 s thus allowing measurements of particle velocities during more than 4 s. Side-wall temperature creep created convective motion in the cell. Its influence was suppressed by choosing sufficiently flat cell geometry. The values of the measured thermophoretic velocities for Knudsen number in the range 0.047–0.89 were found to be between predictions of the classical models of Talbot et al [1] on one hand and Yamamoto and Ishiara [2] on the other hand. Particles of different thermal conductivities (paraffin and NaCl) had about the same velocities. No negative thermophoresis was observed at these conditions for NaCl.     

Nanoparticle traffic on helical tracks: thermophoretic mass transport through carbon nanotubes

Using molecular dynamics simulations, we demonstrate and quantify thermophoretic motion of solid gold nanoparticles inside carbon nanotubes subject to wall temperature gradients ranging from 0.4 to 25 K/nm. For temperature gradients below 1 K/nm, we find that the particles move "on tracks" in a predictable fashion as they follow unique helical orbits depending on the geometry of the carbon nanotubes. These findings markedly advance our knowledge of mass transport mechanisms relevant to nanoscale applications.     

Uncertainty in particulate deposition for 1 microm AMAD particles in an adult lung model

The ICRP 66 lung model may be used to determine dose estimates for members of the public via the inhalation pathway. A significant source of uncertainty in internal dosimetric modelling is due to particulate deposition in regions of the respiratory tract. Uncertainties in estimates of particulate deposition are present because model input parameters have their own inherent variability. These sources of uncertainty need to be examined in an effort to understand better model processes and to estimate better the doses received by individuals exposed through the inhalation pathway. An improved understanding of the uncertainty in particulate deposition will further guide research efforts and improve our ability to quantify internal dose estimates. The ICRP 66 lung deposition model is most sensitive to breathing rate when 1 microm AMAD particles are inhaled by members of the public. Uncertainties in deposition fractions are shown to span an order of magnitude with their distributions varying by gender for a particular lung region. The largest fractional deposition occurs in the deep lung alveolar and extrathoracic regions.     

Numerical investigation of powder dispersion mechanisms in Turbuhaler and the contact electrification effect

Powder dispersion in dry powder inhalers (DPI) is affected by factors such as device design and flow rate, but also electrification due to particle–particle/device collisions. This work presented a CFD-DEM study of powder dispersion in Turbuhaler®, aiming to understand the effect of electrostatic charge on the dispersion mechanisms. The device geometry was reconstructed from CT-scan images of commercial Turbuhaler device. Different work functions were applied to the active pharmaceutical ingredient (API) powder and the device wall. Electrostatic charges were accumulated on the API particles due to contact potential difference (CPD) between the particles and the device wall. Results showed that both the chamber and the spiral mouthpiece played an important role in de-agglomeration of powders caused by particle–wall impactions. With increasing flow rates, the performance of the device was improved with higher emitted dose (ED) and fine particle fractions (FPF). The electrostatic charging of the particles was enhanced with higher CPD and higher flow rates, but the electrostatic charging had a minimum effect on powder dispersion and deposition with slight reduction in ED and FPF. In conclusion, the van der Waals force is still the dominant adhesive inter-particle force, and the dispersion efficiency is affected by the flow rate rather than contact electrification of particles. Future work should focus on the effect of highly charged particles emitted from the inhaler on the deposition in the airway.     

Thermophoretic deposition of aerosol particles in laminar mixed-convection flow in a channel with two heated built-in square cylinders

The thermophoretic deposition of aerosol particles in laminar mixed-convection flow in a channel with two heated built-in square cylinders was studied numerically. The objective of this research was to study the effect of free convection and the distance between cylinders, on deposition of particles. Continuity, momentum and energy equations were solved to determine the velocity and temperature profiles in the channel. The particle trajectories were evaluated by solving the Lagrangian equation of motion that included the drag, Brownian diffusion and thermophoresis forces. It was found that the temperature gradient near the channel wall, in mixed flow regime, is higher than the temperature gradient in forced convection regime. Increasing the temperature gradient increased the effect of thermophoresis on deposition of particles. It was observed that the deposition was increased with the Richardson number. The distance between cylinders is a parameter that influences the deposition of particles. Temperature gradient decreases with increasing the cylinders’ distance; on the other hand, the length of the high temperature gradient zone, which is located in the region between the cylinders where the most deposition occurs, will be increased. These two opposite phenomena cause the fact that at a distance which is four times longer than the cylinders’ length, a maximum cumulative deposition fraction occurs. It was eventually concluded that the thermophoresis and the inertial impaction are dominant deposition mechanisms of particles on the channel wall.     

DEPOSITION OF SUBMICRON AEROSOL PARTICLES DURING INTEGRATED CIRCUIT MANUFACTURING: THEORY

ABSTRACT

Submicron (≤1μm) particle contamination can produce unacceptably low yields in the manufacture of integrated circuits. Calculations were made to predict deposition velocities of 0·01-lOμm particles, incorporating gravitational, dlffusional, and electrostatic effects. The results were summarized in equations that correlate non-dimensional deposition (Sherwood number) with convective-diffusion (Peclet number) and with electrostatics (Boltzmann and Fuchs charge distributions). These equations were used In conjunction with particle size distributions to predict particle deposition. In a companion paper |25| the predictions were shown to compare well with limited experimental data. To reduce deposition product surfaces should not be electrically charged and, where possible, these surfaces should be at higher temperatures than the ambient gas. For quality control purposes, the deposition flux predictions could serve to link the specifications of gas cleanliness with the specifications of surface cleanliness.     

DEPOSITION OF SUBMICRON AEROSOL PARTICLES DURING INTEGRATED CIRCUIT MANUFACTURING: THEORY

Submicron (≤1μm) particle contamination can produce unacceptably low yields in the manufacture of integrated circuits. Calculations were made to predict deposition velocities of 0·01-lOμm particles, incorporating gravitational, dlffusional, and electrostatic effects. The results were summarized in equations that correlate non-dimensional deposition (Sherwood number) with convective-diffusion (Peclet number) and with electrostatics (Boltzmann and Fuchs charge distributions). These equations were used In conjunction with particle size distributions to predict particle deposition. In a companion paper |25| the predictions were shown to compare well with limited experimental data. To reduce deposition product surfaces should not be electrically charged and, where possible, these surfaces should be at higher temperatures than the ambient gas. For quality control purposes, the deposition flux predictions could serve to link the specifications of gas cleanliness with the specifications of surface cleanliness.     

A theoretical approach to the deposition and clearance of fibers with variable size in the human respiratory tract

In the study presented here, a mathematical approach for the deposition and clearance of rigid and chemically stable fibers in the human respiratory tract (HRT) is described in detail. For the simulation of fiber transport and deposition in lung airways an advanced concept of the aerodynamic diameter is applied to a stochastic lung model with individual particle trajectories computed according to a random walk algorithm. Interception of fibrous material at airway bifurcations is considered by implementation of correction factors obtained from previously published numerical approaches to fiber deposition in short bronchial sequences. Fiber clearance is simulated on the basis of a multicompartment model, within which separate clearance scenarios are assumed for the alveolar, bronchiolar, and bronchial lung region and evacuation of fibrous material commonly takes place via the airway and extrathoracic path to the gastrointestinal tract (GIT) or via the transepithelial path to the lymph nodes and blood vessels.Deposition of fibrous particles in the HRT is controlled by the fiber aspect ratio β in as much as particles with diameters <0.1 μm deposit less effectively with increasing β, while larger particles exhibit a positive correlation between their deposition efficiencies and β. A change from sitting to light-work breathing conditions causes only insignificant modifications of total fiber deposition in the HRT, whereas alveolar and, above all, tubular deposition of fibrous particles with a diameter ≥0.1 μm are affected remarkably. For these particles enhancement of the inhalative flow rate results in an increase of the extrathoracic and bronchial deposition fractions. Concerning the clearance of fibers from the HRT, 24-h retention is noticeably influenced by β and, not less important, by the preferential deposition sites of the simulated particles. The significance of β with respect to particle size may be regarded as similar to that determined for the deposition scenarios, while breathing conditions do not have a valuable effect on clearance.     

An experimental investigation of the behavior of solid particles during their motion in smooth and dimpled pipes

Results are given of LDA measurements of averaged and fluctuation velocities of glass particles during their deposition in smooth and dimpled narrow pipes. Experiments reveal a decrease in the axial component of averaged velocity of particles and a significant increase in fluctuation velocities of particles during their motion in a pipe with dimples.     

Tracing Thermal Creep and Thermophoresis in Porous Structures at Low Ambient Pressure and Low Gravity

We carried out two types of drop tower experiments to quantify gas and particle motion induced by temperature gradients inside a porous structure in a low pressure environment. In one setup 400 μm sized particles were traced inside heated channels at pressures of a few Pascal. Their motion is consistent with pure thermophoresis. In the second setup tracer particles were used to track the thermal creep gas flow through a porous dust bed. Here, the flow was traced outside of the dust bed and without thermophoretic motion. The results are consistent with a simple capillary model of the dust bed.     

Vortex Nucleation and the Levitation of Charged Helium II Drops

Intrinsic nucleation of quantized vortices in Helium II can be studied by means of rotating freely suspended superfluid drops at angular velocities above some critical value. The motivation for doing so is described, as well as recent progress in the electrostatic levitation of Helium II drops charged with positive ions. To date, stable levitation has been achieved for drops of order 100–150 micrometers in diameter, with a surface charge density about a factor of ten smaller than Rayleigh limit, and a diameter a similar factor less than the maximum allowed in normal gravity. We discuss the possibility of rotating these drops via the surface charge density and discuss the advantages of a microgravity environment, including the attainment of significantly larger suspended drops. Recent efforts to find optical seed particles for angular velocity measurements are discussed.     

Electrostatic control of particle deposition

A two-dimensional simulation has been conducted in an electrostatic powder-coating booth for controlling particle deposition. In the absent of electrostatic forces, most of particles flow along steamlines and cannot deposit on the target. When electrostatic forces (coulombic and image forces) are added in the motion of the particle, the trajectories change significantly near the target. It is found that image force hardly affects the deposited position of the particle. From the calculated force distribution near the aimed region on the target, coulombic force is found to be larger than drag force when the particle is very close to the target. Particles with small Stokes number having small specific charge deposit exactly on the aimed regions. Furthermore the width of the aimed region also affects the accuracy of the particle deposition.     

Assessment of the upper particle size limit for quantitative analysis of aerosols using laser-induced breakdown spectroscopy

The laser-induced plasma vaporization of individual silica microspheres in an aerosolized air stream was investigated. The upper size limit for complete particle vaporization corresponds to a silica particle diameter of 2.1 microm for a laser pulse energy of 320 mJ, as determined by the deviation from a linear mass response of the silicon atomic emission signal. Comparison of the measured silica particle sampling rates and those predicted based on Poisson sampling statistics and the overall laser-induced plasma volume suggests that the primary mechanism of particle vaporization is related to direct plasma-particle interactions as opposed to a laser beam-particle interaction. Finally, temporal and spatial plasma evolution is discussed in concert with factors that may influence the vaporization dynamics of individual aerosol particles, such as thermophoretic forces and vapor expulsion.     

制备条件对电致变色氧化镍薄膜特性的影响

在充氧气的真空室内 ,用电子束蒸发NiO粉末颗粒的方法分别以 0 1和 0 8nm/s的淀积速率制备了氧化镍薄膜 ,并在不同的环境中对薄膜进行热处理。研究了薄膜结构和电致变色特性与淀积速率的关系 ,发现以较慢和较快速率淀积的薄膜分别具有NiO晶粒的 (2 0 0 )和 (111)不同择优取向 ,前者致色范围较小 ,后者致色范围较大。还研究了热处理对薄膜的结构、动态致色范围、致色效率 ,以及红外光谱特性的影响 ,发现热处理对薄膜的致色效率影响较小 ,然而对动态致色范围的影响很大。     

Predicting the Particle Deposition Characteristics Using a Modified Eulerian Method on a Tilted Surface in the Turbulent Flow

This study uses a v2-f turbulence model with a two phase Eulerian approach. The v2-f model can accurately calculate the near wall fluctuationsm which mainly represent the nonisotropic nature of turbulent flow near the walls. The Eulerian method was modified based on considering the most important mechanisms in the particle deposition rate when compared to the experimental data. The model performance is examined by comparing the rate of particle deposition on a vertical surface with the experimental and numerical data in a turbulent channel flow available in the literature. The model takes into account the effects of lift, turbophoretic, electrostatic, gravitational, and Brownian forces together with turbulent diffusion on the particle deposition rate. Electrostatic forces due to mirror charging and due to charged particles under the influence of an electric field were considered. The influence of the tilt angle on the particle deposition rate was investigated. The results show that, using the modified model with v2-f model predicts the rate of deposition with reasonable accuracy. It is shown that considering the turbophoretic force as the only inertia force and neglecting the lift force, leads to reasonable accuracy in predicting particle deposition rate. It is also observed that when the mirror charging and electric field are present, the electrostatic force has the dominant effect in a wider range of particles’ size. Furthermore, the results show that increasing the Reynolds number at a given tilt angle decreases the rate of particle deposition and the tilt angle has insignificant impact on the particle deposition rate in high shear velocity or high Reynolds number.