Simultaneous effects of adhesion and poly-dispersity on microstructures and geometrical properties in particle deposition

The simultaneous effects of adhesion and polydispersity on packing (or deposit) microstructures and their bulk properties are examined. The results show that the microstructures and bulk properties of the deposits vary sharply with the introduction of even small adhesion and polydispersity. A structural phase diagram is obtained as functions of adhesion and polydispersity. Increases in adhesion lead to noticeable or large fluctuations in packing fractions for polydisperse systems. However, the packing fraction can be stabilized and the fluctuations greatly reduced regardless of the magnitude of the polydispersity index by keeping the adhesion relatively low (i.e., s≤0.1).     

Characterization of particle emissions from consumer fused deposition modeling 3D printers

Particle emissions from multiple fused deposition modeling consumer 3D printers were systematically quantified utilizing an established emission testing protocol (Blue Angel) to allow quantitative exposure assessments for printers operating in different environments. The data are consistent with particle generation from volatilization of the polymer filament as it is heated by the extruder. Typically, as printing begins, a burst of new particle formation leads to the smallest sizes and maximum number concentrations produced throughout the print job. For acrylonitrile butadiene styrene (ABS) filaments, instantaneous concentrations were up to 10⁶ #/cm³ with mean particle sizes of 20 to 40 nm when measured in a well mixed 1 m³ chamber with 1 air change per hour. Particles are continuously formed during printing and the size distribution evolves consistent with vapor condensation and particle coagulation. Particles emitted per mass of filament consumed (particle yield) varied widely due to factors including printer brand, and type and brand of filament. Higher extruder temperatures result in larger emissions. For filament materials tested, average particle number yields ranged from 7.3 × 10⁸ to 5.2 × 10¹⁰ g^?1 (approximately 0.65 to 24 ppm), with trace additives apparently driving the large variations. Nanoparticles (diameters less than 100 nm) dominate number distributions, whereas diameters in the range of 200 to 500 nm contribute most to estimated mass. Because 3D printers are often used in public spaces and personal residences, the general public and particularly susceptible populations, such as children, can be exposed to high concentrations of non-engineered nanoparticles of potential toxicity.

Copyright © 2017 American Association for Aerosol Research     

Calculation of the coefficient of particle capture by polydisperse drops

The effect of the polydispersity of sprayed liquid drops on the dust collection factor is investigated. The dust collection efficiency is compared at various dispersion compositions of sprays obtained using different nozzles.     

Critical particle concentration in electrophoretic deposition

The role of particle concentration in electrophoretic deposition (EPD) was investigated with two different suspension systems. The first system consisted of positively charged TiO₂ nanoparticles dispersed in isopropanol with 1 vol% water. The second system consisted of negatively charged polystyrene (PS) microbeads dispersed in isopropanol. Constant voltage EPD was performed using suspensions with variable particle concentration (0.013–0.43 vol% TiO₂ and 0.06–11.4 vol% PS). Threshold concentration values were identified for both systems after EPD at 100 V (250 V cm^?1) for 1 min. Below these values the deposited mass deviated from the trend dictated by Hamaker's equation. Higher applied voltages and longer deposition times were tested and the results suggested that the threshold concentration did not depend on those parameters. A phenomenological model of particle deposition was proposed, which accounts for the local electrochemical conditions close to the substrate in relation to particle size.     

Fluid‐particle drag in inertial polydisperse gas–solid suspensions

In this article, we extend the low Reynolds number fluid‐particle drag relation proposed by Yin and Sundaresan for polydisperse systems to include the effect of moderate fluid inertia. The proposed model captures the fluid‐particle drag results obtained from lattice‐Boltzmann simulations of bidisperse and ternary suspensions at particle mixture Reynolds numbers ranging from 0 ≤ Re_mix ≤ 40, over a particle volume fraction range of 0.2 ≤ ? ≤ 0.4, volume fraction ratios of 1 ≤ ?_i/?_j ≤ 3, and particle diameter ratios of 1 ≤ d_i/d_j ≤ 2.5. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Dynamics of particle deposition in model fiber filters

Aerosol filtration in model filters composed of an array of identical parallel fibers with equal interfiber distance was studied through simulation and experimentation. The simulation was made using a stochastic model based on principles proposed earlier [7, 8]. The major modification is that particles impacting on collectors or already deposited particles may bounce off and escape collection if the impact velocity is great enough. Good agreement between simulation and experiment was observed, lending credence to the eventual application of the stochastic model as a basis for the study of deposition dynamics. Based on the simulation results and confirmed by experimental data, an empirical correlation relating the increase in collection efficiency with the degree of particle deposition was established.     

The dependence of particle deposition velocity on particle inertia in turbulent pipe flow

Recent measurements of particle deposition velocities on the walls of a pipe in turbulent flow (Liu and Agarwal, 1974) show a decline with increasing particle size beyond a critical particle size. A stochastic model of particle deposition is presented which explains this result. As in other models, the deposition process is composed of turbulent diffusion, together with inertial projection through the boundary layer; in this model, both processes are particle inertia dependent, in opposing ways. The observed decline is due to the increased fractional penetration of the boundary layer with increasing particle size being insufficient to compensate for the reduced rate of transport to that region.

A simple expression is given for the particle deposition velocity in terms of the r.m.s. velocity at that point and the fractional penetration of the boundary layer. The inertial dependence of the particle velocity is expressed in terms of the particle's response to the turbulent velocity fluctuations of its neighbouring fluid by relating the velocity spectral densities of the particle and fluid using a linear dimensionless form of the equation of motion of the particle. The fractional penetration of the boundary layer is based on Stokes' drag with a quiescent fluid.

The deposition profile shows good agreement with the experiments of Liu and Agarwal.     

Fine particle deposition in laminar and turbulent flows

The deposition of fine silica and polystyrene spheres was measured for conditions of laminar and turbulent flow (960 ≤ Re ≤ 16040) in a rectangular channel using image analysis. The plate glass deposition surfaces were rendered positively charged by coating them with a cationic copolymer while, under the water chemistry conditions employed, both types of particles were negatively charged. It was found that, contrary to the results for laminar flow, the initial depositon rates in turbulent flow decreased with increasing Re, indicating that deposition was no longer mass-transfer controlled and that particle attachment played an increasingly important role as Re was raised. Attachment was modelled as a rate process in series with mass transfer in which the attachment rate varies inversely as the square of the friction velocity. Under the conditions of the present experiments, no particle re-entrainment was observed, so that the declining rate of particle accumulation on the wall recorded in each run could only be attributed to a declining deposition rate. Even where asymptotic accumulations were reached, particle coverages never exceeded 3.5%.     

Estimation of thermo- and diffusiophoretic particle deposition

Theories are developed for estimating thermo- and diffusiophoretic deposition of particles in liquid as well as gas phase heat and mass transfer equipment, operating under turbulent conditions. Simple expressions are obtained which give satisfactory agreement with experimental data. It is shown that the phoretic effects can lead to appreciable particle deposition, and the practical implications of this, such as heat exchanger fouling, are discussed. The simultaneous occurrence of these and other deposition mechanisms is also considered.     

The effect of re-entrainment on particle deposition

The effect of particle removal on observed deposition rates is considered by combining previous sublayer analyses. Wall shear stress is shown to be a controlling parameter; below a critical value where removal can occur, deposition varies linearly with time but above this value the observed deposition rate is dependent on the choice of time interval over which measurements are made. For monodisperse particles, a discontinuity in deposition plot is predicted although this may be smoothed out for particles with a wide size distribution. It is suggested that the measurement of the critical wall shear stress may be useful for estimating adhesion forces under dynamic conditions such as occur for redeposition problems.The functional dependence of deposition on wall shear stress for combined deposition and removal has been derived for the cases where diffusional, inertial and gravitational forces are dominant. Comparison with data from the literature confirms the general trends but precise matchings have not been attempted.     

Thermophoretic particle deposition efficiency in turbulent tube flow

This study investigated the thermophoretic particle deposition efficiency numerically. The critical trajectory was used to calculate thermophoretic particle deposition in turbulent tube flow. The numerical results obtained in turbulent flow regime in this study were validated by particle deposition efficiency measurements with monodisperse particles (particle diameter ranges from 0.038 to 0.498 μm) in a tube (1.18 m long, 0.43 cm i.d., stainless-steel tube). The theoretical predictions are found to fit the experimental data of Tsai et al. [Tsai, C. J., J. S. Lin, S. G. Aggarwal, and D. R. Chen, “Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows,” Aerosol Sci. Technol., 38, 131 (2004)] very well in turbulent flows. In addition, an empirical expression has been developed to predict the thermophoretic deposition efficiency in turbulent tube flow.     

An improved model for particle deposition in porous foams

Porous foam provides an inexpensive, light-weight and effective medium to capture physiologically-relevant aerosol fractions. It can be manufactured to have a wide range of properties relevant to aerosol deposition. A series of laboratory experiments were conducted to measure particle penetration though porous foam media of varying pore size and foam length. Both solid and liquid aerosols (0.01–10 μm diameter) were tested using a Sequenzial Mobility Particle Sizer or Aerodynamic Particle Sizer to count and size particles penetrating the foam. With this data, an existing semi-empirical model was improved upon to predict particle penetration through a foam of a given fiber diameter, and thickness. The model is based on three dimensionless parameters (St, Ng, Pe) that account for inertial, gravitational, and diffusive modes of deposition, respectively.     

圆管内水中碳酸钙颗粒趋壁沉积数值模拟

采用雷诺应力模型和拉格朗日方法,研究水平圆管内碳酸钙颗粒析出后的趋壁沉积行为。研究发现,二维建模下采用Fluent计算流场脉动速度偏大,又由于颗粒沉积是三维上的不规律运动,所以采用二维模型模拟颗粒沉积并不准确。文中对碳酸钙颗粒沉积进行三维模拟,比较脉动速度模拟结果,证明三维模拟结果优于二维模拟结果,且在近壁区内十分接近DNS结果。在此基础上,模拟不同粒径(颗粒弛豫时间0.4—10)碳酸钙颗粒的沉积速度,结果与文献中实验和模拟结果符合良好,发现水中涡扩散影响区和颗粒惯性影响区分界对应的颗粒弛豫时间(文中约为4)小于气体中的分界(10—100)。     

Electrochemical deposition of cylindrical Cu/Cu2O microstructures

Cylindrical microstructures of Cu/Cu₂O have been electrochemically produced from the self-oscillating Cu(II)-lactate system using etched ion track polycarbonate membranes as templates. After removal of the polymer, arrays of free-standing cylinders were obtained. The influence of the applied current density and the deposition temperature on the oscillation pattern and quality of the deposited wires were studied in detail for pores having a diameter of about 1000 nm. The widest current density range for oscillations was found at 25 °C. Under these conditions the deposited wires were of equal length and showed smooth contours, while a granular structure was observed at higher temperatures.     

Mathematical models of particle deposition in the human respiratory tract

Several mathematical formalisms have been proposed to calculate particle transport onto airway surfaces: the temporally and spatially discrete Findeisen formalism; the temporally discrete and spatially continuous Altshuler formalism; and the temporally and spatially continuous Taulbee-Yu formalism; they are termed primary deposition models. Models adopting these formalisms are termed secondary deposition models. This review concentrates on the discussion of the general concepts of primary models and their numerical verification and on characteristics of secondary models. Current deposition models predict particle deposition in close agreement with current experimental data.     

Asymmetric human lung morphology induce particle deposition variation

The effect of lung morphology on the heterogeneity of regional ventilation and particle deposition in the bronchial airways is studied using Horsfield's regular-asymmetric lung model. Flow distribution among the airways is calculated by solving the whole tree network, assuming laminar flow hydrodynamic resistances without accounting for gravitationally enhanced preferential airflow distribution. The variation of morphological properties, such as the lung volume and surface area distal to any airway generation, and physiological properties, such as ventilation and particle deposition, is calculated, and fractal dimensions that characterize these properties and processes are computed. The close agreement between the model fractal dimension characterizing ventilation and those found from clinical data assess the validity of the model. It is shown that the fractal dimensions that characterize the morphological properties and the physiological processes are similar, suggesting that all are related and stem from a common underlying attribute—the lung morphology. The variation of particle deposition in the lung, as well as the variation of ventilation and morphological attributes, increases moderately with the lung tree asymmetry. The deposition density, regarded as a key exposure metric or therapeutic index, does not follow a spatial scale-free distribution.     

Thermophoretic acceleration of particle deposition from laminar air streams

A preliminary study is reported of the use of temperature gradients to accelerate the deposition of small particles from laminar air streams. It is shown that appreciable effects can be obtained even with small temperature gradients and particles as large as 30 μm. An empirical correlation is proposed for the thermophoretic force based on the present results and those published previously for smaller particles.