Motion Of Aerosol Particles Research Articles

Reducing the diffusional spreading rate of a Brownian particle by an appropriate non-uniform external force field

The variance of the probabilistic distribution of positions for a Brownian particle acted upon by an external force depends on whether the force field is uniform or not. If the field is uniform, the variance is independent of the field strength and, in fact, it is equal to that found for a pure Brownian process (free particle). If the force field acting on the particle is not uniform, the variance can be larger or smaller than that for a free particle depending on whether the particle, on the average, moves, respectively, toward increasing or decreasing absolute values of the field. This important result may find an immediate application in the design of high-resolution differential mobility analyzers for nanoparticles: the diffusional broadening of the transfer function can be dramatically reduced by simply reversing the direction of the aerosol particle motion between the electrodes.

Boundary effects on creeping motion of an aerosol particle in a non-concentric pore

The boundary effect on translational and rotational mobility coefficients of creeping motion of a spherical aerosol particle in a non-concentric spherical pore is investigated. Both particle and pore surfaces can be frictionally slippery and the particle can be situated at an arbitrary location within the pore. The creeping motion of the aerosol particle is decomposed into two independent components, one parallel with the line of centers of the particle and the pore and the other perpendicular to the line of centers. The parallel and perpendicular components of translational ( M = and M +) and rotational mobility ( N = and N +) coefficients are thoroughly studied as functions of normalized particle size ( a/ b), normalized deviation of the particle center from the pore center ( d/ b), and normalized frictional slip coefficients of both particle (C m ∗) and pore ( C ̂ m ∗) surfaces. Generally, the presence of the pore surfaces tends to retard the particle motion and results in mobility coefficients smaller than unity. There however exist situations in which the mobility coefficients can go higher than unity, signifying a positive boundary surface effect. This positive boundary effect apparently arises from the slippage between surrounding gas and the boundary surfaces, and is found to occur for M +, N = and N + at a high enough frictional slip coefficient at a non-zero d/ b. The presence of pore surface also induces torque and force for particles translating and rotating, respectively, in the direction perpendicular to the line of centers at a non-zero d/ b. The induced mobility function is found to increase moderately with increasing slip coefficient, but decrease sharply with increasing d/ b and a/ b.

LIGHT-INDUCED MOTION OF AEROSOL PARTICLES

LIGHT-INDUCED MOTION OF AEROSOL PARTICLES

Thermophoretic motion of an aerosol particle in a non-concentric pore

The thermophoretic motion of a spherical aerosol particle arbitrarily situated in a spherical pore is studied. The effect exerted by the boundary on the translational and rotational thermophoretic velocities of the particle is investigated by solving the relevant boundary value problem with a boundary collocation method. Variations in particle size, deviation of particle center from pore center, frictional slip coefficients of the particle and pore surfaces, and thermal conductivities of the particle and pore surfaces are studied. Both particle and pore surfaces can have frictional and thermal slip, and discontinuity in temperature field is allowed on both surfaces. Due to system symmetries, there exist only two independent scenarios for the present problem: one with the imposed ambient temperature gradient parallel to the line of centers between particle and pore (parallel case) and the other perpendicular to the line of centers (perpendicular case). It is found that only translational motion can be induced for the parallel case, while both translational and rotational motions occur for the perpendicular case. The thermo-osmotic flow induced by the thermal slip of the pore surface enhances the thermophoretic motion. The physical properties (thermal conductivity, slip coefficient, and temperature jump coefficient) of the particle are found to be more influential than those of the pore. The enhancing effect of particle or pore thermal conductivity reaches its saturation at the thermal conductivity value of about 100. Furthermore, the parallel normalized translational thermophoretic velocity is always greater than the perpendicular one, and the normalized rotational thermophoretic velocity correlates positively with the difference of the two translational velocities.

Continuum regime motion of a growing droplet in opposing thermo-diffusiophoretic and gravitational fields of a thermal diffusion cloud chamber

A model for the motion of aerosol particles by Stefan flow, thermo-diffusiophoresis and gravity in a continuum regime is described, which considers a phase change on the particle surface. It is tested in a thermal diffusion cloud chamber where a droplet formed by nucleation quickly grows and simultaneously moves upwards due to vertical temperature and concentration gradients. Kinetic coefficients are assumed to be constant. Model predictions of the height where the droplet reverses its motion are in satisfactory agreement with the experimental results of Ždı́mal et al. ((1996). Colloids Surfaces A, 106, 119). The droplet motion seems to be predicted well at higher gradients and vapor fluxes, but model underestimates droplet motion at lower ones. For those cases also the free-molecule and transition regimes need to be included.

Effects of inertia on the slow motion of aerosol particles

The translational motion of a spherical particle and a circular cylindrical particle (in the direction normal to its axis) in a quiescent unbounded fluid at small but finite Reynolds number is examined theoretically. The fluid, which may be a slightly rarefied gas, is allowed to slip at the surfaces of the particles. The axisymmetric Navier–Stokes equation for the fluid flow around the sphere and the two-dimensional equation of motion for the flow surrounding the cylinder are solved by using a method of matched asymptotic expansions. The approximate expressions for the drag force exerted by the fluid on the sphere and the cylinder are obtained analytically. For both cases of a sphere and a circular cylinder, the normalized drag force is found to increase monotonically with the Reynolds number and to decrease monotonically with the dimensionless slip coefficient (or the Knudsen number). The resulting formulas presented here include the previous results for a no-slip rigid sphere, a perfect-slip fluid sphere, and a no-slip circular cylinder as special cases.

Thermophoresis of Large Volatile One‐Component Drops

The work is devoted to the theory of motion of aerosol particles in temperature‐inhomogeneous gases. An investigation is made of the influence of the Dufour effect on the thermophoresis rate of large volatile one‐component drops. Estimates of the resultant formula show that this effect exerts an insignificant influence on the velocity of motion of the drops under actual conditions.

Photogrammetric set-up for the analysis of particle motion in aerosol under microgravity conditions

An optical set-up to investigate the motion of aerosol particles under microgravity conditions has been designed and manufactured. It provides accurate measurement of 3D coordinates of up to 1000 particles, discrimination between particles of different nature and estimation of the angular velocity of small aerosol crystals. A photogrammetric technique is applied to track independently moving particles, using the acquisition and processing of stereoscopic image sequences. The optical scheme of the set-up is based on a unique objective that collects the light coming from different directions from the back- and front-illuminated aerosol, and transfers it to four different cameras. Driving parameters for the optical head design and image quality are discussed. The set-up performance was verified on optical test objects, aerosol simulators and also real aerosol under microgravity conditions during parabolic flight. The set-up has been designed for use in the JET experiment on board the Maser 8 sounding rocket. The experiment aims at the first direct observation of the chemojet motion of free-flying small growing crystals. Investigation of chemojet motion along with elaboration of the image analysis technique is focused on the development of highly sensitive, real-time, non-perturbative analysis of the reactions on the surface of aerosol particles.

Effect of lift on the motion of aerosol particles

where h is the rate of shear. This study may be used to analyze particle motion in the viscous sublayer in turbulent flows, with application to particle deposition. The results show that the lift force has a strong effect on the deposition of aerosol particles from turbulent flows onto surfaces (Fichman et al, 1988). The motion of a particle in vertical flow is subject to centrifugal forces in addition to drag and lift. These centrifugal forces modify the condition for the instability of particle motion. The analysis of vertical flow assumes locally simple shear flow, and the criterion for instability is (Pnueli et al, 1997):

Pressure Drop and Interception Efficiency of Multifiber Filters

ABSTRACT The viscous flow fields around multifiber filters have been investigated in a previous paper. The results of the previous work show that the flow becomes periodic immediately after the first fiber array downstream from the entrance if the fibers are arranged uniformly along the flow direction. The characteristics of such flow fields enable the pressure drop and the particle interception efficiency of a multifiber filter to be represented by single-fiber models. The total filtration efficiency, however, cannot be so represented since fibers interact during filtration processes. In this study, the pressure drop and the interception efficiency were investigated by making use of the viscous flow fields modeled in the previous research. The fiber separation ratio was found to have significant effects on pressure drop and efficiency. At a given volume fraction, changes in the fiber separation ratio will result in changes to the patterns of fluid flow and aerosol particle motion. Therefore, the fiber separation ratio significantly affects pressure drop and interception efficiency.

Settling and asymptotic motion of aerosol particles in a cellular flow field

This paper presents a proof that given a dilute concentration of aerosol particles in an infinite, periodic, cellular flow field, arbitrarily small inertial effects are sufficient to induce almost all particles to settle. It is shown that when inertia is taken as a small parameter, the equations of particle motion admit a slow manifold that is globally attracting. The proof proceeds by analyzing the motion on this slow manifold, wherein the flow is a small perturbation of the equation governing the motion of fluid particles. The perturbation is supplied by the inertia, which here occurs as a regular parameter. Further, it is shown that settling particles approach a finite number of attracting periodic paths. The structure of the set of attracting paths, including the nature of possible bifurcations of these paths and the resulting stability changes, is examined via a symmetric one-dimensional map derived from the flow.

On the perturbed equations of motion of aerosol particles in regions of strong curvature of the fluid streamlines

Asymptotic forms are obtained and solved for the equations of motion of aerosol particles in various flow fields. The flows chosen to be treated include regions of strong curvature of the fluid streamlines, and regions where the initial motion of the particles differ from that of the undisturbed flow. It is shown that regular perturbations cannot yield the correct details of the trajectories of the particles in such regions, and that singular perturbations must be applied. The parameters whose magnitude control the phenomena come out to be the Stokes numbers, the curvature of the fluid streamlines, and the stopping distance of the aerosol articles. In principle this analysis is analogous to the derivation of the classical boundary layer equations. However, the classical boundary layer equations are field equations, which describe the flow in an Eulerian fashion, and therefore apply to a certain domain, geographically fixed in the field. The aerosol equations of motion considered here are Lagrangian, and as the particles move they carry with them the equations whose perturbed form must thus be modified by the fluid flow encountered along the trajectories traced by the particles. The forms assumed by the perturbed equations of motion are, therefore, different in the various cases treated. Analytical solutions are obtained for two cases which until now were handled only numerically, i.e., particle motion in viscous stagnation flow, and injection of particles into a laminar boundary layer on a flat plate. The comparison with numerical solutions shows that this new method gives rather good approximations.

Side Drift of Aerosols in a Two-Dimensional Slowly Oscillating Sonic Field

The two-dimensional drifting motion of aerosol particles in vessels under the influence of slow sonic oscillations is analyzed theoretically using the asymptotic small parameter method. The vessel geometry is such that the dimension in the direction of wave propagation does not exceed the sonic wavelength. It was recently shown that in the case of one-dimensional sonic waves, particles move away from the oscillating wall and drift toward the stationary one. The domain considered here is quite different from those previously treated in one-dimensional cases in the sense that the stationary wall of the vessel is slightly curved. It is shown that this curvature causes a drift of aerosol particles in the longitudinal direction, normal to the propagation of the applied sonic field.

Charging-Induced Diffusion of Aerosol Particles Moving under External Electric Fields in Ionized Gases

The stochastic motion of aerosol particles in an external electric field ( E) is investigated in the continuum and free-molecular regimes, under the condition of bipolar charging with radiation sources. The charging process is shown to give rise to a random "diffusive motion" of the particles along the field lines, in addition to their well-known drift motion (migration velocity). The "coefficient of charging-induced diffusion" (Dc) is, in general, a dyadic, but it reduces to a scalar in simple geometries having symmetrical field configurations. Dc is proportional to E 2/n o, where 2n o is the total ion concentration. Specific calculations indicate that Dc can be made several orders of magnitude larger than the intrinsic Brownian diffusivities. Possible methods of measuring Dc in real systems, especially using oscillating electric fields, are discussed. Some applications of this study are mentioned.

29 O 02 Numerical investigation of the influence of different forces on the motion of aerosol particles in a standing sonic field

29 O 02 Numerical investigation of the influence of different forces on the motion of aerosol particles in a standing sonic field

Acoustic Measurement of Aerosol Particles

Measurements of the motion of aerosol particles in an acoustic field were made on particles covering two orders of magnitude (0.3 μm–30 μm) in diameter and one order of magnitude in density. The measurements of both velocity magnitude and phase agree well with the theoretical model presented by Konig in 1891. As a result, the diameter and the density of spherical aerosol particles can be determined simultaneously from the measurement of the particle velocity. The phase and magnitude of the acoustic velocity can be determined by indirect methods, allowing particle sizing to be performed without the use of precision particles for calibration standards

On the drift of aerosol particles in sonic fields

The drifting motion of aerosol particles enclosed in vessels subject to ultrasonic oscillations is analysed theoretically. The geometry considered here is of vessels with dimensions not exceeding the ultrasonic wavelength. This geometry is quite different from previously treated cases, where the flow field dimensions measured many times the ultrasonic wavelength and where the aerosol particles were found to move towards nodes and towards loci of maximum amplitudes, according to their sizes. The results obtained here are rather different and indicate that all particles move away from the oscillating wall and towards the stationary one. These results conform to the motivation of the investigation, which is to protect a clean site from settling particles. The analysis incorporates two-scale expansions methods applied to the multiphase flow equations for a one-dimensional flow model. The results are constructed by the superposition of a slow mean motion of the gas and a fast oscillatory motion associated with the eigenfunctions of the standing waves solution. The combined effect on the particles is a drift in the direction of the stationary wall.

Electric field generation in atmospheric convective cells

An electric field generation mechanism in the atmosphere, formed by the joint action of friction and gravitational forces upon the charged aerosol particles captured into a convective cell is considered. It is shown that the electric field structure may be found by a self-consistent approach, taking into account the influence of the electric field upon the aerosol particle motion. Estimates of electric field amplitudes at different atmospheric heights are given.

Molecular emission following laser evaporation of copper aerosol

Copper aerosol is formed by laser evaporation of copper metal followed by sudden cooling of this vapour in either H 2, N 2 or Ar environment at room temperature. Subsequently, single laser pulses evaporate the aerosol and at the same time excite both atomic and molecular emissions. In contrast to the predominant atomic emission from laser ablated plasma from metal surface, the laser induced emission from copper aerosol is primarily molecular in nature. This is shown to be due to Cu 2: X→II→I laser induced flourescence by 308 nm excimer laser light. In H 2 environment, additional emission of rotationally resolved 0-0 band of A 1Σ +→X 1Σ + is also seen. Because of slow brownian motion of aerosol particles the laser produced molecular emission can be observed even 30 minutes after the production of aerosol is discontinued. The method provides a novel quasi-steady state source for molecular spectroscopy.

On some regularities of aerosol particle motion in electromagnetic fields

The paper is devoted to the results of experiments studying aerosol behaviour in magnetic and electric fields of various configurations. A complete analogy with the behaviour of ferromagnetic particles in uniform magnetic and electric fields is presented. Possible mechanisms of movement of the ferromagnetic particles in the constant magnetic field and the validity of a model with a magnetic charge are discussed.