Diffusion interception

Collection of particles is based on filtration, gravitational and centrifugal sedimentation, inertial impaction and impingement, diffusion, interception, or electrostatic or thermal precipitation (e.g., see Spurny, 1986, Chapter 3). The choice of method depends on a number of parameters such as the composition and size of the particles, the purpose of the sample, and acceptable sampling rates. Table 11.10 summarizes some of the commonly used methods and the size ranges over which they are effective. 

Airborne particles may be delivered to surfaces by wet and dry deposition. Several transport mechanisms, such as turbulent diffusion, precipitation, sedimentation, Brownian diffusion, interception, and inertial migration, influence the dry deposition process of airborne particles. Large particles (dNIOAm) are transported mainly by sedimentation hence, large particulate PAHs tend to be deposited nearer the sources of emission Small particles (dblAm), which behave like gases, are often transported and deposited far from where they originated (Baek et al., 1991 Wu et al., 2005). 

Later Tien and Payatakes (15), in their study of the deposition of colloidal particles, developed a mathematical model which included in one expression the contributions by Brownian diffusion, interception with London attraction, and gravitational field. The most general form of their expression is ... 

Deposition mechanisms in the nose include inertial impaction, sedimentation, diffusion, interception and electrostatic attraction. The structure and physiology of the nasal cavity, with the small cross-section for airflow and sharp curves, suggests that inertial impaction is the most significant mechanism for drag deposition in the nasal cavity. 

Five mechanisms govern particle deposition in lung airways, namely, inertial impaction, gravitational sedimentation, diffusion, interception, and electrostatic attraction.f Electrostatic charges enhance deposition by increasing attractive forces to airway surfaces. Melandri et found that the deposition of... 

This equation must be solved with the diffusion-interception boundary conditions... 

Deposition mechanisms are principles by which particles can be deposited onto airway walls. For inhaled aerosol particles only two major mechanisms are important inertial deposition and sedimentation (see definitions). In the literature also diffusion, interception and electrostatic precipitation are sometimes mentioned as deposition mechanisms, but these mechanisms, if occurring at aU, are of lower relevance. 

Airborne particles entering the respiratory tract will be deposited on the surface of any part of the airways with which they come into contact. Because the surfaces of the airways are always moist, there is a negligible chance of a particle becoming re-entrained in the air after once having made contact with a surface. Particles which do not make contact with the airway surfaces will be exhaled. The deposition of particles in the airways is a complicated process. Zeng et al. (2001) identify five possible mechanisms contributing to the deposition of particles carried into the respiratory tract. These are sedimentation, inertial impaction, diffusion, interception and electrostatic precipitation. After introducing each mechanism, their relative importance in each part of the respiratory tract will be discussed. 

The combination of the effect of inertial impaction, diffusion, interception, and electrokinetic attraction on the cell particle capture and retention to a fibre is called direct adhesion, and the most important mechanism of leucocyte depletion filtration... 

The retention of the capmred blood cells in the nonwoven filter mostly depends on the following mechanisms sieving, direct interception, direct adhesion (including inertial impaction, diffusion, interception and electrokinetic attraction), and indirect adhesion (including bridging and particle re-entrainment). 

The significance of the results presented in Figs. 43 5 is that they allow one to estimate the area over the surface where the flux (transfer rate) become uniform. The mass-transfer-rate constant (reduced flux) measured in this uniformly accessible area and referred to as A is of a particular significance because it can be analyzed theoretically in a much more efficient way than the local flux. Moreover, A can be directly used for determining the significance of various transport mechanisms like diffusion, interception, specific or external force, and so forth. 

The site and magnitude of deposition of particles in the respiratory tract is determined by physical mechanisms and the biology of the subject inhaling the particles. The five most significant mechanisms of deposition are sedimentation, impaction, diffusion, interception, and electrostatic precipitation (Fig. 7). 

For hquid systems v is approximately independent of velocity, so that a plot of JT versus v provides a convenient method of determining both the axial dispersion and mass transfer resistance. For vapor-phase systems at low Reynolds numbers is approximately constant since dispersion is determined mainly by molecular diffusion. It is therefore more convenient to plot H./v versus 1/, which yields as the slope and the mass transfer resistance as the intercept. Examples of such plots are shown in Figure 16. 

The collection technique involves the removal of particles from the air stream. The two principal methods are filtration and impaction. Filtrahon consists of collecting particles on a filter surface by three processes—direct interception, inertial impaction, and diffusion (5). Filtration attempts to remove a very high percentage of the mass and number of particles by these three processes. Any size classification is done by a preclassifier, such as an impactor, before the particle stream reaches the surface of the filter. 

In Figure 7, the resistance to mass transfer term (the (C) term from the Van Deemter curve fit) is plotted against the reciprocal of the diffusivity for both solutes. It is seen that the expected linear curves are realized and there is a small, but significant, intercept for both solutes. This shows that there is a small but, nevertheless, significant contribution from the resistance to mass transfer in the stationary phase for these two particular solvent/stationary phase/solute systems. Overall, however, all the results in Figures 5, 6 and 7 support the Van Deemter equation extremely well. 

The retention efficiency of membranes is dependent on particle size and concentration, pore size and length, porosity, and flow rate. Large particles that are smaller than the pore size have sufficient inertial mass to be captured by inertial impaction. In liquids the same mechanisms are at work. Increased velocity, however, diminishes the effects of inertial impaction and diffusion. With interception being the primary retention mechanism, conditions are more favorable for fractionating particles in liquid suspension. 

Dry The interception and retention by surfaces of gases or particulate matter by diffusion, gravitational settling, or thermal forces. 

Impaction Collection mechanism where the contaminants collide with the surface of the filter by inertia, interception, or Brownian diffusion. 

The principal mechanisms of disposition in dust collectors are (1) gravitational deposition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition, and (5) electrostatic deposition. During the initial operating period, particle deposition takes place mainly by inertial and flow-line interception, diffusion, and gravity. Once the dust layer has been fully established, sieving is probably the dominant deposition mechanism. 

Since radiation arriving at a black surface is completely absorbed, no problems arise from multiple reflections. Radiation is emitted from a diffuse surface in all directions and therefore only a proportion of the radiation leaving a surface arrives at any other given surface. This proportion depends on the relative geometry of the surfaces and this may be taken into account by the view factor, shape factor or configuration F, which is normally written as F, for radiation arriving at surface j from surface i. In this way, F,y, which is, of course, completely independent of the surface temperature, is the fraction of radiation leaving i which is directly intercepted by j. 

The contribution of atmospheric dust to surface dust depends on the dust falling to the earth. This occurs either as dry dust fall or wet washout with rain, snow or hail (1-6,8-10). Dry dust fall occurs by s imentation, impaction, interception or diffusion. Sedimentation, the fall under gravity, may be estimated using Stoke s law which relates the density and diameter of particles to their falling velocity. A particle of density 1.0 g cm"3 and diameter around 0.1 pm would fall with a velocity of around 9 x 10" cm s" ... 

First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. 

The physical meaning of the g (ion) potential depends on the accepted model of an ionic double layer. The proposed models correspond to the Gouy-Chapman diffuse layer, with or without allowance for the Stem modification and/or the penetration of small counter-ions above the plane of the ionic heads of the adsorbed large ions. " The experimental data obtained for the adsorption of dodecyl trimethylammonium bromide and sodium dodecyl sulfate strongly support the Haydon and Taylor mode According to this model, there is a considerable space between the ionic heads and the surface boundary between, for instance, water and heptane. The presence in this space of small inorganic ions forms an additional diffuse layer that partly compensates for the diffuse layer potential between the ionic heads and the bulk solution. Thus, the Eq. (31) may be considered as a linear combination of two linear functions, one of which [A% - g (dip)] crosses the zero point of the coordinates (A% and 1/A are equal to zero), and the other has an intercept on the potential axis. This, of course, implies that the orientation of the apparent dipole moments of the long-chain ions is independent of A. 

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