Brownian motion aerosol deposition

The aerosol model In VICTORIA accounts for the following basic mechanisms (1) condensation or evaporation from aerosol particle surfaces (2) deposition onto structural surfaces (3) agglomeration of aerosol particles (4) and transport of aerosols from one cell to another by advection. The deposition mechanisms modeled are gravitational settling, laminar or turbulent deposition. Brownian motion, thermophoresis, diffusiophoresis, and inertial deposition in curved channels (bends). Agglomeration mechanisms include Brownian motion, relative gravitational motion. Interactions In a shear field, and inertia in a turbulent field. 

Particulate diffusion does not play a significant role in the deposition of pharmaceutical aerosols. However, it is worth noting the mechanism by which diffusion of particles occurs in the lungs. The principle of Brownian motion is responsible for particle deposition under the influence of impaction with gas molecules in the airways. The amplitude of particle displacement is given by the following equation ... 

The deposition of particles on macroscopic surface is the primary goal in CVD processes, bnt rednces the efficiency of vapor phase particle synthesis. Particles can deposit by Brownian motion, bnt in high-temperature reactors, thermophoretic deposition often dominates. Thermophoresis is the migration of small aerosol particles as a resnlt of a temperatnre gradient. It causes particles carried in a hot gas to deposit on a cool surface. Eor small particles, Kn 1, a dimensionless group can be created to describe thermophoresis, Th ... 

The lines in Fig. 7.4 are the results of theoretical calculations, using models of the respiratory tract (Yu Diu, 1982). The points are measurements with radioactive aerosols. Numerous other determinations of fractional deposition in the whole tract have been made, using non-radioactive methods to count the number of particles in the inhaled and exhaled air (Heyder et al., 1986 Schiller et al., 1988). Fractional deposition is least for particles of about 0.2 to 0.5 m diameter. Table 7.1 shows that the combined effect of sedimentation and Brownian motion is then at a minimum. 

A comparison of the results with other data on the deposition of submicrometre aerosols, all related to a tidal volume of 11, is shown in Fig. 1.14. Although the density of the lead particles was greater than that of the other particles, the fractional deposition was similar, except possibly for the 0.5 pm size, because deposition was by Brownian motion. The percentage deposition increases for particles of diameter less than 0.1 /urn, and this means increased uptake of lead, relative to a given PbA, for persons exposed to non-aggregated aerosol, as found alongside major roads. 

When aerosols are in a flow configuration, diffusion by Brownian motion can take place, causing deposition to surfaces, independent of inertial forces. The rate of deposition depends on the flow rate, the particle diffusivity, the gradient in particle concentration, and the geometry of the collecting obstacle. The diffusion processes are the key to the effectiveness of gas filters, as we shall see later. 

When particles experience a mean curvilinear motion and also have Brownian agitation, they are deposited on obstacles by both mechanisms. For very small particles of radii less than 0.1 /xm, Brownian motion dominates particle collection on surfaces. For larger particles, inertial forces dominate. An example of the difference in collection efficiency for spherical collectors of different size is shown in Fig. 3 for different particle diameters and aerosol flow velocity. 

The molecules of a gas are in constant motion and collide with aerosol particles. If these particles are small, they are disturbed by the impact of the gas molecules and move in an irregular fashion described as Brownian motion. Particles moving in this way may encounter the walls of the airways and thus be deposited. The... 

Thus, at very short times, the deposition flux is that resulting from diffusion plus one-half that due to settling, whereas for long times the deposition flux becomes solely the settling flux. For particles of radii 0.1 /rm and 1 tm in air (at 1 atm, 298 K), Xds is about 80 seconds and 0.008 second, respectively, assuming a density of 1 g cm. For times longer than that. Brownian motion does not have any effect on the particle motion. The aerosol number concentration and removal flux are shown in Figures 8.9 and 8.10. The system reaches a... 

Sedimentation velocity is proportional to particle density, but Brownian motion is independent of density. Table 5.6 shows that sedimentation of unit density particles is more effective in causing deposition than Brownian diffusion when dp exceeds 1 pm, whereas the reverse is true if dp is less than 0.5 pm. For this reason, it is appropriate to use the aerodynamic diameter Dp equal to p] dp when this exceeds 1 pm, but the actual diameter for submicrometre aerosol particles. 

Hinds, W. C. 1999. Aerosol Technology Properties, Behavior, and Measurement of Airborne Particles, 2nd ed. New York John Wiley Sons. An upper-division/graduate-level text covering, for example, bioaerosols. Brownian motion and diffusion, respiratory deposition models, measurement, and sampling. 

Other lesser mechanisms that result in aerosol removal by filters are (1) gravitational settling due to the difference in mass of the aerosol and the carrying gas, (2) thermal precipitation due to the temperature gradient between a hot gas stream and the cooler filter medium which causes the particles to be bombarded more vigorously by the gas molecules on the side away from the filter element, and (3) Brownian deposition as the particles are bombarded with gas molecules that may cause enough movement to permit the aerosol to come in contact with the filter element. Browruan motion may also cause some of the particles to miss the filter element because they are moved away from it as they pass by. For practical purposes, only the three mechanisms shown in Fig. 29-1 are normally considered for removal of aerosols from a gas stream. 

For the Brownian-particle approximation, it is assumed that the aerosol system is stable over some experimental time. Particles are stable and their motions are uncorrelated. A special case in which the particles are treated as rigid bodies is frequently termed the Rayleigh gas. Of course, a characteristic feature of an aerosol is its inherent instability due to coagulation and deposition. Therefore, this approximation is limited to experimental times collision... 

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