Stop distance, aerosols

Impaction is caused by the inertial mass of the traveling aerosol particles that forces them to move in a straight-line direction even when the flow of the inhaled air transporting them is bent around a curvature. Hence the particles tend to deposit on obstacles placed in the path of their travel. The inertial mass depends on particle size, density, and velocity. The stopping distance S of a particle having mass mP and initial velocity v0iP is defined according to... 

Since aerosol particles are continually undergoing molecular bombardment, their paths are smooth curves rather than segments of straight lines. It still is possible to define an apparent mean free path for the aerosol particles (Fuchs, 1964). This is the distance traveled by an average particle before it changes its direction of motion by 90°. The apparent mean free path represents the distance traveled by an average particle in a given direction before particle velocity in that direction equals zero. But this is just the stop distance. 

Separate large- and small-drop aerosols can be produced by taking advantage of the different stop distances of the primary and satellite droplets. Aerosols of the original pure liquids can be produced in this way with primary droplet diameters ranging from 6 to 3000 / im and liquid flow rates up to 168 cmVmin. When solutions with volatile solvents are used, the solvents can be evaporated, leaving behind particles whose size depends on the... 

Sheldon started his Ph.D. studies at a time when the field of aerosol science was in its early stages of development. Working with H.F. Johnstone, he focused on how particles in turbulent airflow are deposited on the walls of pipes and ducts. Sheldon made important contributions right from the start he introduced the notion of a stopping distance of a particle injected into stagnant air, and then used this concept to predict particle motion through the viscous boundary layer to the surface. His thesis work laid the foundation for much of the later work on deposition of particles in industrial systems as well as dry deposition from the ambient atmosphere, where turbulent eddies impart velocities normal to the mean flow and enable particles to reach the surface. 

For active devices the situation can be similar to the passive case in terms of the device resistance dictating the inhaled flow rate and hence the flow rate at which the device should be tested. However, just like the pMDI, if the device generates an aerosol that has a velocity relative to the inspired airstieam, some form of stopping distance measurement, such as an oropharyngeal cast or inlet bend, is mandatory. 

Turbulent deposition of particles on surfaces becomes an important process in reactor containments only when particle concentrations have become small. Accurate modeling of turbulent deposition rates has been a topic of debate for some time within the aerosol science community. Models currently available are usually assumed to be adequate in light of the relatively crude understanding of turbulent hydraulics within reactor containments that is now available. These models are based on the study of turbulent flows through pipes. The models are based on the hypothesis that turbulent impulses to aerosol particles can thrust particles across laminar boundary layers adjacent to structural surfaces if the layers are thinner than the stopping distance of the particles. 

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