Aerosols flocculation rate

Fig. XIV-9. Effects of electrolyte on the rate of flocculation of Aerosol MA-stabilized emulsions. (From Ref. 35.)...

Having considered the factors that govern the extent or rate of flocculation in an aerosol, it is also important to consider the effects of flocculation on the properties of an aerosol. Consider a large sphere of diameter Dpl, with its surface covered with small particles of diameter Dpi. If the adhering particles are no more than one layer thick, it can be shown as a good first approximation that... 

Ambipolar charges can give increased deposition but such deposition depends for the most part on image effects. In this sense the rate of deposition will be comparable with the rate of flocculation of ambipolar aerosols. Improved deposition rates by such means are likely to be small. 

Let us consider the case of aerosol suspensions. The suspended particles must be well dispersed so as to penetrate the lung. In this sense, an ideal aerosol suspensiem docs not sediment and form aggregates. However, particles eventually do aggregate, so it is convenient to formulate them in such a way as to promote weak flocculates that can be easily redispersed by shear (e.g agitation of the aerosol ermtainer). The formulation can be adjusted mea.suring viscosity as a function of shear rate or tdiear stress, as depicted in Refs. 6 and 30. 

For simphdty, it is assumed that the density of the gas phase is small compared to that of the particle. For more accurate results, the density difference between particle and gas (p = Pp Pg) should be employed. At 20°C and atmospheric pressure, the viscosity of air is 1.83 X 10 cP (centipoise or g cm s ), so that for an aerosol particle otR = 10 cm and p = 3.0 g cm (e.g., volcanic ash), the rate of fall will be approximately 0.04 cm s. Particles from a plume of ash thrown to an altitude of 10,000 m would (theoretically and neglecting all complicating factors mentioned above) take about 290 days to reach the ground If the particle size grows to 10" cm radius by flocculation, its rate of fall increases to 3.6 cm s"S and the same trip will take about 3.2 days. It is easy to understand, then, why volcanic eruptions and other natural (and unnatural) events that produce high-altitude aerosols can affect not only the color of our sunsets but also other more vital global atmospheric interactions. 

According to Equation (13.6), the rate of fall of an aerosol particle will be proportional to the radius squared and the density of the particle (actually the difference in density between particle and gas), and inversely proportional to the viscosity of the gas. For very small aerosols the correction given in Equation (13.5) can be applied. Obviously, any process that will increase the particle size (e.g., through flocculation) will help to facihtate the removal of the aerosol by gravitational means. The same applies to centrifugal or other processes based on the inertia of the aerosol particle. 

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