Gravitational Settling of an Aerosol Particle

TABLE 9-3 Slip Correction Factor Cc for Spherical Particles in Air at 298 K and 1 atm 

A number of investigators over the years have determined the values for the numerical coefficients used in the expression above. Allen and Raabe (1982) have reanalyzed Millikan s data (based on experiments performed between 1909 and 1923) to produce the updated set of parameters shown above. 

Values of Cc as a function of the particle diameter Dp in air at 25°C are given in Table 9.3. The slip correction factor is generally neglected for particles exceeding 10 pm in diameter, as the correction is less than 2%. On the other hand, the drag force for a 0.1 pm in diameter particle is reduced by almost a factor of 3 as a result of this slip correction. 

Up to this point, we have considered the drag force on a particle moving at a steady velocity u00 through a quiescent fluid. Recall that this case is equivalent to the flow of a 

For a particle falling in a fluid there are two forces acting on it, the gravitational force mpg and the drag force Fdrag. Therefore, for Re 0.1, the equation of motion becomes 

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