Sedimentation Due to Gravity

Sedimentation occurs when particles of sufficient mass are acted on by gravity. The settling velocity of a particle due to gravity is determined by Stokes' Law [232,239]:

= Pd2gCs _ dagCs V 18hT 18h where p and d are the particle density and diameter, respectively, g is the gravitational constant, h is the fluid viscosity, and Cs is the slip correction factor that corrects Stokes' Law for the assumption that the air velocity at the particle surface is zero (which does not hold for particles smaller than 10 mm) [222]. The probability of gravitational deposition is proportional to the particle settling distance and, therefore, increases in proportion to (t * d^). Deposition owing to gravitational settling is significant for particles with aerodynamic diameters larger than 0.5 mm [221] and occurs primarily in the lower bronchial and alveolar regions, due to the significantly decreased velocities compared to the upper airways. Long residence times within the airways and low air velocities—for example, during breath holding—increase particle deposition by sedimentation.

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