Sedimentation [22] of the disperse phase usually takes place under the force of gravity. (It may be caused also by an imposed centrifugal force field). A sphere of diameter D and density d under the influence of gravitational force Fg will have a terminal settling velocity Vts in the laminar region governed by Stoke's law:

18 X h g where g is gravitational acceleration and h is the viscosity of air. It follows that the probability of deposition by sedimentation also increases with increasing particle mobility, similar to inertial impaction.

When the particles become very small so that using the Stokes picture of an object moving in a continuous medium is no longer permissible, the so-called slip correction factor, Cc, must be applied [22]. The particle can "slip" through the medium, and, consequently, its velocity is greater than that predicted by the previous equation.

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