Abstract
Stoke's law F=6πηRυ is valid only in liquids. In order to apply this law to particles suspended in air, the slip correction is needed, especially for particles with diameters less than 1 μ m. The slip correction is included in Stoke's law by F= 6πηRυ 1+A·Kn with A+α+β·e − γ Kn and the Knudsen number (ratio of the particle radius to the mean free path of the gas molecules), η the viscosity of the gas, R the particle radius and and υ the velocity of the particle. For large Knudsen numbers, this equation reduces to F= 6πηR 2υ (α+β)·λ In the present work a simple Monte-Carlo simulation model is used to determine slip corrections in the free molecule regime (Knudsen number Kn ⪢ 1: The velocity of the air molecules are assumed to follow a Maxwellian distribution. The particle moves steadily in the gas, and the molecules impinge on its surface. The impaction points are distributed uniformly over the particle's surface. A simple criterium shows whether a molecule can in fact hit the surface at the selected point. If so, the transferred momentum is calculated. After sufficient iterations the slip correction is obtained by comparing the total transferred momentum with the expression for the Stokes drag force. Since only the free molecule regime is considered, the slipcorrection equals α + β.
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