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Analytical results are presented concerning the force acting on
a spherical particle which is moving towards the wall of a planar plate in
the near-wall region. The problem is assumed to be within the
free-molecule regime, and a combined specular and diffuse reflection at
the plate and/or particle surface is included in the analysis. It has been
shown that the action force on the near-wall particle contains two terms.
The first term depends on the gas pressure and the plate-wall/gas
temperature ratio, but is independent of the speed ratio or the
macroscopic relative velocity between the particle and bulk gas. Only the
second term is directly proportional to the speed ratio or the relative
velocity. Consequently, the action force on the near-wall particle moving
towards the planar plate may be negative or positive, i.e. the particle
may be decelerated or accelerated, depending on the case conditions. As
long as the gas temperature is different from the plate-wall temperature,
the near-wall particle will always experience a force even for the case
without relative motion between the particle and bulk gas.
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