Thermophoresis of a Near-Wall Particle at Great Knudsen Numbers

Analytical results are presented concerning the thermophoretic force acting on a spherical particle suspended near a planar wall at great Knudsen numbers (i.e., for the free-molecule regime). A combined specular and diffuse reflection at the wall and at the particle surface is included in the analysis. It has been shown that the thermophoretic force acting on the near-wall particle depends not only on the temperature gradient or heat flux within the bulk gas, but also on the gas pressure and gas/wall temperature ratio. The particle temperature and the specular reflection fraction at the particle surface do not affect the thermophoretic force. Dependent on the case conditions, the thermophoretic force acting on the near-wall particle may be greater or less than its counterpart without accounting for the near-wall effect.