Comment on "Thermophoresis of a Near-Wall Particle at Great Knudsen Numbers"

An approximate expression for the thermophoretic force on a near-wall particle at large particle Knudsen number is presented. The wall has arbitrary specularity. The derivation of this expression follows that of Chen and Xu (2002) but corrects two errors in their approach. The corrected expression predicts that the near-wall thermophoretic force is only 5-10% greater than the bulk-gas (Waldmann) value and that the greatest difference occurs for a specular wall, whereas the original expression predicts that the force is increased up to 35% and that the greatest difference occurs for a diffuse wall. Independent molecular gas dynamics simulations confirm the corrected expression.