Thermal Force on Spherical Particles in a Rarefied Gas

A theoretical analysis is presented for the thermal force exerted on a spherical particle between two flat plates. The case is considered where the Knudsen number KL, based on the plate spacing, is nonzero. The analysis is based on the assumption that the particle radius is much smaller than the plate spacing and the result is valid for all values of KL from zero to infinity. A moment solution to the Boltzmann equation coupled with a two‐stream Chapman‐Enskog distribution function is employed, and the collision integral is evaluated on the basis of Maxwell molecules.