The effects of Knudsen numbers (Kn), particle sizes, their relative velocities, and particle and gas properties on the drag force acting on micro- and nano-particles, moving in a gas, are investigated using the molecular dynamics approach. The slip correction factor C and the slip parameter A are shown to depend not only on Kn, as commonly assumed, but also on the physical properties of the gases and particles. Quantitative analysis of the ratio between local (near the particle surface) and ambient gas number densities showed that increased gas–particle interactions at micro- and nano-scales lead to an increase in the local gas number density, which leads to an increase in the drag force. A comprehensive potential depth parameter εggp is introduced to take into account both gas–particle and gas–gas interactions. This parameter allowed us to provide a unified framework for interpreting slip correction factor behavior for different particle–gas combinations. A universal predictive approximation of the slip parameter is suggested as a function of εggp and Kn. The prediction of this approximation is shown to be in good agreement with experimental data referring to the drag forces experienced by polystyrene latex particles in nitrogen at Kn10.
Liu et al. (Thu,) studied this question.