Effects of non-local scattering on the aerodynamic coefficients of nano-particles

Most gas-surface interaction models used in rarefied gas dynamics assume that the space and time scale associated with gas-solid interaction are negligibly small. Accordingly, it is assumed that a gas molecule hitting a solid surface is instantaneously re-emitted at the same spatial location. However, as shown by molecular dynamics simulation, the re-emission event usually takes place at a distance of several molecular diameters from the first collision location. It is to expected that diffusion processes on the surface do affect the aerodynamic forces on particles transported by a carrier gas, when the particle size is comparable with the distance covered by a molecule on the solid surface during interaction. In such case, local gas-surface interaction models fail to predict the correct drag on the particle. In this paper a non-local kinetic gas-surface interaction model is applied to compute aerodynamic forces on spherical particles in the free molecular flow regime. Is is shown that effective accommodation coefficients of nanosized particles depend on particle radius.