Wall morphology dependence of rare gas Knudsen diffusion in silica and graphite slit nanopore: A molecular dynamics study

P Magnico, VACUUM, 242, 114756 (2025).

DOI: 10.1016/j.vacuum.2025.114756

Gas/wall collision mechanisms play a key role in Knudsen diffusion process. In particular, the channel wall structure has a major influence in mass transfer. So, we investigate the influence of the wall roughness, anisotropy and porosity on the self-diffusion of helium and neon in nanochannels. Three materials are proposed: graphite and (3-cristobalite and amorphous silica. The study makes it possible to analyze, in function of temperature, the correlation between 1/the ballistic/diffusion transition regime of the surface gas transfer, 2/the transition of the bouncing process to a linear increase of the bounce number with time and 3/the shape of the surface residence time distribution characterized by a Frechet like distribution at short time and an exponential decay at long time. As concerns the amorphous SiO2, the bounce must be redefined owing to the transfer inside the material which is dominated by a cage effect. The anisotropy effect on collision process and Knudsen diffusion is analyzed by means of a tensorial computation of the tangential momentum accommodation coefficient and of the mean square displacement. Using the Langevin at the channel scale and the Arya model, the ballistic/diffusion transition time of the mean square displacement is related to the collision frequency and the collision number required for the velocity to be uncorrelated. A stochastic model confirms the molecular dynamics results with -SiO2 (3 channel: The behavior of the Knudsen diffusion coefficient according to the Arrhenius law and the influence of collision frequency on transition time.

Return to Publications page