Size Dependence of Tangential Momentum Accommodation Coefficient in Nanoconfined Gas Flow

Gas flow in nano- to angstrom-scale channels is dominated by gas-wall scattering rather than intermolecular collisions. Consequently, the tangential momentum accommodation coefficient (TMAC), equivalently the diffuse reflection fraction f, becomes a key parameter for describing confined gas flow. In prior studies, f is commonly treated as a constant determined solely by the gas species and surface material. Here, we employ molecular simulations to obtain f in situ by quantifying tangential momentum changes before and after gas-wall collisions during gas flow in graphene channels with heights spanning nanoscale to angstrom scale. The results show that f exhibits a pronounced size dependence under extreme confinement, thereby invalidating the classical constant f assumption, and can be described by an empirical exponential relation f ( h ). Incorporating f ( h ) into Knudsen theory substantially improves the prediction accuracy of flux, particularly at small heights where constant f model systematically overestimates the flux. Potential energy analysis further indicates that overlap of the interaction fields from opposing walls reshapes the energy landscape across the entire channel, providing a physical origin for the size-dependent behavior of f . We also elucidate the modulation of f ( h ) by molecular interaction parameters and temperature. This work provides crucial insights for accurate modeling and rational design of gas flow in nanoconfined systems.