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.