Rough-Surface-Dominated Knudsen Gas Flow in Heterogeneous Nanochannels

Gas flow in heterogeneous nanochannels with asymmetric wall properties is important for many advanced technologies, yet how dissimilar surfaces synergistically govern the Knudsen gas flow remains elusive. Here, we combine molecular dynamics simulations with grand canonical Monte Carlo methods to systematically investigate gas flow in heterogeneous nanochannels composed of smooth walls (graphene and h-BN) and rough walls (MoS2 and SiO2) with channel heights ranging from the angstrom scale to the nanoscale. The results show that gas flux in heterogeneous nanochannels is consistently lower than the arithmetic mean of the fluxes in the corresponding homogeneous channels and that this deviation is amplified under angstrom-scale extreme confinement. This rough-surface-dominated behavior arises from the nonlinear relationship between the tangential momentum accommodation coefficient and gas flux. Because gas molecules collide with the two walls with nearly equal probability in the Knudsen regime, the tangential momentum accommodation coefficient of heterogeneous channels under weak to moderate confinement is close to the average of the corresponding homogeneous values. This averaged tangential momentum accommodation coefficient, however, yields a flux biased toward the rough channel value rather than the average. In addition, under angstrom-scale confinement, asymmetric wall potentials further amplify the internal energy corrugation, enhancing momentum dissipation, and suppressing flux. These findings reveal the mechanisms of Knudsen gas flow in heterogeneous nanochannels and provide guidance for predictive modeling and rational design of asymmetric nanofluidic devices.