Thermodynamically Dependent Behavior in Gas Transport in Two-Dimensional Graphene Nanochannels.

Gas transport through nanochannels has aroused significant interest in many fields. Recently, "ballistic transport" of gas was observed through a two-dimensional graphene nanochannel, and it causes a peculiar enhancement compared to the predictions of the Knudson theory. Many studies attributed this effect to the specular reflection caused by the atomically smooth surface of the channel. Here, our molecular dynamics simulation, showing consistent results with previous experiments and simulations, reveals an interesting aspect: gas atoms with higher kinetic energies tend to pass the channel more easily. Extensive calculations of the tangential momentum accommodation coefficient considering different velocities on the graphene surface reveal that the attractive force between the gas and the surface atoms plays a more prominent role than the previous view, and gas atoms with more normal kinetic energies will overcome the attraction. Consequently, it indicates that a constant parameter used to balance the specular and diffuse reflection may not be adequate and should be replaced by a function considering the thermodynamic properties of gases.