Effect of the wall structure on nanochannel gas flow: A molecular dynamics study

Abstract

Nonequilibrium molecular dynamics simulations are performed for force-driven argon gas nanochannel flow to investigate the effect of the nanoscopic wall structure on the gas flow characteristics. The monoatomic molecule argon is first used for both the fluid and wall molecules. A face-centered cubic wall structure is first investigated. It is confirmed that the fcc (111) surface structure induces the fastest flow, followed by the fcc (100) and (110) surface structures. Unlike liquid flow in a nanochannel, the density of the monolayer adsorbed on the wall surface greatly depends on the wall configuration. A lattice wall configuration that allows stronger adsorption of molecules induces higher velocity flow. From simulations with different wall molecule bond lengths, the gas flow is affected more by the surface roughness than adsorption of gas molecules to the wall. Because the density close to the wall surface is affected by the surface configuration, the relationship between gas molecule density and the velocity profile is analytically investigated in the early transition regime. The results suggest that the magnitude of the adsorbed monolayer density partially causes a kink in the velocity profile. Other wall molecule configurations are also investigated, such as silicon, diamond, and graphite. It is found that the diamond structure can induce a much larger density peak than silicon owing to the high wall density. This strong adsorption to the wall disrupts the motion of fluid molecules near the wall, which results in less slippage and lower bulk velocity. The graphite structure is comparable to the diamond structure. Finally, the wall-fluid interaction of the graphite wall and argon gas is considered using different interatomic potentials for the wall and the gas. Adsorption becomes weaker and the slippage velocity is significant because the wall surface roughness becomes the dominant factor affecting the flow.