Flow of Aqueous Solutions in Carbon Nanotubes

Abstract

We conduct simulations of water flowing inside carbon nanotubes using non-equilibrium molecular dynamics simulations. A new adaptive forcing scheme is proposed to enforce a mean center of mass velocity. This scheme is compared to the non-adaptive, constant body/gravity force for the flow of methane in a carbon nanotube. The two schemes produce similar streaming velocity profiles and practically identical slip lengths. The wall slip predicted in the present simulations is found to be considerably shorter than the one observed in earlier studies using the constant body/gravity force scheme at considerably lower pressures. This observation is reminiscent of the slip length reduction with pressure increase that has been observed in the case of Couette flow of water. For water flowing through carbon nanotubes with diameters of 2.712, 4.068 and 5.424 nmand with flow speeds of 100 m s-1 we find slip lengths of 10, 12, and 18 nm. In addition, we consider mixtures of nitrogen and water flowing in a (20,20) carbon nanotube with diameter of 2.712 nm. For the mixture we find that the slip length is reduced to 6 nm as compared to 10 nm slip for the pure water. The shorter slip length is attributed to the fact that nitrogen forms droplets at the carbon surface, thus partially shielding the bulk flow from the hydrophobic effect.