Effect of the carbon-nanotube length on water permeability

Effect of the carbon nanotube (CNT) channel length on the water flow through the CNT is studied using molecular dynamics simulations. The water flow is found to decay with the channel length (similar to 1/N-2.3, N is the number of carbon rings along the nanotube axis), much faster than that predicted by a previous continuous-time random walk (CTRW) model (similar to 1/N). This unexpected decay rate of flow is found to result from the weakening of the correlation of the concerted motion of the water molecules inside the CNT. An improved CTRW model is then proposed by taking into account of this effect. Meanwhile, the diffusion constant of water molecules inside CNTs with various lengths is found to be relatively invariant, which results in a relatively constant hopping rate.