Diffusivity of liquid argon in carbon nanotubes

We present molecular dynamics simulation results on the system size dependence and the tube diameter dependence of the self-diffusion coefficient of liquid argon confined in carbon nanotubes. The tube diameter is varied from 1 nm to 10 nm under the condition of fixed temperature and fixed axial pressure. Variation of the diffusion coefficient in a nanoscale diameter range is large and intricate, which is caused by a variety of underlying structures of liquid argon in the carbon nanotube. In a mesoscopic range, as the diameter is increased, the diffusion coefficient gradually increases and approaches the limiting value of the bulk liquid. The rate of approach is significantly slow compared to the rate of approach of the density to the bulk density. A main reason is found to be formation of solid like structure in the vicinity of the cylindrical wall.