Molecular dynamics simulations of liquid flow in and around carbon nanotubes

The advent of carbon nanotube (CNT) synthesis has created exciting new oppor- tunities in fluid dynamic applications as the fluid behaviour can deviate signifi- cantly from conventional continuum expectations. CNTs indicate major potential in nanotechnologies such as seawater desalination. Molecular dynamics (MD) is often the numerical method of choice for fluid dynamics at the nanoscale due to its high level of detail and accuracy. Using the “controllers” of Borg et al.[1], we are able to shrink the computa- tional domain required for molecular dynamic simulations of the external flow of liquid argon past a CNT and significantly increase the simulation’s computational efficiency. We apply three pressure differences across a CNT membrane carrying liquid argon, and compare the results of the pressure-driven flow through the nanotube with hydrodynamic predictions and Navier-Stokes solutions. We find that both fail to accurately predict flow behaviour in this problem. Non-equilibrium molecular dynamics simulations are then used to investigate water transport through (7,7) CNTs, investigating how changing the CNT length affects the internal flow dynamics. We show that, under the same applied pressure difference, an increase in CNT length has a negligible effect on the resulting mass flow rate and fluid flow velocity. Axial profiles of fluid properties demonstrate that entrance and exit effects are significant in the transport of water along CNTs. Large viscous losses in these entrance/exit regions lead into central “developed” regions in longer CNTs where the flow is effectively frictionless. Finally, we investigate how changing the number of structural defects in the wall of a (7,7) single-wall carbon nanotube (CNT) affects the water transport and internal fluid dynamics. Structural defects are modelled as vacancy sites (missing carbon atoms). We find that, while fluid flow rates exceed continuum expectations, increasing numbers of defects lead to significant reductions in fluid velocity and mass flow rate. The inclusion of such defects disrupts the nearly- frictionless water transport commonly attributed to CNTs. The results presented in this thesis are crucial in the development of future nanotechnologies such as CNT membranes for selective material separation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo