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Abstract Water purification membranes comprising aligned, dense arrays of carbon nanotubes (CNTs) have been investigated for more than 10 years. Water transport 2–5 orders of magnitude greater than Hagen-Poiseuille predictions has been observed in CNTs of diameters 0.8–10 nm in a small number of experiments. While the measured flow rates in different experiments substantially disagree with each other, there is a clear opportunity for these membranes to impact filtration technologies. We propose a multiscale computational flow method that combines molecular dynamics (MD) simulations in critical locations of the membrane with a continuum flow resistance model. This provides the flow resistances in a nanotube membrane configuration to enable, for the first time, computationally-efficient macroscopic predictions of flows through laboratory-scale membranes. Our multiscale simulation results of water flow through CNTs are also used to calibrate the Hagen–Poiseuille–Weissberg equation with slip. This study reveals that the slip length, density and viscosity can vary with CNT diameter at sub-2-nm diameters, which would otherwise be challenging to compute using MD alone. Previously published experimental results show either clear agreement or clear disagreement with our multiscale predictions; more work is required to understand this variance for similar flow cases. Graphical abstract fx1 Highlights • A new multiscale technique for water flows in membranes of aligned-nanotubes. • Flow resistances for sub-2-nm carbon nanotubes show non-continuum behaviour. • A modified Hagen-Poiseuille-Weissberg model can be used for lab-scale membranes. • Density, viscosity and slip length are revealed as functions of nanotube diameter. • Some experimental results agree with these predictions; others need reassessment. [ABSTRACT FROM AUTHOR]

The properties of water confined inside nanotubes are of considerable scientific and technological interest. We use molecular dynamics to investigate the structure and average orientation of water flowing within a carbon nanotube. We find that water exhibits biaxial paranematic liquid crystal ordering both within the nanotube and close to its ends. This preferred molecular ordering is enhanced when an axial electric field is applied, affecting the water flow rate through the nanotube. A spatially patterned electric field can minimize nanotube entrance effects and significantly increase the flow rate. [ABSTRACT FROM AUTHOR]

This article compares both new and commonly used boundary conditions for generating pressure-driven water flows through carbon nanotubes in molecular dynamics simulations. Three systems are considered: (1) a finite carbon nanotube membrane with streamwise periodicity and ‘gravity’-type Gaussian forcing, (2) a non-periodic finite carbon nanotube membrane with reservoir pressure control, and (3) an infinite carbon nanotube with periodicity and ‘gravity’-type uniform forcing. Comparison between these focuses on the flow behaviour, in particular the mass flow rate and pressure gradient along the carbon nanotube, as well as the radial distribution of water density inside the carbon nanotube. Similar flow behaviour is observed in both membrane systems, with the level of user input required for such simulations found to be largely dependent on the state controllers selected for use in the reservoirs. While System 1 is simple to implement in common molecular dynamics codes, System 2 is more complicated, and the selection of control parameters is less straightforward. A large pressure difference is required between the water reservoirs in these systems to compensate for large pressure losses sustained at the entrance and exit of the nanotube. Despite a simple set-up and a dramatic increase in computational efficiency, the infinite length carbon nanotube in System 3 does not account for these significant inlet and outlet effects, meaning that a much smaller pressure gradient is required to achieve a specified mass flow rate. The infinite tube set-up also restricts natural flow development along the carbon nanotube due to the explicit control of the fluid. Observation of radial density profiles suggests that this results in over-constraint of the water molecules in the tube. [ABSTRACT FROM PUBLISHER]