Multiscale simulation of water flow through laboratory-scale nanotube membranes

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.