Dipole Alignment of Water Molecules Flowing Through a Carbon Nanotube

Recent years have seen an upsurge of interest in exploring ultrafast transport of water in various nanochannels with potential applications in desalination, separation processes, nanomedicine and energy conversion. However, a quantitative understanding of flow-induced effects in nanochannels is still lacking. We have used molecular dynamics simulations to study pressure-induced flow of water through a (10,10) single-walled carbon nanotube. We find that the dipole moments of water molecules inside the nanotube get aligned by flow, resulting in a net dipole moment in the flow direction. With increasing flow velocity, the net dipole moment first increases and eventually saturates to a constant value. This behavior is qualitatively similar to that in the Langevin theory of paraelectricity with the flow velocity acting as an effective aligning field. We show that the microscopic origin of this behavior is the entry of water molecules inside the nanotube with their dipole vectors preferentially pointing inward along the nanotube axis.