Fluidity and phase transitions of water in hydrophobic and hydrophilic nanotubes

Abstract

We put water flow under scrutiny to report radial distributions of water viscosity within hydrophobic and hydrophilic nanotubes as functions of the water-nanotube interactions ({{\epsilon }}_{sf}), surface wettability (θ), and nanotube size (R) using a proposed hybrid continuum-molecular mechanics. Based on the computed viscosity data, {{\rm{\epsilon }}}_{{\rm{sf}}}/{\rm{\theta }}-{\rm{R}} phase diagram of the phase transitions of confined water in nanotubes is developed. It is revealed that water exhibits different multiphase structures, and the formation of one of these structures depends on {{\rm{\epsilon }}}_{{\rm{sf}}},\,{\rm{\theta }}, R parameters. A drag of water flow at the first water layer is revealed, which is conjugate to sharp increase in the viscosity and formation of an ice phase under severe confinement (R ≤ 3.5 nm) and strong water-nanotube interaction conditions. A vapor/vapor-liquid phase is observed at hydrophobic and hydrophilic interfaces. A state of confinement is revealed at which water exhibits different multiphase structures under the same flow rate. The derived viscosity functions are used to accurately determine factors of flow enhancement/inhibition of confined water.