Friction versus flow enhancement in nanotube structures with heterojunctions

Abstract

Water channels have important applications to many problems, including new purification systems, drug delivery, and many others. Depending on the application, such channels must have two key characteristics in order to be effective. One is that their structures must be more complex than straight tubes, while their second characteristic should be preserving enhancement of volume flow rate of water over what the classical hydrodynamics predicts for straight channels, and has been reported for straight nanotubes. We use both equilibrium and non-equilibrium molecular dynamics simulation to explore the effect of nanojunctions on the rate of water transport in a nanostructured system that consists of connected carbon nanotubes (CNTs) with various radii. This type of nanotube has been fabricated in the past and is of practical importance. We show that the wall-water friction coefficient increases dramatically with the roughness, and can be orders of magnitude larger than the corresponding values in smooth CNTs. Despite the increased friction, however, flow of water in such relatively complex nanochannels is still enhanced significantly over what one obtains in ordinary tubes and channels. Our analysis indicates that, similar to straight CNTs, the entrance/exit dissipation in the system suppresses the effect of the wall-water friction in nanotubes connected by nanojunctions, hence allowing enhancement of water flow