Asymmetric transport of water molecules through a hydrophobic conical channel

Abstract

Unlike macroscale systems, symmetry breaking could lead to surprising results for nanoscale systems. Although great attention has been paid to the transport properties of water molecules through nanochannels in recent years, most of the existing studies are related to symmetric channels (e.g. cylindrical ones). Herein, we use molecular dynamics simulations to study the transport of water through a hydrophobic conical channel. Surprisingly, without any dynamic load, when the channel becomes more asymmetric (the wide radius increases) net water fluxes along the divergent direction are observed, which should be related to the thermal noise. To further explore the asymmetric properties of the conical channel, we then apply the pressure differences for the convergent and divergent directions, respectively. We find that the convergent flux is changed from smaller to larger than the divergent one with the increase of pressure. However, for the salt solution, the convergent flux is coupled to ions due to the blocking effect at low pressures; while the divergent flux is almost independent of ions. These results demonstrate some new physical insights of the ratchet effect of conical channels, and further a pressure controlled water rectification that could have deep implications for designing highly efficient nanoporous materials for sea water desalination.