No abstract

The transport of water through carbon nanotubes (CNTs) is now of great importance in bionanotechnology and of considerable interest for potential nanofluidic applications. In this paper, we show by molecular dynamics simulations that the permeation of single-file water molecules through a CNT can be significantly improved by means of tuning the direction of pressure difference, i.e. introducing an additional lateral pressure to the longitudinal one. The water flow exhibits an interesting maximum behavior with the change of lateral pressure, deciphered by the breakdown of single-file water chain inside the CNT. The translocation time decreases monotonously with the increase of lateral pressure and exhibits a clear bifurcation due to the longitudinal pressure, corresponding to the flow enhancement. Therefore, the lateral pressure will increase the difficulty for water entering, while promotes the water conduction inside the CNT, whose competition ultimately leads to the flow maximum behaviors. Along with the water reducing inside the CNT, the CNT switches between the filling and empty states with the unique distributions of water dipole orientation, density and H-bond number. Our results indicate that tuning the direction of pressure difference should be a significant new strategy for enhancing the water permeability, where the key lies in the breakdown of single-file water chain and are thus insightful for future studies.

Energy-saving water pump and efficient semipermeable membranes are the cores of reverse osmosis technology. Applying nanotechnology to improve the performance is a fashion in recent years. Based on the competitive effect of water’s spontaneous infiltration of two sides of a carbon nanotube, we design a water pump that makes use of the natural permeability by weakening one side’s competitiveness based on a small graphite sheet laying on the membrane. According to molecular dynamic simulations, continues net flux is observed. The motion mode of the sheet is the key for the performance. For the pure Brownian motion without any dynamical load, we find two water molecules per nanosecond flux, while the flux induced by the unidirectional motion can be several times enhanced, depending on the external force. The Brownian motion is similar to the physical mechanism of osmotic pressure, and the unidirectional motion shows great performance that has huge applications for reverse osmosis. Our work creatively proposes a new strategy to pump water molecules crossing though a nanochannel, inspiring for nanofluidic device designers.