Pressure-driven water flow through a carbon nanotube controlled by a lateral electric field

Abstract

Tuning the water flow through nanochannels provides a key to many physicochemical phenomena, such as energy harvesting, desalination, biosensors and so on. An electric field has been proven to be an effective factor to control the water flow, because water molecules form a typical polar liquid that is sensitive to the external field. However, to the best of our knowledge, previous work has mostly focused on electric fields along the channel axis direction, while the effect of a non-axial field is rarely explored. In this work, we use molecular dynamics simulations to study the effect of a lateral electric field on pressure-driven single-file water transport through a (6,6) carbon nanotube (CNT). Of particular interest is that the water flux exhibits a monotonous reduction with an increase in the lateral electric field, suggesting an on–off gating behavior. This is because the lateral electric field disrupts the hydrogen bond connections in a single-file water chain, which ultimately leads to a reduction in water occupancy. Notably, the results strongly depend on the CNT length, where for long CNTs, the single-file water chain can be broken down at small field strengths, resulting in a more drastic reduction in flux and occupancy. This is because a long single-file water chain can be less affected by the outside reservoirs and should be more susceptible to the lateral electric field. The water translocation time, dipole orientation and flipping are also sensitive to the field strength. Furthermore, under high field strength, the increase in pressure difference results in an increase in water occupancy and reformation of the single-file water chain, which ultimately leads to an increase in flux. On the whole, the lateral electric field causes a liquid-to-vapor phase transition for the confined water, while the pressure difference has the opposite function. Our results demonstrate that a lateral electric field strongly affects the dynamics of single-file water chains, and should have great implications in the design of novel gating nanofluidic devices.