Control of water and ion transport through nanochannels is of primary importance for the design of novel nanofluidic devices. In this work, we use molecular dynamics simulations to systematically analyze the coupling transport of water and ions through a carbon nanotube in electric fields. We focus on the role of ionic conditions, including the salt species and concentration, which can significantly regulate the ion and further the water transport. We find that the coupling of water–anions is stronger than water–cations, and thus anions play a dominant role in determining the water transport. Specifically, the water and ion flux both exhibit a linear increase with the field strength, in agreement with recent experimental observations; while the water and ion translocation time show a linear and power law decrease, respectively, yielding to the Langevin predictions. These results strongly depend on the salt species, demonstrated by the ion binding. More surprisingly, with the increase of salt concentration, the anion flux displays a maximum behavior, inducing a similar maximum for water flux; while the cation flux increases almost linearly. These unordinary ion flux behaviors should be due to the channel confinement, since a wider channel exhibits similar behaviors with the previous simulation and experimental work. Detailed discussion based on Poisson-Nernst-Plank equation are further presented for ion transport. Our results reveal deep insights into the coupling transport of water and ions, especially the important role of ionic condition, and are helpful for the design of desalination, ion separation and high flux nanofluidic devices.

Two-dimensional granular flow in a channel with small exit is studied by both experiments and simulations. We first observe the time variation of the transition from dilute flow state to dense flow state by both experiments and simulations. Then we obtain a relationship between the local flow rate and the local packing fraction in the choke area by use of molecular dynamics simulations. The relationship is a continuous function rather than a discontinuous one. The flow rate has a maximum at a moderate packing fraction and the packing fraction is terminated at high value with negative slope. According to the relationship, four flow states--i.e., stable dilute flow state, metastable dilute flow state, unstable dense flow state, and stable dense flow state--are defined for fixed inflow rate. The discontinuities and the complex time variation behavior occurring in the transition between dilute and dense flow states can be attributed to the abrupt variation through unstable flow state.

Water and ion transport through graphene nanochannels has attracted considerable attention thanks to the possibility of dimensional control of the channel sizes down to a single atomic layer. Using molecular dynamics simulations, we systematically analyzed the coupled transport of water and ions in the solutions of LiCl, NaCl, and KCl salts as a function of channel sizes, applied electric fields, and salt concentrations. A universal order of ion flux is found with K + > Cl − > Na + ≈ Li + , and the K + flux is twice as large as those of Na + and Li + , indicating the ion selectivity with such graphene channels. The local structures and transport dynamics within the channels show sensitive dependence on the channel height, forming two-dimensional hydration shells in the low-height limit. The hydration shells of the ions undergo transformation from three-to two-dimensional structures upon entering the narrow channel. A power law relationship between the ion translocation time and electric field is also found and can be well described by the one-dimensional Langevin equation. In addition, the linear relation between the ion flux and concentration agrees well with the one-dimensional Poisson−Nernst−Planck equation. Our results provide insights into the ionic transport through graphene channels and have implications for the design of novel nanofluidic devices for selective ion transport in future applications.

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