Non-equilibrium molecular dynamics simulation of electrokinetic effects on heterogeneous ionic transport in nano-channel

Abstract

The presence of the electric double layer (EDL) near the solid/liquid interface has a great impact on the liquid flowing through nano channels. In this paper, a non-equilibrium molecular dynamics method (NEMD) model is developed to investigate the transport characteristics of the heterogeneous ionic fluid flowing in a 4.672 nm-depth channel due to the electrokinetic effect induced by the EDL. An external perturbing velocity is introduced on the liquid instead of a constant force to study the electrokinetic effects on the flow. The effect of charges adsorbed on channel surfaces on flow resistance is especially considered. Two different charged surfaces with discrete or uniform charge distributions are compared, respectively. Besides Lennard-Jones (L-J) potential energy and Coulomb force, the ion–dipole interaction between the charged particle and the water molecule in the liquid is especially taken into account. According to the simulation results, the liquid density reaches the peak value in the EDL. However, unlike that predicted by the Poisson–Boltzmann theory, the liquid density profile is not exactly an anti-parabola curve, which has a significant fall before the density reaching the peak value in the EDL. It is shown that the charges absorbed on the channel surface have little influence on the liquid density distribution, but they play an important role in the fluid velocity distribution. Meanwhile, the liquid velocity slippage in the EDL emerges, which is well in agreement with the prior experimental data available. Moreover, though the non-dimensional velocities are nearly the same under different external velocity fields, the velocity slippage for the uniformly charged wall is much larger than that for the discretely charged wall. All of these results provide a probable reason why the flow behavior in a nano-channel is particularly different from that in a macro duct.