Nonequilibrium molecular dynamics simulation of electro-osmotic flow in a charged nanopore

Abstract

Nonequilibrium molecular dynamics simulations were performed for Poiseuille and electro-osmotic flow in a charged cylindrical nanopore. The goal was to examine any deviations from continuum flow behavior and to compare and contrast the Poiseuille and electro-osmotic flow situations. The fluid was composed of cationic counterions and nonpolar monatomic solvent molecules. The cylindrical surface of the pore wall was represented by a stochastic scattering boundary condition. The lack of any surface roughness and the computational efficiency of the fluid model enabled the velocity profile near the wall to be measured at very high spatial resolution. The simulation results indicate that both Poiseuille and electro-osmotic flow conform to continuum transport theories except in the first monolayer of fluid at the pore wall. The apparent viscosity in this region was highly nonuniform and exhibited singularities. Despite this, the viscosity profiles obtained from Poiseuille and electro-osmotic flow were in good mutual agreement at all locations. The singularities were caused by a local maximum in the solvent and counterion velocity profiles occurring at the edge of the first monolayer of liquid. This apparent channeling of fluid near the pore wall has been observed in previous studies of Poiseuille flow. The exact cause is not clear, but it may be due to cooperative transport of the fluid molecules facilitated by two-dimensional ordering at the wall.