Effects of surface charge density and distribution on the nanochannel electro-osmotic flow

Surface charge density and distribution dependence of a nanochannel electro-osmotic flow was examined using a molecular dynamics (MD) model. Systems consisting of Na and Cl− ions in water confined between crystalline walls with varying negative charge on inner surfaces in an external electric field were investigated. At low surface charge densities, water flows as expected by common interpretations of electro-osmosis. At intermediate surface charge density, the flow exhibits a maximum. Strongly charged surfaces cause adsorption of counterions, immobilization of the near-wall fluid layers, and subsequent flow reversal. An effect of increase in the viscosity of water near the strongly charged surface was demonstrated. When the discrete −1e charge was distributed on a subgrid of surface atoms, the flow deteriorated and reversed at much lower surface charge densities than when all the surface atoms carried equal partial charge.