Molecular and continuum perspectives on intermediate and flow reversal regimes in electroosmotic transport

Abstract

Electroosmotic slip flows in the Debye–Hückel regime were previously investigated using molecular dynamics and continuum transport perspectives ( J. Phys. Chem. C 2018, 122, 9699). This continuing work focuses on distinct electrostatic coupling regimes, where the variations in electroosmotic flows are elucidated based on Poisson–Fermi and Stokes equations and molecular dynamics simulations. In particular, aqueous NaCl solution in silicon nanochannels are considered under realistic electrochemical conditions, exhibiting intermediate flow and flow reversal regimes with increased surface charge density. Electroosmotic flow exhibits plug flow behavior in the bulk region for channel heights as small as 5 nm. With increased surface charge density, constant bulk electroosmotic flow velocity first increases and then it begins to gradually decrease until flow reversal is observed. In order to capture the flow physics and discrete motions within electric double layer accurately, the continuum model includes overscreening and crowding effects as well as slip contribution and local variations of enhanced viscosity. After extraction of the continuum parameters based on molecular dynamics simulations, good agreement between simulation results and continuum predictions are obtained for surface charges as large as −0.37 C/m2.