Transient electrokinetic transport in micro/nanofluidic systems with sudden expansion and contraction cross sections

Abstract

This study numerically investigates electrokinetic transport in a micro/nanofluidic system by solving the transient Poisson, Nernst-Planck, and Navier-Stokes equations simultaneously. The considered system is a nanochannel connected to two micro channels at its ends. Under various applied electric potential biases, the concentration polarization effect on the fluid flow, induced pressure, and electric current is examined. By comparing with the Donnan equilibrium condition and electroosmotic flow in the microscale dimension, electric body force due to non-zero charge density is the mechanism for producing vortex flow and inducing a positive pressure gradient on the anodic side of the system. The diffusive boundary layer thickness is reduced due to stirring by the generated vortex flow, resulting in over-limiting current when the applied electric potential bias is high. The steady-state current voltage curve indicates that in the Ohmic regime, higher current can be obtained when the surface charge density is large due to higher fluid velocity. In the limiting and over-limiting current regimes, higher electric current can be obtained when the nanochannel is larger with smaller surface density because more ions are available for carrying the current. The nanochannel size effect on the limiting and over-limiting current magnitudes is insignificant when the surface charge density is large.