Accounting for electric double layer and pressure gradient-induced dispersion effects in microfluidic current monitoring

Abstract

Current monitoring (CM) is a common experimental method to determine the zeta potential of a microfluidic channel. Dissimilar solution concentrations and finite electric double layers (EDLs) cause deviations to typical CM curves, diminishing the utility of such a method. Here, we theoretically and experimentally investigate these effects, simulating the full time-dependent Navier–Stokes equation coupled with the Poisson–Boltzmann distribution for small ions and dilute systems and validating with experiments in both micro- and nanochannels. We find that current monitoring in finite EDL nanochannel systems and small concentration ratios can be approximated with a simple function, predicting ζ-potential to within 10 % even when the surface charge is unknown. However, as the concentrations used become more and more dissimilar, dispersion due to induced pressure gradients greatly affects the shapes of the current monitoring curves. We show that typical microchannel and nanochannel experiments can easily span a wide range of dispersion regimes, from pure axial diffusion to pure convective dispersion. We explain the shapes of the curves based on the regime of dispersion and give practical guidelines for CM with high-concentration ratios.