Effect of overlapping double layers and pH on electrokinetics in a microchannel

Abstract

Electroosmosis in porous media has a very wide range of applications such as soil remediation, oil recovery, subsurface exploration, drug delivery in tissues. In this research, we focus on solving Poisson-Boltzmann equation in a rectangular channel with different boundary, geometrical and pH conditions in order to capture the effective factors in electroosmotic flow (EOF) in a microchannel. Based on the results, the electroosmotic flow varies significantly with overlapping condition of double layers, represented by a scaling factor, which is the ratio of half of the height of a channel to Debye length, referred to asK ̅. The other factor is the aspect ratio of the channel, which is the ratio of width to the height of the channel. Results show that the effect of thickness of the electric double layer on EOF is crucial, while the aspect ratio was found to have less effect on EOF. The pH effects were also taken into account as the changes in pH in known to have significant effect on the state of EOF and electroosmotic permeability can change by orders of magnitude. The results show a decline in EO permeability as the pH increases, if the K ̅ effect on electroosmotic permeability is still important. As known, the linear Poisson-Boltzmann equation cannot be used for larger zeta potential values as well as constant charge boundary condition. Thus, we showed the effect of ignorance of this nonlinearity for a large zeta potential 60mV. Results show that if a linear Poisson-Boltzmann equation for a large zeta potential condition is used, the electroosmotic permeability will be underestimated and the average electric potential will be overestimated. Moreover, results showed that solving the linear Poisson-Boltzmann equation for constant charge condition would not provide realistic results. The results of this study not only can be used to understand the fundamentals of electroosmosis in porous media but also will be used for upscaling this phenomenon to Darcy scale using a pore network modeling approach.