The electroviscous effect on time-periodic pressure-driven flow in a pH-regulated slit nanochannel

Abstract

The surface charge plays a crucial role in electrokinetic flow within micro/nanochannels. The charge on silica surfaces originates from the protonation and deprotonation of the silanol functional groups. Instead of being set at a constant value, the surface charge density in this paper depends on the solution pH and ion concentration. Analytical and semi-analytical solutions for the electric potential, velocity, and flow rate of time-periodic pressure-driven flow in a pH-regulated slit nanochannel have been derived. The results show that the solution pH and background salt concentration significantly influence the electric potential. At high pH values and low background salt concentrations, there is a notable reduction in velocity amplitude. The electroviscous effect increases with the distance from the pH value to the isoelectric point and decreases with increasing the background salt concentration. The electroviscous effect is more pronounced in channels with smaller heights. The thinner the electric double layer is, the smaller the electroviscous effect will be. Generally, the pressure frequency does not significantly influence the electroviscous effect because the oscillating Reynolds number is much less than unity in nanochannels.