Electroviscous Effects in Stationary Solid Phase Suspensions

Flowing through porous media is a matter of interest in different research fields such as medicine, engineering and science. The spontaneous appearance of ionic distribution at the solid liquid interface gives place to a reduction in the flow rate, which is generally named electroviscous effect. However, this should be differentiated in two more specific effects, the primary effect due to the distortion of ionic clouds, and the secondary effect due to the overlapping of ionic clouds. Theoretical and experimental works have not always been clearly conducted in order to separate both effects. Instead, they have been globally grouped. The purpose of this review is to revise theoretical and experimental bibliography on the electroviscous effect in stationary solid phase suspensions (porous plugs, membranes, microchannels, capillaries). The main conclusions of this brief revision are: (i) when ionic clouds are relatively small, it is possible to accept that only the primary effect is the cause for the apparent increase of the viscosity of the liquid phase when it is forced to flow relative to the stationary solid phase; (ii) although theory predicts a maximum for the variation of the overall electroviscous effect vs the relative size of the ionic cloud, it has been experimentally observed but not properly reasoned that its existence depends on the salt type; and (iii) it is necessary to justify why, if the fluid is non-Newtonian, electrokinetic parameters dominate the characteristics of the flow due to high pressure gradients, but the rheological parameters are more decisive when the flow is generated by low pressure gradients.

Analysis of streaming potential flow and electroviscous effect in a shear-driven charged slit microchannel

Investigating the flow behavior in microfluidic systems has become of interest due to the need for precise control of the mass and momentum transport in microfluidic devices. In multilayered-flows, precise control of the flow behavior requires a more thorough understanding as it depends on multiple parameters. The following paper proposes a microfluidic system consisting of an aqueous solution between a moving plate and a stationary wall, where the moving plate mimics a charged oil–water interface. Analytical expressions are derived by solving the nonlinear Poisson–Boltzmann equation along with the simplified Navier–Stokes equation to describe the electrokinetic effects on the shear-driven flow of the aqueous electrolyte solution. The Debye–Huckel approximation is not employed in the derivation extending its compatibility to high interfacial zeta potential. Additionally, a numerical model is developed to predict the streaming potential flow created due to the shear-driven motion of the charged upper wall along with its associated electric double layer effect. The model utilizes the extended Nernst–Planck equations instead of the linearized Poisson–Boltzmann equation to accurately predict the axial variation in ion concentration along the microchannel. Results show that the interfacial zeta potential of the moving interface greatly impacts the velocity profile of the flow and can reverse its overall direction. The numerical results are validated by the analytical expressions, where both models predicted that flow could reverse its overall direction when the interfacial zeta potential of the oil–water is above a certain threshold value. Finally, this paper describes the electroviscous effect as well as the transient development of electrokinetic effects within the microchannel.

Direct Measurements of Electroviscous Phenomena in Nafion Membranes

Investigation of electroviscous effects is of interest to technologies that exploit transport of ions through ion exchange membranes, charged capillaries, and porous media. When ions move through such media due to a hydrostatic pressure difference, they interact with the fixed charges, leading to an increased hydraulic resistance. Experimentally this is observed as an apparent increase in the viscosity of the solution. Electroviscous effects are present in all electrochemical membrane-based processes ranging from nanofiltration to fuel-cells and redox flow batteries. Direct measurements of electroviscous effects varying the applied ionic current through Nafion membranes have, to the best of the authors’ knowledge, not yet been reported in literature. In the current study, electroviscous phenomena in different Nafion ion exchange membranes are measured directly with a method where the volume permeation is measured under constant trans-membrane pressure difference while varying the ion current density in the membrane. The direct measurement of the electroviscous effect is compared to the one calculated from the phenomenological transport equations and measured transport coefficients. Within the experimental uncertainty, there is a good agreement between the two values for all membranes tested. We report here an electroviscous effect for all Nafion membranes tested to be κH?κH−1=1.15−0.052+0.035.

Electroviscous Effect of Power Law Fluids in a Slit Microchannel With Asymmetric Wall Zeta Potentials

ABSTRACTThis work analyzes the pressure driven flow of a power law fluid in a slit microchannel of asymmetric walls with electroviscous effects. The steady state Cauchy momentum and the Poisson-Boltzmann equation are solved for the velocity and the potential distribution inside the microchannel. The Debye-Huckel approximation as applicable for low zeta potentials is not made in the present work. The unknown streaming potential is solved by casting the governing equations as an optimization problem using COMSOL Multiphysics. This proposed method is very robust and can be used for a wide variety of cases. It is found that the asymmetry of the zeta potential at the two walls plays an important role on the streaming potential developed. There is a unique zeta potential ratio at which the streaming potential exhibits a maxima for both Debye-Huckel parameter and the power law index. Shear thinning fluids exhibit a stronger dependency of the streaming potential on asymmetry of the zeta potential than shear thickening fluids. For Newtonian fluids narrow slit microchannels develop larger streaming potentials compared to wider microchannels for a given asymmetry.