Electrokinetic flow through porous media composed of fine cylindrical capillaries

Abstract

Electrokinetic flow through fine cylindrical capillaries was investigated experimentally, using dilute solutions of potassium iodide in purified water as the electrolyte. Two types of porous media were studied: a fused array of borosilicate glass capillaries each 2 μm in diameter and 1.91 μm long with a moderatly high zeta potential; and a polycarbonate filter membrane containing nearly uniform circular pores each about 0.1 μm in diameter and 12 μm long, with a relatively low zeta potential. The electrokinetic radius was varied by varying the concentration of potassium iodide in the solution, the conductivity of which was continuously monitored. Streaming potentials were measured using platinum electrodes for the glass disc and silver iodide electrodes for the polycarbonate membrane. An apparent effect of pressure drop on the ratio of streaming potential to pressure drop for the polycarbonate membrane, which could serve as an explanation for the non-Darcy flow behavior reported by others for low liquid velocities through certain porous media, was rationalized as fortuitous by invoking arguments recently presented by Ball and Fuerstenau [7]. The electroviscous retardataion effect was measured for the glass disc by comparing the product of pressure drop and displacement time for a fixed volume of liquid with the corresponding product obtained under conditions of high electrokinetic radius (small double layer thickness). The results for both streaming potential and electroviscous retardation effect are compared with the theory of Rice and Whitehead [1] for low zeta potentials, as well as with the more recent theory of Levine et al. [2] for all zeta potentials. The data support the latter theory more than the former.