title: Electrokinetics in undeveloped flows.
abstract: For the correct interpretation of results of tangential electrokinetic measurements with porous materials, in particular, composite/asymmetric membranes on porous supports, it is necessary to have the data available for various channel heights. In some kinds of equipment, the variation of channel height is technically possible only for a range of relatively large heights. This communication shows that under these conditions, the fluid flow can become undeveloped and the conventional approaches to the interpretation of electrokinetic measurements should be modified accordingly. In particular, the dependence of streaming-current coefficient on the channel height becomes sub-linear. If the experimental data are available only for larger channel heights, this can be mistakenly taken for the manifestation of contribution of porous sub-structure to the streaming current. In this communication, we investigate electrokinetic phenomena in undeveloped flows both numerically and experimentally. We confirm that the aforementioned sub-linearity occurs for nonporous as well as porous substrates. We also demonstrate that the channel heights estimated from the volume flow rate by using numerical simulations of undeveloped flows are in very good agreement with the reference values obtained from the electrical conductance (in contrast to the values estimated by using the conventional approach of Hagen-Poiseuille equation). The numerical fitting of channel-height dependences of streaming-current coefficient enables us to separate the contributions of external surface and porous sub-structure (in case of porous substrates) and obtain quite reasonable values of (effective) zeta-potentials in both cases. Nonetheless, the accuracy of experimental data deteriorates with increasing channel height, so it is generally advisable to vary the heights within a range below 100-150 μm.
authors.affiliation: ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain; Department of Chemical Engineering, Polytechnic University of Catalonia, Av. Diagonal 647, 08028 Barcelona, Spain. Electronic address: andriy.yaroshchuk@upc.edu.
authors.forename: Andriy
authors.initials: A
authors.lastname: Yaroshchuk
authors.forename: Edxon Eduardo Licón
authors.initials: EE
authors.lastname: Bernal
authors.forename: Thomas
authors.initials: T
authors.lastname: Luxbacher
keywords: Channel height; Electrokinetic; Porous substrate; Tangential; Undeveloped flow