Dispersion in Electro Osmotic Flow Through a Slit Channel With Axial Step Changes of Zeta Potential

Abstract

An analytical study is presented in this paper on hydrodynamic dispersion due to steady electroosmotic flow (EOF) in a slit microchannel with longitudinal step changes of خ¶ potential. The channel wall is periodically patterned with alternating stripes of distinct خ¶ potentials. Existing studies in the literature have considered dispersion in EOF with axial nonuniformity of خ¶ potential only in the limiting case where the length scale for longitudinal variation is much longer than the crosssectional dimension of the channel. Hence, the existing theories on EOF dispersion subject to nonuniform charge distributions are all based on the lubrication approximation, by which crosssectional mixing is ignored. In the present study, the general case where the length of one periodic unit of wall pattern (which involves a step change of خ¶ potential) is comparable with the channel height, as well as the longwave limiting case, are investigated. The problem for the hydrodynamic dispersion coefficient is solved numerically in the general case, and analytically in the longwave lubrication limit. The dispersion coefficient and the plate height are found to have strong, or even nonmonotonic, dependence on the controlling parameters, including the period length of the wall pattern, the area fraction of the EOFsuppressing region, the Debye parameter, the Pأ©clet number, and the ratio of the two خ¶ potentials.