Dynamic electroosmotic flow and solute dispersion through a nanochannel filled with an electrolyte surrounded by a layer of a dielectric and immiscible liquid

Abstract

The present article deals with the modulation of oscillatory electroosmotic flow (EOF) and solute dispersion across a nanochannel filled with an electrolyte solution surrounded by a layer of a dielectric liquid. The dielectric permittivity of the liquid layer adjacent to supporting rigid walls is taken to be lower than that of the electrolyte solution. Besides, the aforesaid liquid layer may bear additional mobile charges, e.g., free lipid molecules, charged surfactant molecules etc., which in turn lead to a nonzero charge along the liquid–liquid interface. Such a layer of a dielectric liquid resembles the membrane of various biological cells. An AC voltage is applied to generate the fluid motion. Note that among others, the major advantage of AC voltage is that it can suppress the formation of gas bubbles that are often very detrimental in flow through microdevices. Considering the combined impact of ion partitioning and ion steric effects, we have studied the EOF modulation and its impact on the dispersion of the solute band of given width placed initially at the middle of the channel. The full scale numerical results for flow modulation induced by an AC electric field and its impact on the solute transport are presented considering a wide range of pertinent parameters. It is observed that the molar concentration of additional charge present in the dielectric liquid layer and its thickness, interfacial charge, and concentration of the bulk electrolyte, ion partitioning and ion steric effects, frequency of oscillatory electric field, channel height etc., have a substantial impact on the flow modulation, effective dispersion coefficient as well as broadening of the solute band across the channel. We have further highlighted the impact of the Péclet number on the transport and dispersion of solutes. Along with the numerical results, several benchmark analytical results under various limits are deduced for electrostatic potential, flow velocity and various quantities associated with the dispersion process.