Microfluidic technology, with its miniaturization, high-throughput, and low sample consumption characteristics, has become a core technology in the fields of chemical sensing, targeted drug delivery, and biomolecular separation. Electroosmotic flow, as a key driving mechanism in microfluidics, can effectively enhance mass transport and separation effciency by modulating the flow field structure through periodic oscillation. Building on this, the present study reveals the intrinsic coupling mechanisms among oscillating electric fields, flow field structures, and solute transport. Moreover, through parameter regulation, it enables the active design and optimization of mixing, reaction, and separation processes in microfluidic chips.
In this study, the flow characteristics of the periodically oscillating electroosmotic flow and the resulting mass transport and separation mechanisms are investigated for a class of micro-nano fluids in rectangular microchannels under high Zeta potential conditions. The finite difference method and numerical integration are used to calculate the electric double layer potential, velocity field, concentration field, and the spatiotemporal average mass transport rate, respectively. The effects of relevant parameters, such as the wall Zeta potential, Debye length, Womersley number, and Schmidt number are analyzed on both the fluid flow characteristics and the resulting mass transport and separation mechanisms. The results show that: (1) the velocity profile is significantly regulated by the Womersley number, when the Womersley number is small, the flow exhibits a quasi-steady plug-like profile with uniform velocity distribution in the channel center; as the Womersley number increases, inertial effects dominate, leading to phase lag, shear layers, and localized flow reversal in the velocity distribution; (2) high wall Zeta potential enhances the electroosmotic driving force, but maintaining a fixed tidal displacement results in a decrease in the Peclet number, thereby attenuating the convective effect; (3) the analysis for the concentration field reveals that mass transport is governed by the convectiondiffusion balance, with concentration gradients highly concentrated near the walls and the center concentration gradients approaching zero; a smaller Debye length results in a more localized electroosmotic forcing near the walls, leading to sharper concentration gradients; a larger Debye length produces smoother concentration gradients; under asymmetric wall Zeta potential, the concentration distribution exhibits spatial asymmetry, with a steeper gradient on the side of higher Zeta potential; (4) quantification of spatiotemporal average mass transport rates shows that a larger tidal displacement amplifies convective contributions, further increasing the mass transport rate; an asymmetric Zeta potential configuration enhances mass transport by inducing flow asymmetry; the transport rate increases with the Womersley number, and the species with a larger Schmidt number (smaller diffusion coeffcient) exhibit higher transport rates, and a crossover phenomenon is observed; this indicates that at specific frequencies, the transport rate curves of different diffusive species intersect, thus enabling the possibility of species separation.