Mass Transport and Separation of Species due to Oscillatory Electroosmotic and Pressure-Driven Flows

The mass transport and solute separation by combined oscillatory electroosmotic and pressure-driven flows are theoretically studied. With the assumptions of dilute solution, non-overlapping of electrical double layers, small Zeta potential, and fully-developed imcompressible flow, the governing equations for the electrical, velocity, and concentration fields are linearized and solved analytically. The dimensionless mass flow rate is then calculated under the condition of constant tidal displacement. The four primary dimensionless parameters are Womersley number, Schmidt number, dimensionless Debye length, and averaged tidal displacement.
The results indicate that the dimensionless mass flow rate due to the oscillatory electroosmotic flow increases with Womersley number, Schmidt number, averaged tidal displacement, and decreases with dimensionless Debye length while the dimensionless mass flow rate due to the oscillatory pressure-driven flow increase only with Womersley number, Schmidt number, averaged tidal displacement, and remains the same with dimensionless Debye length. Moreover, the oscillatory pressure-driven flow possesses better dimensionless mass flow rate than the oscillatory electroosmotic flow for small values of Womersley number. With large values of Womersley number, the situation is reversed.
The mass transport rate varies, in general, with solutes even in the same solvent and with the same oscillation frequency. For low frequency, the solute with large diffusivity possesses large dimensionless mass flow rate. As the oscillation frequency increases, the transport of the solute with larger diffusivity, and thus smaller Schmidt number, becomes mild while if increases rapidly again when frequency keeps increasing, resulting in the so called cross-over phenomenon. Consequently, the solute separation becomes possible.