Enhancement of mass transport and separation of species by oscillatory electroosmotic flows

Abstract

Mass transport driven by oscillatory electroosmotic flows (EOF) in a two-dimensional micro-channel is studied theoretically. The results indicate that the velocity and concentration distributions across the channel-width become more and more non-uniform as the Womersley number W , or the oscillation frequency, increases. It is also revealed that, with a constant tidal displacement, the total mass transport rate increases with the Womersley number W due to both the stronger convective and the transverse dispersion effects. The total mass transport rate also increases with the tidal displacement (with a fixed oscillation frequency) because of the associated stronger convective effects. The cross-over phenomenon of the mass transport rates for different species becomes possible with sufficiently large Debye lengths and at sufficiently large values of W . Consequently, with proper choices of the Debye length, oscillation frequency and tidal displacement, oscillatory EOF may become a good candidate for the first-step separation of the mass species.

Dispersion in electroosmotic flow generated by oscillatory electric field interacting with oscillatory wall potentials

“…Therefore, higher the frequency, the smaller the value ofd: An order unity of the oscillation parameterd ¼ Oð1Þ is assumed here. This is consistent with the frequency values used or reported by Huang and Lai (2006), Kuo et al (2008), and Ramon et al (2011). The inverse Debye length or the Debye-Hückel parameterk characterizes the thickness of the EDL.…”

“…Note that our unsteady EOF is subjected to the constraint of an upper frequency limit for the validity of the static Poisson-Boltzmann equation. The theoretical bound of this model is the same as that of, among others, Huang and Lai (2006), Kuo et al (2008), and Ramon et al (2011). These authors have already looked into the time scale for the development of the Debye layer when the flow and/or electric fields are time oscillating.…”

“…These authors have already looked into the time scale for the development of the Debye layer when the flow and/or electric fields are time oscillating. Huang and Lai (2006) and Ramon et al (2011) have remarked that, although EOF is achievable for a wide range of frequencies, it is desirable if the frequency, x/2p, is kept below 1 MHz to avoid EDL relaxation effects. Kuo et al (2008) used the effective capacitance-resistance model to estimate the upper frequency to be of the range 0.3-0.8 MHz, depending on the electrolyte concentration and the dimension of the channel.…”

“…The Schmidt number appearing in the expression forD Tw ; is the ratio of kinematic viscosity to molecular diffusivity. For aqueous solvents, the Schmidt number is of the order 10 3 (McEldoon and Datta 1992;Huang and Lai 2006), which is the value chosen here for the numerical calculations.…”

“…Time periodic EOF is also known as AC EO, and is driven by an alternating electric field which has potential applications in biotechnology and separation science. Huang and Lai (2006) have presented an analytical study of the enhanced mass transfer in an oscillatory EOF, within a parallel-plate microchannel configuration. Mass transfer in time periodic EOF through charged micro/nanochannel was discussed by Bhattacharyya and Nayak (2008).…”