Abstract
A set of dimensionless groups along with constraints on their values are formulated to serve as an indication of the parametric combinations, leading to minimal enhancement of streamwise passive species transport in time-periodic electro-osmotic microchannel flows with asymmetric hydrodynamic slip and zeta potential boundaries. While the slip lengths and zeta potentials are mostly treated as uncoupled in the electrokinetic transport literature, we incorporate into the analysis wall surface charge/potential coupled apparent slip lengths and find that more than one minimal enhancement instances may be achieved during a zeta potential sweep, provided that the uncharged surface slip length ratio is large enough.
Highlights
Transport of uncharged, chemically inert passive mass species in electro-osmotically driven time-periodic oscillatory flows is currently drawing significant attention from the microfluidics community due to its potential yet promising role in the design and development of integrable mixing, dispersion, or transport enhancement devices for micro-total-analysis systems applications.1–14 Enhanced transport of the passive species can be achieved in zero net flow rate steady oscillatory electro-osmosis when a constant species concentration gradient is arranged in parallel to the longitudinal streamwise direction of the flow.4–12 Of the various versatile physics involved, the Taylor–Aris dispersion effect15–22 is likely the key to explaining the underlying mechanism responsible for this novel phenomenon
Eqs. (2) and (3) along with the results presented in Figs. 1–3 are found to be consistent with the minimized effective dispersion coefficients obtained by Ng21 for steady pressure driven flows with uncharged, asymmetric slippery channel walls, and those by Medina et al.3 for pulsatile electro-osmosis subject to no-slip but asymmetric wall zeta potentials
These results suggest that surface charge or potential coupled apparent hydrodynamic slip lengths may likely play a crucial role in the dispersion and transport of passive mass species in time-periodic electro-osmosis or related electrokinetic flows
Summary
Chemically inert passive mass species in electro-osmotically driven time-periodic oscillatory flows is currently drawing significant attention from the microfluidics community due to its potential yet promising role in the design and development of integrable mixing, dispersion, or transport enhancement devices for micro-total-analysis systems (μTAS) applications. Enhanced transport of the passive species can be achieved in zero net flow rate steady oscillatory electro-osmosis when a constant species concentration gradient is arranged in parallel to the longitudinal streamwise direction of the flow. Of the various versatile physics involved, the Taylor–Aris dispersion effect is likely the key to explaining the underlying mechanism responsible for this novel phenomenon. The results presented are likely relevant to the minimal dispersion required in microfluidic separation techniques and to the preferred enhanced dispersion in microfluidic mixing and transport applications. can be deduced from the characteristics of the plug velocity profile They are as follows: the velocity distribution is (almost) uniform throughout the microchannel width (h), the Debye thickness (1/κ) is much smaller than the microchannel width (i.e., κh ≫ 1), and the viscous momentum diffusional time scale (tv = h2/ν, with ν = 10−6 m2/s being the electrolyte kinematic viscosity) should be much less than the oscillation period (tp = 2π/ω, with ω being the angular frequency) of the flow. Notice that the characteristic HS velocity including the hydrodynamic slip lengths can be found in the works
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